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• Is there a way to split a widescreen monitor in to two or more virtual monitors?

  - by Mike Thompson

  Like most developers I have grown to love dual monitors. I won't go into all the reasons for their goodness; just take it as a given. However, they are not perfect. You can never seem to line them up "just right". You always end up with the monitors at slight funny angles. And of course the bezel always gets in the way. And this is with identical monitors. The problem is much worse with different monitors -- VMWare's multi monitor feature won't even work with monitors of differnt resolutions. When you use multiple monnitors, one of them becomes your primary monitor of focus. Your focus may flip from one monitor to the other, but at any point in time you are usually focusing on only one monitor. There are exceptions to this (WinDiff, Excel), but this is generally the case. I suggest that having a single large monitor with all the benefits of multiple smaller monitors would be a better solution. Wide screen monitors are fantastic, but it is hard to use all the space efficiently. If you are writing code you are generally working on the left-hand side of the window. If you maximize an editor on a wide-screen monitor the right-hand side of the window will be a sea of white. Programs like WinSplit Revolution will help to organise your windows, but this is really just addressing the symptom, not the problem. Even with WinSplit Revolution, when you maximise a window it will take up the whole screen. You can't lock a window into a specific section of the screen. This is where virtual monitors comes in. What would be really nice is a video driver that sits on top of the existing driver, but allows a single monitor to be virtualised into multiple monitors. Control Panel would see your single physical monitor as two or more virtual monitors. The software could even support a virtual bezel to emphasise what is happening, or you could opt for seamless mode. Programs like WinSplit Revolution and UltraMon would still work. This virtual video driver would allow you to slice & dice your physical monitor into as many virtual monitors as you want. Does anybody know if such software exists? If not, are there any budding Windows display driver guru's out there willing to take up the challenge? I am not after the myriad of virtual desktop/window manager programs that are available. I get frustrated with these programs. They seem good at first but they usually have some strange behaviour and don't work well with other programs (such as WinSplit Revolution). I want the real thing!

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• When to call glEnable(GL_FRAMEBUFFER_SRGB)?

  - by Steven Lu

  I have a rendering system where I draw to an FBO with a multisampled renderbuffer, then blit it to another FBO with a texture in order to resolve the samples in order to read off the texture to perform post-processing shading while drawing to the backbuffer (FBO index 0). Now I'd like to get some correct sRGB output... The problem is the behavior of the program is rather inconsistent between when I run it on OS X and Windows and this also changes depending on the machine: On Windows with the Intel HD 3000 it will not apply the sRGB nonlinearity but on my other machine with a Nvidia GTX 670 it does. On the Intel HD 3000 in OS X it will also apply it. So this probably means that I'm not setting my GL_FRAMEBUFFER_SRGB enable state at the right points in the program. However I can't seem to find any tutorials that actually tell me when I ought to enable it, they only ever mention that it's dead easy and comes at no performance cost. I am currently not loading in any textures so I haven't had a need to deal with linearizing their colors yet. To force the program to not simply spit back out the linear color values, what I have tried is simply comment out my glDisable(GL_FRAMEBUFFER_SRGB) line, which effectively means this setting is enabled for the entire pipeline, and I actually redundantly force it back on every frame. I don't know if this is correct or not. It certainly does apply a nonlinearization to the colors but I can't tell if this is getting applied twice (which would be bad). It could apply the gamma as I render to my first FBO. It could do it when I blit the first FBO to the second FBO. Why not? I've gone so far as to take screen shots of my final frame and compare raw pixel color values to the colors I set them to in the program: I set the input color to RGB(1,2,3) and the output is RGB(13,22,28). That seems like quite a lot of color compression at the low end and leads me to question if the gamma is getting applied multiple times. I have just now gone through the sRGB equation and I can verify that the conversion seems to be only applied once as linear 1/255, 2/255, and 3/255 do indeed map to sRGB 13/255, 22/255, and 28/255 using the equation 1.055*C^(1/2.4)+0.055. Given that the expansion is so large for these low color values it really should be obvious if the sRGB color transform is getting applied more than once. So, I still haven't determined what the right thing to do is. does glEnable(GL_FRAMEBUFFER_SRGB) only apply to the final framebuffer values, in which case I can just set this during my GL init routine and forget about it hereafter?

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• How to draw a filled envelop like a cone on OpenGL (using GLUT)?

  - by ashishsony

  Hi, I am relatively new to OpenGL programming...currently involved in a project that uses freeglut for opengl rendering... I need to draw an envelop looking like a cone (2D) that has to be filled with some color and some transparency applied. Is the freeglut toolkit equipped with such an inbuilt functionality to draw filled geometries(or some trick)?? or is there some other api that has an inbuilt support for filled up geometries.. Thanks. Best Regards. Edit1: just to clarify the 2D cone thing... the envelop is the graphical interpretation of the coverage area of an aircraft during interception(of an enemy aircraft)...that resembles a sector of a circle..i should have mentioned sector instead.. and glutSolidCone doesnot help me as i want to draw a filled sector of a circle...which i have already done...what remains to do is to fill it with some color... how to fill geometries with color in opengl?? Thanks. Edit2: Ok thanks for replying...all the answers posted to this questions can work for my problem in a way.. But i would definitely would want to know a way how to fill a geometry with some color. Say if i want to draw an envelop which is a parabola...in that case there would be no default glut function to actually draw a filled parabola(or is there any??).. So to generalise this question...how to draw a custom geometry in some solid color?? Thanks. Edit3: The answer that mstrobl posted works for GL_TRIANGLES but for such a code: glBegin(GL_LINE_STRIP); glColor3f(0.0, 0.0, 1.0); glVertex3f(0.0, 0.0, 0.0); glColor3f(0.0, 0.0, 1.0); glVertex3f(200.0, 0.0, 0.0); glColor3f(0.0, 0.0, 1.0); glVertex3f(200.0, 200.0, 0.0); glColor3f(0.0, 0.0, 1.0); glVertex3f(0.0, 200.0, 0.0); glColor3f(0.0, 0.0, 1.0); glVertex3f(0.0, 0.0, 0.0); glEnd(); which draws a square...only a wired square is drawn...i need to fill it with blue color. anyway to do it? if i put some drawing commands for a closed curve..like a pie..and i need to fill it with a color is there a way to make it possible... i dont know how its possible for GL_TRIANGLES... but how to do it for any closed curve?? Thanks.

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• GLKit Memory Leak copywithZone

  - by TommyT39

  Running the instruments utility against the game I'm writing shows a bunch of memory leaks related to copy with Zone when I cycle through an array and draw some simple cube objects. Im not sure the best way to track this down as I'm new to OpenGL programming. My program is using ARC and is set to build for IOS 5. I am initializing GLKit to use OPenGl 2.0 and using the BafeEffect so I don't have to write my own shaders etc.. This shouldn't be rocket science. Im guessing that I must be not releasing something within the draw function. Below is the code to my draw function. Could you guys take a look and see if anything stands out as the problem? One other thing to note is that I'm using 15 different textures, the cubes can be 1 of 15 different ones. I have a property set on the cube class for the texture and I set it as I create the cube in there array. But I do load all 15 when my programs view did load starts.They are small .jps files that are less than 75k each and each cube uses the same texture all the way around so shouldn't be too big of an issue. Here is the code to my draw function: - (void)draw { GLKMatrix4 xRotationMatrix = GLKMatrix4MakeXRotation(rotation.x); GLKMatrix4 yRotationMatrix = GLKMatrix4MakeYRotation(rotation.y); GLKMatrix4 zRotationMatrix = GLKMatrix4MakeZRotation(rotation.z); GLKMatrix4 scaleMatrix = GLKMatrix4MakeScale(scale.x, scale.y, scale.z); GLKMatrix4 translateMatrix = GLKMatrix4MakeTranslation(position.x, position.y, position.z); GLKMatrix4 modelMatrix = GLKMatrix4Multiply(translateMatrix,GLKMatrix4Multiply(scaleMatrix,GLKMatrix4Multiply(zRotationMatrix, GLKMatrix4Multiply(yRotationMatrix, xRotationMatrix)))); GLKMatrix4 viewMatrix = GLKMatrix4MakeLookAt(0, 0, 1, 0, 0, -5, 0, 1, 0); effect.transform.modelviewMatrix = GLKMatrix4Multiply(viewMatrix, modelMatrix); effect.transform.projectionMatrix = GLKMatrix4MakePerspective(0.125*M_TAU, 1.0, 2, 0); effect.texture2d0.name = wallTexture.name; [effect prepareToDraw]; glEnable(GL_DEPTH_TEST); glEnable(GL_CULL_FACE); glEnableVertexAttribArray(GLKVertexAttribPosition); glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 0, triangleVertices); glEnableVertexAttribArray(GLKVertexAttribTexCoord0); glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, 0, textureCoordinates); glDrawArrays(GL_TRIANGLES, 0, 18); glDisableVertexAttribArray(GLKVertexAttribPosition); glDisableVertexAttribArray(GLKVertexAttribTexCoord0); }

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• Multi-threaded Pooled Allocators

  - by Darren Engwirda

  I'm having some issues using pooled memory allocators for std::list objects in a multi-threaded application. The part of the code I'm concerned with runs each thread function in isolation (i.e. there is no communication or synchronization between threads) and therefore I'd like to setup separate memory pools for each thread, where each pool is not thread-safe (and hence fast). I've tried using a shared thread-safe singleton memory pool and found the performance to be poor, as expected. This is a heavily simplified version of the type of thing I'm trying to do. A lot has been included in a pseudo-code kind of way, sorry if it's confusing. /* The thread functor - one instance of MAKE_QUADTREE created for each thread */ class make_quadtree { private: /* A non-thread-safe memory pool for int linked list items, let's say that it's * something along the lines of BOOST::OBJECT_POOL */ pooled_allocator<int> item_pool; /* The problem! - a local class that would be constructed within each std::list as the * allocator but really just delegates to ITEM_POOL */ class local_alloc { public : //!! I understand that I can't access ITEM_POOL from within a nested class like //!! this, that's really my question - can I get something along these lines to //!! work?? pointer allocate (size_t n) { return ( item_pool.allocate(n) ); } }; public : make_quadtree (): item_pool() // only construct 1 instance of ITEM_POOL per // MAKE_QUADTREE object { /* The kind of data structures - vectors of linked lists * The idea is that all of the linked lists should share a local pooled allocator */ std::vector<std::list<int, local_alloc>> lists; /* The actual operations - too complicated to show, but in general: * * - The vector LISTS is grown as a quadtree is built, it's size is the number of * quadtree "boxes" * * - Each element of LISTS (each linked list) represents the ID's of items * contained within each quadtree box (say they're xy points), as the quadtree * is grown a lot of ID pop/push-ing between lists occurs, hence the memory pool * is important for performance */ } }; So really my problem is that I'd like to have one memory pool instance per thread functor instance, but within each thread functor share the pool between multiple std::list objects.

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• Algorithm to split an article without breaking the reading flow or HTML code

  - by Victor Stanciu

  Hello, I have a very large database of articles, of varying lengths. The articles have HTML elements in them. I have to insert some ads (simple <script> elements) in the body of each article when it is displayed (I know, I hate ads that interrupt my reading too). Now, the problem is that each ad must be inserted at about the same position in each article. The simplest solution is to simply split the article on a fixed number of characters (without breaking words), and insert the ad code. This, however, runs the risk of inserting the ad in the middle of a HTML tag. I could go the regex way, but I was thinking about the following solution, using JS: Establish a character count threshold. For example, "the add should be inserted at about 200 words" Set accepted deviations in each direction, say -20, +20 characters. Loop through each text node inside the article, and while doing so, keep count of the total number of characters so far Once the count exceeds the threshold, make the following decision: 4.1. If count exceeds the threshold by a value lower that the positive accepted deviation (for example, 17 characters), insert the ad code just after the current text node. 4.2. If the count is greater than the sum of the threshold and the deviation, roll back to the previous text node, and make the same decision, only this time use the previous count and check if it's lower than the difference between the threshold and the deviation, and if not, insert the ad between the current node and the previous one. 4.3. If the 4.1 and 4.2 fail (which means that the previous node reached a too low character count and the current node a too high one), insert the ad after whatever character count is needed inside the current element. I know it's convoluted, but it's the first thing out of my mind and it has the advantage that, by trying to insert the ad between text nodes, perhaps it will not break the flow of the article as bad as it would if I would just stick it in (like the final 4.3 case) Here is some pseudo-code I put together, I don't trust my english-explaining skills: threshold = 200 deviation = 20 current_count = 0 for each node in article_nodes { previous_count = current_count current_count = current_count + node.length if current_count < threshold { continue // next interation } if current_count > threshold + deviation { if previous_count < threshdold - deviation { // insert ad in current node } else { // insert ad between the current and previous nodes } } else { // insert ad after the current node } break; } Am I over-complicating stuff, or am I missing a simpler, more elegant solution?

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• How to cache queries in EJB and return result efficient (performance POV)

  - by Maxym

  I use JBoss EJB 3.0 implementation (JBoss 4.2.3 server) At the beginning I created native query all the time using construction like Query query = entityManager.createNativeQuery("select * from _table_"); Of couse it is not that efficient, I performed some tests and found out that it really takes a lot of time... Then I found a better way to deal with it, to use annotation to define native queries: @NamedNativeQuery( name = "fetchData", value = "select * from _table_", resultClass=Entity.class ) and then just use it Query query = entityManager.createNamedQuery("fetchData"); the performance of code line above is two times better than where I started from, but still not that good as I expected... then I found that I can switch to Hibernate annotation for NamedNativeQuery (anyway, JBoss's implementation of EJB is based on Hibernate), and add one more thing: @NamedNativeQuery( name = "fetchData2", value = "select * from _table_", resultClass=Entity.class, readOnly=true) readOnly - marks whether the results are fetched in read-only mode or not. It sounds good, because at least in this case of mine I don't need to update data, I wanna just fetch it for report. When I started server to measure performance I noticed that query without readOnly=true (by default it is false) returns result with each iteration better and better, and at the same time another one (fetchData2) works like "stable" and with time difference between them is shorter and shorter, and after 5 iterations speed of both was almost the same... The questions are: 1) is there any other way to speed query using up? Seems that named queries should be prepared once, but I can't say it... In fact if to create query once and then just use it it would be better from performance point of view, but it is problematic to cache this object, because after creating query I can set parameters (when I use ":variable" in query), and it changes query object (isn't it?). well, is here any way to cache them? Or named query is the best option I can use? 2) any other approaches how to make results retrieveng faster. I mean, for instance I don't need those Entities to be attached, I won't update them, all I need is just fetch collection of data. Maybe readOnly is the only available way, so I can't speed it up, but who knows :) P.S. I don't ask about DB performance, all I need now is how not to create query all the time, so use it efficient, and to "allow" EJB to do less job with the same result concerning data returning.

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• WPF: Improving Performance for Running on Older PCs

  - by Phil Sandler

  So, I'm building a WPF app and did a test deployment today, and found that it performed pretty poorly. I was surprised, as we are really not doing much in the way of visual effects or animations. I deployed on two machines: the fastest and the slowest that will need to run the application (the slowest PC has an Intel Celeron 1.80GHz with 2GB RAM). The application ran pretty well on the faster machine, but was choppy on the slower machine. And when I say "choppy", I mean the cursor jumped even just passing it over any open window of the app that had focus. I opened the Task Manager Performance window, and could see that the CPU usage jumped whenever the app had focus and the cursor was moving over it. If I gave focus to another (e.g. Excel), the CPU usage went back down after a second. This happened on both machines, but the choppiness was only noticeable on the slower machine. I had very limited time to tinker on the deployment machines, so didn't do a lot of detailed testing. The app runs fine on my development machine, but I also see the CPU spiking up to 10% there, just running the cursor over the window. I downloaded the WPF performance tool from MS and have been tinkering with it (on my dev machine). The docs say this about the "Frame Rate" metric in the Perforator tool: For applications without animation, this value should be near 0. The app is not doing any heavy animation, but the frame rate stays near 50 when the cursor is over any window. The screens I tested on have column headers in a grid that "highlight" and buttons that change color and appearance when scrolled over. Even moving the mouse on blank areas of the windows cause the same Frame rate and CPU usage (doesn't seem to be related to these minor animations). (Also, I am unable to figure out how to get anything but the two default tools--Perforator and Visual Profiler--installed into the WPF performance tool. That is probably a separate question). I also have Redgate's profiling tool, but I'm not sure if that can shed any light on rendering performance. So, I realize this is not an easy thing to troubleshoot without specifics or sample code (which I can't post). My questions are: What are some general things to look for (or avoid) in the code to improve performance? What steps can I take using the WPF performance tool to narrow down the problem? Is the PC spec listed above (Intel Celeron 1.80GHz with 2GB RAM) too slow to be running even vanilla WPF applications?

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• How to use Mozilla ActiveX Control without registry

  - by Andrew McKinlay

  I've been using the IE Browser component that is part of Windows. But I'm running into problems with security settings. For example, users get security warnings on pages with Javascript. So I'm looking at using the Mozilla ActiveX control instead. It's especially nice because it has a compatible interface. It works well if I let it install the control in the registry. But my users don't always have administrator rights to install things in the registry. So I'm trying to figure out how to use the control without registry changes. I'm using DllGetClassObject to get the class factory (IID_ICLASSFACTORY) and then CoRegisterClassObject to register it. All the API calls appear to succeed. And when I create an AtlAxWin window with the CLSID, it also appears to work. But when I try to call Navigate on the AtlAxGetControl it doesn't work - the interface doesn't have Navigate. I would show the code but it's in an obscure language (Suneido) so it wouldn't mean much. An example in C or C++ would be easy for me to translate. Or an example in another dynamic language like Python or Ruby might be helpful. Obviously I'm doing something wrong. Maybe I'm passing the wrong thing to CoRegisterClassObject? The MSDN documentation isn't very clear on what to pass and I haven't found any good examples. Or if there is another approach, I'm ok with that too. Note: I'm using the AtlAxWin window class so I'm not directly creating the control and can't use this approach. Another option is registry free com with a manifest. But again, I couldn't find a good example, especially since I'm not using Visual Studio. I tried to use the MT manifest tool, but couldn't figure it out. I don't think I can use DLL redirection since that doesn't get around the registry issue AFAIK. Another possibility is using WebKit but it seems even harder to use.

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• Patterns: Local Singleton vs. Global Singleton?

  - by Mike Rosenblum

  There is a pattern that I use from time to time, but I'm not quite sure what it is called. I was hoping that the SO community could help me out. The pattern is pretty simple, and consists of two parts: A singleton factory, which creates objects based on the arguments passed to the factory method. Objects created by the factory. So far this is just a standard "singleton" pattern or "factory pattern". The issue that I'm asking about, however, is that the singleton factory in this case maintains a set of references to every object that it ever creates, held within a dictionary. These references can sometimes be strong references and sometimes weak references, but it can always reference any object that it has ever created. When receiving a request for a "new" object, the factory first searches the dictionary to see if an object with the required arguments already exits. If it does, it returns that object, if not, it returns a new object and also stores a reference to the new object within the dictionary. This pattern prevents having duplicative objects representing the same underlying "thing". This is useful where the created objects are relatively expensive. It can also be useful where these objects perform event handling or messaging - having one object per item being represented can prevent multiple messages/events for a single underlying source. There are probably other reasons to use this pattern, but this is where I've found this useful. My question is: what to call this? In a sense, each object is a singleton, at least with respect to the data it contains. Each is unique. But there are multiple instances of this class, however, so it's not at all a true singleton. In my own personal terminology, I tend to call the factory method a "global singleton". I then call the created objects "local singletons". I sometimes also say that the created objects have "reference equality", meaning that if two variables reference the same data (the same underlying item) then the reference they each hold must be to the same exact object, hence "reference equality". But these are my own invented terms, and I am not sure that they are good ones. Is there standard terminology for this concept? And if not, could some naming suggestions be made? Thanks in advance...

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• Implementing coroutines in Java

  - by JUST MY correct OPINION

  This question is related to my question on existing coroutine implementations in Java. If, as I suspect, it turns out that there is no full implementation of coroutines currently available in Java, what would be required to implement them? As I said in that question, I know about the following: You can implement "coroutines" as threads/thread pools behind the scenes. You can do tricksy things with JVM bytecode behind the scenes to make coroutines possible. The so-called "Da Vinci Machine" JVM implementation has primitives that make coroutines doable without bytecode manipulation. There are various JNI-based approaches to coroutines also possible. I'll address each one's deficiencies in turn. Thread-based coroutines This "solution" is pathological. The whole point of coroutines is to avoid the overhead of threading, locking, kernel scheduling, etc. Coroutines are supposed to be light and fast and to execute only in user space. Implementing them in terms of full-tilt threads with tight restrictions gets rid of all the advantages. JVM bytecode manipulation This solution is more practical, albeit a bit difficult to pull off. This is roughly the same as jumping down into assembly language for coroutine libraries in C (which is how many of them work) with the advantage that you have only one architecture to worry about and get right. It also ties you down to only running your code on fully-compliant JVM stacks (which means, for example, no Android) unless you can find a way to do the same thing on the non-compliant stack. If you do find a way to do this, however, you have now doubled your system complexity and testing needs. The Da Vinci Machine The Da Vinci Machine is cool for experimentation, but since it is not a standard JVM its features aren't going to be available everywhere. Indeed I suspect most production environments would specifically forbid the use of the Da Vinci Machine. Thus I could use this to make cool experiments but not for any code I expect to release to the real world. This also has the added problem similar to the JVM bytecode manipulation solution above: won't work on alternative stacks (like Android's). JNI implementation This solution renders the point of doing this in Java at all moot. Each combination of CPU and operating system requires independent testing and each is a point of potentially frustrating subtle failure. Alternatively, of course, I could tie myself down to one platform entirely but this, too, makes the point of doing things in Java entirely moot. So... Is there any way to implement coroutines in Java without using one of these four techniques? Or will I be forced to use the one of those four that smells the least (JVM manipulation) instead?

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• dynamic module creation

  - by intuited

  I'd like to dynamically create a module from a dictionary, and I'm wondering if adding an element to sys.modules is really the best way to do this. EG context = { a: 1, b: 2 } import types test_context_module = types.ModuleType('TestContext', 'Module created to provide a context for tests') test_context_module.__dict__.update(context) import sys sys.modules['TestContext'] = test_context_module My immediate goal in this regard is to be able to provide a context for timing test execution: import timeit timeit.Timer('a + b', 'from TestContext import *') It seems that there are other ways to do this, since the Timer constructor takes objects as well as strings. I'm still interested in learning how to do this though, since a) it has other potential applications; and b) I'm not sure exactly how to use objects with the Timer constructor; doing so may prove to be less appropriate than this approach in some circumstances. EDITS/REVELATIONS/PHOOEYS/EUREKAE: I've realized that the example code relating to running timing tests won't actually work, because import * only works at the module level, and the context in which that statement is executed is that of a function in the testit module. In other words, the globals dictionary used when executing that code is that of main, since that's where I was when I wrote the code in the interactive shell. So that rationale for figuring this out is a bit botched, but it's still a valid question. I've discovered that the code run in the first set of examples has the undesirable effect that the namespace in which the newly created module's code executes is that of the module in which it was declared, not its own module. This is like way weird, and could lead to all sorts of unexpected rattlesnakeic sketchiness. So I'm pretty sure that this is not how this sort of thing is meant to be done, if it is in fact something that the Guido doth shine upon. The similar-but-subtly-different case of dynamically loading a module from a file that is not in python's include path is quite easily accomplished using imp.load_source('NewModuleName', 'path/to/module/module_to_load.py'). This does load the module into sys.modules. However this doesn't really answer my question, because really, what if you're running python on an embedded platform with no filesystem? I'm battling a considerable case of information overload at the moment, so I could be mistaken, but there doesn't seem to be anything in the imp module that's capable of this. But the question, essentially, at this point is how to set the global (ie module) context for an object. Maybe I should ask that more specifically? And at a larger scope, how to get Python to do this while shoehorning objects into a given module?

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• Get UITableView to scroll to the selected UITextField and Avoid Being Hidden by Keyboard

  - by Lauren Quantrell

  I have a UITextField in a table view on a UIViewController (not a UITableViewController). If the table view is on a UITableViewController, the table will automatically scroll to the textField being edited to prevent it from being hidden by the keyboard. But on a UIViewController it does not. I have tried for a couple of days reading through multiple ways to try to accomplish this and I cannot get it to work. The closest thing that actually scrolls is: -(void) textFieldDidBeginEditing:(UITextField *)textField { // SUPPOSEDLY Scroll to the current text field CGRect textFieldRect = [textField frame]; [self.wordsTableView scrollRectToVisible:textFieldRect animated:YES]; } However this only scrolls the table to the topmost row. What seems like an easy task has been a couple of days of frustration. I am using the following to construct the tableView cells: - (UITableViewCell *)tableView:(UITableView *)aTableView cellForRowAtIndexPath:(NSIndexPath *)indexPath { NSString *identifier = [NSString stringWithFormat: @"%d:%d", [indexPath indexAtPosition: 0], [indexPath indexAtPosition:1]]; UITableViewCell *cell = [aTableView dequeueReusableCellWithIdentifier:identifier]; if (cell == nil) { cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:identifier] autorelease]; cell.accessoryType = UITableViewCellAccessoryNone; UITextField *theTextField = [[UITextField alloc] initWithFrame:CGRectMake(180, 10, 130, 25)]; theTextField.adjustsFontSizeToFitWidth = YES; theTextField.textColor = [UIColor redColor]; theTextField.text = [textFieldArray objectAtIndex:indexPath.row]; theTextField.keyboardType = UIKeyboardTypeDefault; theTextField.returnKeyType = UIReturnKeyDone; theTextField.font = [UIFont boldSystemFontOfSize:14]; theTextField.backgroundColor = [UIColor whiteColor]; theTextField.autocorrectionType = UITextAutocorrectionTypeNo; theTextField.autocapitalizationType = UITextAutocapitalizationTypeNone; theTextField.clearsOnBeginEditing = NO; theTextField.textAlignment = UITextAlignmentLeft; //theTextField.tag = 0; theTextField.tag=indexPath.row; theTextField.delegate = self; theTextField.clearButtonMode = UITextFieldViewModeWhileEditing; [theTextField setEnabled: YES]; [cell addSubview:theTextField]; [theTextField release]; } return cell; } I suspect I can get the tableView to scroll properly if I can somehow pass the indexPath.row in the textFieldDidBeginEditing method? Any help is appreciated.

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• Delphi Unit local variables - how to make each instance unique?

  - by Justin

  Ok, this, I'm sure is something simple that is easy to do. The problem : I've inherited scary spaghetti code and am slowly trying to better it when new features need adding - generally when a refactor makes adding the new feature neater. I've got a bunch of code I'm packing into a single unit which, in different places in the application, controls the same physical thing in the outside world. The control appears in several places in the application and operates slightly differently in each instance. What I've done is to create a unit with all of the features I need which I can simply drop, as a frame, into each form that requires it. Each form then uses the unit's interface methods to customise the behaviour for each instance. The problem within the problem : In the unit in question (the frame) I have a variable declared in the IMPLEMENTATION section - local to the unit. I also have a procedure, declared in the TYPE section which takes an argument and assigns that argument to the local variable in question - each form passes a unique variable to each instance of the frame/unit. What I want it to do is for each instance of the frame to keep its own version of that variable, different from the others, and use that to define how it operates. What seems to be happening, however, is that all instances are using the same value, even if I explicitly pass each instance a different variable. ie: Unit FlexibleUnit; interface uses //the uses stuff type TFlexibleUnit=class(TFrame) //declarations including procedure makeThisInstanceX(passMeTheVar:integer); private // public // end; implementation uses //the uses var myLocalVar; procedure makeThisInstanceX(passMeTheVar:integer); begin myLocalVar:=passMeTheVar; end; //other procedures using myLocalVar //etc to the end; Now somewhere in another Form I've dropped this Frame onto the Design pane, sometimes two of these frames on one Form, and have it declared in the proper places, etc. Each is unique in that : ThisFlexibleUnit : TFlexibleUnit; ThatFlexibleUnit : TFlexibleUnit; and when I do a: ThisFlexibleUnit.makeThisInstanceX(var1); //want to behave in way "var1" ThatFlexibleUnit.makeThisInstanceX(var2); //want to behave in way "var2" it seems that they both share the same variable "myLocalVar". Am I doing this wrong, in principle? If this is the correct method then it's a matter of debugging what I have (which is too huge to post) but if this is not correct in principle then is there a way to do what I am suggesting? Thanks in advance, Stack Overflow - you guys (and gals!) are legendary.

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• Rails learn's confusion

  - by Steve

  This is a beginner's rails learning confusion. When I learn rails, from time to time, I feel frustrated on rails' principle "Convention over Configuration". Rails uses heavily on conventions. A lot of them are just naming conventions. If I forget a convention, I will either use the wrong naming and get unexpected result or get things magically done but don't understand how. Sometimes, I think of configuration. At least configuration lists everything clearly and nothing is in fog. In rails, there seems a hidden, dark contract between you and the machine. If you follow the contract, you communicate well. But a beginner usually forgets items listed on the contract and this usually leads to confusion. That's why when I first pick up rails, I feel like it is somehow difficult to learn. Besides, there are many other things that could be new to a learner, such as using git, using plugins from community, using RESTful routing style, using RSpec. All these are new and come together in learning ruby and rails. This definitely adds up difficulties for a beginner. In contrast, if you learn php, it wouldn't be that bad. You can forget many things and focus on learning php itself. You don't need to learn database handling if you know SQL already(in rails, you need to learn a whole new concept migration), you don't have to learn a new decent unit test(in rails, usually they teach RSpec along the way because rails is agile and you should learn test-driven development in the early learning stage), you don't have to learn a new version control(in rails, you will be taught about git anyway), you don't have to use complicated plugins(in rails, they usually use third-party plugins in textbook examples! what the hell? why not teach how to do a simplified similar thing in rails?), you don't have to worry RESTful style. All in all, when I learn php, I learn it quick and soon I start to write things myself. Learning php is similar to learning C/java. It tastes like those traditional languages. When I learn rails, it is more difficult. And I need to learn ruby as well (I believe many of you learn ruby just because of rails). Does anyone have the similar feeling as I have? How do you overcome it and start to master rails? Hints will be welcomed. Thank you.

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• Should I skip authorization, with CanCan, of an action that instantiates a resource?

  - by irkenInvader

  I am writing a web app to pick random lists of cards from larger, complete sets of cards. I have a Card model and a CardSet model. Both models have a full RESTful set of 7 actions (:index, :new, :show, etc). The CardSetsController has an extra action for creating random sets: :random. # app/models/card_set.rb class CardSet < ActiveRecord::Base belongs_to :creator, :class_name => "User" has_many :memberships has_many :cards, :through => :memberships # app/models/card.rb class Card < ActiveRecord::Base belongs_to :creator, :class_name => "User" has_many :memberships has_many :card_sets, :through => :memberships I have added Devise for authentication and CanCan for authorizations. I have users with an 'editor' role. Editors are allowed to create new CardSets. Guest users (Users who have not logged in) can only use the :index and :show actions. These authorizations are working as designed. Editors can currently use both the :random and the :new actions without any problems. Guest users, as expected, cannot. # app/controllers/card_sets_controller.rb class CardSetsController < ApplicationController before_filter :authenticate_user!, :except => [:show, :index] load_and_authorize_resource I want to allow guest users to use the :random action, but not the :new action. In other words, they can see new random sets, but not save them. The "Save" button on the :random action's view is hidden (as designed) from the guest users. The problem is, the first thing the :random action does is build a new instance of the CardSet model to fill out the view. When cancan tries to load_and_authorize_resource a new CardSet, it throws a CanCan::AccessDenied exception. Therefore, the view never loads and the guest user is served a "You need to sign in or sign up before continuing" message. # app/controllers/card_sets_controllers.rb def random @card_set = CardSet.new( :name => "New Set of 10", :set_type => "Set of 10" ) I realize that I can tell load_and_authorize_resource to skip the :random action by passing :except => :random to the call, but that just feels "wrong" for some reason. What's the "right" way to do this? Should I create the new random set without instantiating a new CardSet? Should I go ahead and add the exception?

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• java instanceof not finding method

  - by Razvan N

  I have a problem with java instanceof. I have a class called Employee and several others that extend this one, for example - Manager. I also created another class,EmployeeStockPlan, where I wanted to test if instanceof is finding which object I am using. But when I am calling a method from the new class, I have this error: The method grantStock(Manager) is undefined for the type Loader. Sorry, I am somehow new to some thing in java, I hope I am not asking dumb questions. The Employee class: package com.example.domain; public class Employee { private int empId; private String name; private String ssn; private double salary; public Employee(int empId, String name, String ssn, double salary) { // constructor // method; this.empId = empId; this.name = name; this.ssn = ssn; this.salary = salary; } public void setName(String newName) { if (newName != null) { this.name = newName; } } public void raiseSalary(double increase) { this.salary += increase; } public String getName() { return name; } public double getSalary() { return salary; } public String getDetails() { return "Employee id: " + empId + "\n" + "Employee name: " + name; } } The Manager class: package com.example.domain; public class Manager extends Employee { private String deptName; public Manager(int empId, String name, String ssn, double salary, String dept) { super(empId, name, ssn, salary); this.deptName = dept; } public String getDeptName() { return deptName; } public String getDetails() { return super.getDetails() + "\n" + "Department: " + deptName; } } The EmployeeStockPlan class: package com.example.domain; public class EmployeeStockPlan { public void grantStock(Employee e) { // nothing calculated, just simulating; System.out.println("This is an employee!"); if (e instanceof Manager) { // process Manager stock grant System.out.println("This is a manager!"); } else { // error - instance of Engineer? System.out.println("Not an engineer!"); } return; } } The main class: EmployeeStockPlan esp = new EmployeeStockPlan(); Manager m = new Manager (12421, "Manager1", "111-4254-521", 2430, "Marketing1"); grantStock(m);

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• Uploading files to varbinary(max) in SQL Server -- works on one server, not the other

  - by pjabbott

  I have some code that allows users to upload file attachments into a varbinary(max) column in SQL Server from their web browser. It has been working perfectly fine for almost two years, but all of a sudden it stopped working. And it stopped working on only the production database server -- it still works fine on the development server. I can only conclude that the code is fine and there is something up with the instance of SQL Server itself. But I have no idea how to isolate the problem. I insert a record into the ATTACHMENT table, only inserting non-binary data like the title and the content type, and then chunk-upload the uploaded file using the following code: // get the file stream System.IO.Stream fileStream = postedFile.InputStream; // make an upload buffer byte[] fileBuffer; fileBuffer = new byte[1024]; // make an update command SqlCommand fileUpdateCommand = new SqlCommand("update ATTACHMENT set ATTACHMENT_DATA.WRITE(@Data, NULL, NULL) where ATTACHMENT_ID = @ATTACHMENT_ID", sqlConnection, sqlTransaction); fileUpdateCommand.Parameters.Add("@Data", SqlDbType.Binary); fileUpdateCommand.Parameters.AddWithValue("@ATTACHMENT_ID", newId); while (fileStream.Read(fileBuffer, 0, fileBuffer.Length) > 0) { fileUpdateCommand.Parameters["@Data"].Value = fileBuffer; fileUpdateCommand.ExecuteNonQuery(); <------ FAILS HERE } fileUpdateCommand.Dispose(); fileStream.Close(); Where it says "FAILS HERE", it sits for a while and then I get a SQL Server timeout error on the very first iteration through the loop. If I connect to the development database instead, everything works fine (it runs through the loop many, many times and the commit is successful). Both servers are identical (SQL Server 9.0.3042) and the schemas are identical as well. When I open Activity Monitor right after the timeout to see what's going it, it says the last command is (@Data binary(1024),@ATTACHMENT_ID decimal(4,0))update ATTACHMENT set ATTACHMENT_DATA.WRITE(@Data, NULL, NULL) where ATTACHMENT_ID = @ATTACHMENT_ID which I would expect but it also says it has a status of "Suspended" and a wait type of "PAGEIOLATCH_SH". I looked this up and it seems to be a bad thing but I can't find anything specific to my stuation. Ideas?

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• How to obtain the first cluster of the directory's data in FAT using C# (or at least C++) and Win32A

  - by DarkWalker

  So I have a FAT drive, lets say H: and a directory 'work' (full path 'H:\work'). I need to get the NUMBER of the first cluster of that directory. The number of the first cluster is 2-bytes value, that is stored in the 26th and 27th bytes of the folder enty (wich is 32 bytes). Lets say I am doing it with file, NOT a directory. I can use code like this: static public string GetDirectoryPtr(string dir) { IntPtr ptr = CreateFile(@"H:\Work\dover.docx", GENERIC_READ, FILE_SHARE_READ | FILE_SHARE_WRITE, IntPtr.Zero, OPEN_EXISTING, 0,//FILE_FLAG_BACKUP_SEMANTICS, IntPtr.Zero); try { const uint bytesToRead = 2; byte[] readbuffer = new byte[bytesToRead]; if (ptr.ToInt32() == -1) return String.Format("Error: cannot open direcotory {0}", dir); if (SetFilePointer(ptr, 26, 0, 0) == -1) return String.Format("Error: unable to set file pointer on file {0}", ptr); uint read = 0; // real count of read bytes if (!ReadFile(ptr, readbuffer, bytesToRead, out read, 0)) return String.Format("cant read from file {0}. Error #{1}", ptr, Marshal.GetLastWin32Error()); int result = readbuffer[0] + 16 * 16 * readbuffer[1]; return result.ToString();//ASCIIEncoding.ASCII.GetString(readbuffer); } finally { CloseHandle(ptr); } } And it will return some number, like 19 (quite real to me, this is the only file on the disk). But I DONT need a file, I need a folder. So I am puttin FILE_FLAG_BACKUP_SEMANTICS param for CreateFile call... and dont know what to do next =) msdn is very clear on this issue http://msdn.microsoft.com/en-us/library/aa365258(v=VS.85).aspx It sounds to me like: "There is no way you can get a number of the folder's first cluster". The most desperate thing is that my tutor said smth like "You are going to obtain this or you wont pass this course". The true reason why he is so sure this is possible is because for 10 years (or may be more) he recieved the folder's first cluster number as a HASH of the folder's addres (and I was stupid enough to point this to him, so now I cant do it the same way) PS: This is the most spupid task I have ever had!!! This value is not really used anythere in program, it is only fcking pointless integer.

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• Is this postgres function cost efficient or still have to clean

  - by kiranking

  There are two tables in postgres db. english_all and english_glob First table contains words like international,confidential,booting,cooler ...etc I have written the function to get the words from english_all then perform for loop for each word to get word list which are not inserted in anglish_glob table. Word list is like I In Int Inte Inter .. b bo boo boot .. c co coo cool etc.. for some reason zwnj(zero-width non-joiner) is added during insertion to english_all table. But in function I am removing that character with regexp_replace. Postgres function for_loop_test is taking two parameter min and max based on that I am selecting words from english_all table. function code is like DECLARE inMinLength ALIAS FOR $1; inMaxLength ALIAS FOR $2; mviews RECORD; outenglishListRow english_word_list;--custom data type eng_id,english_text BEGIN FOR mviews IN SELECT id,english_all_text FROM english_all where wlength between inMinLength and inMaxLength ORDER BY english_all_text limit 30 LOOP FOR i IN 1..char_length(regexp_replace(mviews.english_all_text,'(?)$','')) LOOP FOR outenglishListRow IN SELECT distinct on (regexp_replace((substring(mviews.english_all_text from 1 for i)),'(?)$','')) mviews.id, regexp_replace((substring(mviews.english_all_text from 1 for i)),'(?)$','') where regexp_replace((substring(mviews.english_all_text from 1 for i)),'(?)$','') not in(select english_glob.english_text from english_glob where i=english_glob.wlength) order by regexp_replace((substring(mviews.english_all_text from 1 for i)),'(?)$','') LOOP RETURN NEXT outenglishListRow; END LOOP; END LOOP; END LOOP; END; Once I get the word list I will insert that into another table english_glob. My question is is there any thing I can add to or remove from function to make it more efficient. edit Let assume english_all table have words like footer,settle,question,overflow,database,kingdom If inMinLength = 5 and inmaxLength=7 then in the outer loop footer,settle,kingdom will be selected. For above 3 words inner two loop will apply to get words like f,fo,foo,foot,foote,footer,s,se,set,sett,settl .... etc. In the final process words which are bold will be entered into english_glob with another parameter like 1 to denote it is a proper word and stored in the another filed of english_glob table. Remaining word will be stored with another parameter 0 because in the next call words which are saved in database should not be fetched again. edit2: This is a complete code CREATE TABLE english_all ( id serial NOT NULL, english_all_text text NOT NULL, wlength integer NOT NULL, CONSTRAINT english_all PRIMARY KEY (id), CONSTRAINT english_all_kan_text_uq_id UNIQUE (english_all_text) ) CREATE TABLE english_glob ( id serial NOT NULL, english_text text NOT NULL, is_prop integer default 1, CONSTRAINT english_glob PRIMARY KEY (id), CONSTRAINT english_glob_kan_text_uq_id UNIQUE (english_text) ) insert into english_all(english_text) values ('ant'),('forget'),('forgive'); on function call with parameter 3 and 6 fallowing rows should fetched a an ant f fo for forg forge forget next is insert to another table based on above row insert into english_glob(english_text,is_prop) values ('a',1),('an',1), ('ant',1),('f',0), ('fo',0),('for',1), ('forg',0),('forge',1), ('forget',1), on function call next time with parameter 3 and 7 fallowing rows should fetched.(because f,fo,for,forg are all entered in english_glob table) forgi forgiv forgive

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• How to change button's image in visual c++ at run time?

  - by karikari

  After trying and error for many times, I decided to ask here. My objective is I wanted to change the feature of my IE toolbar button. The button is firstly setup by IE at IE startup using the function CRebarHandler::onSetRedraw and CRebarHandler::setButtonMenu2(). And then, I create a call from another cpp file, to call CRebarHandler::setButtonMenu2(). I intent to change just the button's image. I assigned the ID of the image correctly. But somehow it does not work. When I put other code inside this function,like a code for writing to file, it is proven work. Means, it is properly being called from the other file. But the thing is, the code for the button inside CRebarHandler::setButtonMenu2() seems does not work. Need help. Here is the code I am working on (I modify John Lister's button code): LRESULT CRebarHandler::onSetRedraw(UINT uMsg, WPARAM wParam, LPARAM lParam, BOOL& bHandled){ bHandled=false; if (m_ieVer==6){ if (!m_hWndToolbar) scanForToolbarSlow(); if (m_hWndToolbar){ findButton(m_hWndToolbar); if (m_buttonID>0) setButtonMenu(); } } return S_OK; } void CRebarHandler::setButtonMenu(){ HIMAGELIST hImageList = ImageList_Create(32, 32,ILC_COLOR16 | ILC_MASK,1, 0); HINSTANCE module = _AtlBaseModule.GetResourceInstance(); TBBUTTONINFO inf; inf.cbSize=sizeof(inf); inf.dwMask = TBIF_IMAGE; char psBuffer[128]; FILE *pPipe; float f = 0; pPipe = _popen("javaw -jar c:\\simmetrics.jar c:\\chtml.txt c:\\thtml.txt", "rt" ); char* p = fgets(psBuffer, 128, pPipe); std::istringstream iss(p); iss >> f; if (f > 0.9) { inf.iImage = 1; SendMessage(m_hWndToolbar, TB_SETBUTTONINFO, m_buttonID, (LPARAM)(&inf)); iss.clear(); f = 0; } else { inf.iImage = 2; SendMessage(m_hWndToolbar, TB_SETBUTTONINFO, m_buttonID, (LPARAM)(&inf)); iss.clear(); f = 0; } iss.clear(); f = 0; } void CRebarHandler::setButtonMenu2(){ TBBUTTONINFO inf; inf.cbSize=sizeof(inf); inf.dwMask = TBIF_IMAGE; inf.iImage = 1; //green SendMessage(NULL, TB_SETBUTTONINFO, m_buttonID, (LPARAM)(&inf)); }

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• SSL Authentication with Certificates: Should the Certificates have a hostname?

  - by sixtyfootersdude

  Summary JBoss allows clients and servers to authenticate using certificates and ssl. One thing that seems strange is that you are not required to give your hostname on the certificate. I think that this means if Server B is in your truststore, Sever B can pretend to be any server that they want. (And likewise: if Client B is in your truststore...) Am I missing something here? Authentication Steps (Summary of Wikipeida Page) Client Server ================================================================================================= 1) Client sends Client Hello ENCRIPTION: None - highest TLS protocol supported - random number - list of cipher suites - compression methods 2) Sever Hello ENCRIPTION: None - highest TLS protocol supported - random number - choosen cipher suite - choosen compression method 3) Certificate Message ENCRIPTION: None - 4) ServerHelloDone ENCRIPTION: None 5) Certificate Message ENCRIPTION: None 6) ClientKeyExchange Message ENCRIPTION: server's public key => only server can read => if sever can read this he must own the certificate - may contain a PreMasterSecerate, public key or nothing (depends on cipher) 7) CertificateVerify Message ENCRIPTION: clients private key - purpose is to prove to the server that client owns the cert 8) BOTH CLIENT AND SERVER: - use random numbers and PreMasterSecret to compute a common secerate 9) Finished message - contains a has and MAC over previous handshakes (to ensure that those unincripted messages did not get broken) 10) Finished message - samething Sever Knows The client has the public key for the sent certificate (step 7) The client's certificate is valid because either: it has been signed by a CA (verisign) it has been self-signed BUT it is in the server's truststore It is not a replay attack because presumably the random number (step 1 or 2) is sent with each message Client Knows The server has the public key for the sent certificate (step 6 with step 8) The server's certificate is valid because either: it has been signed by a CA (verisign) it has been self-signed BUT it is in the client's truststore It is not a replay attack because presumably the random number (step 1 or 2) is sent with each message Potential Problem Suppose the client's truststore has certs in it: Server A Server B (malicous) Server A has hostname www.A.com Server B has hostname www.B.com Suppose: The client tries to connect to Server A but Server B launches a man in the middle attack. Since server B: has a public key for the certificate that will be sent to the client has a "valid certificate" (a cert in the truststore) And since: certificates do not have a hostname feild in them It seems like Server B can pretend to be Server A easily. Is there something that I am missing?

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• Finding contained bordered regions from Excel imports.

  - by dmaruca

  I am importing massive amounts of data from Excel that have various table layouts. I have good enough table detection routines and merge cell handling, but I am running into a problem when it comes to dealing with borders. Namely performance. The bordered regions in some of these files have meaning. Data Setup: I am importing directly from Office Open XML using VB6 and MSXML. The data is parsed from the XML into a dictionary of cell data. This wonks wonderfully and is just as fast as using docmd.transferspreadsheet in Access, but returns much better results. Each cell contains a pointer to a style element which contains a pointer to a border element that defines the visibility and weight of each border (this is how the data is structured inside OpenXML, also). Challenge: What I'm trying to do is find every region that is enclosed inside borders, and create a list of cells that are inside that region. What I have done: I initially created a BFS(breadth first search) fill routine to find these areas. This works wonderfully and fast for "normal" sized spreadsheets, but gets way too slow for imports into the thousands of rows. One problem is that a border in Excel could be stored in the cell you are checking or the opposing border in the adjacent cell. That's ok, I can consolidate that data on import to reduce the number of checks needed. One thing I thought about doing is to create a separate graph that outlines the cells using the borders as my edges and using a graph algorithm to find regions that way, but I'm having trouble figuring out how to implement the algorithm. I've used Dijkstra in the past and thought I could do similar with this. So I can span out using no endpoint to search the entire graph, and if I encounter a closed node I know that I just found an enclosed region, but how can I know if the route I've found is the optimal one? I guess I could flag that to run a separate check for the found closed node to the previous node ignoring that one edge. This could work, but wouldn't be much better performance wise on dense graphs. Can anyone else suggest a better method? Thanks for taking the time to read this.

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• Learning... anything really

  - by WebDevHobo

  I'm particularly interested in Windows PowerShell, but here's a somewhat more general complaint: When asking for help on learning something new, be it a small subject on PHP or understanding a class in Java, what usually happens is that people direct me towards the documentation pages. What I'm looking for is somewhat of a course. A deep explanation of why something works the way it does. I know my basic programming, like Java and C#. I've never seen C or C++, though I have seen a bit of assembler. I know what the Stack and Heap are, how boxing and unboxing works, why you have to deep-copy an array instead of copying the pointer and some other things. Windows PowerShell on the other hand, I know nothing about. And I notice that when reading the small document or some code, I usually forget what it does or why it works. What I am looking for is preferably, a nice tutorial that explains the beginnings, the concepts, and goes to more difficult things at a steady pace. The only thing documentation can do is explain what a function does. That's no good to me since I don't know what I want to do yet. I could read about a thousand functions, and forget about most of them, because I don't need to implement them right after it. Randomly wandering through the documentation doesn't do me any good. So conclude, what is a good tutorial on Windows Powershell? One which explains in clear language what is happening, one which builds on previous things learned. I don't think googling this is a good idea. Doing a Google search on this would turn up numerous tutorials. And experience tells me that you have to look long and hard to find the gem you're looking for. That's why I'm asking here. Because this is the place where you can find more experienced people. Many of the PowerShell guys among you will know the good ones already, and by asking you, I avoid wasting time that could be spent learning. So to summarize: I will not google this!

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• Using Core Data Concurrently and Reliably

  - by John Topley

  I'm building my first iOS app, which in theory should be pretty straightforward but I'm having difficulty making it sufficiently bulletproof for me to feel confident submitting it to the App Store. Briefly, the main screen has a table view, upon selecting a row it segues to another table view that displays information relevant for the selected row in a master-detail fashion. The underlying data is retrieved as JSON data from a web service once a day and then cached in a Core Data store. The data previous to that day is deleted to stop the SQLite database file from growing indefinitely. All data persistence operations are performed using Core Data, with an NSFetchedResultsController underpinning the detail table view. The problem I am seeing is that if you switch quickly between the master and detail screens several times whilst fresh data is being retrieved, parsed and saved, the app freezes or crashes completely. There seems to be some sort of race condition, maybe due to Core Data importing data in the background whilst the main thread is trying to perform a fetch, but I'm speculating. I've had trouble capturing any meaningful crash information, usually it's a SIGSEGV deep in the Core Data stack. The table below shows the actual order of events that happen when the detail table view controller is loaded: Main Thread Background Thread viewDidLoad Get JSON data (using AFNetworking) Create child NSManagedObjectContext (MOC) Parse JSON data Insert managed objects in child MOC Save child MOC Post import completion notification Receive import completion notification Save parent MOC Perform fetch and reload table view Delete old managed objects in child MOC Save child MOC Post deletion completion notification Receive deletion completion notification Save parent MOC Once the AFNetworking completion block is triggered when the JSON data has arrived, a nested NSManagedObjectContext is created and passed to an "importer" object that parses the JSON data and saves the objects to the Core Data store. The importer executes using the new performBlock method introduced in iOS 5: NSManagedObjectContext *child = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; [child setParentContext:self.managedObjectContext]; [child performBlock:^{ // Create importer instance, passing it the child MOC... }]; The importer object observes its own MOC's NSManagedObjectContextDidSaveNotification and then posts its own notification which is observed by the detail table view controller. When this notification is posted the table view controller performs a save on its own (parent) MOC. I use the same basic pattern with a "deleter" object for deleting the old data after the new data for the day has been imported. This occurs asynchronously after the new data has been fetched by the fetched results controller and the detail table view has been reloaded. One thing I am not doing is observing any merge notifications or locking any of the managed object contexts or the persistent store coordinator. Is this something I should be doing? I'm a bit unsure how to architect this all correctly so would appreciate any advice.

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