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• How to change the state of a singleton in runtime

  - by user34401

  Consider I am going to write a simple file based logger AppLogger to be used in my apps, ideally it should be a singleton so I can call it via public class AppLogger { public static String file = ".."; public void logToFile() { // Write to file } public static log(String s) { AppLogger.getInstance().logToFile(s); } } And to use it AppLogger::log("This is a log statement"); The problem is, what is the best time I should provide the value of file since it is a just a singleton? Or how to refactor the above code (or skip using singleton) so I can customize the log file path? (Assume I don't need to write to multiple at the same time) p.s. I know I can use library e.g. log4j, but consider it is just a design question, how to refactor the code above?

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• Event Aggregator.. not getting a response, how to determine completion?

  - by Duncan_m

  I'm rewriting a vehicle tracking application, a google maps based thing.. The users are able to search for a vehicle by typing a few characters of the vehicles "callsign". My application is based around a sort of "event bus" within Backbone.. when a search occurs I send a message on the bus saying something like "does anyone match this?".. If a marker matches the search term it responds with a sort of "yes, I match!".. My challenge arises when no-one matches, I get no response.. it feels a little hacky to "wait a little while" and check if a response has been recieved.. The application is based around Backbone.js and using the Event Aggregator pattern described in the answer to this question on Stack Overflow: http://stackoverflow.com/questions/7708195/access-function-in-one-view-from-another-in-backbone-js Is there a well defined design pattern that might assist me here? Sending a request for a response and not getting any responses?

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• When designing an application around Model-View-Controller (MVC), what is in your toolbox?

  - by ericgorr

  There are a lot of great explanations for what the Model-View-Controller design pattern is, but I am having trouble finding good resources showing how to use it in practice. So, when you are starting a new application (doesn't matter what it is), what is in your toolbox? For example, it was suggested that using UML collaboration diagrams ( http://www.objectmentor.com/resources/articles/umlCollaborationDiagrams.pdf ) can be useful when designing an application around MVC, although, I am not certain exactly how or why this might be the case...? So, what is in your toolbox for MVC?

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• Distinguishing between sets of status reports

  - by user1769486

  I am working on an internal database monitoring system and am at a point where I sort of hit the wall in terms of application design. Basically I have an extensible plugin architecture where I shall have an OK, a warning or an error upon running a db verification. My first question whether it is sufficient to have only one status reported with an optional status message or provide the ability to have more than one returned (with attached messages) and then calculate an aggregated overall status. In particular in the latter case my second issue would be how to distinguish between two verification reports with the same status code (as it can come from different triggers). I would need to do this to see whether some change happened between the current and last verification. I could simply have string comparisons of the attached status messages mentioned above but that does not seem very reliable.

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• WPF UserControl Style results in error message

  - by Didier

  Hi I'm new in WPF so I try to create a WPF UserControl. My problem is about the style of the user control I've Created. On a design time I got this error message Can only base on a Style with target type that is base type 'RichTextBox'. at System.Windows.Style.Seal() at System.Windows.StyleHelper.UpdateStyleCache(FrameworkElement fe, FrameworkContentElement fce, Style oldStyle, Style newStyle, Style& styleCache) at System.Windows.FrameworkElement.OnStyleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e) at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e) at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args) at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType) at System.Windows.DependencyObject.InvalidateProperty(DependencyProperty dp) at System.Windows.FrameworkElement.UpdateStyleProperty() at System.Windows.TreeWalkHelper.InvalidateStyleAndReferences(DependencyObject d, ResourcesChangeInfo info, Boolean containsTypeOfKey) at System.Windows.TreeWalkHelper.OnResourcesChanged(DependencyObject d, ResourcesChangeInfo info, Boolean raiseResourceChangedEvent) at System.Windows.TreeWalkHelper.OnResourcesChangedCallback(DependencyObject d, ResourcesChangeInfo info) at System.Windows.DescendentsWalker1._VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.VisitNode(FrameworkElement fe) at System.Windows.DescendentsWalker1.VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren) at System.Windows.DescendentsWalker1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren) at System.Windows.DescendentsWalker1.IterateChildren(DependencyObject d) at System.Windows.DescendentsWalker1._VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.VisitNode(FrameworkElement fe) at System.Windows.DescendentsWalker1.VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren) at System.Windows.DescendentsWalker1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren) at System.Windows.DescendentsWalker1.IterateChildren(DependencyObject d) at System.Windows.DescendentsWalker1._VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.VisitNode(FrameworkElement fe) at System.Windows.DescendentsWalker1.VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren) at System.Windows.DescendentsWalker1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren) at System.Windows.DescendentsWalker1.IterateChildren(DependencyObject d) at System.Windows.DescendentsWalker1._VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.VisitNode(FrameworkElement fe) at System.Windows.DescendentsWalker1.VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren) at System.Windows.DescendentsWalker1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren) at System.Windows.DescendentsWalker1.IterateChildren(DependencyObject d) at System.Windows.DescendentsWalker1._VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.VisitNode(FrameworkElement fe) at System.Windows.DescendentsWalker1.VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren) at System.Windows.DescendentsWalker1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren) at System.Windows.DescendentsWalker1.IterateChildren(DependencyObject d) at System.Windows.DescendentsWalker1._VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.VisitNode(FrameworkElement fe) at System.Windows.DescendentsWalker1.VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren) at System.Windows.DescendentsWalker1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren) at System.Windows.DescendentsWalker1.IterateChildren(DependencyObject d) at System.Windows.DescendentsWalker1._VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.VisitNode(FrameworkElement fe) at System.Windows.DescendentsWalker1.VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren) at System.Windows.DescendentsWalker1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren) at System.Windows.DescendentsWalker1.IterateChildren(DependencyObject d) at System.Windows.DescendentsWalker1._VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.VisitNode(FrameworkElement fe) at System.Windows.DescendentsWalker1.VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren) at System.Windows.DescendentsWalker1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren) at System.Windows.DescendentsWalker1.IterateChildren(DependencyObject d) at System.Windows.DescendentsWalker1._VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.VisitNode(FrameworkElement fe) at System.Windows.DescendentsWalker1.VisitNode(DependencyObject d) at System.Windows.DescendentsWalker1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren) at System.Windows.DescendentsWalker1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren) at System.Windows.DescendentsWalker1.IterateChildren(DependencyObject d) at System.Windows.DescendentsWalker`1.StartWalk(DependencyObject startNode, Boolean skipStartNode) at System.Windows.TreeWalkHelper.InvalidateOnResourcesChange(FrameworkElement fe, FrameworkContentElement fce, ResourcesChangeInfo info) at System.Windows.ResourceDictionary.NotifyOwners(ResourcesChangeInfo info) at System.Windows.ResourceDictionary.EndInit() at MS.Internal.Host.Designer.OnAppResourcesChanged(Object sender, EventArgs e) at MS.Internal.Host.Designer.get_View() at MS.Internal.Designer.VSDesigner.Load() at MS.Internal.Designer.VSIsolatedDesigner.VSIsolatedView.Load() at MS.Internal.Designer.VSIsolatedDesigner.VSIsolatedDesignerFactory.Load(IsolatedView view) at MS.Internal.Host.Isolation.IsolatedDesigner.BootstrapProxy.LoadDesigner(IsolatedDesignerFactory factory, IsolatedView view) at MS.Internal.Host.Isolation.IsolatedDesigner.BootstrapProxy.LoadDesigner(IsolatedDesignerFactory factory, IsolatedView view) at MS.Internal.Host.Isolation.IsolatedDesigner.Load() at MS.Internal.Designer.DesignerPane.LoadDesignerView() And at a run time An error Message type XamlParseException Occurs and the message is: Cannot create instance of 'RichTextBox' defined in assembly 'PresentationFramework, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. Can only base on a Style with target type that is base type 'RichTextBox'. Error at object 'System.Windows.Controls.Grid' in markup file 'NameSpace;component/usercontrols /myusercontrol.xaml' Line 125 Position 14. My user control have 3 richtextboxes 1 textbox and 3 dropdownlist and about 10 buttons. I think The problem is about to define the style of my user control, can anyone help me to do this. Thanks

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• Organizing an entity system with external component managers?

  - by Gustav

  I'm designing a game engine for a top-down multiplayer 2D shooter game, which I want to be reasonably reuseable for other top-down shooter games. At the moment I'm thinking about how something like an entity system in it should be designed. First I thought about this: I have a class called EntityManager. It should implement a method called Update and another one called Draw. The reason for me separating Logic and Rendering is because then I can omit the Draw method if running a standalone server. EntityManager owns a list of objects of type BaseEntity. Each entity owns a list of components such as EntityModel (the drawable representation of an entity), EntityNetworkInterface, and EntityPhysicalBody. EntityManager also owns a list of component managers like EntityRenderManager, EntityNetworkManager and EntityPhysicsManager. Each component manager keeps references to the entity components. There are various reasons for moving this code out of the entity's own class and do it collectively instead. For example, I'm using an external physics library, Box2D, for the game. In Box2D, you first add the bodies and shapes to a world (owned by the EntityPhysicsManager in this case) and add collision callbacks (which would be dispatched to the entity object itself in my system). Then you run a function which simulates everything in the system. I find it hard to find a better solution to do this than doing it in an external component manager like this. Entity creation is done like this: EntityManager implements the method RegisterEntity(entityClass, factory) which registers how to create an entity if that class. It also implements the method CreateEntity(entityClass) which would return an object of type BaseEntity. Well now comes my problem: How would the reference to a component be registered to the component managers? I have no idea how I would reference the component managers from a factory/closure.

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• Missing Operating System after trying to upgrade to Ubuntu 11

  - by Mauricio

  there! After trying to upgrade from Ubuntu 10.04 to 11, the upgrading process stopped when running and then I got an "out of disk, grub rescue" message when booting. After running Boot Repair, I got this results. Now I get "Missing Operating System" whent trying to boot. Bellow I show some results from some commands I gather from help foruns, but I still reached no solution. Could you please help me? Any enlightment will be very helpful! Disk Utility says "Disk has a few bad sectors". When trying to run the Self-test I get "FAILED (Read)" Here we have what Gparted says about the /dev/sda1 partition (ext4): Flags: boot Status: not mounted Warning: e2label: Attempt to read block from filesystem resulted in short read while trying to open /dev/sda1Couldn`t find valid filesystem superblockUnable to read the contents of this filesystem! From sudo fdisk -lI got: Disk /dev/sda: 320.1 GB, 320072933376 bytes 255 heads, 63 sectors/track, 38913 cylinders, total 625142448 sectorsUnits = sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 512 bytesI/O size (minimum/optimal): 512 bytes / 512 bytesDisk identifier: 0x000e0596 Device Boot Start End Blocks Id System/dev/sda1 * 2048 607428607 303713280 83 Linux/dev/sda2 607430654 625141759 8855553 5 Extended/dev/sda5 607430656 625141759 8855552 82 Linux swap / SolarisDisk /dev/sdb: 320.1 GB, 320072933376 bytes255 heads, 63 sectors/track, 38913 cylinders, total 625142448 sectorsUnits = sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 512 bytesI/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x000c3c41 Device Boot Start End Blocks Id System /dev/sdb1 * 63 625137344 312568641 c W95 FAT32 (LBA) " and fromsudo fdisk /dev/sda1I got fdisk: unable to read /dev/sda1: Inappropriate ioctl for device` From sudo mount /dev/sda1 /mntI got: mount: wrong fs type, bad option, bad superblock on /dev/sda1, missing codepage or helper program, or other error In some cases useful info is found in syslog - try dmesg | tail or so From sudo update-grubI got: error: cannot read from `/dev/sda'. /usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?).

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• 11.10 system crashes for no apparent reason [closed]

  - by varanoid

  I'm relatively new to Linux, and while I know a fair amount about computers and programming, it certainly isn't my specialty. I have a dual boot with Windows 7, and it has been working very well for me until recently. Just randomly the computer will freeze. The last time I was smart enough to keep the System Monitor open when it happened, and it looks like at the time of freezing "bash" and a bunch of other processes that I don't recognize seem to have flooded the memory. So it looks like my memory is getting overloaded and this is what is crashing the system, but I honestly have no idea what could be doing it. Generally I have a bunch of programs running, but they don't take much RAM or CPU: Transmission, Libre Office Writer, Firefox, Empathy, and Banshee. Sometimes I also have Text Editor and Terminal open, but it crashes regardless. When it crashes, it seems that all of the programs are working fine but things like windows, the taskbar, and operations like Alt+Tab just stop working properly or at all. Sometimes the mouse and keyboard freeze and I have to power off manually. Other than that I don't know what the problem is. The only irregularity I've experienced is that I can't download "Debian package management system" even though other updates download fine.

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• Wrappers/law of demeter seems to be an anti-pattern...

  - by Robert Fraser

  I've been reading up on this "Law of Demeter" thing, and it (and pure "wrapper" classes in general) seem to generally be anti patterns. Consider an implementation class: class Foo { void doSomething() { /* whatever */ } } Now consider two different implementations of another class: class Bar1 { private static Foo _foo = new Foo(); public static Foo getFoo() { return _foo; } } class Bar2 { private static Foo _foo = new Foo(); public static void doSomething() { _foo.doSomething(); } } And the ways to call said methods: callingMethod() { Bar1.getFoo().doSomething(); // Version 1 Bar2.doSomething(); // Version 2 } At first blush, version 1 seems a bit simpler, and follows the "rule of Demeter", hide Foo's implementation, etc, etc. But this ties any changes in Foo to Bar. For example, if a parameter is added to doSomething, then we have: class Foo { void doSomething(int x) { /* whatever */ } } class Bar1 { private static Foo _foo = new Foo(); public static Foo getFoo() { return _foo; } } class Bar2 { private static Foo _foo = new Foo(); public static void doSomething(int x) { _foo.doSomething(x); } } callingMethod() { Bar1.getFoo().doSomething(5); // Version 1 Bar2.doSomething(5); // Version 2 } In both versions, Foo and callingMethod need to be changed, but in Version 2, Bar also needs to be changed. Can someone explain the advantage of having a wrapper/facade (with the exception of adapters or wrapping an external API or exposing an internal one).

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• dynamic behavior of factory class

  - by manu1001

  I have a factory class that serves out a bunch of properties. Now, the properties might come either from a database or from a properties file. This is what I've come up with. public class Factory { private static final INSTANCE = new Factory(source); private Factory(DbSource source) { // read from db, save properties } private Factory(FileSource source) { // read from file, save properties } // getInstance() and getProperties() here } What's a clean way of switching between these behaviors based on the environment. I want to avoid having to recompile the class each time.

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• QuestionOrAnswer model?

  - by Mark

  My site has Listings. Users can ask Questions about listings, and the author of the listing can respond with an Answer. However, the Answer might need clarification, so I've made them recursive (you can "answer" an answer). So how do I set up the database? The way I have it now looks like this (in Django-style models): class QuestionOrAnswer(Model): user = ForeignKey(User, related_name='questions') listing = ForeignKey(Listing, related_name='questions') parent = models.ForeignKey('self', null=True, blank=True, related_name='children') message = TextField() But what bugs me is that listing is now an attribute of the answers as well (it doesn't need to be). What happens if the database gets mangled and an answer belongs to a different listing than its parent question? That just doesn't make any sense. We can separate it with polymorphism: QuestionOrAnswer user message created updated Question(QuestionOrAnswer) shipment Answer(QuestionOrAnswer) parent = ForeignKey(QuestionOrAnswer) And that ought to work, but now ever question and answer is split into 2 tables. Is it worth this overhead for clearly defined models?

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• Help naming a class that has a single public method called Execute()

  - by devoured elysium

  I have designed the following class that should work kind of like a method (usually the user will just run Execute()): public abstract class ??? { protected bool hasFailed = false; protected bool hasRun = false; public bool HasFailed { get { return hasFailed; } } public bool HasRun { get { return hasRun; } } private void Restart() { hasFailed = false; hasRun = false; } public bool Execute() { ExecuteImplementation(); bool returnValue = hasFailed; Restart(); return returnValue; } protected abstract void ExecuteImplementation(); } My question is: how should I name this class? Runnable? Method(sounds awkward)?

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• To identify the classes for uml diagrams?

  - by user106535

  I want to implement a software engineering project based on "crime management system". The main modules are: visitors, users, administrator. The main events that are taking place are: registration, report complaint, report crime, report most wanted, view status of reported crime. So could you please help me to identify the classes that are to be used in this project and help me to draw the class diagram?

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• Alternatives to a leveling system

  - by Bane

  I'm currently designing a rough prototype of a mecha fighting game. These are the basics I came up with: Multiplayer (matchmaking for up to 10 people, for now) Browser based (HTML5) 2D (<canvas>) Persistent (as in, players have accounts and don't have to use a new mech each time they start a match) Players earn money upon destroying another mech, which is used to buy parts (guns, armor, boosters, etc) Simplicity (both of the game itself, and of the development of said game) No "leveling" (as in, players don't get awarded with XP) The last part is bothering me. At first, I wanted to have players gain experience points (XP) when destroying other mechs, but gaining two things at once (money and XP) seemed to be in conflict with my last point, which is simplicity. If I were to have a leveling system, that would require additional development. But, the biggest problem is that I simply couldn't fit it anywhere! Adding levels would require adding meaning to these levels, and most of the things that I hoped to achieve could already be achieved with the money mechanic I introduced. So I decided to drop leveling off completely. That, in turn, removed a fairly popular and robust mean of progression in games from my game (not that I would use it well anyway). Is there another way of progression in games, aside from leveling and XP points, that wouldn't get rendered redundant by my money mechanic, would be somehow meaningful (even on a symbolic level), and wouldn't be in conflict with my last point, which is simplicity?

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• Recommendations for a web-based help system

  - by ggkmath

  Hi Experts, I'm putting together a fairly large GUI. I'm a tech person, but more on the hardware side, not software. I'm wondering what software package would be best suited to enable me to generate a web-based help-system. Preferably it takes care of a lot of coding, allowing me to focus on the content. For example, the user would click on a link in the GUI when they have a question, that brings them to a web-based Help Guide, for example, providing an overview of how to use the GUI, perhaps a searchable index (key-word based index), table of contents, etc, for navigating through the Help Guide. My first thought was to program everything in XHTML using Dreamweaver, but my layout requirements are fairly modest (just figures and text, maybe a few equations), and I'd prefer not to spend a lot of time concentrating on the programming. Was wondering if any software existed that made generating web-based navigatable pages easy to create/publish. Again, I'm not really a programmer, so if there's something obvious out there, I'm probably not aware of it. Any advice much appreciated! Thanks in advance.

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• Material System

  - by Towelie

  I'm designing Material/Shader System (target API DX10+ and may be OpenGL3+, now only DX10). I know, there was a lot of topics about this, but i can't find what i need. I don't want to do some kind of compilation/parsing scripts in real-time. So there some artist-created material, written at some analog of CG. After it compiled to hlsl code and after to final shader. Also there are some hard-coded ConstantBuffers, like cbuffer EveryFrameChanging { float4x4 matView; float time; float delta; } And shader use shared constant buffers to get parameters. For each mesh in the scene, getting needs and what it can give (normals, binormals etc.) and finding corresponding permutation of shader or calculating missing parts. Also, during build calculating render states and the permutations or hash for this shader which later will be used for sorting or even giving the ID from 0 to ShaderCount w/o gaps to it for sorting. FinalShader have only 1 technique and one pass. After it for each Mesh setting some shader and it's good to render. some pseudo code SetConstantBuffer(ConstantBuffer::PerFrame); foreach (shader in FinalShaders) SetConstantBuffer(ConstantBuffer::PerShader, shader); SetRenderState(shader); foreach (mesh in shader.GetAllMeshes) SetConstantBuffer(ConstantBuffer::PerMesh, mesh); SetBuffers(mesh); Draw(); class FinalShader { public: UUID m_ID; RenderState m_RenderState; CBufferBindings m_BufferBindings; } But i have no idea how to create this CG language and do i really need it?

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• Minimize useless tweaking of a numeric app

  - by Potatoswatter

  I'm developing a numeric application (nonlinear optimizer), with a zillion knobs to tweak and rising. It's not my first foray into this domain, but this time there are even more variables in the code and I'm on a tight schedule. Don't want to waste time fiddling. Days or even months can potentially be wasted adjusting variables, recompiling, and reprocessing benchmark datasets. The resulting data is viewed and trouble spots are checked. The overall quality of the solution is reported by the program but the meaning of the report could change over time. (Numeric units for the report are one thing I'm trying to nail down.) One main problem is organizing result files to identify each with specific code changes. Note taking can be a pain, is there software to help with this? Are there agreed best practices to making this kind of development cycle reliably move forward? The solver package converges to its optimal solution with mechanical determination, but I'm all too familiar with the way an excess of design decisions can mire development.

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• Prioritize compiler functionality/tasks, when designing a new language

  - by Mahdi

  Well, the question should be so hard to ask and I expect couple of down votes, however, I'm really interested to have your ideas and recommendations. :) I've already made a very simple compiler, with a few and limited functionality. Now I'm getting more on it to make it more like a real-world compiler. I definitely need to start over 'cause I've much more experience and ideas in this area rather a few years ago. So, I want to know, right now, from the very first step again, which tasks/features for the new compiler should implement first and which tasks has lower priority rather than others? For example, I'd say, first I'd go to decide about the object-oriented structure for the new language, but you might say, hey, just go for a compiler that could define a variable, when you finished that, then start thinking about OOP designs ... I prefer to hear the pros and cons for your suggestions also. Actually I like to start from Bottom to Top, where I could add simplest tasks first, and later adding more complex ones, but I'm totally open for any new ideas, and really appreciate that. Also please consider that I'm thinking about the design concepts. Actually I expect answers like: Priority from Highest to Lowest: variables, because .... functions, because .... loops, because .... ... Not: define a syntax for your new language, and start parsing your source code ...

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• Struggling with the Single Responsibility Principle

  - by AngryBird

  Consider this example: I have a website. It allows users to make posts (can be anything) and add tags that describe the post. In the code, I have two classes that represent the post and tags. Lets call these classes Post and Tag. Post takes care of creating posts, deleting posts, updating posts, etc. Tag takes care of creating tags, deleting tags, updating tags, etc. There is one operation that is missing. The linking of tags to posts. I am struggling with who should do this operation. It could fit equally well in either class. On one hand, the Post class could have a function that takes a Tag as a parameter, and then stores it in a list of tags. On the other hand, the Tag class could have a function that takes a Post as a parameter and links the Tag to the Post. The above is just an example of my problem. I am actually running into this with multiple classes that are all similar. It could fit equally well in both. Short of actually putting the functionality in both classes, what conventions or design styles exist to help me solve this problem. I am assuming there has to be something short of just picking one? Maybe putting it in both classes is the correct answer?

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• From a DDD perspective is a report generating service a domain service or an infrastructure service?

  - by Songo

  Let assume we have the following service whose responsibility is to generate Excel reports: class ExcelReportService{ public String generateReport(String fileFormatFilePath, ResultSet data){ ReportFormat reportFormat = new ReportFormat(fileFormatFilePath); ExcelDataFormatterService excelDataFormatterService = new ExcelDataFormatterService(); FormattedData formattedData = excelDataFormatterService.format(data); ExcelFileService excelFileService = new ExcelFileService(); String reportPath= excelFileService.generateReport(reportFormat,formattedData); return reportPath; } } This is pseudo code for the service I want to design where: fileFormatFilePath: path to a configuration file where I'll keep the format of my excel file (headers, column widths, number of columns,..etc) data: the actual records returned from the database. This data can't be used directly coz I might need to make further calculations to the data before inserting them to the excel file. ReportFormat: Value object to hold the report format, has methods like getHeaders(), getColumnWidth(),...etc. ExcelDataFormatterService: a service to hold any logic that need to be applied to the data returned from the database before inserting it to the file. FormattedData: Value object the represents the formatted data to be inserted. ExcelFileService: a wrapper top the 3rd party library that generates the excel file. Now how do you determine whether a service is an infrastructure or domain service? I have the following 3 services here: ExcelReportService, ExcelDataFormatterService and ExcelFileService?

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• Implementing a ILogger interface to log data

  - by Jon

  I have a need to write data to file in one of my classes. Obviously I will pass an interface into my class to decouple it. I was thinking this interface will be used for testing and also in other projects. This is my interface: //This could be used by filesystem, webservice public interface ILogger { List<string> PreviousLogRecords {get;set;} void Log(string Data); } public interface IFileLogger : ILogger { string FilePath; bool ValidFileName; } public class MyClassUnderTest { public MyClassUnderTest(IFileLogger logger) {....} } [Test] public void TestLogger() { var mock = new Mock<IFileLogger>(); mock.Setup(x => x.Log(Is.Any<string>).AddsDataToList()); //Is this possible?? var myClass = new MyClassUnderTest(mock.Object); myClass.DoSomethingThatWillSplitThisAndLog3Times("1,2,3"); Assert.AreEqual(3,mock.PreviousLogRecords.Count); } This won't work I don't believe as nothing is storing the items so is this possible using Moq and also what do you think of the design of the interface?

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• Handling Types for Real and Complex Matrices in a BLAS Wrapper

  - by mga

  I come from a C background and I'm now learning OOP with C++. As an exercise (so please don't just say "this already exists"), I want to implement a wrapper for BLAS that will let the user write matrix algebra in an intuitive way (e.g. similar to MATLAB) e.g.: A = B*C*D.Inverse() + E.Transpose(); My problem is how to go about dealing with real (R) and complex (C) matrices, because of C++'s "curse" of letting you do the same thing in N different ways. I do have a clear idea of what it should look like to the user: s/he should be able to define the two separately, but operations would return a type depending on the types of the operands (R*R = R, C*C = C, R*C = C*R = C). Additionally R can be cast into C and vice versa (just by setting the imaginary parts to 0). I have considered the following options: As a real number is a special case of a complex number, inherit CMatrix from RMatrix. I quickly dismissed this as the two would have to return different types for the same getter function. Inherit RMatrix and CMatrix from Matrix. However, I can't really think of any common code that would go into Matrix (because of the different return types). Templates. Declare Matrix<T> and declare the getter function as T Get(int i, int j), and operator functions as Matrix *(Matrix RHS). Then specialize Matrix<double> and Matrix<complex>, and overload the functions. Then I couldn't really see what I would gain with templates, so why not just define RMatrix and CMatrix separately from each other, and then overload functions as necessary? Although this last option makes sense to me, there's an annoying voice inside my head saying this is not elegant, because the two are clearly related. Perhaps I'm missing an appropriate design pattern? So I guess what I'm looking for is either absolution for doing this, or advice on how to do better.

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• Parameterized Django models

  - by mgibsonbr

  In principle, a single Django application can be reused in two or more projects, providing functionality relevent to both. That implies that the same database structure (tables and relations) will be re-created identically in different databases, and most times this is not a problem (assuming the projects/databases are unrelated - for instance when someone downloads a complete app to use in their own projects). Sometimes, however, the models must be "tweaked" a little to better fit the problem needs. This can be accomplished by forking the app, but I wondered if there wouldn't be a better option in cases where the app designer can anticipate the most common customizations. For instance, if I have a model that could relate to another as one-to-one or one-to-many, I could specify the unique property as a parameter, that can be specified in the project's settings: class This(models.Model): other = models.ForeignKey(Other, unique=settings.OTHER_TO_THIS) Or if a model can relate to many others, I could create an intermediate table for each of them (thus enforcing referential integrity) instead of using generic fks: for related in settings.MODELS_RELATED_TO_OTHER: model_name = '%s_Other' % related globals()[model_name] = type(model_name, (models.Model,) { me:models.ForeignKey(find_model_class(related)), other:models.ForeignKey(Other), # Some other properties all intersection tables must have }) Etc. Let me stress out that I'm not proposing to change the models at runtime nor anything like that; once the parameters were defined and syncdb called for the first time, those parameters are not to be changed again (unless you're doing a schema migration). Is this a good design? Are there better ways to accomplish the same thing, or maybe drawbacks I coulnd't anticipate? This technique is meant to be used sparingly (only on apps meant to be reused in wildly different contexts, and only when a specific need of customization can be detected while the app model is being designed).

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• Best practice to collect information from child objects

  - by Markus

  I'm regularly seeing the following pattern: public abstract class BaseItem { BaseItem[] children; // ... public void DoSomethingWithStuff() { StuffCollection collection = new StuffCollection(); foreach(child c : children) c.AddRequiredStuff(collection); // do something with the collection ... } public abstract void AddRequiredStuff(StuffCollection collection); } public class ConcreteItem : BaseItem { // ... public override void AddRequiredStuff(StuffCollection collection) { Stuff stuff; // ... collection.Add(stuff); } } Where I would use something like this, for better information hiding: public abstract class BaseItem { BaseItem[] children; // ... public void DoSomethingWithStuff() { StuffCollection collection = new StuffCollection(); foreach(child c : children) collection.AddRange(c.RequiredStuff()); // do something with the collection ... } public abstract StuffCollection RequiredStuff(); } public class ConcreteItem : BaseItem { // ... public override StuffCollection RequiredStuff() { StuffCollection stuffCollection; Stuff stuff; // ... stuffCollection.Add(stuff); return stuffCollection; } } What are pros and cons of each solution? For me, giving the implementation access to parent's information is some how disconcerting. On the other hand, initializing a new list, just to collect the items is a useless overhead ... What is the better design? How would it change, if DoSomethingWithStuff wouldn't be part of BaseItem but a third class? PS: there might be missing semicolons, or typos; sorry for that! The above code is not meant to be executed, but just for illustration.

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• Classes as a compilation unit

  - by Yannbane

  If "compilation unit" is unclear, please refer to this. However, what I mean by it will be clear from the context. Edit: my language allows for multiple inheritance, unlike Java. I've started designing+developing my own programming language for educational, recreational, and potentially useful purposes. At first, I've decided to base it off Java. This implied that I would have all the code be written inside classes, and that code compiles to classes, which are loaded by the VM. However, I've excluded features such as interfaces and abstract classes, because I found no need for them. They seemed to be enforcing a paradigm, and I'd like my language not to do that. I wanted to keep the classes as the compilation unit though, because it seemed convenient to implement, familiar, and I just liked the idea. Then I noticed that I'm basically left with a glorified module system, where classes could be used either as "namespaces", providing constants and functions using the static directive, or as templates for objects that need to be instantiated ("actual" purpose of classes in other languages). Now I'm left wondering: what are the benefits of having classes as compilation units? (Also, any general commentary on my design would be much appreciated.)

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