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• What is a good design pattern / lib for iOS 5 to synchronize with a web service?

  - by Junto

  We are developing an iOS application that needs to synchronize with a remote server using web services. The existing web services have an "operations" style rather than REST (implemented in WCF but exposing JSON HTTP endpoints). We are unsure of how to structure the web services to best fit with iOS and would love some advice. We are also interested in how to manage the synchronization process within iOS. Without going into detailed specifics, the application allows the user to estimate repair costs at a remote site. These costs are broken down by room and item. If the user has an internet connection this data can be sent back to the server. Multiple photographs can be taken of each item, but they will be held in a separate queue, which sends when the connection is optimal (ideally wifi). Our backend application controls the unique ids for each room and item. Thus, each time we send these costs to the server, the server echoes the central database ids back, thus, that they can be synchronized in the mobile app. I have simplified this a little, since the operations contract is actually much larger, but I just want to illustrate the basic requirements without complicating matters. Firstly, the web service architecture: We currently have two operations: GetCosts and UpdateCosts. My assumption is that if we used a strict REST architecture we would need to break our single web service operations into multiple smaller services. This would make the services much more chatty and we would also have to guarantee a delivery order from the app. For example, we need to make sure that containing rooms are added before the item. Although this seems much more RESTful, our perception is that these extra calls are expensive connections (security checks, database calls, etc). Does the type of web api (operation over service focus) determine chunky vs chatty? Since this is mobile (3G), are we better handling lots of smaller messages, or a few large ones? Secondly, the iOS side. What is the current advice on how to manage data synchronization within the iOS (5) app itself. We need multiple queues and we need to guarantee delivery order in each queue (and technically, ordering between queues). The server needs to control unique ids and other properties and echo them back to the application. The application then needs to update an internal database and when re-updating, make sure the correct ids are available in the update message (essentially multiple inserts and updates in one call). Our backend has a ton of business logic operating on these cost estimates. We don't want any of this in the app itself. Currently the iOS app sends the cost data, and then the server echoes that data back with populated ids (and other data). The existing cost data is deleted and the echoed response data is added to the client database on the device. This is causing us problems, because any photos might not have been sent, but the original entity tree has been removed and replaced. Obviously updating the costs tree rather than replacing it would remove this problem, but I'm not sure if there are any nice xcode libraries out there to do such things. I welcome any advice you might have.

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• Strategy to prevent players from seeing through walls in an online FPS?

  - by geneotech

  Why do we still moan on wallhackers in multiplayer first-person shooters ? Isn't it possible to perform occlusion culling for all players server-side ? For example, send player xyz information to client only when the player is visible in client's frustum and not occluded by any object ? Even if the collision-geometry is very simplified, most of the time cheater won't receive tactical information. Why not do this ?

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• Query something and return the reason if nothing has been found

  - by Daniel Hilgarth

  Assume I have a Query - as in CQS that is supposed to return a single value. Let's assume that the case that no value is found is not exceptional, so no exception will be thrown in this case. Instead, null is returned. However, if no value has been found, I need to act according to the reason why no value has been found. Assuming that the Query knows the reason, how would I communicate it to the caller of the Query? A simple solution would be not return the value directly but a container object that contains the value and the reason: public class QueryResult { public TValue Value { get; private set; } public TReason ReasonForNoValue { get; private set; } } But that feels clumsy, because if a value is found, ReasonForNoValue makes no sense and if no value has been found, Value makes no sense. What other options do I have to communicate the reason? What do you think of one event per reason? For reference: This is going to be implemented in C#.

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• What is the best approach to solve a factory method problem which has to be an instance?

  - by Iago

  I have to add new funcionality in a web service legacy project and I'm thinking what is the best approach for a concrete situation. The web service is simple: It receives a XML file, unmarshalling, generates response's objects, marshalling and finally it sends the response as a XML file. For every XML files received, the web service always responds with the same XML structure. What I have to do is to generate a different XML file according to the XML received. So I have a controller class which has all marshalling/unmarshalling operations, but this controller class has to be an instance. Depending on XML received I need some marshalling methods or others. Trying to make few changes on legacy source, what is the best approach? My first approach was to do a factory method pattern with the controller class, but this class has to be an instance. I want to keep, as far as it goes, this structure: classController.doMarshalling(); I think this one is a bit smelly: if(XMLReceived.isTypeOne()) classController.doMarshallingOne(); else if(XMLReceived.isTypeTwo()) classController.doMarshallingTwo(); else if(XMLReceived.isTypeThree()) classController.doMarshallingThree(); else if ... I hope my question is well understood

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• prism and multiple screens

  - by Avi

  OK - I am studying Prism a little because of a "free weekend" offer on Pluralsight. As this is proving too complex for me, I went to the Prism book and looked at the forward, and this is what it said: What comes after “Hello, World?” WPF and Silverlight developers are blessed with an abundance of excellent books... There’s no lack of tutorials on Model-View-ViewModel ... But they stop short of the guidance you need to deliver a non-trivial application in full. Your first screen goes well. You add a second screen and a third. Because you started your solution with the built-in “Navigation Application Template,” adding new screens feels like hanging shirts on a closet rod. You are on a roll. Until the harsh reality of real application requirements sets in. As it happens, your application has 30 screens not three. There’s no room on that closet rod for 30 screens. Some screens are modal pop-ups; you don’t navigate to a pop-up. Screens become interdependent such that user activity in one screen triggers changes that propagate throughout the UI. Some screens are optional; others are visible only to authorized users. Some screens are permanent, while other screens can be opened and closed at will. You discover that navigating back to a previously displayed screen creates a new instance. That’s not what you expected and, to your horror, the prior instance is gone along with the user’s unsaved changes. Now the issue is, I don't relate to this description. I've never been a UI programmer, but same as everyone else I'm using Windows apps such as MS-Office, and web sites such as Amazon, Facebook and StackExchange. And I look at these and I don't see many "so many screens" issues! Indeed, the only applications having many windows I can think of is Visual Studio. Maybe also Visio, a little. But take Word - You have a ribbon and a main window. Or take Facebook: You have those lists on the left (Favorites, Lists, Groups etc.), the status middle, the adds and then the Contacts sidebar. But it's only one page. Of course, I understand that in enterprise scenarios there are dashboad applications where multiple segments of the screen are updated from multiple non-related services. This I dig. But other scenarios? So - What am I missing? What is the "multiple screens" monster Pirsm is supposed to be the silver bullet solution for? Shoud I invest in studying Prism in addition to learning WPF or ASP.NET MVC?

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• How to store prices that have effective dates?

  - by lal00

  I have a list of products. Each of them is offered by N providers. Each providers quotes us a price for a specific date. That price is effective until that provider decides to set a new price. In that case, the provider will give the new price with a new date. The MySQL table header currently looks like: provider_id, product_id, price, date_price_effective Every other day, we compile a list of products/prices that are effective for the current day. For each product, the list contains a sorted list of the providers that have that particular product. In that way, we can order certain products from whoever happens to offer the best price. To get the effective prices, I have a SQL statement that returns all rows that have date_price_effective >= NOW(). That result set is processed with a ruby script that does the sorting and filtering necessary to obtain a file that looks like this: product_id_1,provider_1,provider_3,provider8,provider_10... product_id_2,provider_3,provider_2,provider1,provider_10... This works fine for our purposes, but I still have an itch that a SQL table is probably not the best way to store this kind of information. I have that feeling that this kind of problema has been solved previously in other more creative ways. Is there a better way to store this information other than in SQL? or, if using SQL, is there a better approach than the one I'm using?

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• Concurrency pattern of logger in multithreaded application

  - by Dipan Mehta

  The context: We are working on a multi-threaded (Linux-C) application that follows a pipeline model. Each module has a private thread and encapsulated objects which do processing of data; and each stage has a standard form of exchanging data with next unit. The application is free from memory leak and is threadsafe using locks at the point where they exchange data. Total number of threads is about 15- and each thread can have from 1 to 4 objects. Making about 25 - 30 odd objects which all have some critical logging to do. Most discussion I have seen about different levels as in Log4J and it's other translations. The real big questions is about how the overall logging should really happen? One approach is all local logging does fprintf to stderr. The stderr is redirected to some file. This approach is very bad when logs become too big. If all object instantiate their individual loggers - (about 30-40 of them) there will be too many files. And unlike above, one won't have the idea of true order of events. Timestamping is one possibility - but it is still a mess to collate. If there is a single global logger (singleton) pattern - it indirectly blocks so many threads while one is busy putting up logs. This is unacceptable when processing of the threads are heavy. So what should be the ideal way to structure the logging objects? What are some of the best practices in actual large scale applications? I would also love to learn from some of the real designs of large scale applications to get inspirations from!

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• Should I limit my type name suffix vocabulary when using OOP?

  - by Den

  My co-workers tend to think that it is better to limit non-domain type suffixes to a small fixed set of OOP-pattern inspired words, e.g.: *Service *Repository *Factory *Manager *Provider I believe there is no reason to not extend that set with more names, e.g. (some "translation" to the previous vocabulary is given in brackets): *Distributor (= *DistributionManager or *SendingService) *Generator *Browser (= *ReadonlyRepositoryService) *Processor *Manipulator (= *StateMachineManager) *Enricher (= *EnrichmentService) (*) denotes some domain word, e.g. "Order", "Student", "Item" etc. The domain is probably not complex enough to use specialized approaches such as DDD which could drive the naming.

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• How to had operation with character/items on binary with concrete operations on C++?

  - by Piperoman

  I have the next problem. A item can had a lot of states: NORMAL = 0000000 DRY = 0000001 HOT = 0000010 BURNING = 0000100 WET = 0001000 COLD = 0010000 FROZEN = 0100000 POISONED= 1000000 A item can had some states at same time but not all of them Is impossible to be dry and wet at same time. If you COLD a WET item, it turns into FROZEN. If you HOT a WET item, it turns into NORMAL A item can be BURNING and POISON Etc. I have try to set binary flags to states, and use AND to set operation to combine different states, checking before if is possible or not to do it, or change to another status. Exist a concrete patron to solve this problem efficiently without had a interminable switch that check every states with everynew states? It is relative easy to check 2 different states, but if exist a third state it is not trivial to do.

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• How should I architect a personal schedule manager that runs 24/7?

  - by Crawford Comeaux

  I've developed an ADHD management system for myself that's attempting to change multiple habits at once. I know this is counter to conventional wisdom, but I've tried the conventional for years & am now trying it my way. (just wanted to say that to try and prevent it from distracting people from the actual question) Anyway, I'd like to write something to run on a remote server that monitors me, helps me build/avoid certain habits, etc. What this amounts to is a system that: runs 24/7 may have multiple independent tasks to run at once may have tasks that require other tasks to run first lets tasks be scheduled by specific time, recurrence (ie. "run every 5 mins"), or interval (ie. "run from 2pm to 3pm") My first naive attempt at this was just a single PHP script scheduled to run every minute by cron (language was chosen in order to use a certain library, but no longer necessary). The logic behind when to run this or that portion of code got hairy pretty quick. So my question is how should I approach this from here? I'm not tied to any one language, though I'm partial to python/javascript. Thoughts: Could be done as a set of scripts that include a scheduling mechanism with one script per bit of logic...but the idea just feels wrong to me. Building it as a daemon could be helpful, but still unsure what to do about dozens of if-else statements for detecting the current time

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• Syncing client and server CRUD operations using json and php

  - by Justin

  I'm working on some code to sync the state of models between client (being a javascript application) and server. Often I end up writing redundant code to track the client and server objects so I can map the client supplied data to the server models. Below is some code I am thinking about implementing to help. What I don't like about the below code is that this method won't handle nested relationships very well, I would have to create multiple object trackers. One work around is for each server model after creating or loading, simply do $model->clientId = $clientId; IMO this is a nasty hack and I want to avoid it. Adding a setCientId method to all my model object would be another way to make it less hacky, but this seems like overkill to me. Really clientIds are only good for inserting/updating data in some scenarios. I could go with a decorator pattern but auto generating a proxy class seems a bit involved. I could use a generic proxy class that uses a __call function to allow for original object data to be accessed, but this seems wrong too. Any thoughts or comments? $clientData = '[{name: "Bob", action: "update", id: 1, clientId: 200}, {name:"Susan", action:"create", clientId: 131} ]'; $jsonObjs = json_decode($clientData); $objectTracker = new ObjectTracker(); $objectTracker->trackClientObjs($jsonObjs); $query = $this->em->createQuery("SELECT x FROM Application_Model_User x WHERE x.id IN (:ids)"); $query->setParameters("ids",$objectTracker->getClientSpecifiedServerIds()); $models = $query->getResults(); //Apply client data to server model foreach ($models as $model) { $clientModel = $objectTracker->getClientJsonObj($model->getId()); ... } //Create new models and persist foreach($objectTracker->getNewClientObjs() as $newClientObj) { $model = new Application_Model_User(); .... $em->persist($model); $objectTracker->trackServerObj($model); } $em->flush(); $resourceResponse = $objectTracker->createResourceResponse(); //Id mappings will be an associtave array representing server id resources with client side // id. //This method Dosen't seem to flexible if we want to return additional data with each resource... //Would have to modify the returned data structure, seems like tight coupling... //Ex return value: //[{clientId: 200, id:1} , {clientId: 131, id: 33}];

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• What is a good design model for my new class?

  - by user66662

  I am a beginning programmer who, after trying to manage over 2000 lines of procedural php code, now has discovered the value of OOP. I have read a few books to get me up to speed on the beginning theory, but would like some advice on practical application. So,for example, let's say there are two types of content objects - an ad and a calendar event. what my application does is scan different websites (a predefined list), and, when it finds an ad or an event, it extracts the data and saves it to a database. All of my objects will share a $title and $description. However, the Ad object will have a $price and the Event object will have $startDate. Should I have two separate classes, one for each object? Should I have a 'superclass' with the $title and $description with two other Ad and Event classes with their own properties? The latter is at least the direction I am on now. My second question about this design is how to handle the logic that extracts the data for $title, $description, $price, and $date. For each website in my predefined list, there is a specific regex that returns the desired value for each property. Currently, I have an extremely large switch statement in my constructor which determines what website I am own, sets the regex variables accordingly, and continues on. Not only that, but now I have to repeat the logic to determine what site I am on in the constructor of each class. This doesn't feel right. Should I create another class Algorithms and store the logic there for each site? Should the functions of to handle that logic be in this class? or specific to the classes whos properties they set? I want to take into account in my design two things: 1) I will add different content objects in the future that share $title and $description, but will have their own properties, so, I want to be able to easily grow these as needed. 2) I will add more websites constantly (each with their own algorithms for data extraction) so I would like to plan efficienty managing and working with these now. I thought about extending the Ad or Event class with 'websiteX' class and store its functions there. But, this didn't feel right either as now I have to manage 100s of little website specific class files. Note, I didn't know if this was the correct site or stackoverflow was the better choice. If so, let me know and I'll post there.

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• Who can change the View in MVC?

  - by Luke

  I'm working on a thick client graph displaying and manipulation application. I'm trying to apply the MVC pattern to our 3D visualization component. Here is what I have for the Model, View, and Controller: Model - The graph and it's metadata. This includes vertices, edges, and the attributes of each. It does not contain position information, icons, colors, or anything display related. View - This would commonly be called a scene graph. It includes the 3D display information, texture information, color information, and anything else that is related specifically to the visualization of the model. Controller - The controller takes the view and displays it in a Window using OpenGL (but it could potentially be any 3D graphics package). The application has various "layouts" that change the position of the vertices in the display. For instance, one layout may arrange the vertices in a circle. Is it common for these layouts to access and change the view directly? Should they go through the Controller to access the View? If they go through the Controller, should they just ask for direct access to the View or should each change go through the controller? I realize this is a bit different from the standard MVC example where there a finite number of Views. In this case, the View can change in an infinite number of ways. Perhaps I'm shattering some basic principle of MVC here. Thanks in advance!

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• When designing a job queue, what should determine the scope of a job?

  - by Stuart Pegg

  We've got a job queue system that'll cheerfully process any kind of job given to it. We intend to use it to process jobs that each contain 2 tasks: Job (Pass information from one server to another) Fetch task (get the data, slowly) Send task (send the data, comparatively quickly) The difficulty we're having is that we don't know whether to break the tasks into separate jobs, or process the job in one go. Are there any best practices or useful references on this subject? Is there some obvious benefit to a method that we're missing? So far we can see these benefits for each method: Split Job lease length reflects job length: Rather than total of two Finer granularity on recovery: If we lose outgoing connectivity we can tell them all to retry The starting state of the second task is saved to job history: Helps with debugging (although similar logging could be added in single task method) Single Single job to be scheduled: Less processing overhead Data not stale on recovery: If the outgoing downtime is quite long, the pending Send jobs could be outdated

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• Best Method of function parameter validation

  - by Aglystas

  I've been dabbling with the idea of creating my own CMS for the experience and because it would be fun to run my website off my own code base. One of the decisions I keep coming back to is how best to validate incoming parameters for functions. This is mostly in reference to simple data types since object validation would be quite a bit more complex. At first I debated creating a naming convention that would contain information about what the parameters should be, (int, string, bool, etc) then I also figured I could create options to validate against. But then in every function I still need to run some sort of parameter validation that parses the parameter name to determine what the value can be then validate against it, granted this would be handled by passing the list of parameters to function but that still needs to happen and one of my goals is to remove the parameter validation from the function itself so that you can only have the actual function code that accomplishes the intended task without the additional code for validation. Is there any good way of handling this, or is it so low level that typically parameter validation is just done at the start of the function call anyway, so I should stick with doing that.

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• ASP.NET design not SOLID

  - by w0051977

  SOLID principles are described here: http://en.wikipedia.org/wiki/SOLID_%28object-oriented_design%29 I am developing a large ASP.NET app. The previous developer created a few very large classes each with lots of different purposes. It is very difficult to maintain and extend. The classes are deployed to the web server along with the code behind files etc. I want to share a small amount of the app with another application. I am considering moving all of the classes of the ASP.NET web app to a DLL, so the small subset of functionality can be shared. I realise it would be better to only share the classes which contain code to be shared but because of the dependencies this is proving to be very difficult e.g. class A contains code that should be shared, however class A contains references to classes B, C, D, E, F, G etc, so class A cannot be shared on its own. I am planning to refactor the code in the future. As a temporary solution I am planning to convert all the classes into a single class library. Is this a bad idea and if so, is there an alternative? as I don't have time to refactor at the moment.

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• Using "prevent execution of method" flags

  - by tpaksu

  First of all I want to point out my concern with some pseudocode (I think you'll understand better) Assume you have a global debug flag, or class variable named "debug", class a : var debug = FALSE and you use it to enable debug methods. There are two types of usage it as I know: first in a method : method a : if debug then call method b; method b : second in the method itself: method a : call method b; method b : if not debug exit And I want to know, is there any File IO or stack pointer wise difference between these two approaches. Which usage is better, safer and why?

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• What's the best way to expose a Model object in a ViewModel?

  - by Angel

  In a WPF MVVM application, I exposed my model object into my viewModel by creating an instance of Model class (which cause dependency) into ViewModel. Instead of creating separate VM properties, I wrap the Model properties inside my ViewModel Property. My model is just an entity framework generated proxy class: public partial class TblProduct { public TblProduct() { this.TblPurchaseDetails = new HashSet<TblPurchaseDetail>(); this.TblPurchaseOrderDetails = new HashSet<TblPurchaseOrderDetail>(); this.TblSalesInvoiceDetails = new HashSet<TblSalesInvoiceDetail>(); this.TblSalesOrderDetails = new HashSet<TblSalesOrderDetail>(); } public int ProductId { get; set; } public string ProductCode { get; set; } public string ProductName { get; set; } public int CategoryId { get; set; } public string Color { get; set; } public Nullable<decimal> PurchaseRate { get; set; } public Nullable<decimal> SalesRate { get; set; } public string ImagePath { get; set; } public bool IsActive { get; set; } public virtual TblCompany TblCompany { get; set; } public virtual TblProductCategory TblProductCategory { get; set; } public virtual TblUser TblUser { get; set; } public virtual ICollection<TblPurchaseDetail> TblPurchaseDetails { get; set; } public virtual ICollection<TblPurchaseOrderDetail> TblPurchaseOrderDetails { get; set; } public virtual ICollection<TblSalesInvoiceDetail> TblSalesInvoiceDetails { get; set; } public virtual ICollection<TblSalesOrderDetail> TblSalesOrderDetails { get; set; } } Here is my ViewModel: public class ProductViewModel : WorkspaceViewModel { #region Constructor public ProductViewModel() { StartApp(); } #endregion //Constructor #region Properties private IProductDataService _dataService; public IProductDataService DataService { get { if (_dataService == null) { if (IsInDesignMode) { _dataService = new ProductDataServiceMock(); } else { _dataService = new ProductDataService(); } } return _dataService; } } //Get and set Model object private TblProduct _product; public TblProduct Product { get { return _product ?? (_product = new TblProduct()); } set { _product = value; } } #region Public Properties public int ProductId { get { return Product.ProductId; } set { if (Product.ProductId == value) { return; } Product.ProductId = value; RaisePropertyChanged("ProductId"); } } public string ProductName { get { return Product.ProductName; } set { if (Product.ProductName == value) { return; } Product.ProductName = value; RaisePropertyChanged(() => ProductName); } } private ObservableCollection<TblProduct> _productRecords; public ObservableCollection<TblProduct> ProductRecords { get { return _productRecords; } set { _productRecords = value; RaisePropertyChanged("ProductRecords"); } } //Selected Product private TblProduct _selectedProduct; public TblProduct SelectedProduct { get { return _selectedProduct; } set { _selectedProduct = value; if (_selectedProduct != null) { this.ProductId = _selectedProduct.ProductId; this.ProductCode = _selectedProduct.ProductCode; } RaisePropertyChanged("SelectedProduct"); } } #endregion //Public Properties #endregion // Properties #region Commands private ICommand _newCommand; public ICommand NewCommand { get { if (_newCommand == null) { _newCommand = new RelayCommand(() => ResetAll()); } return _newCommand; } } private ICommand _saveCommand; public ICommand SaveCommand { get { if (_saveCommand == null) { _saveCommand = new RelayCommand(() => Save()); } return _saveCommand; } } private ICommand _deleteCommand; public ICommand DeleteCommand { get { if (_deleteCommand == null) { _deleteCommand = new RelayCommand(() => Delete()); } return _deleteCommand; } } #endregion //Commands #region Methods private void StartApp() { LoadProductCollection(); } private void LoadProductCollection() { var q = DataService.GetAllProducts(); this.ProductRecords = new ObservableCollection<TblProduct>(q); } private void Save() { if (SelectedOperateMode == OperateModeEnum.OperateMode.New) { //Pass the Model object into Dataservice for save DataService.SaveProduct(this.Product); } else if (SelectedOperateMode == OperateModeEnum.OperateMode.Edit) { //Pass the Model object into Dataservice for Update DataService.UpdateProduct(this.Product); } ResetAll(); LoadProductCollection(); } #endregion //Methods } Here is my Service class: class ProductDataService:IProductDataService { /// <summary> /// Context object of Entity Framework model /// </summary> private MaizeEntities Context { get; set; } public ProductDataService() { Context = new MaizeEntities(); } public IEnumerable<TblProduct> GetAllProducts() { using(var context=new R_MaizeEntities()) { var q = from p in context.TblProducts where p.IsDel == false select p; return new ObservableCollection<TblProduct>(q); } } public void SaveProduct(TblProduct _product) { using(var context=new R_MaizeEntities()) { _product.LastModUserId = GlobalObjects.LoggedUserID; _product.LastModDttm = DateTime.Now; _product.CompanyId = GlobalObjects.CompanyID; context.TblProducts.Add(_product); context.SaveChanges(); } } public void UpdateProduct(TblProduct _product) { using (var context = new R_MaizeEntities()) { context.TblProducts.Attach(_product); context.Entry(_product).State = EntityState.Modified; _product.LastModUserId = GlobalObjects.LoggedUserID; _product.LastModDttm = DateTime.Now; _product.CompanyId = GlobalObjects.CompanyID; context.SaveChanges(); } } public void DeleteProduct(int _productId) { using (var context = new R_MaizeEntities()) { var product = (from c in context.TblProducts where c.ProductId == _productId select c).First(); product.LastModUserId = GlobalObjects.LoggedUserID; product.LastModDttm = DateTime.Now; product.IsDel = true; context.SaveChanges(); } } } I exposed my model object in my viewModel by creating an instance of it using new keyword, also I instantiated my DataService class in VM. I know this will cause a strong dependency. So: What's the best way to expose a Model object in a ViewModel? What's the best way to use DataService in VM?

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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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• Design: How to model / where to store relational data between classes

  - by Walker

  I'm trying to figure out the best design here, and I can see multiple approaches, but none that seems "right." There are three relevant classes here: Base, TradingPost, and Resource. Each Base has a TradingPost which can offer various Resources depending on the Base's tech level. Where is the right place to store the minimum tech level a base must possess to offer any given resource? A database seems like overkill. Putting it in each subclass of Resource seems wrong--that's not an intrinsic property of the Resource. Do I have a mediating class, and if so, how does it work? It's important that I not be duplicating code; that I have one place where I set the required tech level for a given item. Essentially, where does this data belong? P.S. Feel free to change the title; I struggled to come up with one that fits.

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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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• What design pattern do you use to support graceful fallback on old platforms?

  - by JoJo

  Let's say I need to add a drop shadow behind a box. Some old platforms do not support drop shadows, so I have to fake it by putting an image behind the box. Here's the pseudo code of how I'm currently handling this fallback: if (dropShadowsAreSupported) { box.addDropShadow("black"); } else { box.putImageBehindIt("gaussianBlur.png"); } Is this the right way to handle it? It seems too amateur to me. Is there a better design pattern?

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• Does MVC apply only to web

  - by Deeptechtons

  It is almost and instantaneous whenever I talk to developers about Model View Controller (MVC) they say you make a request to a url the server builds a entity (MODEL) and provides you with visual representation of that model. So does this mean MVC is only for the web or have I been meeting people who are just developers who employ MVC for writing web applications? Are there usages for MVC on desktop style applications? I for one am new to paradigm and would like to know of any super-set to MVC

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• How to solve cyclic dependencies in a visitor pattern

  - by Benjamin Rogge

  When programming at work we now and then face a problem with visitors and module/project dependencies. Say you have a class A in a module X. And there are subclasses B and C in module Y. That means that module Y is dependent on module X. If we want to implement a visitor pattern to the class hierarchy, thus introducing an interface with the handle Operations and an abstract accept method in A, we get a dependency from module Y to module X, which we cannot allow for architectural reasons. What we do is, use a direct comparison of the types (i.e. instanceof, since we program in Java), which is not satisfying. My question(s) would be: Do you encounter this kind of problem in your daily work (or do we make poor architectural choices) and if so, how is your approach to solve this?

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• doing a full permutation search and replace on a string

  - by user73307

  I'm writing an app that does something like a custom number (licence) place generator tool where if I ask for the plate "robin" it will suggest I try: r0bin rob1n r0b1n Are there any published algorithms which can do this? It has to be able to handle replacing single letters with multiples, e.g. m with rn and vise-versa and not fall over if it replaces an i with an l then comes to check the l and replaces it back to an i. The list of what gets swapped with what is going to be user input but I'm not expecting a huge list, possibly 10 pairs at most. I'll be implementing this in Ruby or Python but I should be able to convert code from any other language.

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