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  • Representing complex object dependencies

    - by max
    I have several classes with a reasonably complex (but acyclic) dependency graph. All the dependencies are of the form: class X instance contains an attribute of class Y. All such attributes are set during initialization and never changed again. Each class' constructor has just a couple parameters, and each object knows the proper parameters to pass to the constructors of the objects it contains. class Outer is at the top of the dependency hierarchy, i.e., no class depends on it. Currently, the UI layer only creates an Outer instance; the parameters for Outer constructor are derived from the user input. Of course, Outer in the process of initialization, creates the objects it needs, which in turn create the objects they need, and so on. The new development is that the a user who knows the dependency graph may want to reach deep into it, and set the values of some of the arguments passed to constructors of the inner classes (essentially overriding the values used currently). How should I change the design to support this? I could keep the current approach where all the inner classes are created by the classes that need them. In this case, the information about "user overrides" would need to be passed to Outer class' constructor in some complex user_overrides structure. Perhaps user_overrides could be the full logical representation of the dependency graph, with the overrides attached to the appropriate edges. Outer class would pass user_overrides to every object it creates, and they would do the same. Each object, before initializing lower level objects, will find its location in that graph and check if the user requested an override to any of the constructor arguments. Alternatively, I could rewrite all the objects' constructors to take as parameters the full objects they require. Thus, the creation of all the inner objects would be moved outside the whole hierarchy, into a new controller layer that lies between Outer and UI layer. The controller layer would essentially traverse the dependency graph from the bottom, creating all the objects as it goes. The controller layer would have to ask the higher-level objects for parameter values for the lower-level objects whenever the relevant parameter isn't provided by the user. Neither approach looks terribly simple. Is there any other approach? Has this problem come up enough in the past to have a pattern that I can read about? I'm using Python, but I don't think it matters much at the design level.

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  • In database table design, how does "Virtual Goods" affect table design -- should we create an instan

    - by Jian Lin
    When we design a database table for a DVD rental company, we actually have a movie, which is an abstract idea, and a physical DVD, so for each rental, we have a many-to-many table with fields such as: TransactionID UserID DvdID RentedDate RentalDuration AmountPaid but what about with virtual goods? For example, if we let a user rent a movie online for 3 days, we don't actually have a DVD, so we may have a table: TransactionID UserID MovieID RentedDate RentalDuration AmountPaid should we create a record for each instance of "virtual good"? For example, what if this virtual good (the movie) can be authorized to be watched on 3 devices (with 3 device IDs), then should we then create a virtual good record in the VirtualGoods table, each with a VirtualGoodID and then another table that has VirtualGoodID DeviceID to match up the movie with the DeviceIDs? We can also just use the TransactionID as the VirtualGoodID. Are there circumstances where we may want to create a record to record this "virtual good" in a VirtualGoods table?

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  • Which is the better C# class design for dealing with read+write versus readonly

    - by DanM
    I'm contemplating two different class designs for handling a situation where some repositories are read-only while others are read-write. (I don't foresee any need to a write-only repository.) Class Design 1 -- provide all functionality in a base class, then expose applicable functionality publicly in sub classes public abstract class RepositoryBase { protected virtual void SelectBase() { // implementation... } protected virtual void InsertBase() { // implementation... } protected virtual void UpdateBase() { // implementation... } protected virtual void DeleteBase() { // implementation... } } public class ReadOnlyRepository : RepositoryBase { public void Select() { SelectBase(); } } public class ReadWriteRepository : RepositoryBase { public void Select() { SelectBase(); } public void Insert() { InsertBase(); } public void Update() { UpdateBase(); } public void Delete() { DeleteBase(); } } Class Design 2 - read-write class inherits from read-only class public class ReadOnlyRepository { public void Select() { // implementation... } } public class ReadWriteRepository : ReadOnlyRepository { public void Insert() { // implementation... } public void Update() { // implementation... } public void Delete() { // implementation... } } Is one of these designs clearly stronger than the other? If so, which one and why? P.S. If this sounds like a homework question, it's not, but feel free to use it as one if you want :)

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  • What do you do if you reach a design dead-end in evolutionary methods like Agile or XP?

    - by Dipan Mehta
    As I was reading Martin Fowler's famous blog post Is Design Dead?, one of the striking impressions I got is that given the fact that in Agile Methodology and Extreme Programming, the design as well as programming is evolutionary, there are always points where things need to get refactored. It may be possible that when a programmer's level is good, and they understand design implications and don't make critical mistakes, the code continues to evolve. However, in a normal context, what is the ground reality in this context? In a normal day given some significant development goes into product, and when critical change occurs in requirement isn't it a constraint that how much ever we wish, fundamental design aspects cannot be modified? (without throwing away major part of the code). Is it not quite likely that one reaches dead-end on any further possible improvement on design and requirements? I am not advocating any non-Agile practice here, but I want to know from people who practice agile or iterative or evolutionary development methods, as for their real experiences. Have you ever reached such dead-ends? How have you managed to avoid it or escaped it? Or are there measures to ensure that design remains clean and flexible as it evolves?

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  • Should database-models (conceptual or physical) be reviewed by DBAs?

    - by user61852
    Where I work, new applications that are being developed that will use their own relational database, must have their database-models (conceptual, then physical ) reviewed and aproved by DBAs. Things looked after are normalization, antipatterns, table and column naming standards, etc. Is this really a DBA's responsability to do this ? or should it be, in a greater extend, the responsability of app designers and architects ?

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  • DDD: Service or Repository

    - by tikhop
    I am developing an app in DDD manner. And I have a little problem with it. I have a Fare (airline fare) and FareRepository objects. And at some point I should load additional fare information and set this information to existing Fare. I guess that I need to create an Application Service (FareAdditionalInformationService) that will deal with obtaining data from the server and than update existing Fare. However, some people said me that it is necessary to use FareRepository for this problem. I don't know wich place is better for my problem Service or Repository.

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  • Architecture guidelines for a "single page web-app"

    - by Matt Roberts
    I'm going to start a side project to build a "single page" web application. The application needs to be real-time, sending updates to the clients as changes happen. Are there any good resources for best-practice approaches wrt the architecture for these kinds of applications. The best resource I've found so far is the trello architecture article here: http://blog.fogcreek.com/the-trello-tech-stack/ To me, this architecture, although very sexy, is probably over-engineered for my specific needs - although I do have similar requirements. I'm wondering if I need to bother with a sub/pub at the server side, could I not just push updates from the server when something happens (e.g. when the client sends an update to the server, write the update to the db, and then send an update to the clients). Tech-wise, I'm probably looking to build this out in Node.JS or maybe Ruby, although the architecture guidelines should to some extent apply to any underlying server technologies.

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  • Good Software Architecture book or material?

    - by Inder Kumar Rathore
    I am a programmer and there is always a word going around about the architecture of the application/software. I want to gain some knowledge about how to develop good architecture. I know it is something that comes with the experience but I need some start so that I can practice it and get some good experience. So Please refer a good book for architecture. I know "Head first design patterns" is there, should I go for it or is there some good books also. Thanks

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  • When to use Aspect Oriented Architecture (AOA/AOD)

    When is it appropriate to use aspect oriented architecture? I think the only honest answer to this question is that it depends on the context for which the question is being asked. There really are no hard and fast rules regarding the selection of an architectural model(s) for a project because each model provides good and bad benefits. Every system is built with a unique requirements and constraints. This context will dictate when to use one type of architecture over another or in conjunction with others. To me aspect oriented architecture models should be a sub-phase in the architectural modeling and design process especially when creating enterprise level models. Personally, I like to use this approach to create a base architectural model that is defined by non-functional requirements and system quality attributes.   This general model can then be used as a starting point for additional models because it is targets all of the business key quality attributes required by the system.Aspect oriented architecture is a method for modeling non-functional requirements and quality attributes of a system known as aspects. These models do not deal directly with specific functionality. They do categorize functionality of the system. This approach allows a system to be created with a strong emphasis on separating system concerns into individual components. These cross cutting components enables a systems to create with compartmentalization in regards to non-functional requirements or quality attributes.  This allows for the reduction in code because an each component maintains an aspect of a system that can be called by other aspects. This approach also allows for a much cleaner and smaller code base during the implementation and support of a system. Additionally, enabling developers to develop systems based on aspect-oriented design projects will be completed faster and will be more reliable because existing components can be shared across a system; thus, the time needed to create and test the functionality is reduced.   Example of an effective use of Aspect Oriented ArchitectureIn my experiences, aspect oriented architecture can be very effective with large or more complex systems. Typically, these types of systems have a large number of concerns so the act of defining them is very beneficial for reducing the system’s complexity because components can be developed to address each concern while exposing functionality to the other system components. The benefits to using the aspect oriented approach as the starting point for a system is that it promotes communication between IT and the business due to the fact that the aspect oriented models are quality attributes focused so not much technical understanding is needed to understand the model.An example of this can be in developing a new intranet website. Common Intranet Concerns: Error Handling Security Logging Notifications Database connectivity Example of a not as effective use of Aspect Oriented ArchitectureAgain in my experiences, aspect oriented architecture is not as effective with small or less complex systems in comparison.  There is no need to model concerns for a system that has a limited amount of them because the added overhead would not be justified for the actual benefits of creating the aspect oriented architecture model.  Furthermore, these types of projects typically have a reduced time schedule and a limited budget.  The creation of the Aspect oriented models would increase the overhead of a project and thus increase the time needed to implement the system. An example of this is seen by creating a small application to poll a network share for new files and then FTP them to a new location.  The two primary concerns for this project is to monitor a network drive and FTP files to a new location.  There is no need to create an aspect model for this system because there will never be a need to share functionality amongst either of these concerns.  To add to my point, this system is so small that it could be created with just a few classes so the added layer of componentizing the concerns would be complete overkill for this situation. References:Brichau, Johan; D'Hondt, Theo. (2006) Aspect-Oriented Software Development (AOSD) - An Introduction. Retreived from: http://www.info.ucl.ac.be/~jbrichau/courses/introductionToAOSD.pdf

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  • Resources for popular domain models

    - by Songo
    I have come across many situations where I had to build a system for a library or a clinic or other popular domains. The thing is a domain model for a library was probably done 1000 times already with different level of details of course. Here is an example. Is there a popular website or community where one can find ready made domain models for popular systems? The whole purpose I'm trying to achieve is to quickly get a grasp of the domain I'm modeling and customize it to my needs. Re-inventing the wheel seems really absurd when the same system might have been modeled properly previously. Note I know Google might sound like the perfect source, but there is a repository out there that people can post there models, so that others can share them.

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  • DDD and filtering

    - by tikhop
    I am developing an app in ddd maner. So I have a complex domain model. Suppose I have a Fare object and Airline. Each Airline should contain several or much more Fares. My UI should represent Model (only small part of complex model) as a list of Airline, when the user select the Airline, I must show the list of Fares. User can filtering the Fares (by travel time, cost, etc.). What is the appropriate place for filtering Fares and Airlines? I am assuming that I should do it in ViewModel. Like: My domain model has wrapped with Service Layer - UI works with ViewModel - ViewModel obtain data from Service Layer filtering it and create DTO objects for UI. Or I'm wrong?

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  • Simple Architecture Verification

    - by Jean Carlos Suárez Marranzini
    I just made an architecture for an application with the function of scoring, saving and loading tennis games. The architecture has 2 kinds of elements: components & layers. Components: Standalone elements that can be consumed by other components or by layers. They might also consume functionality from the model/bottom layer. Layers: Software components whose functionality rests on previous layers (except for the model layer). -Layers: -Models: Data and it's behavior. -Controllers: A layer that allows interaction between the views and the models. -Views: The presentation layer for interacting with the user. -Components: -Persistence: Makes sure the game data can be stored away for later retrieval. -Time Machine: Records changes in the game through time so it's possible to navigate the game back and forth. -Settings: Contains the settings that determine how some of the game logic will apply. -Game Engine: Contains all the game logic, which it applies to the game data to determine the path the game should take. This is an image of the architecture (I don't have enough rep to post images): http://i49.tinypic.com/35lt5a9.png The requierements which this architecture should satisfy are the following: Save & load games. Move through game history and see how the scoreboard changes as the game evolves. Tie-breaks must be properly managed. Games must be classified by hit-type. Every point can be modified. Match name and player names must be stored. Game logic must be configurable by the user. I would really appreciate any kind of advice or comments on this architecture. To see if it is well built and makes sense as a whole. I took the idea from this link. http://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller

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  • Are the technologies used in an application part of the architecture, or do they represent implementation/detailed design details?

    - by m3th0dman
    When designing and writing documentation for a project an architecture needs to be clearly defined: what are the high-level modules of the system, what are their responsibilities, how do they communicate with each other, what protocols are used etc. But in this list, should the concrete technologies be specified or this is actually an implementation detail and need to be specified at a lower level? For example, consider a distributed application that has two modules which communicate asynchronously via AMQP protocol, mediated by a message broker. The fact that these modules use the Spring AMQP library for sending and receiving messages is a fact that needs to be specified in the architecture or is a lower-level detailed design/implementation detail?

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  • Do you leverage the benefits of the open-closed principle?

    - by Kaleb Pederson
    The open-closed principle (OCP) states that an object should be open for extension but closed for modification. I believe I understand it and use it in conjunction with SRP to create classes that do only one thing. And, I try to create many small methods that make it possible to extract out all the behavior controls into methods that may be extended or overridden in some subclass. Thus, I end up with classes that have many extension points, be it through: dependency injection and composition, events, delegation, etc. Consider the following a simple, extendable class: class PaycheckCalculator { // ... protected decimal GetOvertimeFactor() { return 2.0M; } } Now say, for example, that the OvertimeFactor changes to 1.5. Since the above class was designed to be extended, I can easily subclass and return a different OvertimeFactor. But... despite the class being designed for extension and adhering to OCP, I'll modify the single method in question, rather than subclassing and overridding the method in question and then re-wiring my objects in my IoC container. As a result I've violated part of what OCP attempts to accomplish. It feels like I'm just being lazy because the above is a bit easier. Am I misunderstanding OCP? Should I really be doing something different? Do you leverage the benefits of OCP differently? Update: based on the answers it looks like this contrived example is a poor one for a number of different reasons. The main intent of the example was to demonstrate that the class was designed to be extended by providing methods that when overridden would alter the behavior of public methods without the need for changing internal or private code. Still, I definitely misunderstood OCP.

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  • Building a template engine - starting point

    - by Anirudh
    We're building a Django-based project with a template component. This component will be separate from the project as such and can be Django/Python, Node, Java or whatever works. The template has to be rendered into HTML. The templates will contain references to objects with properties that are defined in the DB, say, a Bus. For eg, it could be something like [object type="vehicle" weight="heavy"] and it would have to pull a random object from the DB fulfilling the criteria : type="vehicle" weight="heavy" (bus/truck/jet) and then substitute that tag with an image, say, of a Bus. Also it would have to be able to handle some processing. Eg: What is [X type="integer" lte="10"] + [Y type="integer" lte="10"] [option X+Y correct_ans="true"] [option X-Y correct_ans="false"] [option X+y+1 correct_ans="false"] The engine would be expected to fill in a random integer value <= 10 for X and Y and show radioboxes for each of the options. Would also have to store the fact that the first option is the correct answer. Does it to make sense to write something from the scratch? Or is it better to use an existing templating system (like Django's own templating system) as a starting point? Any suggestions on how I can approach this?

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  • DDD / Layers and legacy systems

    - by CSM
    I have to refactor a complex C# app (many dialogs, mixed logic and so on). There is a part managing the communication with special hardware equipments (sending commands and receive data via asynchronous c# callbacks). The code is "spaghetti" with mixed UI/Logic/Communication/etc and my task is to split the layers in a DDD sense. So, to which layer belongs a callback driver routine? The callbacks are creating "bubbles" in the system, up to the UI layer and because of this I cannot enforce the essential principle that any element of a layer depends only on other elements in the same layer or on elements of the layers "beneath" it. Thank you in advance.

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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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  • Can manager classes be a sign of bad architecture?

    - by Paul
    Lately I've begun to think that having lots of manager classes in your design is a bad thing. The idea hasn't matured enough for me to make a compelling argument, but here's a few general points: I found it's a lot harder for me to understand systems that rely heavily on "managers". This is because, in addition to the actual program components, you also have to understand how and why the manager is used. Managers, a lot of the time, seem to be used to alleviate a problem with the design, like when the programmer couldn't find a way to make the program Just WorkTM and had to rely on manager classes to make everything operate correctly. Of course, mangers can be good. An obvious example is an EventManager, one of my all time favorite constructs. :P My point is that managers seem to be overused a lot of the time, and for no good reason other than mask a problem with the program architecture. Are manager classes really a sign of bad architecture?

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  • MVVM application architecture, where to put dependency injection configuration class, BusinessLayer and Common interfaces?

    - by gt.guybrush
    Planning my architecture for an MVVM application I come to this: MyApp.UI View MyApp.BusinessLayer ViewModel MyApp.DataAccessLayer RepositoryImplEF MyApp.DomainLayer DomainObject RepositoryInterface MyApp.Common Logging Security Utility (contains some reflection method used by many levels) CustomException MyApp.UnitTest I was inspired by Domain-driven-desing, test-driven-development and onion architecture but not sure to have done all well. I am not sure of a couple of things: where to put dependency injection configuration class? In the common project? where to put BusinessLayer interfaces? in Domain layer? where to put Common interfaces? in Domain layer? But Common in referenced from domain (for some reflection utilities and for DI if the response to 1. is yes) and circular reference isn't good

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  • Is it better to return NULL or empty values from functions/methods where the return value is not present?

    - by P B
    I am looking for a recommendation here. I am struggling with whether it is better to return NULL or an empty value from a method when the return value is not present or cannot be determined. Take the following two methods as an examples: string ReverseString(string stringToReverse) // takes a string and reverses it. Person FindPerson(int personID) // finds a Person with a matching personID. In ReverseString(), I would say return an empty string because the return type is string, so the caller is expecting that. Also, this way, the caller would not have to check to see if a NULL was returned. In FindPerson(), returning NULL seems like a better fit. Regardless of whether or not NULL or an empty Person Object (new Person()) is returned the caller is going to have to check to see if the Person Object is NULL or empty before doing anything to it (like calling UpdateName()). So why not just return NULL here and then the caller only has to check for NULL. Does anyone else struggle with this? Any help or insight is appreciated.

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  • Object oriented wrapper around a dll

    - by Tom Davies
    So, I'm writing a C# managed wrapper around a native dll. The dll contains several hundred functions. In most cases, the first argument to each function is an opaque handle to a type internal to the dll. So, an obvious starting point for defining some classes in the wrapper would be to define classes corresponding to each of these opaque types, with each instance holding and managing the opaque handle (passed to its constructor) Things are a little awkward when dealing with callbacks from the dll. Naturally, the callback handlers in my wrapper have to be static, but the callbacks arguments invariable contain an opaque handle. In order to get from the static callback back to an object instance, I've created a static dictionary in each class, associating handles with class instances. In the constructor of each class, an entry is put into the dictionary, and this entry is then removed in the Destructors. When I receive a callback, I can then consult the dictionary to retrieve the class instance corresponding to the opaque reference. Are there any obvious flaws to this? Something that seems to be a problem is that the existence static dictionary means that the garbage collector will not act on my class instances that are otherwise unreachable. As they are never garbage collected, they never get removed from the dictionary, so the dictionary grows. It seems I might have to manually dispose of my objects, which is something absolutely would like to avoid. Can anyone suggest a good design that allows me to avoid having to do this?

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  • Abstract Design Pattern implementation

    - by Pathachiever11
    I started learning design patterns a while ago (only covered facade and abstract so far, but am enjoying it). I'm looking to apply the Abstract pattern to a problem I have. The problem is: Supporting various Database systems using one abstract class and a set of methods and properties, which then the underlying concrete classes (inheriting from abstract class) would be implementing. I have created a DatabaseWrapper abstract class and have create SqlClientData and MSAccessData concrete class that inherit from the DatabaseWrapper. However, I'm still a bit confused about how the pattern goes as far as implementing these classes on the Client. Would I do the following?: DatabaseWrapper sqlClient = new SqlClientData(connectionString); This is what I saw in an example, but that is not what I'm looking for because I want to encapsulate the concrete classes; I only want the Client to use the abstract class. This is so I can support for more database systems in the future with minimal changes to the Client, and creating a new concrete class for the implementations. I'm still learning, so there might be a lot of things wrong here. Please tell me how I can encapsulate all the concrete classes, and if there is anything wrong with my approach. Many Thanks! PS: I'm very excited to get into software architecture, but still am a beginner, so take it easy on me. :)

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  • How do we keep dependent data structures up to date?

    - by Geo
    Suppose you have a parse tree, an abstract syntax tree, and a control flow graph, each one logically derived from the one before. In principle it is easy to construct each graph given the parse tree, but how can we manage the complexity of updating the graphs when the parse tree is modified? We know exactly how the tree has been modified, but how can the change be propagated to the other trees in a way that doesn't become difficult to manage? Naturally the dependent graph can be updated by simply reconstructing it from scratch every time the first graph changes, but then there would be no way of knowing the details of the changes in the dependent graph. I currently have four ways to attempt to solve this problem, but each one has difficulties. Nodes of the dependent tree each observe the relevant nodes of the original tree, updating themselves and the observer lists of original tree nodes as necessary. The conceptual complexity of this can become daunting. Each node of the original tree has a list of the dependent tree nodes that specifically depend upon it, and when the node changes it sets a flag on the dependent nodes to mark them as dirty, including the parents of the dependent nodes all the way down to the root. After each change we run an algorithm that is much like the algorithm for constructing the dependent graph from scratch, but it skips over any clean node and reconstructs each dirty node, keeping track of whether the reconstructed node is actually different from the dirty node. This can also get tricky. We can represent the logical connection between the original graph and the dependent graph as a data structure, like a list of constraints, perhaps designed using a declarative language. When the original graph changes we need only scan the list to discover which constraints are violated and how the dependent tree needs to change to correct the violation, all encoded as data. We can reconstruct the dependent graph from scratch as though there were no existing dependent graph, and then compare the existing graph and the new graph to discover how it has changed. I'm sure this is the easiest way because I know there are algorithms available for detecting differences, but they are all quite computationally expensive and in principle it seems unnecessary so I'm deliberately avoiding this option. What is the right way to deal with these sorts of problems? Surely there must be a design pattern that makes this whole thing almost easy. It would be nice to have a good solution for every problem of this general description. Does this class of problem have a name?

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  • Is component-based design an architectural pattern or design pattern?

    - by xEnOn
    When using the component-based paradigm in game development with engines like Unity, is component-based design an architectural pattern, or a design pattern? Can I even say that component-based design is my "main" architectural pattern for my game? I see architectural patterns as being more high-level than design pattern. The component-based design in game development's context (like with Unity engine) seems to fit as an architectural pattern to me. However, on some sites, I read that component-based design is a behavioural pattern, much like other behavioural design patterns, and not so much like an architectural pattern like MVC.

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  • Objective-c design advice for use of different data sources, swapping between test and live

    - by user200341
    I'm in the process of designing an application that is part of a larger piece of work, depending on other people to build an API that the app can make use of to retrieve data. While I was thinking about how to setup this project and design the architecture around it, something occurred to me, and I'm sure many people have been in similar situations. Since my work is depending on other people to complete their tasks, and a test server, this slows work down at my end. So the question is: What's the best practice for creating test repositories and classes, implementing them, and not having to depend on altering several places in the code to swap between the test classes and the actual repositories / proper api calls. Contemplate the following scenario: GetDataFromApiCommand *getDataCommand = [[GetDataFromApiCommand alloc]init]; getDataCommand.delegate = self; [getDataCommand getData]; Once the data is available via the API, "GetDataFromApiCommand" could use the actual API, but until then a set of mock data could be returned upon the call of [getDataCommand getData] There might be multiple instances of this, in various places in the code, so replacing all of them wherever they are, is a slow and painful process which inevitably leads to one or two being overlooked. In strongly typed languages we could use dependency injection and just alter one place. In objective-c a factory pattern could be implemented, but is that the best route to go for this? GetDataFromApiCommand *getDataCommand = [GetDataFromApiCommandFactory buildGetDataFromApiCommand]; getDataCommand.delegate = self; [getDataCommand getData]; What is the best practices to achieve this result? Since this would be most useful, even if you have the actual API available, to run tests, or work off-line, the ApiCommands would not necessarily have to be replaced permanently, but the option to select "Do I want to use TestApiCommand or ApiCommand". It is more interesting to have the option to switch between: All commands are test and All command use the live API, rather than selecting them one by one, however that would also be useful to do for testing one or two actual API commands, mixing them with test data. EDIT The way I have chosen to go with this is to use the factory pattern. I set up the factory as follows: @implementation ApiCommandFactory + (ApiCommand *)newApiCommand { // return [[ApiCommand alloc]init]; return [[ApiCommandMock alloc]init]; } @end And anywhere I want to use the ApiCommand class: GetDataFromApiCommand *getDataCommand = [ApiCommandFactory newApiCommand]; When the actual API call is required, the comments can be removed and the mock can be commented out. Using new in the message name implies that who ever uses the factory to get an object, is responsible for releasing it (since we want to avoid autorelease on the iPhone). If additional parameters are required, the factory needs to take these into consideration i.e: [ApiCommandFactory newSecondApiCommand:@"param1"]; This will work quite well with repositories as well.

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