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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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• What is the proper name for this design pattern in Python?

  - by James

  In Python, is the proper name for the PersonXXX class below PersonProxy, PersonInterface, etc? import rest class PersonXXX(object): def __init__(self,db_url): self.resource = rest.Resource(db_url) def create(self,person): self.resource.post(person.data()) def get(self): pass def update(self): pass def delete(self): pass class Person(object): def __init__(self,name, age): self.name = name self.age = age def data(self): return dict(name=self.name,age=self.age)

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• Strategies to Defeat Memory Editors for Cheating - Desktop Games

  - by ashes999

  I'm assuming we're talking about desktop games -- something the player downloads and runs on their local computer. Many are the memory editors that allow you to detect and freeze values, like your player's health. How do you prevent cheating? What strategies are effective to combat this kind of cheating? I'm looking for some good ones. Two I use that are mediocre are: Displaying values as a percentage instead of the number (eg. 46/50 = 92% health) A low-level class that holds values in an array and moves them with each change

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• Rails: Law of Demeter Confusion

  - by user2158382

  I am reading a book called Rails AntiPatterns and they talk about using delegation to to avoid breaking the Law of Demeter. Here is their prime example: They believe that calling something like this in the controller is bad (and I agree) @street = @invoice.customer.address.street Their proposed solution is to do the following: class Customer has_one :address belongs_to :invoice def street address.street end end class Invoice has_one :customer def customer_street customer.street end end @street = @invoice.customer_street They are stating that since you only use one dot, you are not breaking the Law of Demeter here. I think this is incorrect, because you are still going through customer to go through address to get the invoice's street. I primarily got this idea from a blog post I read: http://www.dan-manges.com/blog/37 In the blog post the prime example is class Wallet attr_accessor :cash end class Customer has_one :wallet # attribute delegation def cash @wallet.cash end end class Paperboy def collect_money(customer, due_amount) if customer.cash < due_ammount raise InsufficientFundsError else customer.cash -= due_amount @collected_amount += due_amount end end end The blog post states that although there is only one dot customer.cash instead of customer.wallet.cash, this code still violates the Law of Demeter. Now in the Paperboy collect_money method, we don't have two dots, we just have one in "customer.cash". Has this delegation solved our problem? Not at all. If we look at the behavior, a paperboy is still reaching directly into a customer's wallet to get cash out. EDIT I completely understand and agree that this is still a violation and I need to create a method in Wallet called withdraw that handles the payment for me and that I should call that method inside the Customer class. What I don't get is that according to this process, my first example still violates the Law of Demeter because Invoice is still reaching directly into Customer to get the street. Can somebody help me clear the confusion. I have been searching for the past 2 days trying to let this topic sink in, but it is still confusing.

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• Designing web-based plugin systems correctly so they don't waste as many resources?

  - by Xeoncross

  Many CMS systems which rely on third parties for much of their code often build "plugin" or "hooks" systems to make it easy for developers to modify the codebase's actions without editing the core files. This usually means an Observer or Event design pattern. However, when you look at systems like wordpress you see that on every page they load some kind of bootstrap file from each of the plugin's folders to see if that plugin will need to run that request. Its this poor design that causes systems like wordpress to spend many extra MB's of memory loading and parsing unneeded items each page. Are there alternative ways to do this? I'm looking for ideas in building my own. For example, Is there a way to load all this once and then cache the results so that your system knows how to lazy-load plugins? In other words, the system loads a configuration file that specifies all the events that plugin wishes to tie into and then saves it for future requests? If that also performs poorly, then perhaps there is a special file-structure that could be used to make educated guesses about when certain plugins are unneeded to fullfil the request. Any ideas? If anyone wants an example of the "plugin" concept you can find one here.

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• Decorator not calling the decorated instance - alternative design needed

  - by Daniel Hilgarth

  Assume I have a simple interface for translating text (sample code in C#): public interface ITranslationService { string GetTranslation(string key, CultureInfo targetLanguage); // some other methods... } A first simple implementation of this interface already exists and simply goes to the database for every method call. Assuming a UI that is being translated at start up this results in one database call per control. To improve this, I want to add the following behavior: As soon as a request for one language comes in, fetch all translations from this language and cache them. All translation requests are served from the cache. I thought about implementing this new behavior as a decorator, because all other methods of that interface implemented by the decorater would simple delegate to the decorated instance. However, the implementation of GetTranslation wouldn't use GetTranslation of the decorated instance at all to get all translations of a certain language. It would fire its own query against the database. This breaks the decorator pattern, because every functionality provided by the decorated instance is simply skipped. This becomes a real problem if there are other decorators involved. My understanding is that a Decorator should be additive. In this case however, the decorator is replacing the behavior of the decorated instance. I can't really think of a nice solution for this - how would you solve it? Everything is allowed, even a complete re-design of ITranslationService itself.

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• Implementing the transport layer for a SIP UAC

  - by Jonathan Henson

  I have a somewhat simple, but specific, question about implementing the transport layer for a SIP UAC. Do I expect the response to a request on the same socket that I sent the request on, or do I let the UDP or TCP listener pick up the response and then route it to the correct transaction from there? The RFC does not seem to say anything on the matter. It seems that especially using UDP, which is connection-less, that I should just let the listeners pick up the response, but that seems sort of counter intuitive. Particularly, I have seen plenty of UAC implementations which do not depend on having a Listener in the transport layer. Also, most implementations I have looked at do not have the UAS receiving loop responding on the socket at all. This would tend to indicate that the client should not be expecting a reply on the socket that it sent the request on. For clarification: Suppose my transport layer consists of the following elements: TCPClient (Sends Requests for a UAC via TCP) UDPClient (Sends Requests for a UAC vid UDP) TCPSever (Loop receiving Requests and dispatching to transaction layer via TCP) UDPServer (Loop receiving Requests and dispatching to transaction layer via UDP) Obviously, the *Client sends my Requests. The question is, what receives the Response? The *Client waiting on a recv or recvfrom call on the socket it used to send the request, or the *Server? Conversely, the *Server receives my requests, What sends the Response? The *Client? doesn't this break the roles of each member a bit?

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• Who should control navigation in an MVVM application?

  - by SonOfPirate

  Example #1: I have a view displayed in my MVVM application (let's use Silverlight for the purposes of the discussion) and I click on a button that should take me to a new page. Example #2: That same view has another button that, when clicked, should open up a details view in a child window (dialog). We know that there will be Command objects exposed by our ViewModel bound to the buttons with methods that respond to the user's click. But, what then? How do we complete the action? Even if we use a so-called NavigationService, what are we telling it? To be more specific, in a traditional View-first model (like URL-based navigation schemes such as on the web or the SL built-in navigation framework) the Command objects would have to know what View to display next. That seems to cross the line when it comes to the separation of concerns promoted by the pattern. On the other hand, if the button wasn't wired to a Command object and behaved like a hyperlink, the navigation rules could be defined in the markup. But do we want the Views to control application flow and isn't navigation just another type of business logic? (I can say yes in some cases and no in others.) To me, the utopian implementation of the MVVM pattern (and I've heard others profess this) would be to have the ViewModel wired in such a way that the application can run headless (i.e. no Views). This provides the most surface area for code-based testing and makes the Views a true skin on the application. And my ViewModel shouldn't care if it displayed in the main window, a floating panel or a child window, should it? According to this apprach, it is up to some other mechanism at runtime to 'bind' what View should be displayed for each ViewModel. But what if we want to share a View with multiple ViewModels or vice versa? So given the need to manage the View-ViewModel relationship so we know what to display when along with the need to navigate between views, including displaying child windows / dialogs, how do we truly accomplish this in the MVVM pattern?

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• Dependency injection and IOC containers in a closed project

  - by Puckl

  Does it make sense to assemble my project with dependency injection containers if I am the only one who will use the code of that project? The question came up when I read this IOC Article http://martinfowler.com/articles/injection.html The justification for using dependency injection in this article is that friends can reuse a class, and replace depending classes with their own classes because they get injected and not instantiated in the class. I would only use it to inject objects where they are needed instead of passing them through layers to their target. (Which is not so bad I learned here: Is it bad practice to pass instances through several layers?) (Maybe I will reuse parts of the project, who knows, but I don´t know if that is a good justification)

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• Pattern for performing game actions

  - by Arkiliknam

  Is there a generally accepted pattern for performing various actions within a game? A way a player can perform actions and also that an AI might perform actions, such as move, attack, self-destruct, etc. I currently have an abstract BaseAction which uses .NET generics to specify the different objects that get returned by the various actions. This is all implemented in a pattern similar to the Command, where each action is responsible for itself and does all that it needs. My reasoning for being abstract is so that I may have a single ActionHandler, and AI can just queue up different action implementing the baseAction. And the reason it is generic is so that the different actions can return result information relevant to the action (as different actions can have totally different outcomes in the game), along with some common beforeAction and afterAction implementations. So... is there a more accepted way of doing this, or does this sound alright?

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• Absolute Top Programming Tips [closed]

  - by Eric

  I'm very intersted in the stuff that REALLY makes a critical difference to career in programming, other than intrinsic stuff like how smart your are, where you were born, etc... Some ideas: 1) Best approach to managing small, medium, and large teams. 2) Most important books to read. 3) Most important skills to know. 4) Correct balance of learning theory vs. just writing code. 5) A good approach to estimating time and cost of a project. 6) Etc... Please limit your answers. If you see somebody has already written your idea, please just vote for their response. I'd like to see what the community thinks are the true indicators of a successful career in our field.

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• Modular enterprise architecture using MVC and Orchard CMS

  - by MrJD

  I'm making a large scale MVC application using Orchard. And I'm going to be separating my logic into modules. I'm also trying to heavily decouple the application for maximum extensibility and testability. I have a rudimentary understanding of IoC, Repository Pattern, Unit of Work pattern and Service Layer pattern. I've made myself a diagram. I'm wondering if it is correct and if there is anything I have missed regarding an extensible application. Note that each module is a separate project. Update So I have many UI modules that use the db module, that's why they've been split up. There are other services the UI modules will use. The UI modules have been split up because they will be made over time, independent of each other.

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• Is having a class have a handleAction(type) method bad practice?

  - by zhenka

  My web application became a little too complicated to do everything in a controller so I had to build large wrapper classes for ORM models. The possible actions a user can trigger are all similar and after a certain point I realized that the best way to go would be to just have constructor method receive action type as a parameter to take care of the small differences internally, as opposed to either passing many arguments or doing a lot of things in the controller. Is this a good practice? I can't really give details for privacy issues.

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• What is the simplest human readable configuration file format?

  - by Juha

  Current configuration file is as follows: mainwindow.title = 'test' mainwindow.position.x = 100 mainwindow.position.y = 200 mainwindow.button.label = 'apply' mainwindow.button.size.x = 100 mainwindow.button.size.y = 30 logger.datarate = 100 logger.enable = True logger.filename = './test.log' This is read with python to a nested dictionary: { 'mainwindow':{ 'button':{ 'label': {'value':'apply'}, ... }, 'logger':{ datarate: {'value': 100}, enable: {'value': True}, filename: {'value': './test.log'} }, ... } Is there a better way of doing this? The idea is to get XML type of behavior and avoid XML as long as possible. The end user is assumed almost totally computer illiterate and basically uses notepad and copy-paste. Thus the python standard "header + variables" type is considered too difficult. The dummy user edits the config file, able programmers handle the dictionaries. Nested dictionary is chosen for easy splitting (logger does not need or even cannot have/edit mainwindow parameters).

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• Recommened design pattern to handle multiple compression algorithms for a class hierarchy

  - by sgorozco

  For all you OOD experts. What would be the recommended way to model the following scenario? I have a certain class hierarchy similar to the following one: class Base { ... } class Derived1 : Base { ... } class Derived2 : Base { ... } ... Next, I would like to implement different compression/decompression engines for this hierarchy. (I already have code for several strategies that best handle different cases, like file compression, network stream compression, legacy system compression, etc.) I would like the compression strategy to be pluggable and chosen at runtime, however I'm not sure how to handle the class hierarchy. Currently I have a tighly-coupled design that looks like this: interface ICompressor { byte[] Compress(Base instance); } class Strategy1Compressor : ICompressor { byte[] Compress(Base instance) { // Common compression guts for Base class ... // if( instance is Derived1 ) { // Compression guts for Derived1 class } if( instance is Derived2 ) { // Compression guts for Derived2 class } // Additional compression logic to handle other class derivations ... } } As it is, whenever I add a new derived class inheriting from Base, I would have to modify all compression strategies to take into account this new class. Is there a design pattern that allows me to decouple this, and allow me to easily introduce more classes to the Base hierarchy and/or additional compression strategies?

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• Are there any examples of a temporal field/object updater?

  - by Bryan Agee

  The system in question has numerous examples of temporal objects and fields--ones which are a certain variable at a certain point in time. An example of this would be someone's rate of pay--there are different answers depending on when you ask and what the constraints might be; eg, can there ever be more than one of a certain temporal object concurrently, etc. Ideally, there would be an object that handles those constraints when a new state/stateful object is introduced; when a new value is set, it would prevent creating negative ranges and overlaps. Martin Fowler has written some great material on this (such as this description of Temporal Objects) , but what I've found of it tends to be entirely theoretic, with no concrete implementations. PHP is the target language, but examples in any language would be most helpful.

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• C# WebForms and Ninject

  - by ipohfly

  I'm re-working on the design of an existing application which is build using WebForms. Currently the plan is to work it into a MVP pattern application while using Ninject as the IoC container. The reason for Ninject to be there is that the boss had wanted a certain flexibility within the system so that we can build in different flavor of business logic in the model and let the programmer to choose which to use based on the client request, either via XML configuration or database setting. I know that Ninject have no need for XML configuration, however I'm confused on how it can help to dynamically inject the dependency into the system? Imagine I have a interface IMember and I need to bind this interface to the class decided by a xml or database configuration at the launch of the application, how can I achieve that?

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• MVC + 3 tier; where ViewModels come into play?

  - by mikhairu

  I'm designing a 3-tiered application using ASP.NET MVC 4. I used the following resources as a reference. CodeProject: MVC + N-tier + Entity Framework Separating data access in ASP.NET MVC I have the following desingn so far. Presentation Layer (PL) (main MVC project, where M of MVC was moved to Data Access Layer): MyProjectName.Main Views/ Controllers/ ... Business Logic Layer (BLL): MyProjectName.BLL ViewModels/ ProjectServices/ ... Data Access Layer (DAL): MyProjectName.DAL Models/ Repositories.EF/ Repositories.Dapper/ ... Now, PL references BLL and BLL references DAL. This way lower layer does not depend on the one above it. In this design PL invokes a service of the BLL. PL can pass a View Model to BLL and BLL can pass a View Model back to PL. Also, BLL invokes DAL layer and DAL layer can return a Model back to BLL. BLL can in turn build a View Model and return it to PL. Up to now this pattern was working for me. However, I've ran into a problem where some of my ViewModels require joins on several entities. In the plain MVC approach, in the controller I used a LINQ query to do joins and then select new MyViewModel(){ ... }. But now, in the DAL I do not have access to where ViewModels are defined (in the BLL). This means I cannot do joins in DAL and return it to BLL. It seems I have to do separate queries in DAL (instead of joins in one query) and BLL would then use the result of these to build a ViewModel. This is very inconvenient, but I don't think I should be exposing DAL to ViewModels. Any ideas how I can solve this dilemma? Thanks.

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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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• Is there a good design pattern for this messaging class?

  - by salonMonsters

  Is there a good design pattern for this? I want to create a messaging class. The class will be passed: the type of message (eg. signup, signup confirmation, password reminder etc) the client's id The class needs to then look up the client's messaging preferences in the db (whether they want communication by email, sms or both) Then depending on the client's preference it will format the message for the medium (short version for sms, long form for email) and send it through our mail or sms provider's API. Because the fact that we want to be able to change out email and sms providers if need be I wondered if the Command Pattern would be a good choice.

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• Access functions from user control without events?

  - by BornToCode

  I have an application made with usercontrols and a function on main form that removes the previous user controls and shows the desired usercontrol centered and tweaked: public void DisplayControl(UserControl uControl) I find it much easier to make this function static or access this function by reference from the user control, like this: MainForm mainform_functions = (MainForm)Parent; mainform_functions.DisplayControl(uc_a); You probably think it's a sin to access a function in mainform, from the usercontrol, however, raising an event seems much more complex in such case - I'll give a simple example - let's say I raise an event from usercontrol_A to show usercontrol_B on mainform, so I write this: uc_a.show_uc_b+= (s,e) => { usercontrol_B uc_b = new usercontrol_B(); DisplayControl(uc_b); }; Now what if I want usercontrol_B to also have an event to show usercontrol_C? now it would look like this: uc_a.show_uc_b+= (s,e) => { usercontrol_B uc_b = new usercontrol_B(); DisplayControl(uc_b); uc_b.show_uc_c += (s2,e2) => {usercontrol_C uc_c = new usercontrol_C(); DisplayControl(uc_c);} }; THIS LOOKS AWFUL! The code is much simpler and readable when you actually access the function from the usercontrol itself, therefore I came to the conclusion that in such case it's not so terrible if I break the rules and not use events for such general function, I also think that a readable usercontrol that you need to make small adjustments for another app is preferable than a 100% 'generic' one which makes my code look like a pile of mud. What is your opinion? Am I mistaken?

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• Philosophy behind the memento pattern

  - by TheSilverBullet

  I have been reading up on memento pattern from various sources of the internet. Differing information from different sources has left me in confusion regarding why this pattern is actually needed. The dofactory implementation says that the primary intention of this pattern is to restore the state of the system. Wiki says that the primary intention is to be able to restore the changes on the system. This gives a different impact - saying that it is possible for a system to have memento implementation with no need to restore. And that ability of restore is a feature of this. OODesign says that It is sometimes necessary to capture the internal state of an object at some point and have the ability to restore the object to that state later in time. Such a case is useful in case of error or failure. So, my question is why exactly do we use this one? Is it to save previous states - or to promote encapsulation between the Caretaker and the Memento? Why is this type of encapsulation so important? Edit: For those visiting, check out this Implementation!

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• Designing a Business Rule Engine

  - by Nisha_Roy

  I have a requirement where there are 10 Rules to be applied on data in excel. If Rule 1 and Rule 2 fails rest of the rules are not checked. But if Rule 1 and Rule 2 passes the rest of all the Rules should be verified and if any errors found- they should be logged. Is there any design pattern which I can use to keep this Rule Engine flexible for adding these 10 rules and Closed for any additional chains in the Current Rule. I was thinking of something like a Decorator Pattern. Will this help me achieve that?

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• What are DRY, KISS, SOLID, etc. classified as?

  - by Morgan Herlocker

  Is something like DRY a design pattern, a methodology, or something in between? They do not have specific implementations that could neccessarily be demonstrated(even if you can easily demonstrate a case NOT using something like KISS... see The Daily WTF for a plethora of examples), nor do they fully explain a development process like a methodology generally would. Where does that leave these types of "rule of thumb"'s?

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• Authorization design-pattern / practice?

  - by Lawtonfogle

  On one end, you have users. On the other end, you have activities. I was wondering if there is a best practice to relate the two. The simplest way I can think of is to have every activity have a role, and assign every user every role they need. The problem is that this gets really messy in practice as soon as you go beyond a trivial system. A way I recently designed was to have users who have roles, and roles have privileges, and activities require some combinations of privileges. For the trivial case, this is more complex, but I think it will scale better. But after I implemented it, I felt like it was overkill for the system I had. Another option would be to have users, who have roles, and activities require you to have a certain role to perform with many activities sharing roles. A more complex variant of this would given activities many possible roles, which you only needed one of. And an even more complex variant would be to allow logical statements of role ownership to use an activity (i.e. Must have A and (B exclusive or C) and must not have D). I could continue to list more, but I think this already gives a picture. And many of these have trade offs. But in software design, there are oftentimes solutions, while perhaps not perfect in every possible case, are clearly top of the pack to an extent it isn't even considered opinion based (i.e. how to store passwords, plain text is worse, hashing better, hashing and salt even better, despite the increased complexity of each level) (i.e. 2, Smart UI designs for applications are bad, even if it is subjective as to what the best design is). So, is there a best practice for authorization design that is not purely opinion based/subjective?

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