I am developing a large-ish application in WPF/WCF/NHibernate/etc. and have implemented the MVP pattern (although this question is still relevant to MVC) as the core architecture.
It feels quite natural to extend and add functionality as well as to come back and make changes on certain bits and pieces, as far as the core architecture is concerned (controllers, views, etc).
But at times the code-behind-ness of custom user controls that I create feels as if it "breaks" the MVC/MVP paradigm implemented, in that code concerns leak in the design and design concerns leak in the code. Let me clarify again, this is only for user controls. It is my personal opinion that this code-behind model (for both ASP.NET and WPF) is a 'Bad Thing', but no matter what my opinion, I'm stuck with it.
What are your recommendations for best practices in such a scenario? How do you handle such concerns? Do you for instance work around the code-behind-ness of custom controls and if so how??
Since you are using WPF, you should really look into the MVVM (Model-View-ViewModel) pattern. It is a form of the Presentation Model (PM) pattern discussed by Martin Fowler. WPF is very binding-oriented, and provides a very powerful and rich data binding framework for XAML. Using MVVM, you can completely and entirely decouple your ViewModels from your Views, allowing truly POCO UI development that offers the ultimate in separation of concerns and unit testability.
With MVVM, you will be able to modularize and decouple all of your views, including Windows, UserControls, etc., from the code that drives them. You should have no logic in Code Behind other than what is automatically generated for you. Some things are a little tricky at first, but the following links should get you started. The key things to learn are the MVVM pattern itself, Data Binding, Routed Events and Commands, and Attached Behaviors:
MVVM
Data Binding
Attached Behaviors
Attached Commands (VERY USEFUL!)
Routed Commands
Routed Events
WPF + MVVM has a bit of a learning curve up front, but once you get over the initial hurdle, you will never, ever want to look back. The composability, lose coupling, data binding, and raw power of WPF and MVVM are astonishing. You'll have more freedom with your UI than you ever had before, and you will rarely, if ever, have to actually bother with code behind.
I happen to like code-behinds (yet another personal opinion), but they work only as long as they do nothing but facilitate interactions between control events and the rest of the application. I'll admit that I've seen a lot of counter-examples, though. I even wrote a few of them....
Really, all the code-behind should do is "oh, someone clicked this button; there's probably something that wants to know about that." PRISM (from MS patterns and practices) provides a lot of architectural infrastructure for WPF and Silverlight; that includes a publish/subscribe interface that allows the controls and the code-behinds to simply publish an event while not even being aware of possible subscribers, or what the subscribers might do with the event. PRISM also adds commands for Silverlight.
A common variant of MVC for WPF and Silverlight is MVVM (Model, View, ViewModel). The ViewModel makes data available to the user controls in some form that is most useful (such as ObservableCollections, to facilitate two-way binding).
Custom Controls are there to display stuff. In that regard they are no different than a button or a drop down combo box. The trick is that don't let them handle stuff directly. They need to send stuff through the View Interface and the Presenter need to likewise interact with them through the view interface.
Think of it this way. If you ignored MVP the custom control would interact with the model in specific ways. what you doing with MVP is taking those way and defining them with the View Interface. Yes you are adding an extra call layer but the advantage is that you thoroughly document how it interacting with the rest of the system. Plus you get the advantage of being able to rip it out and replace with something entirely different. Because all the new thing needs to do is the implement it's portion of the view interface.
If you have a specific example I can illustrate better.
Related
I’m utilizing the Windows.UI.Composition framework for the first time in my app. My app is built on MVVM Light. I'm updating the app to add more transition animations between elements.
The prevailing wisdom with MVVM is that you should keep your UI code in XAML as much as possible, binding visual states to the ViewModel properties, etc. But all of the Windows.UI.Composition material and samples I’ve been seeing, define the UI manipulations in the code-behind instead.
Let’s use a show/hide scenario as an example. I have a bool property in my ViewModel like ShowTheBox. I bind TheBox’s Visibility property to the ViewModel property. The Box will show or hide automatically based on changes in my ViewModel.
Now, using Windows.UI.Composition, I want to add fade-in/fadeout animations to the visibility changes of TheBox. Where is the best place to put that C# code and how do I bind that transition to my ShowTheBox property?
I wouldn't agree that point of MVVM is to keep UI code in XAML exclusively. The point of MVVM, as any other layer separation pattern, is to separate UI layer from app's logic. MVVM just adds its own flavors in form of bindings.
So I think that when you have complex animations and other UI related stuff, it's perfectly fine to put them in code-behind. But before doing that, you might want to try to extract as much of your animations as you can to custom controls which will facilitate your doubts a little.
EDIT:
Making a lot of your UI logic bound directly to ViewModel properties isn't always a good solution. Layer separation exists for a reason, so when you compose your layers, imaging that you're writing a Xamarin app and have common ViewModels, but different Views for different platforms. Now you're not even sure if on another platform is gonna have those animations or not. Maybe a flow that takes one screen on UWP will take two screens on iOS, or something else. To have a property "IsVisible" which serves only for one of many views isn't making much sense now, does it? So you have to find some common denomination for ViewModel and move everything else to UI layers.
At the end of the day, MVVM is just a pattern which helps you write a better code. It's understandable to want to stick to all the good practices as much as possible, but if it doesn't make sense for your app - is it worth it?
I'm trying to learn MVVM and WPF and I'm using the MVVM Light Toolkit. Here's what I'm not fully understanding and maybe it's due to an incorrect architecture of my UI.
What I'm trying to accomplish is pretty simple actually. This is a utility application by the way. I want a window that serves as the 'controller' so-to-say that has a set of buttons. Each button should change the content of a frame. Example: one button loads a 'screen' ( or a 'view' if you will ) that allows the user to configure an 'Agency' which is a custom object. Another button loads a list of Users from the Agency that was in the first 'screen'. This 'Users' view needs to also be loaded in the same frame. In fact, as of right now, the window with all the buttons really is only responsible for loading the 'screens' in the frame. The meat of the application will be within all the separate 'screens'
What I am not understanding is 1) how to let each screen/view know about each other since one is dependent upon the other. It seems that in MVVM the ViewModel shouldn't know about anything. But in my case, I need to pass information around ( such as my Agency ).
If I can get some hints on what I need to look into, that would be great.
Thanks!
Some ideas that might connect some of the dots:
You'll most likely have one viewmodel per view ("screen").
Each viewmodel will contain all of the logic for its corresponding view
Viewmodels can and will know about the models (Agency, Users)
Viewmodels can communicate with each other via the Messenger in MVVM Light
Think of MVVM Light's Messenger as an "application-wide eventing system". When you send a message out from one view model, any other view model can be listening for that message/event and react to it as needed.
Does that help at all? Keep your thoughts coming and I'll keep commenting and I'm sure the community will as well :)
Few things:
each of your screens, should be separate view (eg. user control or new window - I suppose you've done that already)
every part of model (eg. Agency, User) you want to display in your application, should be wrapped with its dedicated view model
your views don't really need to know about each other; you can use commands or events on view models to get rid of those dependencies
view model only needs to know about one thing: model it's building on
it's good to think about view as really simple class, with one single responsibility of rendering content; no logic, no code behind (unless it's purely UI/display related) is something to follow
You can try to prepare your models first (if you haven't done that already), then make view models for them (thinking what properties of models you want to expose to views) and once that's ready, build your views basing on view models. Other way around is also viable option - pick whichever feels more natural to you.
One more thing: since you mentioned you can display several screens in one (I assume) main area, think about equipping your view models with something along the lines of bool IsCurrentlyActive property. This way, you can easily show/hide views with button clicks and still utilize binding mechanism.
They shouldn't know about each other. That is what the Messenger is for controllers and views subscribe to the events they are interested in. That way they don't need to know or care where they event originated.
Hmm Kendrick is faster. What he said.
Also it sounds like you kind of want an Outlook type interface, some navigation that loads other views. I had the same question a while ago. How to do Regions in WPF without Prism?
To better understand the MVVM pattern look at this article: WPF Apps With The Model-View-ViewModel Design Pattern
Also I advice you to look at Caliburn Micro framework.
I am having a go at refactoring my Winforms code into MVC pattern. I have never used this pattern before.
Obviously the GUI will be the view, the controller will be the 'middle tier' which is invoked by any user interaction with the GUI, and the model performs the requried tasks and informs the view of any status changes.
My question is, with the model, I am assuming that can span a great number of classes and is not confined to one 'model' class? Also, can these three sections all be within the same assembly?
Thanks.
for Winforms i wouldnt suggest MVC - id suggest MVVM
try this tutorial http://weblogs.asp.net/dwahlin/archive/2010/09/30/silverlight-sessions-coming-to-devconnections-las-vegas-november-1-4.aspx
this article mentions Silverlight but the MVVM pattern is generic and can be applied to Winforms
as pointed out by Roger Lipscombe - MVP may also work - try this for information on that http://davybrion.com/blog/2010/08/mvp-in-silverlightwpf-architectural-overview/ - again specific to Silverlight in this light but as its a pattern it can be adapted
For Winforms I would suggest learning about the MVP (Model/View/Presenter) and the MVC pattern. Although others have suggested MVVM might be a good idea I disagree - MVVM takes advantage of data binding offered in WPF and although Winforms supports binding to some extent, it's not as binding centric as the WPF architecture/object model.
The 'Model' layer can consist of many classes and I would always use the 'Single Responsibility Principle' as well as other Solid principles when modelling the classes within this layer of your architecture.
Useful links:
SRP - http://en.wikipedia.org/wiki/Single_responsibility_principle
SOLID - http://en.wikipedia.org/wiki/Solid_(object-oriented_design)
MVP - http://en.wikipedia.org/wiki/Model-view-presenter
No, model is not confined to one model class. In model you usually represent your database, and other data-related stuff. Controllers are responsible for most of the actions.
And yes, all this component will land in one dll. Bet there will be a lot of other files, like view files, which are not always compiled in MVC (but you can force that).
You might want to think about making a 'Model' class as an interface. Then all of your specific models implement that interface but share common methods (such as update, delete, etc.)
They can definitely be written in the same assembly. Your folder structure (strictly), should follow a Models/Views/Controllers structure, and place the code files underneath those respectively.
If you decide to try out the MVP pattern, which is a good choice for Winforms, check out MVC#, a framework for building MVP applications. It's simple and good.
Maybe you are interested in the approach I am heading for to combine the MVC/ MVP pattern with Databinding with fluent interfaces. mvc and databinding, what is the best approach?
If MVC, MVP or MVVM is used is from my point of view a matter of perspective. They will all lead to an abstraction of data, logic and visualization of the data.
What is the difference between MVP VS MVVM? Why we are using MVP even though we have three layers: business, data access and presentation? Is there any specific reason to divide the Presentation layer into MVP?
MVP and MVVM are both derivatives of MVC. MVC is a pattern that separates the user presentation and interaction from the internal representation.
This requires three layers, since tying the user interaction/presentation directly to the internal representation will cause both to bend to conform to each other. In your application, you described these layers as the Presentation, the Business layer, and the Data Access layer. With only those very loose descriptions, you could potentially be describing any of the MVC derivatives, or the original MVC pattern itself.
The key differences between each of the derivatives are the dependencies each layer takes on the other layers, and how tightly they are bound to each other. This article has some details on the differences, though of course it shouldn't be considered authoritative:
http://nirajrules.wordpress.com/2009/07/18/mvc-vs-mvp-vs-mvvm/
"... MVVM is attractive for platforms which support bi-directional binding with less effort. Also a minor tradeoff is ViewModel unlike Presenter can stand on its own (Presenter normally requires a View’s interface)."
We use at our company projects of WPF desktop application MVP instead of the built in MVVM for the main reason that in MVP the presenter is the main entry point that knows everything and no one knows about the presenter.
For each View the presenter which have one responsibility which is taking interactions from the IView interfaces by subscribing to events that the View triggers.
The presenter updates the View by a properties that encapsulates the internal View controls like TextBox with string properity and GridView with any Collection property. The constructor of the MainPresenter class will look something like this MainPresenter(IMainView, IEmployeeStore, IOtherDependency,..)
The Constructor of MainView class will look like this MainView(IPartialViewIfExists,..) that means the view does not know anything about the Presenter or anything else outside the View layer (which is the opposite of MVVM that enforces the MainView to directly couple the MainViewModel to automate the
two way databinding).
That clean loosely coupling architecture which the MVP provides is really powerful and flexible which enables the ability for the following:
Your application can replace the GUI with anytime without changing anything in the presenter, you can also change the GUI technology to something else like WinForms or something.
You can separate your GUIs in a separate project that doesn't require any dependencies of your main application like the presenters and dataAccesses
Your View can be used for any other application witch is useful for general GUIs.
You can unit test the views, the presenters and the data access classes easily.
The ViewModel in MVVM doesn't know about the View but I don't think that is helpful since it is responsible for the View. The View shouldn't know about the presenter which handles business logic and that's what exactly the MVP provides (or the way that we implement MVP).
That doesn't mean that MVVM is bad. MVVM is a good architecture and faster to code and easier to start with since it is already implemented in WPF and Xamarin but as I explained we prefer MVP for listed reasons.
In general MVP is cleaner and more scalable but requires more knowledge and coding experience and have to be implemented manually. MVVM is already there, it is easy to use and lets you implement faster but provides coupling and has some limitations. They all have their pros and cons and it depends on your need.
I have a WPF application which was written in C#. This application hasn't been written with any particular design pattern in mind, but as I have learnt .NET I've realised that the MVVM model would be suitable. Thus, I'd like to start converting the code.
This will be the first time I've used MVVM, and whilst I'm willing to get stuck in, I'm finding it difficult to find solid MVVM examples online where an ADO.NET Data Service is the Model and XAML is the View. I'd like to look over some examples before setting off on the process of converting my own app to make sure I have correctly understood what I am doing!
Can anyone recommend a small (but non-trivial) example application with code which uses WPF, ADO.NET Data Services and the MVVM model?
I recommend starting with any example that uses MVVM with WPF, and there are many. The fact is that a clean implementation of MVVM will not have any true data access code in it -- data access should be handled by another, abstracted layer (see MVVM where to put Data Access Layer?).
Work on designing a viewmodel that encapsulates all of the data and interaction that your (already existing) views require. Clean out your codebehind and get your view binding to your viewmodel.
Once you have that going, you can worry about how to get your objects to and from a persistence store, but the actual work of doing so does not belong in the M, V, or VM.
I know that there are tons of examples with data access right in the viewmodel or even the model, but those are meant to be quick illustrations that don't require tangents to address dependency injection, facades, etc.
Find any nontrivial example of MVVM in WPF, and when you get to the part where they deal directly with data access, remind yourself that at that point you'll be using an abstraction of persistence.