I understand that dependencies properties serve a major purpose in WPF. However I do not get the reasons behind the restriction that in binding, the target property must be a dependency property. Why can't it be any property?
If you're interested, you might open up Reflector and take a look at some of the code related to dependency properties and the binding system in the framework. There is a lot of tricky stuff going on to allow robust, performant resolution of property paths and propagation of changes to dependency properties. Having a standard infrastructure also allows for management of more complex use cases, such as updating a dependency property from more than one source and resolving the priorities. This comes up frequently when animating a property that has its default set by a style, for example.
The other nice thing with dependency properties is that they internally encapsulate a lot of behavior (such as notification, validation and coercion), which means if you see a dependency property, you know that certain behavior will definitely be supported. This is in contrast with INotifyPropertyChanged, where the class implementer may or may not be supporting the interface as advertised. This means less work for class developers.
A simple search in google with DependencyProperty yields some results, which you might find relevant. For instance: http://blog.hackedbrain.com/articles/UnderstandingDependencyObjectAndDependencyProperty.aspx
And I guess one of the reasons would be context. Binding is built into the infrastructure of WPF, but a C# class property does not belong there. Inorder for the WPF infrastructure to find bindable properties you must declare them in code. Also if you declare your properties, you give important metadata for the WPF infrastrtucture.
Although I do agree that it would be easier if it were possible to bind to regular properties, but this is certanly a case of don't bother your precious brain with minor details...
Related
Is localization a responsibility of the View or of the ViewModel? Initially, I thought that it clearly belongs into the VM, as it is just data that needs to be displayed by the view. What exactly needs to be displayed is not important to the view. In addition, I have made the experience that XAML is more brittle than ViewModel code. But in a discussion today, some people strongly believed that localization is the responsibility of the view.
Here's some of the advantages I see for both versions:
Advantages of putting them into the View:
ViewModel is oblivious of localization
You can see the resource key in the XAML
Less code
Advantages of putting them into the ViewModel:
View is oblivious of localization
The View does not need to know anything except for it's ViewModel
It's easier to combine and create more complex strings.
In a Wpf application that uses the MVVM pattern, should the localizable elements (string resources) go into the view, or into the viewmodel? Why? What other advantages and disadvantages are there for the two approaches?
Some background info after comments: Assume that the Localization-backend is based on resx (not LocBaml). In addition, assume there was a framework that (View-Variant) could either transparently replace resource-ids in the view with the string, or that (ViewModel-Variant) would automatically generated INotifyPropertyChanged events for localized properties on the ViewModel.
However, I am mainly interested in arguments why it's better from a conceptual or cleaner-code point of view, disregarding the backend.
Some resources belong to the View, and some resources belong to the ViewModel. I do not think there is a strict rule in that matter. Use your own judgment. Personally, I'm sharing the ViewModel's resource file with the View.
To correctly implement localization in your WPF Application as it was designed, you need to follow the set procedure, so there is not really a choice like you are suggesting. For one thing. you'll need to set the Uid property on all of your UI controls, so that clearly can't be done in a view model. Furthermore, it is common to put all localized string values into separate dlls, so again, you can't do that in a view model.
I don't have the time to describe exactly how to do it right now. Instead, for full details about localization in WPF, please see the WPF Globalization and Localization Overview page on MSDN.
So I stumbled upon a problem while learning MVVM. I had a TreeView that contained TextBlocks which I wanted to perform an action on when I double clicked any of the TextBlocks in the TreeView. I started to learn about Behaviors, and I have a great example of how a behavior is implemented but the example does not connect the Behavior to a ViewModel at all. So in other words, if I double click on the TextBlock, I have the Behavior class that catches it but I don't have any ViewModel to perform any actions.
Could someone take a moment and explain how these tie in? I was reviewing this article:
http://msdn.microsoft.com/en-us/library/gg430869(v=pandp.40).aspx
But I didn't seem to grasp what I was looking for.
MVVM concept provide us a decoupling mechanism in WPF application which means no more code in xaml.cs file. Attached behavior is different thing. It has not relation with MVVM.
But because if we have scenarios where I cant use MVVM e.g. Select the text of TextBox on double click. Which is a behavior you want to add on textbox.
You will prefer implement the double click functionality in xaml.cs file as it not reusable and also tightly coupled.
This is where behavior come into picture. We will create the behavior for TextBox and will attach it. Now you can attach this behavior to as many controls you want.
EDIT:
if you are using WPF 4.5. you can look Markup Extensions for events
If you want to do it with attached behavior. Create an attached behavior of double click event which has Command dependency property. Your double click behavior just raise the command attached and in xaml bind the command with viewmodel which I expect you know how.
Hope, I am able to answer you comment.
D J has a good answer, though I'd like to insert some additional thoughts to this discussion.
In my experience, view behaviors (whether code-behind, attached behaviors, blend behaviors, or custom control logic) are often useful when the view does not depend on a view-model to function properly. If a view (UserControl, Window, Page, etc.) does not logically make sense in the absence of a view-model, it might make more sense to remove behavior from the view and move it to the view-model.
While we can do pretty much anything under the sun with all types of behaviors, it is often not wise to. MVVM, for good reason, limits what we should do so that we can observe separation of concerns to improve our application's cohesiveness and to decouple our classes. These two things are what software maintainability is all about, and these concepts become increasingly important as the application grows into enterprise software.
It is important to think about the concerns that the behavior has. Understanding this will help us find a properly suited place for it. Here are some questions to help know where it belongs:
Does the behavior interact with a business domain model (this is the first 'M' in MVVM)?
Behavior that has a dependency (even a loosely coupled one) on a domain model should likely belong in the view-model (or service), and not in the view. For instance, if the behavior needs to save to or read from an external device (e.g. a database), it has a dependency that the view should never have. Wrap this logic in a service function.
Or if not using a heavily layered architecture, put this inside of a view-model.
Does the behavior interact with application services, domain services, infrastructure services, etc.?
For the same reasons as the prior answer, the behavior likely belongs in the view-model or a service class. The view should have no explicit knowledge of services or domain model objects as it would muddy up its responsibilities (or concerns) that the view has. A view should only be concerned the visual/physical aspect of the user UI. Many views should define a contract (i.e. a view-model interface) that it binds to in order to operate correctly.
Will the behavior need to be reused across different views of the same kind?
This is a bit of a tricky question. Many times we foresee being able to have a different presentation for the same content. In effect, the view in these cases is a thin wrapper around some structure. For instance, suppose for a e-mail application we have a summarized view for received e-mails as well as a detailed view. Both views might need to support the same behavior (e.g. delete, reply, forward). Because we are reusing the behavior across different views of the same kind, then the behavior should belong in a common, reusable place. View-model logic is a good place for this.
Will the behavior need to be reused across views of different kinds?
When the behavior needs to be reused across different kinds of views (e.g. TextBox, ComboBox), we likely need an attached behavior. Usually, we can know this because the views are so diverse that it is not possible for them to share a view-model interface. Given that the behavior is concerned with view-related responsibilities, then custom control logic, code-behind, attached behaviors, or blend behaviors are all suitable places.
The question is stated in the subject: are the advantages of Model-View-ViewModel (MVVM) pattern worth the overhead?
In many cases, implementing the view model involves quite significant overhead of duplicating the Model properties and sometimes synchronization between Model and ViewModel data members. For example, currently in Silverlight 4 & WCF RIA, View Models are not generated (if the developer follows the MVVM pattern, it is up to him to create the view models, often duplicating the corresponding Model's properties at ViewModel, that do nothing significant but refer to Model as the storage).
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
Effectively that is what the PresentationModel is for. Which MVVM is strongly based on. The difference is that the ViewModel is the model for the view and not the model for the data. So you are concerned around more how the view behaves with the data.
If you have a simple UI that all it does is present the model then I would suggest expose the Model on a property of the ViewModel and bind to that. Make sure though the model does implement INotifyPropertyChanged etc.
The power of the ViewModel is when you have things to do in response to a user action. The ViewModel can then support Commands, calling out to services and validation and thus leaving the Model as a data container
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
In the simple cases, this is all the ViewModel is doing - wrapping up the Model so that its extended in a way that's consumable by the View. If your Model can be bound directly, you're welcome to do so.
That being said, there is more to the ViewModel layer than just wrapping the model - this is also where the application specific logic - ie: the application's plumbing, will occur. Something has to make the requests from the Model classes correctly and compose together the logic.
If you are concerned about extra work, you can always create a ViewModelBase (INotifyPropertyChanged , Errors/Validation, generic stuff) to be inherited by your ViewModel, it will minimize things that you think may cost you time to replicate. And also, Silverlight/Wpf provides us with binding which greatly reduces our coding, besides the fact that XAML also does that by providing functionalities through markup. Besides that you can further the design by using screens, controllers, etc.
For me, I do not see any "overhead" with regards to using MVVM; if there were, it'd be worth it. It properly deals with the Separation of Concerns. It provides a good platform for development especially in teams where people may take care of different aspects of the application without affecting other team members codes (especially between developers and designers).
Hope this helps
Benefits of MVVM
Reduced complexity.
Isolation of Designing and Development.
Dependency injection.
Major advantage is when you have a Well MVVM structured Windows Phone application and want to develop same for Windows Metro Desktop, Only thing u want to concentarte on design as the same view model can be used as it is.
Hope it helps.
I am thinking about the design of a WPF or Silverlight application. I am planning to use MVC (or another such design pattern)
Witch ever of the design patterns I choose, I need to connect to view to the model (or presenter) – is databinding a good way of doing this?
(In the past with WinForms applications I have found that Databinding give lots of problem in the long run and does not fulfil its promise. It is the same with WPF and Siverlight?)
Yes, you should definitely definitely use data binding. While WinForms and ASP.NET were always a struggle to get anything data bound consistently and in a maintainable manner, Silverlight and WPF are built from the ground up for data binding pleasure.
Binding is two-way so you don't have to write tedious plumbing code to move data in and out of your model. Just implement INotifable and away you go.
Converters allow you to write code to handle the way things are bound if the defaults aren't working. Using converters (which are dead-simple to write) you can bind booleans to visibility settings, strings to images, integers to background colors, and so on. The sky's the limit.
Patterns such as MVVM are perfect for the rich data-binding support in WPF and Silverlight. MVVM lets you have the best of both worlds: loosely coupled code together with data binding.
Element binding lets you bind the property one element to the property of another element. Together with converters, this gives you impressive power to do things like bind the current position of a slider control to the selected index of a list control. Both ways.
Deep binding means you can bind to the property of a property of your model. Not that you always should, but you can.
Binding is almost magical in its dynamic-ness. As long as your model continues to support the same bound properties, binding will continue to work even if the static type of the model changes. Binding is also crazy flexible. You can bind to collections, interfaces, complex objects, (almost) anything you like.
DataContexts can be used to set up data-binding at a page, control, or container level. Children of the container then inherit the same data-context. This lets you bind once at the page level and then use binding paths for the rest of the page.
I recommend you take a look at the Model-View ViewModel (MVVM) pattern. Here is a very good video you should take a look at: Jason Dolinger on Model-View-ViewModel. Two-way data binding in WPF is very powerful.
be it WPF or Adobe Flex or Winforms , Databinding will always give issues when the application becomes complex. I would prefer to avoid data binding for easier debugging. But data binding runs all around WPF that we can't avoid. Doing data binding in XAML takes away control from the developer.
I think , if we keep the data binding in the code , its much easier to debug.
Imagine , MVVM without data binding , it will look messy. A design pattern that takes advantage of technology is good but a design that is totally dependent on a particular feature is a disaster.
Data binding in WPF goes far beyond what you could achieve in Winforms. It is intrinsic to the platform and prevalent throughout. I would argue you can't understand WPF without understanding its data binding system.
It's not without its pitfalls, to be sure. Broken bindings are often not as obvious as you may like, but improvements have been made to help you identify and flag these issues.
I just converted all my settings from AppSettings to ConfigurationSections. It definitely cleaned things up, but I'm having difficulties with the preferences window. I want to use bindings in my WPF window.
Should I store each of the ConfigurationSections in a dependency property and bind to the ConfigurationSection's properties?
Should I use ObjectDataProvider's that calls the ConfigurationManager.GetSection?
Is there another way I can do this?
Off-topic: I find the bindings in WPF to be really powerful, but it's sometimes a bit confusing or difficult to create the bindings. I wish there was better documentation for XAML.
You don't need to do anything special - you can databind to types with plain old properties. All the stuff about dependency properties are only for WPF controls themselves. When it comes to the model against which you bind, there are no particular constraints. You can bind to Plain Old C# Objects (POCOs), although implementing INotifyPropertyChanged is an advantage.
However, instead of binding directly to your Domain objects (it sounds like your ConfigurationSections fit that role), it is often a good idea to explicitly create a ViewModel that deals with view-specific responsibilities while encapsulating the real Domain objects.
Josh Smith's article Patterns: WPF Apps With The Model-View-ViewModel Design Pattern is an excellent introduction to proper databinding in WPF.