Is it generally desirable to implement INotifyPropertyChanged on Model classes, ViewModel classes, or both? Is it possible to implement on Model only ,not on Viewmodel? If not possible model then why
Basic rule is - There is no hard and fast of any architecture, you can modify things to suit your needs that's why some architectures are more desirable.
for your exact needs go through this
and in this discussion, there are enough points to cover both arguments, see which one matches your project..
this might help you to implement..
You must understand the meaning of INotifyPropertyChanged. It's purpose is to raise notification from target to source when you define Binding in WPF. DependencyProperty and INotifyPropertyChanged are related for autoupdation for the concept of binding. If you need to bind the property on a viewmodel to view you have to implement a notification mechanism to notify to UI if there is change in Viewmodel. Same rule is applied for model to View.
Suppose you want to code in .CS file like Viewmodel.Name =" my new name" and expect that TextBox should display the changed name.
<TextBox Text="{Binding ViewModel.Name} "/>
Here view model need to implement INotifyPropertyChanged.
<TextBox Text="{Binding Model.Name} "/>
here model need to implement INotifyPropertyChanged
Hope it clarifies.
It might be worth noting in this discussion that Microsoft themselves add INPC in their own models e.g. the classes automatically generated when you susbscribe to a web service. I've seen it various other places as well, including a vague recollection of it somewhere in their EF stuff. I myself add it automatically to my DAL entities using Castle Proxy so that I don't have to duplicate everything in the view model, although you can also use things like Fody etc to add it to the IL in a post-processing step.
I need a reference to a Visual (one element of a XAML View Window) in my ViewModel to work with VisualTreeHelper methods like VisualTreeHelper.GetDescendantBounds(Visual reference) but I do not want to break MVVM rules and just name the viewport3d and provide it as a reference when instancing my ViewModel.
Currently I am binding the geometry as content like this to my ViewModel:
<Viewport3D>
<ModelVisual3D Content="{Binding SceneContent.Content}"/>
</Viewport3D>
But I do not see the MVVM possibility to get the containing ModelVisual3D into my ViewModel. Is there a standard (may be data-binding) approach to this in MVVM applications?
Using MVVM, we don't 'get view elements'. If you need to do something with a UI element, then that has nothing to do with MVVM. If you need to use the VisualTreeHelper.GetDescendantBounds method, then once again, that has absolutely nothing to do with MVVM... why do so many people claim to use MVVM, but know nothing about it?
Therefore, your question is invalid. It is entirely appropriate in situations like these for you to use the code behind. In fact, this is a perfect example of when we should use the code behind when following the MVVM methodology. If it is only UI related, then it has no purpose being in a view model, so simply don't put it there.
Please read the answer to the What are MVVM limitations? question to get some further insight into MVVM.
I'm still getting comfortable with WPF, so this might be a silly question. I'm tracking the source of a tab control issue and discovered that I don't know the answer to the question in the title.
My Mvvm-Lite project template put the usual in the Windows opening tag:
DataContext="{Binding Source={StaticResource Locator}, Path=Main}"
And from somewhere else (I really don't remember) I have this handy static resource:
<ObjectDataProvider x:Key="src" ObjectType="vm:MainViewModel" />
While I'm troubleshooting my tab control issue (I won't bore you with those details here) I realized that I didn't know if these two ways of pointing to the view model confilicted in any way. Two separate instances of the view model? I don't think so cause the ViewModelLocator is holding a static reference to Main. Anyway, I just wanted to be sure that I haven't made a "greenie" mistake.
Thanks,
Jim
I am pretty sure that this is going to create two different instances. You can easily verify this by placing a breakpoint in the MainViewModel constructor and running your application in debug mode.
ObjectDataProvider was introduced before MVVM was popular. I would argue that many objects in WPF are kind of obsoleted by MVVM, because now we create these objects and properties in code in the ViewModel layer. In your case, I guess that you probably don't need the ObjectDataProvider, but can use the ViewModelLocator.Main everywhere instead.
Cheers,
Laurent
I want to know why we should use MVVM to implement Silverlight app. What're it's advantages?
We don't do Unit Test for ViewModel, so I want other reasons.
Below are my questions about some advantages people usually say:
1.Loosely Coupled : When we use MVVM , a view rely on ViewModel but not a view, why it's loosely coupled?
2.If I provide public methods in a code-behind, they can also provide reusability.
Well, the unit-testability of the view-model is a significant advantage, so you'll miss out on that benefit. Regarding the other two:
Loosely coupled
Yes, the view does rely on the view-model. They have to be connected in some way to accomplish the function of the application. As a result, they cannot be uncoupled. The only choices are tightly-coupled or loosely-coupled or somewhere in between. With MVVM your view-model interacts with your view in a very limited way: basically just objects, properties and commands. Compare this to doing everything in the code-behind where the view and its control are essentially inseparable.
Re-use
If any code in your code-behind is re-usable enough to merit being public, it can be taken out of the code-behind and put into a general-purpose class. The problem is that what's left after that is not reusable.
If you don't want to go into the MVVM deep dive, then you can get some of the benefits of MVVM by focusing on databinding. After you learn the benefits of databinding, you can reconsider the other benefits of MVVM.
We don't do Unit Test for ViewModel,
With MVVM, it isn't just about unit testing ViewModel. Ideally, your VM should be very thin and only have properties needed by view. So, it isn't really necessary to test the VM.
But, without a VM, how do you do your your feature/functional testing across layers? In Silverlight, to facilitate testing you should use commands, instead of writing code in code-behind files. This allows you to simulate button click and other GUI events while unit testing. Using MVVM pattern along with commands, you can test all of C# code (not xaml), right up to UI (Converter, VMs, etc).
If I provide public methods in a
code-behind, they can also provide
reusability.
Not going into details of how that is a bad design, I want to ask you, How does that provide reusablity? If you create a user control, then the code-behind class is a control? You want to create instances of your control and use them? This is like saying that why do we need member methods, we can just create public static methods and access them. I have a strong opinion that if we don't want to use the automatic binding provided by WPF/Silverlight, then it is better NOT to use these technologies. And to exploit the full capabilities of binding, MVVM is essential.
why it's loosely coupled?
VM is very much part of your view. It is not decoupled from the view. As I have said, your VM should be as thin as possible with only public properties needed by your view. Your business logic will be decoupled from your view (and VM).
I think this is one of the best resources available, in case you want to use and contrast the usage of MVVM vs. ANY other pattern or no pattern.
http://karlshifflett.wordpress.com/2010/11/07/in-the-box-ndash-mvvm-training/
I can answer how I use MVVM pattern.
MVVM is better in the following scenarios:
1 If several controls are bound with a single property.
MVVM:
<TextBlock x:Name="text1" Visibility="{Binding IsSomePropertyTrue, Converter={StaticResource VisibilityConverter}"/>
<TextBlock x:Name="text2" Visibility="{Binding IsSomePropertyTrue, Converter={StaticResource VisibilityConverter}"/>
I can quickly add a similar control or remove an existing control.
Compare with code-behind:
public string IsSomePropertyTrue
{
set
{
//...
text1.Visibility = value;
text2.Visibility = value;
}
}
2 Instead of a multi-converter
public Brush StateColor
{
get
{
if (this.State == State.Edited && this.IsPriority)
return new SolidColorBrush(Color.FromArgb(255, 0, 255, 0));
//...
}
}
<sdk:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<TextBlock Background="{Binding StateColor}" Text="{Binding State}"/>
</DataTemplate>
</sdk:DataGridTemplateColumn.CellTemplate>
3 As an item model in controls like ListBox or DataGrid. For example, if I want to create a list of items with a remove button near each item, I will create a ItemView control and a ItemViewModel class.
<ItemsControl ItemsSource="{Binding SomeItems}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<view:ItemView DataContext="{Binding}"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
4 Copying a data from one view to another:
public JournalEntryViewModel(SalesOrderViewModel vm) {}
5 ViewModel can inherit CLR-classes and implement interfaces (INotifyPropertyChanged or INotifyDataErrorInfo).
Also I use MVVM for replacing events with commands or properties. And use of ViewModels forces to call properties by intelligible names.
I was an early adopter for WPF and I can tell you what made me choose MVVM (and this more or less applies to Silverlight as well). For the project I was working on, I had to create a screen that allowed users to subscribe to notifications within the system. This was a 3 step process:
The user had to search for the item they wanted to be notified about
They had to select the item and fill out additional options regarding the subscription
The system had to provide a summary and allow the user to confirm or edit the subscription.
After implementing the functionality the first time (without MVVM), I was told that we need to exclude from the search items that were already subscribed to by the user.
After making that fix, I was informed that we needed to give the user a live preview of the subscription based on options.
By then I started noticing that some of these changes could be extracted and made easier if I didn't have to deal with manipulating the UI as I changed the logic. I had never intentionally followed MVVM but I realized that the abstraction that I made closely matched the MVVM pattern.
So this is why I recommend the pattern. It simplifies the task of changing the logic that drives the UI by separating it from the UI itself. I also would recommend that you hold off implementing it until you need it. There is a cost to using MVVM but it is amortized over the cost of changing the UI logic.
Without MVVM, your Silverlight app code very soon will turn into unmanageable mess
What is also interesting in MVVM is dynamic automatic binding.
Imagine, that your view model has properties like this: bool IsFirstNameVisible, bool IsFirstNameEnabled, string FirstName, double FirstNameWidth etc.
In your XAML file, you define TextBox with x:Name = "FirstName" and call your dynamic MVVM-binder. It inspects your view model class via reflection, looks what properties you have defined and binds them dynamically to similar properties of control with the same name, applying value converters if needed.
In this case, your get very clean XAML, without kilometer-long data-binding expressions and converters.
That is what my MVVM library does.
Separation of Conerns people. Separation of Concerns.
Forget unit testing (I love it; but that's not the thing here). Forget reusability (do you really re-use view models? No, let's be real here).
It's about Separation of Concerns and putting code and logic where it belongs. The whole idea is maintainability; being able to make changes to the code as it evolves over time without breaking other stuff and without turning the whole thing into a big pile of spaghetti.
MVVM, done properly, allows your code to be separated into logical portions that make sense and allow for reasy refactoring and change as the app's requirements change. It's easier to find where something is when you need to make a change. Trying writing any halfway complex application and then leaving it alone for a month, then coming back to it and trying to make significant changes. A properly structured application with judicious use of MVVM is way easier to grok after a layoff, and it's way easier to make changes.
Does anybody have an idea how to change screens (views) in a MVVM View-First-Approach (The view instantiates the ViewModel:
DataContext="{Binding Source={StaticResource VMLocator},
Path=Find[EntranceViewModel]}"
)
For example:
In my MainWindow (Shell) I show a entrance view with a Button "GoToBeach".
<Window>
<DockPanel>
<TextBox DockPanel.Dock="Top" Text="{Binding Title}" />
<view.EntranceView DockPanel.Dock="Top" />
</DockPanel>
</Window>
When the button is clicked I want to get rid of the "EntranceView" and show the "BeachView".
I am really curious if somebody knows a way to keep the View-First Approach and change the screen (view) to the "BeachView".
I know there are several ways to implement it in a ViewModel-First Approach, but that is not the question.
Perhabs I missed something in my mvvm investigation and can't see the wood for the trees... otherwise i am
hoping for a inspiring discussion.
One possibility would be to have all views in the (MainWindow(Shell) and using Triggers for their visibility. But having a lot of different screens (views) all declared in the MainWindow doesnt feel right for me...
This question came up while reading this nice way of using MEF with MVVM I found on John Papas Blog: Simple ViewModel Locator for MVVM: The Patients Have Left the Asylum . But as nice as this marriage of view and viewmodel is, it seems like there is no way to change screens that satisfies me. :)
So in my opinion if you have a lot of screens(views) you better use a ViewModel-First-Approach...
This looks like it might help:
Creating a ViewModel : do it before or after model data is available?
Failing that, how about creating the ViewModel once only at startup, and assigning it to each View window as it's created (rather than creating a new ViewModel each time). Then just close the first View and open up a new View as required, reassigning the single ViewModel instance.
You may want to look at Prism (i.e. the composite application library). Prism facilitates navigation between views via the region manager. This might be overkill for your application and can take a while to get your head around. Prism also allows you to develop using the MVVM pattern as well.
You can find more information about prism and prism navigation in the Prism documentation.