Why should I use MVVM in Silverlight app?

问题:

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.

回答1:

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.

回答2:

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).

回答3:

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/

回答4:

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.

回答5:

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:

1. The user had to search for the item they wanted to be notified about
2. They had to select the item and fill out additional options regarding the subscription
3. 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.

回答6:

Without MVVM, your Silverlight app code very soon will turn into unmanageable mess

回答7:

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.

回答8:

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.