Sunday, February 6, 2011

Using ViewModel in Views (The MVVM way)

The article says how to use one/more ViewMiodels in a View. Also, multiple Views using same ViewModel. Read this post to create a ViewModel.

There are several ways of using ViewModel in a XAML file based on the different scenarios.

Case 1: Initialize a ViewModel without any constructor parameters.

Read this post. This article also expects the reader to be familiar with Expression Blend.

Case 2: Initialize a ViewModel with constructor parameters.

xmlns:local="clr-namespace:****your namespace***"
<ObjectDataProvider ObjectType="{x:Type local:ViewModel}" x:Key="theViewModel">            <ObjectDataProvider.ConstructorParameters>

Notice the System namespace declaration. We can use any data type from System namespace as a constructor parameter. If you have custom data types you can pass them too as a constructor parameter; by declaring namespace in XAML file.

Case 3: In the above two cases, the object can be accessed on the C# side (as a Resource). However, if you update the object in C# side, the ViewModel values are NOT reflected on the XAML side. Now what??!  We can declare a ViewModel object on the C# side and declare it as DataContext for the XAML side. Here is how to do it.

public partial class MainWindow : Window
private ViewModel theViewModel = new ViewModel();

public MainWindow()
this.DataContext = theViewModel;

private void TextBox_TextChanged(object sender, System.Windows.Controls.TextChangedEventArgs e)
theViewModel.FirstName = "I am still typing";

In this way, any updates from XAML side would be seen on C# side too. But, if you use Expression Blend, the ViewModel is not seen in the designer, so you have to write the binding code manually. I wouldn’t suggest this way, but sometime its not up to your choice!

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