Silverlight4 如何實現DataContextChanged事件，自定義事件

參考：http://www.cnblogs.com/allanli/archive/2012/04/09/2439008.html

在WPF中，任何control的Data Context變化的時候，都會顯示的拋出一個事件，但是在Silverlight 4 中，卻沒有類似的功能。為了滿足需要，我們可以自己來實現。

public interface IDataContextChangedHandler<T> where T : FrameworkElement{void OnDataContextChanged(T sender, DependencyPropertyChangedEventArgs e);}public static class DataContextChangedHelper<T> where T : FrameworkElement, IDataContextChangedHandler<T>{public static readonly DependencyProperty InternalDataContextProperty =DependencyProperty.Register("InternalDataContext",typeof(Object), typeof(T), new PropertyMetadata(OnDataContextChanged));public static void Bind(T control){control.SetBinding(InternalDataContextProperty, new Binding());}private static void OnDataContextChanged(object sender, DependencyPropertyChangedEventArgs e){T control = (T)sender;control.OnDataContextChanged(control, e);}}

如何使用：
在自定義control的構造函數中設置binding即可。參考的文檔沒有說清楚如何使用，我是看下邊的英文原版，才有的靈感，差點放棄了 by dacong

View Code public class SilverlightContol1 :UserControl,IDataContextChangedHandler<SilverlightContol1> {public Gauge() {InitializeComponent();DataContextChangedHelper<SilverlightContol1>.Bind(this); } ?? public void DataContextChanged(object sender, DependencyPropertyChangedEventArgs e){MessageBox.Show("DataContext Changed event"); ?? } }

還有個英文版的參考

http://www.codeproject.com/Articles/38559/Silverlight-DataContext-Changed-Event?fid=1544514&df=90&mpp=25&noise=3&prof=True&sort=Position&view=Quick&spc=Relaxed

One known issue with Silverlight is that the DataContext bound to a control may change, but there is no readily available change event. Unlike WPF, you don't have an explicit event to register with in order to track changes.

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One known issue with Silverlight is that the DataContext bound to a control may change, but there is no readily available change event. Unlike WPF, you don't have an explicit event to register with in order to track changes. This becomes a problem in controls like the DataGrid control which reuses the same control instances for each page. Even though fresh data is bound, if your control isn't aware that the data context changed, it will keep stale content.

If you search online you'll find the solution is simple: you create a dependency property that is actually based on the data context (call it a "dummy" property) and then register for changes to that property. I was glad to find the solution but wanted something a little more reusable (remember, I like the DRY principle: don't repeat yourself, so when I find myself writing the same line of code more than once I have to go back and refactor).

The solution? I was able to find something that I think works well and involves an interface and a static class.

First, I want to identify when a control should be aware of changes to DataContext and also provide a method to call when this happens. That was easy enough. I created IDataContextChangedHandler and defined it like this:

Hide ? Copy Code public interface IDataContextChangedHandler<T> where T: FrameworkElement {void DataContextChanged(T sender, DependencyPropertyChangedEventArgs e); }

As you can see, it is a simple interface. A method is called with the sender (which will presumably be the control itself) and the arguments for a dependency property changed event. It is typed to T, of course.

Next, I used generics to create a base class that manages the "fake" dependency property:

Hide ? Copy Code public static class DataContextChangedHelper<T> where T: FrameworkElement, IDataContextChangedHandler<T> {private const string INTERNAL_CONTEXT = "InternalDataContext"; public static readonly DependencyProperty InternalDataContextProperty =DependencyProperty.Register(INTERNAL_CONTEXT,typeof(Object),typeof(T),new PropertyMetadata(_DataContextChanged));private static void _DataContextChanged(object sender, DependencyPropertyChangedEventArgs e){T control = (T)sender;control.DataContextChanged(control, e);}public static void Bind(T control){control.SetBinding(InternalDataContextProperty, new Binding());} }

As you can see, the class does a few things and works for any framework element, which is a "basic building block" that supports binding. It is typed to the FrameworkElement but also requires that the target implements IDataContextChangedHandler. It creates a dependency property. Because the data context can be any object, the type of the dependency is object, but the type of the parent is the framework element itself ("T"). When something happens to the property, it will invoke _DataContextChanged.

The event handler is sent the control that raised the event as well as the arguments for the old and new properties in the data context. We simply cast the sender back to its original type of T. Then, because we know it implements IDataContextChangedHandler, we can simply call DataContextChanged.

Finally, there is a static call to bind the control itself.

Now let's put the pieces together. Let's say you have a control that makes a gauge based on a data value, and you want to put the control in the grid. You need to know when the DataContext changes, because you will update your gauge. The control will look like this:

Hide ? Copy Code public partial class Gauge : IDataContextChangedHandler<Gauge> {public Gauge() {InitializeComponent();DataContextChangedHelper<Gauge>.Bind(this); }public void DataContextChanged(Gauge sender, DependencyPropertyChangedEventArgs e){if (e.NewValue != null){int gaugeLevel = (int)e.NewLevel;_UpdateImage(gaugeLevel);} } }

And there you have it - to register for the data context changing, we simply implemented IDataContextChangedHandler and then registered by calling Bind in our constructor.

需要補充的是， 這個功能在Silverlight 5中已經自帶。

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