自從我上次寫到關于標記擴展的時候已經有一段時間了......?Visual Studio 11 Developer Preview的發布給WPF帶來了一些新功能，讓我有理由再次使用它們。我要在這里討論的功能可能不是最令人印象深刻的，但它填補了以前版本的空白：支持事件標記擴展。

到目前為止，可以在XAML中使用標記擴展來為屬性賦值，但我們無法做到這一點來訂閱事件。在WPF 4.5中，現在有可能。所以這里是我們可以用它做的一個小例子...

當使用MVVM模式時，我們通常通過綁定機制將ViewModel的命令與視圖的控件關聯起來。這種方法通常運行良好，但它有一些缺點：

• 它在ViewModel中引入了很多樣板代碼
• 并不是所有的控件都有一個Command屬性（實際上大多數不是），當這個屬性存在時，它只對應于控件的一個事件（例如點擊一個按鈕）。沒有真正簡單的方法將其他事件“綁定”到ViewModel的命令

能夠直接將事件綁定到ViewModel方法會很好，如下所示：

1 2	<Button Content="Click me" ????????Click="{my:EventBinding OnClick}" />

使用OnClickViewModel中定義的方法：

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public void OnClick(object sender, EventArgs e) { ????MessageBox.Show("Hello world!"); }

那么，現在可以！這是一個概念證明...下面EventBindingExtension顯示的類首先獲取DataContext控件，然后在該對象上查找指定的方法DataContext，并最終返回此方法的委托：

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using System; using System.Collections.Generic; using System.ComponentModel; using System.Diagnostics; using System.Linq; using System.Reflection; using System.Windows; using System.Windows.Markup; ????public class EventBindingExtension : MarkupExtension ????{ ????????public EventBindingExtension() { } ????????public EventBindingExtension(string eventHandlerName) ????????{ ????????????this.EventHandlerName = eventHandlerName; ????????} ????????[ConstructorArgument("eventHandlerName")] ????????public string EventHandlerName { get; set; } ????????public override object ProvideValue(IServiceProvider serviceProvider) ????????{ ????????????if (string.IsNullOrEmpty(EventHandlerName)) ????????????????throw new ArgumentException("The EventHandlerName property is not set", "EventHandlerName"); ????????????var target = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget)); ????????????EventInfo eventInfo = target.TargetProperty as EventInfo; ????????????if (eventInfo == null) ????????????????throw new InvalidOperationException("The target property must be an event"); ????????????? ????????????object dataContext = GetDataContext(target.TargetObject); ????????????if (dataContext == null) ????????????????throw new InvalidOperationException("No DataContext found"); ????????????var handler = GetHandler(dataContext, eventInfo, EventHandlerName); ????????????if (handler == null) ????????????????throw new ArgumentException("No valid event handler was found", "EventHandlerName"); ????????????return handler; ????????} ????????#region Helper methods ????????static object GetHandler(object dataContext, EventInfo eventInfo, string eventHandlerName) ????????{ ????????????Type dcType = dataContext.GetType(); ????????????var method = dcType.GetMethod( ????????????????eventHandlerName, ????????????????GetParameterTypes(eventInfo)); ????????????if (method != null) ????????????{ ????????????????if (method.IsStatic) ????????????????????return Delegate.CreateDelegate(eventInfo.EventHandlerType, method); ????????????????else ????????????????????return Delegate.CreateDelegate(eventInfo.EventHandlerType, dataContext, method); ????????????} ????????????return null; ????????} ????????static Type[] GetParameterTypes(EventInfo eventInfo) ????????{ ????????????var invokeMethod = eventInfo.EventHandlerType.GetMethod("Invoke"); ????????????return invokeMethod.GetParameters().Select(p => p.ParameterType).ToArray(); ????????} ????????static object GetDataContext(object target) ????????{ ????????????var depObj = target as DependencyObject; ????????????if (depObj == null) ????????????????return null; ????????????return depObj.GetValue(FrameworkElement.DataContextProperty) ?????????????????? depObj.GetValue(FrameworkContentElement.DataContextProperty); ????????} ????????#endregion ????}

這個類可以像上面例子中所示的那樣使用。

現在，這個標記擴展有一個令人討厭的限制：DataContext必須在調用之前設置ProvideValue，否則將無法找到事件處理程序方法。一種解決方案可能是訂閱DataContextChanged事件以在DataContext設置后查找方法，但同時我們仍然需要返回一些內容......并且我們不能返回null，因為它會導致異常（因為您無法訂閱具有空處理程序的事件）。所以我們需要返回一個從事件簽名中動態生成的假處理程序。它讓事情變得更加困難......但它仍然是可行的。

這是實現這種改進的第二個版本：

？

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using System; using System.Collections.Generic; using System.ComponentModel; using System.Diagnostics; using System.Linq; using System.Reflection; using System.Reflection.Emit; using System.Windows; using System.Windows.Markup; ????public class EventBindingExtension : MarkupExtension ????{ ????????private EventInfo _eventInfo; ????????public EventBindingExtension() { } ????????public EventBindingExtension(string eventHandlerName) ????????{ ????????????this.EventHandlerName = eventHandlerName; ????????} ????????[ConstructorArgument("eventHandlerName")] ????????public string EventHandlerName { get; set; } ????????public override object ProvideValue(IServiceProvider serviceProvider) ????????{ ????????????if (string.IsNullOrEmpty(EventHandlerName)) ????????????????throw new ArgumentException("The EventHandlerName property is not set", "EventHandlerName"); ????????????var target = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget)); ????????????var targetObj = target.TargetObject as DependencyObject; ????????????if (targetObj == null) ????????????????throw new InvalidOperationException("The target object must be a DependencyObject"); ????????????_eventInfo = target.TargetProperty as EventInfo; ????????????if (_eventInfo == null) ????????????????throw new InvalidOperationException("The target property must be an event"); ????????????object dataContext = GetDataContext(targetObj); ????????????if (dataContext == null) ????????????{ ????????????????SubscribeToDataContextChanged(targetObj); ????????????????return GetDummyHandler(_eventInfo.EventHandlerType); ????????????} ????????????var handler = GetHandler(dataContext, _eventInfo, EventHandlerName); ????????????if (handler == null) ????????????{ ????????????????Trace.TraceError( ????????????????????"EventBinding: no suitable method named '{0}' found in type '{1}' to handle event '{2'}", ????????????????????EventHandlerName, ????????????????????dataContext.GetType(), ????????????????????_eventInfo); ????????????????return GetDummyHandler(_eventInfo.EventHandlerType); ????????????} ????????????return handler; ????????????? ????????} ????????#region Helper methods ????????static Delegate GetHandler(object dataContext, EventInfo eventInfo, string eventHandlerName) ????????{ ????????????Type dcType = dataContext.GetType(); ????????????var method = dcType.GetMethod( ????????????????eventHandlerName, ????????????????GetParameterTypes(eventInfo.EventHandlerType)); ????????????if (method != null) ????????????{ ????????????????if (method.IsStatic) ????????????????????return Delegate.CreateDelegate(eventInfo.EventHandlerType, method); ????????????????else ????????????????????return Delegate.CreateDelegate(eventInfo.EventHandlerType, dataContext, method); ????????????} ????????????return null; ????????} ????????static Type[] GetParameterTypes(Type delegateType) ????????{ ????????????var invokeMethod = delegateType.GetMethod("Invoke"); ????????????return invokeMethod.GetParameters().Select(p => p.ParameterType).ToArray(); ????????} ????????static object GetDataContext(DependencyObject target) ????????{ ????????????return target.GetValue(FrameworkElement.DataContextProperty) ?????????????????? target.GetValue(FrameworkContentElement.DataContextProperty); ????????} ????????static readonly Dictionary<Type, Delegate> _dummyHandlers = new Dictionary<Type, Delegate>(); ????????static Delegate GetDummyHandler(Type eventHandlerType) ????????{ ????????????Delegate handler; ????????????if (!_dummyHandlers.TryGetValue(eventHandlerType, out handler)) ????????????{ ????????????????handler = CreateDummyHandler(eventHandlerType); ????????????????_dummyHandlers[eventHandlerType] = handler; ????????????} ????????????return handler; ????????} ????????static Delegate CreateDummyHandler(Type eventHandlerType) ????????{ ????????????var parameterTypes = GetParameterTypes(eventHandlerType); ????????????var returnType = eventHandlerType.GetMethod("Invoke").ReturnType; ????????????var dm = new DynamicMethod("DummyHandler", returnType, parameterTypes); ????????????var il = dm.GetILGenerator(); ????????????if (returnType != typeof(void)) ????????????{ ????????????????if (returnType.IsValueType) ????????????????{ ????????????????????var local = il.DeclareLocal(returnType); ????????????????????il.Emit(OpCodes.Ldloca_S, local); ????????????????????il.Emit(OpCodes.Initobj, returnType); ????????????????????il.Emit(OpCodes.Ldloc_0); ????????????????} ????????????????else ????????????????{ ????????????????????il.Emit(OpCodes.Ldnull); ????????????????} ????????????} ????????????il.Emit(OpCodes.Ret); ????????????return dm.CreateDelegate(eventHandlerType); ????????} ????????private void SubscribeToDataContextChanged(DependencyObject targetObj) ????????{ ????????????DependencyPropertyDescriptor ????????????????.FromProperty(FrameworkElement.DataContextProperty, targetObj.GetType()) ????????????????.AddValueChanged(targetObj, TargetObject_DataContextChanged); ????????} ????????private void UnsubscribeFromDataContextChanged(DependencyObject targetObj) ????????{ ????????????DependencyPropertyDescriptor ????????????????.FromProperty(FrameworkElement.DataContextProperty, targetObj.GetType()) ????????????????.RemoveValueChanged(targetObj, TargetObject_DataContextChanged); ????????} ????????private void TargetObject_DataContextChanged(object sender, EventArgs e) ????????{ ????????????DependencyObject targetObj = sender as DependencyObject; ????????????if (targetObj == null) ????????????????return; ????????????object dataContext = GetDataContext(targetObj); ????????????if (dataContext == null) ????????????????return; ????????????var handler = GetHandler(dataContext, _eventInfo, EventHandlerName); ????????????if (handler != null) ????????????{ ????????????????_eventInfo.AddEventHandler(targetObj, handler); ????????????} ????????????UnsubscribeFromDataContextChanged(targetObj); ????????} ????????#endregion ????}

所以這是我們可以做的事情，這要感謝這個新的WPF功能。我們也可以設想一個行為系統，類似于我們可以對附加屬性進行的操作，例如在事件發生時執行標準操作。有很多可能的應用程序，我把它留給你找到它們.

from:https://www.thomaslevesque.com/2011/09/23/wpf-4-5-subscribing-to-an-event-using-a-markup-extension/

轉載于:https://www.cnblogs.com/Chary/p/No0000130.html

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