本帖最后由 Clever-he 于 2016-1-7 10:28 编辑

我们可以看到通过Behaviors绑定了Command，在Tapped事件发生时触发ChangeTitleCommand。

我们再来对比一下系统控件Button的写法：

[mw_shl_code=csharp,true]<Button Command="{x:Bind ChangeTitleCommand}">I am Button</Button>[/mw_shl_code]

在“讲述人”模式下，点击上面这个Button按钮，“讲述人”除了会念出“I am Button Button.”这句话以外，还会补充一句“Double tap to activate.”这时双击Button将会触发ChangeTitleCommand。

其中不一样的地方无非就是Button自带有名为“Command”的，类型为ICommand的依赖属性（DependencyProperty）：

[mw_shl_code=csharp,true]public System.Windows.Input.ICommand Command { get; set; }
Member of Windows.UI.Xaml.Controls.Primitives.ButtonBase[/mw_shl_code]

而我们自定义的CanReadGrid，则是通过附加属性（Attached Property）来获得绑定Command的能力。

附件属性也是一种特殊的依赖属性，二者殊归同路。既然Button通过依赖属性可以做到的事情，附加属性一样可以完成。

想要弄明白Button的Command是如何被调用的，最简单的办法就是去查看源码呗：

[mw_shl_code=csharp,true]public class ButtonAutomationPeer : ButtonBaseAutomationPeer, IInvokeProvider
{
///<summary>Initializes a new instance of the <see cref="T:System.Windows.Automation.Peers.ButtonAutomationPeer" /> class.</summary> ///<param name="owner">The element associated with this automation peer.</param> public ButtonAutomationPeer(Button owner) : base(owner)
{
}

///<summary>Gets the name of the control that is associated with this UI Automation peer.</summary> ///<returns>A string that contains "Button".</returns> protectedoverridestring GetClassNameCore()
{
return"Button";
}

///<summary>Gets the control type of the element that is associated with the UI Automation peer.</summary> ///<returns> /// <see cref="F:System.Windows.Automation.Peers.AutomationControlType.Button" />.</returns> protectedoverride AutomationControlType GetAutomationControlTypeCore()
{
return AutomationControlType.Button;
}

///<summary>Gets the object that supports the specified control pattern of the element that is associated with this automation peer.</summary> ///<returns>If <paramref name="patternInterface" /> is <see cref="F:System.Windows.Automation.Peers.PatternInterface.Invoke" />, this method returns a this pointer, otherwise this method returns null.</returns> ///<param name="patternInterface">A value in the enumeration.</param> publicoverrideobject GetPattern(PatternInterface patternInterface)
{
if (patternInterface == PatternInterface.Invoke)
{
returnthis;
}
returnbase.GetPattern(patternInterface);
}

///<summary>This type or member supports the Windows Presentation Foundation (WPF) infrastructure and is not intended to be used directly from your code.</summary> void IInvokeProvider.Invoke()
{
if (!base.IsEnabled())
{
thrownew ElementNotEnabledException();
}
base.Dispatcher.BeginInvoke(DispatcherPriority.Input, new DispatcherOperationCallback(delegate(object param)
{
((Button)base.Owner).AutomationButtonBaseClick();
returnnull;
}), null);
}
}[/mw_shl_code]

果不其然发现了上一篇我们提到的GetClassNameCore，GetAutomationControlTypeCore，GetPattern三个方法。另外还有一个奇怪的void IInvokeProvider.Invoke()。这货看名字也能猜出来是干啥的啦，这货竟然去调了Button类里的Click方法……

知道真相的我眼泪流出来……搞啥呢，那我在这里调一下Command.Execute不就成了！
首先我们给CanReadGrid添加ExecuteCommand方法，该方法通过DependencyObject的GetValue方法一层层拿到Command，然后执行Execute。

[mw_shl_code=csharp,true]public class CanReadGrid : Grid
{
protectedoverride AutomationPeer OnCreateAutomationPeer()
{
returnnew GridAutomationPeer((this));
}

publicvoid ExecuteCommand()
{
var behaviors = Interaction.GetBehaviors(this);
var actions = behaviors[0].GetValue(EventTriggerBehavior.ActionsProperty) as ActionCollection;
var command = actions[0].GetValue(InvokeCommandAction.CommandProperty) as ICommand;
command.Execute(null);
}
}
[/mw_shl_code]

第二步就是完善GridAutomatioPeer，这里需要注意的是IInvokeProvider这个接口，通过Button的源码推测具有Action的控件需要实现这个接口的Invoke方法来执行操作。我们也是在Invoke方法里来调用CanReadGrid类里的ExecuteCommand方法。

[mw_shl_code=csharp,true]public class GridAutomationPeer : FrameworkElementAutomationPeer, IInvokeProvider
{
public GridAutomationPeer(Grid owner)
: base(owner)
{

}

publicasyncvoid Invoke()
{
await CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.Normal,
() => {
((CanReadGrid)base.Owner).ExecuteCommand();
});
}

protectedoverrideobject GetPatternCore(PatternInterface patternInterface)
{
if (patternInterface == PatternInterface.Invoke)
{
returnthis;
}

returnnull;
}
}
[/mw_shl_code]

大功告成！开启“讲述人”模式来验证成果吧！

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