上一篇我们提到如何让“讲述人”读出自定义的CanReadGrid，但“讲述人”仍然无法识别CanReadGrid上绑定的Command。XAML代码如下：
<StackPanel>
<TextBlock Text="{x:Bind Title,Mode=OneWay}" Foreground="White"></TextBlock>
<local:CanReadGrid Background="Red" AutomationProperties.Name="Can read gird" Height="100">
<Interactivity:Interaction.Behaviors>
<Core:EventTriggerBehavior EventName="Tapped">
<Core:InvokeCommandAction Command="{x:Bind ChangeTitleCommand}"/>
</Core:EventTriggerBehavior>
</Interactivity:Interaction.Behaviors>
</local:CanReadGrid>
</StackPanel>
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：
<Button Command="{x:Bind ChangeTitleCommand}">I am Button</Button>
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（Dependency Property）：
public System.Windows.Input.ICommand Command { get; set; }
Member of Windows.UI.Xaml.Controls.Primitives.ButtonBase
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：
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>
protected override string 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>
protected override 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>
public override object GetPattern(PatternInterface patternInterface)
{
if (patternInterface == PatternInterface.Invoke)
{
return this;
}
return base.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())
{
throw new ElementNotEnabledException();
}
base.Dispatcher.BeginInvoke(DispatcherPriority.Input, new DispatcherOperationCallback(delegate(object param)
{
((Button)base.Owner).AutomationButtonBaseClick();
return null;
}), null);
}
}
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。
public class CanReadGrid : Grid
{
protected override AutomationPeer OnCreateAutomationPeer()
{
return new GridAutomationPeer((this));
}
public void 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);
}
}
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。
public class GridAutomationPeer : FrameworkElementAutomationPeer, IInvokeProvider
{
public GridAutomationPeer(Grid owner)
: base(owner)
{
}
public async void Invoke()
{
await CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.Normal,
() =>
{
((CanReadGrid)base.Owner).ExecuteCommand();
});
}
protected override object GetPatternCore(PatternInterface patternInterface)
{
if (patternInterface == PatternInterface.Invoke)
{
return this;
}
return null;
}
}
大功告成！开启“讲述人”模式来验证成果吧！
结尾插播个广告，本篇内容100%原创，当时俺翻烂了Google的搜索页面、中英文各种blog也木有讲如何让“讲述人”调用Command，逼的俺自由发挥啊。特地写了这篇造福全人类，各位不要吝啬点个推荐哦，虽然“讲述人”并没有什么卵用……