UWP开发砸手机系列（二）—— “讲述人”识别自定义控件Command

　　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[].GetValue(EventTriggerBehavior.ActionsProperty) as ActionCollection;
            var command = actions[].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……