如何响应ContentPage子ContentView的ControlTemplate属性变更？

页面里有个ContentView，通过DataTriggers绑定ViewModel的布尔属性BoolValueFromViewModel，根据状态加载不同的静态ControlTemplate，代码如下：

<ContentPage.Resources>
    <ControlTemplate x:Key="template1">
     <local:AnotherContentView />
     <!-- 需要调用AnotherContentView的某个方法 -->
    </ControlTemplate>
    <ControlTemplate x:Key="template2">
    </ControlTemplate>
  </ContentPage.Resources>

  <ContentView x:Name="templatedView" >
    <ContentView.Triggers>
      <DataTrigger TargetType="ContentView" Binding="{Binding BoolValueFromViewModel}" Value="True">
        <Setter Property="ControlTemplate" Value="{StaticResource template1}"/>
      </DataTrigger>
      <DataTrigger TargetType="ContentView" Binding="{Binding BoolValueFromViewModel}" Value="False">
        <Setter Property="ControlTemplate" Value="{StaticResource template2}"/>
      </DataTrigger>
    </ContentView.Triggers>
  </ContentView>

AnotherContentView是自定义控件，里面有个LoopGaugeDrawing方法，负责每隔若干毫秒重绘GraphicsView，实现随时间递减的仪表盘。当这个控件显示时必须调用该方法，但试了OnApplyTemplate和OnAppearing都没生效。

LoopGaugeDrawing方法代码：

public async Task LoopGaugeDrawing()
{
    do
    {
        _progressGauge.TargetTime = Duration;
        _progressGauge.ElapsedTime = Duration.Subtract(RemainingTime);

        ProgressView.Invalidate();
        await Task.Delay(TimeSpan.FromMilliseconds(Constants.TimerTickMilliseconds));
    }
    while (!ConditionFromViewModel);
}

方法1：重写AnotherContentView的OnParentSet方法 #

控件被加载到模板中时，父元素会被设置，利用这个时机启动任务，同时处理控件被移除时的任务终止：

private Task _drawingTask;

protected override void OnParentSet()
{
    base.OnParentSet();
    // 父元素存在（控件显示）且任务未启动时，开始绘制循环
    if (Parent != null && _drawingTask == null)
    {
        _drawingTask = LoopGaugeDrawing();
    }
    // 父元素为null（控件被移除）时，终止循环
    else if (Parent == null && _drawingTask != null)
    {
        ConditionFromViewModel = true;
        _drawingTask = null;
    }
}

方法2：监听ContentView的TemplateApplied事件 #

在页面中监听模板切换完成事件，找到模板内的AnotherContentView并调用方法：

1. 先给template1里的控件加个名字：

<ControlTemplate x:Key="template1">
    <local:AnotherContentView x:Name="anotherView" />
</ControlTemplate>

2. 给ContentView绑定TemplateApplied事件：

<ContentView x:Name="templatedView" TemplateApplied="TemplatedView_TemplateApplied">
    <!-- 原有触发器代码不变 -->
</ContentView>

3. 后台实现事件处理：

private Task _currentDrawingTask;
private AnotherContentView _previousAnotherView;

private void TemplatedView_TemplateApplied(object sender, EventArgs e)
{
    var contentView = sender as ContentView;
    var anotherView = contentView.FindByName<AnotherContentView>("anotherView");
    
    // 切换到template1时启动任务
    if (anotherView != null && _currentDrawingTask == null)
    {
        _previousAnotherView = anotherView;
        _currentDrawingTask = anotherView.LoopGaugeDrawing();
    }
    // 切换到template2时终止任务
    else if (anotherView == null && _previousAnotherView != null)
    {
        _previousAnotherView.ConditionFromViewModel = true;
        _currentDrawingTask = null;
        _previousAnotherView = null;
    }
}

方法3：通过ViewModel发送消息通知 #

利用MessagingCenter在ViewModel属性变化时，通知AnotherContentView启动或停止任务：

1. 在AnotherContentView中订阅消息：

private Task _drawingTask;

public AnotherContentView()
{
    InitializeComponent();
    MessagingCenter.Subscribe<YourViewModel, bool>(this, "ToggleGaugeDrawing", (vm, isActive) =>
    {
        if (isActive && _drawingTask == null)
        {
            _drawingTask = LoopGaugeDrawing();
        }
        else if (!isActive && _drawingTask != null)
        {
            ConditionFromViewModel = true;
            _drawingTask = null;
        }
    });
}

protected override void Dispose(bool disposing)
{
    if (disposing)
    {
        MessagingCenter.Unsubscribe<YourViewModel, bool>(this, "ToggleGaugeDrawing");
    }
    base.Dispose(disposing);
}

2. 在ViewModel的BoolValueFromViewModel属性变更时发送消息：

private bool _boolValueFromViewModel;
public bool BoolValueFromViewModel
{
    get => _boolValueFromViewModel;
    set
    {
        if (_boolValueFromViewModel != value)
        {
            _boolValueFromViewModel = value;
            OnPropertyChanged();
            MessagingCenter.Send(this, "ToggleGaugeDrawing", value);
        }
    }
}

内容的提问来源于stack exchange，提问作者Toto