Maui/Xamarin：切换系统主题后AppShell标签未重新渲染问题

我的Shell菜单使用以下代码实现FlyoutFooter：

<Shell.FlyoutFooter>
    <VerticalStackLayout Padding="10,0,0,0" Margin="0,0,0,10" HeightRequest="50">
        <Label Text="{Binding UserEmail}" HorizontalOptions="Center" TextColor="{AppThemeBinding Dark=White, Light=Black}" FontFamily="ook" FontSize="Medium"/>
        <Label Text="{Binding AppVersion, StringFormat='V. {0}'}" TextColor="{AppThemeBinding Dark=White, Light=Black}" FontFamily="ook" HorizontalOptions="Center" FontSize="Medium"/>
    </VerticalStackLayout>
</Shell.FlyoutFooter>

这段代码可正常运行，但切换手机主题后不重启直接返回应用时，FlyoutFooter内的标签文字颜色不会随主题变更。奇怪的是Shell菜单里其他元素都能正确重新渲染。

我尝试通过绑定Appearing事件处理（MVVM模式），代码如下：

<Shell.Behaviors>
    <MTK:EventToCommandBehavior EventName="Appearing"  Command="{Binding ReLoadLogsCommand}"/>
</Shell.Behaviors>

但应用恢复时该事件不会触发，此方案无效。请问还有哪些修复方案？

以下几种方案可尝试解决该主题切换不更新的问题：

• 监听主题变更事件，强制刷新绑定上下文
  在ViewModel中监听应用主题变更事件，当主题切换时手动重置FlyoutFooter的绑定上下文，触发UI刷新：

  public YourViewModel()
  {
      Application.Current.RequestedThemeChanged += OnRequestedThemeChanged;
  }
  
  private void OnRequestedThemeChanged(object sender, AppThemeChangedEventArgs e)
  {
      var mainShell = Application.Current.MainPage as Shell;
      if (mainShell?.FlyoutFooter is VerticalStackLayout footerLayout)
      {
          var currentContext = footerLayout.BindingContext;
          footerLayout.BindingContext = null;
          footerLayout.BindingContext = currentContext;
      }
  }
  

• 使用全局样式替代直接绑定
  在资源字典中定义适配主题的Label样式，再给FlyoutFooter内的Label应用该样式，主题变更时样式会自动更新：

  <ResourceDictionary>
      <Style x:Key="FooterLabelStyle" TargetType="Label">
          <Setter Property="TextColor" Value="{AppThemeBinding Dark=White, Light=Black}"/>
          <Setter Property="FontFamily" Value="ook"/>
          <Setter Property="FontSize" Value="Medium"/>
          <Setter Property="HorizontalOptions" Value="Center"/>
      </Style>
  </ResourceDictionary>
  

  修改FlyoutFooter的Label代码：

  <Label Text="{Binding UserEmail}" Style="{StaticResource FooterLabelStyle}"/>
  <Label Text="{Binding AppVersion, StringFormat='V. {0}'}" Style="{StaticResource FooterLabelStyle}"/>
  

• 改用FlyoutFooterTemplate定义内容
  把FlyoutFooter替换为FlyoutFooterTemplate形式，模板化的内容在主题变更时可能会自动触发重新渲染：

  <Shell.FlyoutFooterTemplate>
      <DataTemplate>
          <VerticalStackLayout Padding="10,0,0,0" Margin="0,0,0,10" HeightRequest="50">
              <Label Text="{Binding UserEmail}" HorizontalOptions="Center" TextColor="{AppThemeBinding Dark=White, Light=Black}" FontFamily="ook" FontSize="Medium"/>
              <Label Text="{Binding AppVersion, StringFormat='V. {0}'}" TextColor="{AppThemeBinding Dark=White, Light=Black}" FontFamily="ook" HorizontalOptions="Center" FontSize="Medium"/>
          </VerticalStackLayout>
      </DataTemplate>
  </Shell.FlyoutFooterTemplate>
  

• 添加刷新触发器属性
  在ViewModel中新增一个用于触发UI刷新的属性，主题变更时更新该属性，通过绑定让Label重新渲染：

  private int _themeRefreshTrigger;
  public int ThemeRefreshTrigger
  {
      get => _themeRefreshTrigger;
      set => SetProperty(ref _themeRefreshTrigger, value);
  }
  
  private void OnRequestedThemeChanged(object sender, AppThemeChangedEventArgs e)
  {
      ThemeRefreshTrigger++;
  }
  

  给Label添加绑定触发器：

  <Label Text="{Binding UserEmail}" TextColor="{AppThemeBinding Dark=White, Light=Black}" 
         FontFamily="ook" FontSize="Medium" HorizontalOptions="Center"
         Tag="{Binding ThemeRefreshTrigger}"/>
  

内容的提问来源于stack exchange，提问作者innom