如何在外层标签正确使用RelativeSource避免重复绑定ViewModel？

要解决重复编写Source={RelativeSource AncestorType={x:Type vm:MainViewModel}}绑定代码的问题，核心是保留SwipeView的BindingContext为集合项（保证{Binding .}能正常获取当前列表项），同时让命令绑定到上级的MainViewModel，避免重复代码。

方法一：通过x:Reference直接绑定到MainPage的BindingContext #

先在MainPage的XAML里给根元素命名：

<ContentPage ...
             x:Name="MainPageRoot"
             xmlns:local="clr-namespace:Todo.CustomControls"
             xmlns:vm="using:Todo.ViewModel"
             x:DataType="vm:MainViewModel"
             >
    <!-- 其余内容不变 -->
</ContentPage>

然后修改自定义MySwipeView的XAML，通过引用MainPage的BindingContext绑定命令：

<SwipeView ...
    x:Class="Todo.CustomControls.MySwipeView"
    xmlns:vm="clr-namespace:Todo.ViewModel"
    >
    <SwipeView.LeftItems>
        <SwipeItems>
            <SwipeItem 
                Text="Delete" 
                Command="{Binding BindingContext.DeleteCommand, Source={x:Reference MainPageRoot}}"
                CommandParameter="{Binding .}"/>
        </SwipeItems>
    </SwipeView.LeftItems>
    <Grid Padding="0,5">
        <Frame >
            <Frame.GestureRecognizers>
                <TapGestureRecognizer
                Command="{Binding BindingContext.TapCommand, Source={x:Reference MainPageRoot}}"
                CommandParameter="{Binding .}"/>
            </Frame.GestureRecognizers>
            <Label Text="{Binding .}" FontSize="24"/>
        </Frame>
    </Grid>
</SwipeView>

方法二：给自定义控件添加绑定属性（推荐） #

这种方式符合MVVM规范，将命令作为控件的依赖属性从外部传入：

1. 修改MySwipeView后台代码，添加依赖属性：

public partial class MySwipeView : SwipeView
{
    public static readonly BindableProperty DeleteCommandProperty =
        BindableProperty.Create(nameof(DeleteCommand), typeof(ICommand), typeof(MySwipeView));

    public ICommand DeleteCommand
    {
        get => (ICommand)GetValue(DeleteCommandProperty);
        set => SetValue(DeleteCommandProperty, value);
    }

    public static readonly BindableProperty TapCommandProperty =
        BindableProperty.Create(nameof(TapCommand), typeof(ICommand), typeof(MySwipeView));

    public ICommand TapCommand
    {
        get => (ICommand)GetValue(TapCommandProperty);
        set => SetValue(TapCommandProperty, value);
    }

    public MySwipeView()
    {
        InitializeComponent();
    }
}

2. 修改MySwipeView的XAML，绑定到自身的依赖属性：

<SwipeView ...
    x:Class="Todo.CustomControls.MySwipeView"
    xmlns:vm="clr-namespace:Todo.ViewModel"
    x:Name="ThisSwipeView"
    >
    <SwipeView.LeftItems>
        <SwipeItems>
            <SwipeItem 
                Text="Delete" 
                Command="{Binding DeleteCommand, Source={x:Reference ThisSwipeView}}"
                CommandParameter="{Binding .}"/>
        </SwipeItems>
    </SwipeView.LeftItems>
    <Grid Padding="0,5">
        <Frame >
            <Frame.GestureRecognizers>
                <TapGestureRecognizer
                Command="{Binding TapCommand, Source={x:Reference ThisSwipeView}}"
                CommandParameter="{Binding .}"/>
            </Frame.GestureRecognizers>
            <Label Text="{Binding .}" FontSize="24"/>
        </Frame>
    </Grid>
</SwipeView>

3. 在MainPage的CollectionView中使用控件时，传入MainViewModel的命令：

<CollectionView ...  >
    <CollectionView.ItemTemplate>
        <DataTemplate x:DataType="{x:Type x:String}">
            <local:MySwipeView 
                DeleteCommand="{Binding DeleteCommand, Source={RelativeSource AncestorType={x:Type vm:MainViewModel}}}"
                TapCommand="{Binding TapCommand, Source={RelativeSource AncestorType={x:Type vm:MainViewModel}}}"
                />
        </DataTemplate>
    </CollectionView.ItemTemplate>
</CollectionView>

方法三：复用RelativeSource绑定资源 #

如果不想修改后台代码，可以在自定义控件中定义一个绑定资源，复用RelativeSource配置：

<SwipeView ...
    x:Class="Todo.CustomControls.MySwipeView"
    xmlns:vm="clr-namespace:Todo.ViewModel"
    >
    <SwipeView.Resources>
        <Binding x:Key="MainVmBinding" Source="{RelativeSource AncestorType={x:Type vm:MainViewModel}}"/>
    </SwipeView.Resources>
    <SwipeView.LeftItems>
        <SwipeItems>
            <SwipeItem 
                Text="Delete" 
                Command="{Binding DeleteCommand, Source={StaticResource MainVmBinding}}"
                CommandParameter="{Binding .}"/>
        </SwipeItems>
    </SwipeView.LeftItems>
    <Grid Padding="0,5">
        <Frame >
            <Frame.GestureRecognizers>
                <TapGestureRecognizer
                Command="{Binding TapCommand, Source={StaticResource MainVmBinding}}"
                CommandParameter="{Binding .}"/>
            </Frame.GestureRecognizers>
            <Label Text="{Binding .}" FontSize="24"/>
        </Frame>
    </Grid>
</SwipeView>

为什么直接设置SwipeView的BindingContext无效？ #

SwipeView处于CollectionView的ItemTemplate中，默认BindingContext是当前的集合项（此处为string类型的列表项）。如果直接给SwipeView设置BindingContext为MainViewModel，会覆盖默认值，导致CommandParameter="{Binding .}"和Label Text="{Binding .}"无法获取到当前列表项的内容，因此这种方法不可行。

内容的提问来源于stack exchange，提问作者D G