Xamarin Forms自定义Shell标题视图ViewModel绑定失效问题求助

我正在为Xamarin Forms Shell应用制作可定制、可复用的标题栏模板。目前大部分功能已实现，但部分绑定无法正常工作。

需求很明确：实现一个标准标题栏模板，可在任意页面按需配置，效果如下：

希望在ContentPage中能像这样快速配置，只需指定按钮的图标源和命令，其余样式自动适配：

<ContentPage
    x:DataType="viewmodels:MyViewModel"
    ... >

    <Shell.TitleView>
        
        <tv:CustomAppShellTitleView>
                        
            <tv:CustomAppShellTitleView.LeftButton>
                <tv:TitleViewButtonInfo Command="{Binding SomeCommandHere}" Source="{StaticResource SomeIconHere}" />
            </tv:CustomAppShellTitleView.LeftButton>

            <tv:CustomAppShellTitleView.Title>SomeTitle</tv:CustomAppShellTitleView.Title>

            <tv:CustomAppShellTitleView.RightButton1>
                <tv:TitleViewButtonInfo Command="{Binding SomeCommandHere}" Source="{StaticResource SomeIconHere}" />
            </tv:CustomAppShellTitleView.RightButton1>

            <tv:CustomAppShellTitleView.RightButton2>
                <tv:TitleViewButtonInfo Command="{Binding SomeCommandHere}" Source="{StaticResource SomeIconHere}" />
            </tv:CustomAppShellTitleView.RightButton2>

            <tv:CustomAppShellTitleView.RightButton3>
                <tv:TitleViewButtonInfo Command="{Binding SomeCommandHere}" Source="{StaticResource SomeIconHere}" />
            </tv:CustomAppShellTitleView.RightButton3>
            
        </tv:CustomAppShellTitleView>
        
    </Shell.TitleView>
...

所有ContentPage都使用ViewModel，希望通过ViewModel中的数据控制标题栏行为（比如按钮命令、可见性等）。

TitleViewButtonInfo实现 #

为了减少重复代码，我把4个按钮的可绑定属性封装到TitleViewButtonInfo类中，代码如下：

using System.Windows.Input;
using Xamarin.Forms;

namespace MyNamespace.Views.TitleViews
{
    public class TitleViewButtonInfo : BindableObject
    {
        public static readonly BindableProperty CommandProperty = BindableProperty.Create(nameof(Command), typeof(ICommand), typeof(TitleViewButtonInfo), default(ICommand));
        public ICommand Command
        {
            get => (ICommand)GetValue(CommandProperty);
            set => SetValue(CommandProperty, value);
        }

        public static readonly BindableProperty SourceProperty = BindableProperty.Create(nameof(Source), typeof(ImageSource), typeof(TitleViewButtonInfo), default(ImageSource));
        public ImageSource Source
        {
            get => (ImageSource)GetValue(SourceProperty);
            set
            {
                SetValue(SourceProperty, value);
                OnPropertyChanged(nameof(IsVisible));
            }
        }

        public bool IsVisible => Source != null;
    }
}

CustomAppShellTitleView实现 #

以下是标题栏模板的核心实现，为简化展示，省略了右侧按钮的代码：

XAML代码 #

<ContentView
    x:Class="MyNamespace.Views.TitleViews.CustomAppShellTitleView"
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:local="clr-namespace:MyNamespace.Views.TitleViews"
    x:Name="customAppShellTitleView"
    x:DataType="local:CustomAppShellTitleView">
    <ContentView.Resources>
        <ResourceDictionary>
            <x:Double x:Key="ButtonSize">32</x:Double>
        </ResourceDictionary>
    </ContentView.Resources>
    <ContentView.Content>
        <Grid
            Padding="0,0,10,0"
            RowSpacing="0"
            VerticalOptions="CenterAndExpand">

            <Grid.ColumnDefinitions>
                <!--  左侧按钮  -->
                <ColumnDefinition Width="Auto" />
                <!--  标题  -->
                <ColumnDefinition Width="*" />
                <!--  右侧按钮组  -->
                <ColumnDefinition Width="Auto" />
            </Grid.ColumnDefinitions>

            <!--  左侧按钮  -->
            <ImageButton
                Grid.Column="0"
                Command="{Binding LeftButton.Command, Source={x:Reference Name=customAppShellTitleView}}"
                HeightRequest="{StaticResource ButtonSize}"
                HorizontalOptions="Start"
                IsVisible="{Binding LeftButton.IsVisible, Source={x:Reference Name=customAppShellTitleView}}"
                Source="{Binding LeftButton.Source, Source={x:Reference Name=customAppShellTitleView}}"
                WidthRequest="{StaticResource ButtonSize}" />

            <!--  标题文本  -->
            <Label
                Grid.Column="1"
                HorizontalOptions="StartAndExpand"
                LineBreakMode="TailTruncation"
                MaxLines="1"
                Text="{Binding Title, Source={x:Reference Name=customAppShellTitleView}}"
                VerticalOptions="CenterAndExpand" />

            <!--  右侧按钮组  -->
            <StackLayout
                Grid.Column="2"
                Orientation="Horizontal"
                Spacing="10">
                <!--  按钮1、2、3的代码在此处  -->
            </StackLayout>
        </Grid>
    </ContentView.Content>
</ContentView>

后台代码 #

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace MyNamespace.Views.TitleViews
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class CustomAppShellTitleView : ContentView
    {
        // 左侧按钮属性
        public static readonly BindableProperty LeftButtonProperty = BindableProperty.Create(
            nameof(LeftButton),
            typeof(TitleViewButtonInfo),
            typeof(CustomAppShellTitleView),
            new TitleViewButtonInfo());

        public TitleViewButtonInfo LeftButton
        {
            get => (TitleViewButtonInfo)GetValue(LeftButtonProperty);
            set => SetValue(LeftButtonProperty, value);
        }

        // 右侧按钮1、2、3的属性定义与左侧按钮一致，此处省略

        #region 标题属性
        public static readonly BindableProperty TitleProperty = BindableProperty.Create(nameof(Title), typeof(string), typeof(CustomAppShellTitleView), default(string));
        
        public string Title
        {
            get => (string)GetValue(TitleProperty);
            set => SetValue(TitleProperty, value);
        }
        #endregion

        public CustomAppShellTitleView()
        {
            InitializeComponent();
            BindingContext = this;
        }
    }
}

核心问题 #

目前标题和按钮图标能正常显示，但按钮绑定到ViewModel中的ICommand后点击无响应——而相同的命令绑定到普通按钮时完全正常，说明不是ViewModel与XAML的匹配问题。

已尝试的方法 #

我在TitleViewButtonInfo.CommandProperty和ViewModel构造函数中设置命令的位置加了断点：由于视图初始化早于ViewModel的实例化和BindingContext设置，第一次触发时Command为null是正常的，但ViewModel设置命令后，CommandProperty的setter从未再次触发。即使ViewModel中的ICommand已经赋值，标题栏也无法感知到变化。我尝试手动触发Command的OnPropertyChanged，但没有效果。

请问如何让标题栏感知到ViewModel中Command的变化？我肯定在某个基础概念上理解有误，导致标题栏的属性只能被设置一次。

内容的提问来源于stack exchange，提问作者ktom