Xamarin.Forms 仅绑定更新 ViewModel 属性与默认值

Question

在 Xamarin.Forms 中考虑以下有点复杂的情况（在Xamarin.Forms模拟器上测试）：

• 一个继承自ContentPage并包含BindableProperty的GenericPage超类
• Page class 继承自GenericPage ，其ViewModel使用OneWayToSource绑定模式绑定到BindableProperty
• 一个继承自ContentView并包含BindableProperty的GenericControl超类
• 一个Control class 继承自GenericControl ，其ControlViewModel使用OneWayToSource绑定模式绑定到BindableProperty
• The Control class is embedded in the Page class using XAML and the BindableProperty of GenericControl is bound to the Property from the ViewModel class using a OneWay binding mode

我可以验证从Page到GenericControl的BindableProperty的“连接”确实有效，因为在GenericControl中使用GenericPage中的BindableProperty的默认值调用propertyChanged方法。 我还可以验证从GenericControl到ControlViewModel的“连接”是否正在工作，因为ControlViewModel中的属性设置器是使用GenericControl中的BindableProperty的默认值调用的。

但是，由于某种原因，到达GenericControl中的BindableProperty的更改（来自GenericPage的默认值或外部设置）不会传播到ControlViewModel 。

---

完整代码位于： https://github.com/mlxyz/Xamarin-Forms-Binding-Repro

通用页面：

 public static readonly BindableProperty TestProperty =
            BindableProperty.Create(nameof(Test), typeof(Vector3), typeof(GenericPage), new Vector3(1, 2, 3));

        public Vector3 Test
        {
            get => (Vector3)GetValue(TestProperty);
            set => SetValue(TestProperty, value);
        }

页：

<views:GenericPage Test="{Binding Test, Mode=OneWayToSource}" x:Name="Root">
<views:GenericPage.BindingContext>
        <viewModels:ViewModel />
    </views:GenericPage.BindingContext>
<ContentPage.Content>

   <controls:Control Test="{Binding Source={x:Reference Root}, Path=BindingContext.Test, Mode=OneWay}" />
</ContentPage.Content>
</views:GenericPage>

视图模型：

public Vector3 Test
        {
            get => _test;
            set
            {
                _test = value;
                OnPropertyChanged();
            }
        }

通用控制：

// bindable property and associated property is defined basically the same as in GenericPage except for propertyChanged property set and different default values
        private static void PropertyChanged(BindableObject bindable, object oldvalue, object newvalue)
        {
            System.Diagnostics.Debug.WriteLine("property changed"); // <- this is called with default values from GenericPage (1,2,3)
        }

控制：

<controls:GenericControl Test="{Binding Test, Mode=OneWayToSource}">
    <controls:GenericControl.BindingContext>
        <viewModels:ControlViewModel />
    </controls:GenericControl.BindingContext>
</controls:GenericControl>

控制视图模型：

 public Vector3 Test
        {
            get => _test;
            set => _test = value; // <- this gets called only with default values from `GenericControl` (4,5,6)
        }

Answer 1

我检查了你的样品并找出了原因。

当你设置Control的BindingContext

<controls:GenericControl.BindingContext>
     <viewModels:ControlViewModel />
</controls:GenericControl.BindingContext>

Control和Page1之间的绑定将不再起作用。您需要直接处理Control.xaml.cs中的逻辑。

另外，既然你已经将Page1的BindingContext设置为ViewModel ，为什么还要将Test绑定到自身呢？

Test="{Binding Test, Mode=OneWayToSource}"

您应该将所有绑定源放在 ViewModel 中并在其中处理逻辑。

注意：使用数据绑定时，超类中的属性在子类中不可用。 因此，您需要再次在子类中定义它们。

所以你可以像下面这样修改代码。 我向 Page1 添加了一个按钮。 单击它时，Test 的值会发生变化。

在第1页.xaml

<?xml version="1.0" encoding="utf-8"?>

<views:GenericPage xmlns="http://xamarin.com/schemas/2014/forms"
                   xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                   xmlns:d="http://xamarin.com/schemas/2014/forms/design"
                   xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                   xmlns:controls="clr-namespace:ReproProjectBindingIssue.Controls;assembly=ReproProjectBindingIssue"
                   xmlns:views="clr-namespace:ReproProjectBindingIssue.Views;assembly=ReproProjectBindingIssue"
                   xmlns:viewModels="clr-namespace:ReproProjectBindingIssue.ViewModels;assembly=ReproProjectBindingIssue"
                   mc:Ignorable="d"
                   x:Class="ReproProjectBindingIssue.Views.Page1" x:Name="Root">
    <views:GenericPage.BindingContext>
        <viewModels:ViewModel />
    </views:GenericPage.BindingContext>
    <ContentPage.Content>
        <StackLayout VerticalOptions="CenterAndExpand" HorizontalOptions="Center">
            <Button Text="Change Value"
                   VerticalOptions="CenterAndExpand"
                   HorizontalOptions="CenterAndExpand"
                    Command="{Binding command}"/>

            <controls:Control Test="{Binding Source={x:Reference Root}, Path=BindingContext.Test, Mode=OneWay}" />
        </StackLayout>
    </ContentPage.Content>
</views:GenericPage>

在视图模型中

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Numerics;
using System.Runtime.CompilerServices;
using System.Text;
using System.Windows.Input;
using ReproProjectBindingIssue.Annotations;
using Xamarin.Forms;

namespace ReproProjectBindingIssue.ViewModels
{
    public class ViewModel : INotifyPropertyChanged
    {

        public ICommand command { get; set; }

        private Vector3 _test;

        public event PropertyChangedEventHandler PropertyChanged;

        public Vector3 Test
        {
            get => _test;
            set
            {
                _test = value;
                OnPropertyChanged();
            }
        }

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }


        public ViewModel()
        {
            Test = new Vector3(9,10,11);

            command = new Command(()=> {

                Test = new Vector3(12, 13, 14);

            });

        }

    }
}

在控制中。xaml

<?xml version="1.0" encoding="UTF-8"?>

<controls:GenericControl xmlns="http://xamarin.com/schemas/2014/forms"
                         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                         xmlns:d="http://xamarin.com/schemas/2014/forms/design"
                         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                         xmlns:controls="clr-namespace:ReproProjectBindingIssue.Controls;assembly=ReproProjectBindingIssue"
                         xmlns:viewModels="clr-namespace:ReproProjectBindingIssue.ViewModels;assembly=ReproProjectBindingIssue"
                         mc:Ignorable="d"

                         x:Name="CustomView"


                         x:Class="ReproProjectBindingIssue.Controls.Control" Test="{Binding Test, Mode=OneWayToSource}">
    <controls:GenericControl.BindingContext>
        <viewModels:ControlViewModel />
    </controls:GenericControl.BindingContext>
    <ContentView.Content>
        <StackLayout VerticalOptions="CenterAndExpand" HorizontalOptions="CenterAndExpand">
            <Label  x:Name="label"/>
        </StackLayout>
    </ContentView.Content>
</controls:GenericControl>

在 Control.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using System.Text;
using System.Threading.Tasks;
using ReproProjectBindingIssue.ViewModels;
using ReproProjectBindingIssue.Views;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace ReproProjectBindingIssue.Controls
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class Control : GenericControl
    {
        public Control()
        {
            InitializeComponent();
        }


        public static readonly BindableProperty TestProperty =
            BindableProperty.Create(nameof(Test), typeof(Vector3), typeof(Control), new Vector3(4, 5, 6), propertyChanged: PropertyChanged);

        public Vector3 Test
        {
            get => (Vector3)GetValue(TestProperty);
            set => SetValue(TestProperty, value);
        }

        private static void PropertyChanged(BindableObject bindable, object oldvalue, object newvalue)
        {
            var control = bindable as Control;

            Vector3 value = (Vector3)newvalue;

            control.label.Text = "X:" + value.X + " Y:" + value.Y + " Z:" + value.Z;
        }

    }
}