Xamarin.Forms 仅绑定更新 ViewModel 属性与默认值

Question

Consider the following somewhat complex case in Xamarin.Forms (tested on iOS simulator):

• One GenericPage superclass inheriting from ContentPage and containing a BindableProperty
• One Page class inheriting from GenericPage with a ViewModel which is bound to the BindableProperty using a OneWayToSource binding mode
• One GenericControl superclass inheriting from ContentView and containing a BindableProperty
• One Control class inheriting from GenericControl with a ControlViewModel which is bound to the BindableProperty using a OneWayToSource binding mode
• 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

I can verify that the "connection" from Page to the BindableProperty of GenericControl indeed works, as the propertyChanged method is invoked in GenericControl with the default value from the BindableProperty in GenericPage . I can also verify that the "connection" from GenericControl to ControlViewModel is working as the property setter in ControlViewModel is called with the default value from the BindableProperty in GenericControl .

However, for some reason, the changes which arrive at the BindableProperty in GenericControl (either the default values from GenericPage or externally set) are not propagated to the ControlViewModel .

---

Full Code is available under: https://github.com/mlxyz/Xamarin-Forms-Binding-Repro

GenericPage:

 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);
        }

Page:

<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>

ViewModel:

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

GenericControl:

// 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)
        }

Control:

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

ControlViewModel:

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

Answer 1

I checked your sample and found out the cause.

When you set the BindingContext of Control

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

The binding between Control and Page1 will not work any more.You need to handle the logic in Control.xaml.cs directly.

In addition, since you had set the BindingContext of Page1 as ViewModel , why did you still binding the Test to itself?

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

You should put all binding source in ViewModel and handle logic in it.

Note: The property in superclass will not available in subclass when using data-binding. So you need to define them in subclass one more time.

So you could modify the code like following. I add a button to Page1. When clicking it, the value of Test will change.

in Page1.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>

in ViewModel

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);

            });

        }

    }
}

in Control.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>

in 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;
        }

    }
}