子属性更新调用它的父的`OnPropertyChanged`答案

【问题标题】：Child properties update calling it's parent's `OnPropertyChanged`子属性更新调用它的父的`OnPropertyChanged`
【发布时间】：2018-08-06 04:07:53
【问题描述】：

我正在尝试创建一个 XF 组件，它的某些属性是继承自 BindableObject 的类型。为了说明，我有一个类Shadow 具有double Radius 和Color ShadowColor 属性和一个类MyBoxText，它具有一个bool IsLoading 和一个Shadow Ghost 属性。

我的 View 和它的 Bindings 按预期工作，但我的自定义渲染器有问题：

当我更改Ghost 属性时，我需要重绘整个视图（MyBoxText 控件的），以直观地更新阴影颜色，例如。

这是一些 mcve：

类代码：

public class MyBoxText : Label /* It's bindable by inheritance */
{
    #region Properties
    public static readonly BindableProperty IsLoadingProperty = BindableProperty.Create(nameof(IsLoading), typeof(bool), typeof(MyBoxText), false) ;
    public bool IsLoading
    {
        get { return (bool)GetValue(IsLoadingProperty); }
        set { SetValue(IsLoadingProperty, value); }
    }

    public static readonly BindableProperty GhostProperty = BindableProperty.Create(nameof(Ghost), typeof(Shadow), typeof(MyBoxText), null) ;
    public Shadow Ghost
    {
        get { return (Shadow)GetValue(GhostProperty); }
        set { SetValue(GhostProperty, value); }
    }
    #endregion
}

public class Shadow : BindableObject /* It's explictly bindable */
{
    #region Properties
    public static readonly BindableProperty ShadowColorProperty = BindableProperty.Create(nameof(ShadowColor), typeof(Color), typeof(Shadow), Color.Black) ;
    public Color ShadowColor
    {
        get { return (Color)GetValue(ShadowColorProperty); }
        set { SetValue(ShadowColorProperty, value); }
    }

    public static readonly BindableProperty ShadowRadiusProperty = BindableProperty.Create(nameof(ShadowRadius), typeof(double), typeof(Shadow), 20) ;
    public double ShadowRadius
    {
        get { return (double)GetValue(ShadowRadiusProperty); }
        set { SetValue(ShadowRadiusProperty, value); }
    }
    #endregion

    public Shadow()
    {

    }
}

我的渲染器代码是这样的：

public class MyBoxText : LabelRenderer
{
    public MyBoxText()
    {
        SetWillNotDraw(false);
    }

    public override void Draw(Canvas canvas)
    {
        MyBoxText myView = (MyBoxText)this.Element;

        // Some drawing logic
    }

    protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        base.OnElementPropertyChanged(sender, e);

        if (e.PropertyName == MyBoxText.IsLoadingProperty.PropertyName  ||
            e.PropertyName == MyBoxText.GhostProperty.PropertyName )
            Invalidate();
    }
}

问题是，当我更改 Ghost.ShadowColor 属性时，我的“OnElementPropertyChanged”覆盖没有被调用，并且视图在屏幕上保持旧颜色。

有没有办法将孩子的“属性更新”事件传播到父视图“属性已更改”或其他方式来实现这一点？

【问题讨论】：

标签： c# xamarin binding xamarin.android custom-controls

【解决方案1】：

问题是，当我更改 Ghost.ShadowColor 属性时，不会调用我的“OnElementPropertyChanged”覆盖，并且视图会在屏幕上保持旧颜色。有没有办法将孩子的“属性更新”事件传播到父视图“属性已更改”或其他方式来实现这一点？

是的，有办法。由于您的 Shadow 继承自 BindableObject，它实现了 INotifyPropertyChanged 接口。你可以设置通知ShadowColor更改：

将OnPropertyChanged() 添加到Shadow.cs 中ShadowColor 的Setter：

public class Shadow : BindableObject /* It's explictly bindable */
{
    #region Properties
    public static readonly BindableProperty ShadowColorProperty = BindableProperty.Create(nameof(ShadowColor), typeof(Color), typeof(Shadow), Color.Black);
    public Color ShadowColor
    {
        get { return (Color)GetValue(ShadowColorProperty); }
        set { SetValue(ShadowColorProperty, value);
            //Notify the ShadowColorProperty Changed
            OnPropertyChanged();
        }
    }
   ...
}

像这样修改你的MyBoxText.cs：

public class MyBoxText : Label /* It's bindable by inheritance */
{
 ...

    public static readonly BindableProperty GhostProperty = BindableProperty.Create(nameof(Ghost), typeof(Shadow), typeof(MyBoxText), null);
    public Shadow Ghost
    {
        get { return (Shadow)GetValue(GhostProperty); }
        set {
            //register the ShadowColor change event
            value.PropertyChanged += ShadowColor_PropertyChanged;
            SetValue(GhostProperty, value); }
    }

    private void ShadowColor_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        //unregister the event
        this.Ghost.PropertyChanged -= ShadowColor_PropertyChanged;
        //set this.Ghost to a new object with new ShadowColor to trigger the OnPropertyChanged
        this.Ghost = new Shadow
        {
            ShadowColor = (sender as Shadow).ShadowColor,
            ShadowRadius = Ghost.ShadowRadius
        };
    }
}

【讨论】：

它看起来确实是一个解决方案，完全符合我的需要，但它有一个小错误：当我从 XAML 的绑定中设置 MyBoxText.Ghost 时，它没有命中 set方法（我们开始听的地方）...我会根据这个解决方案进行一些尝试，我会告诉你的。感谢您的时间和精力。
我也绕过了在get 方法中删除和添加侦听器的错误。现在可以正常使用了，谢谢。我根据您的回答制作了一个单独的课程以重用该解决方案，我将在此处发布它只是为了进一步重用。

【解决方案2】：

感谢猫王的回答，我明白了。根据他的想法，我进行了一些更改以在其他组件上重用它，我现在分享它以防其他人需要这样的东西。

我认为这样使用它可以得到更简洁的代码：

public class MyBoxText : Label /* It's bindable by inheritance */
{
    // Added this as private property
    private ChangingPropagator changingPropagator;
    private ChangingPropagator ChangingPropagator
    {
        get
        {
            if (changingPropagator == null)
                changingPropagator = new ChangingPropagator(this, OnPropertyChanged, nameof(Shadow.ShadowColor), nameof(Shadow.ShadowRadius));

            return changingPropagator;
        }
    }

    #region Properties
    public static readonly BindableProperty IsLoadingProperty = BindableProperty.Create(nameof(IsLoading), typeof(bool), typeof(MyBoxText), false) ;
    public bool IsLoading
    {
        get { return (bool)GetValue(IsLoadingProperty); }
        set { SetValue(IsLoadingProperty, value); }
    }

    public static readonly BindableProperty GhostProperty = BindableProperty.Create(nameof(Ghost), typeof(Shadow), typeof(MyBoxText), null) ;
    public Shadow Ghost
    {
        // Here I use the ChangingPropagator's Getter and Setter instead of the deafult ones:
        get { return ChangingPropagator.GetValue<Shadow>(GhostProperty); }
        set { ChangingPropagator.SetValue(GhostProperty,ref value); }
    }
    #endregion
}

这是ChangingPropagator 类：

public class ChangingPropagator
{
    string[] listenedProperties = new string[0];
    Action<string> changesNotifyer = null;
    BindableObject propagationRootObject = null;
    List<KeyValuePair<string, object>> propagationProperties = new List<KeyValuePair<string, object>>();

    public ChangingPropagator(BindableObject bindableObject, Action<string> onPropertyChangedMethod, params string[] propertyToListenTo)
    {
        changesNotifyer = onPropertyChangedMethod;
        propagationRootObject = bindableObject;
        listenedProperties = propertyToListenTo ?? listenedProperties;

        // ToDo: Add some consistency checks
    }

    public void AddPropertyToListenTo(params string[] propertyName)
    {
        listenedProperties = listenedProperties.Union(propertyName).ToArray();
    }

    // I need handle it here too 'cause when I use the child `Ghost` property coming from XAML binding, it didn't hit the `set` method
    public T GetValue<T>(BindableProperty property)
    {
        var value = propagationRootObject?.GetValue(property);

        if (value != null)
        {
            INotifyPropertyChanged bindableSubObject = (value as INotifyPropertyChanged);

            if (bindableSubObject != null)
            {
                bindableSubObject.PropertyChanged -= PropagatorListener;
                bindableSubObject.PropertyChanged += PropagatorListener;

                if (!propagationProperties.Any(a => a.Key == property.PropertyName))
                    propagationProperties.Add(new KeyValuePair<string, object>(property.PropertyName, value));
            }
        }

        return (T)value;
    }

    public void SetValue<T>(BindableProperty property, ref T value)
    {
        var oldValue = propagationRootObject?.GetValue(property);

        if (oldValue != null)
        {
            INotifyPropertyChanged bindableSubObject = (value as INotifyPropertyChanged);

            if (bindableSubObject != null)
                bindableSubObject.PropertyChanged -= PropagatorListener;
        }

        if (value != null)
        {
            INotifyPropertyChanged bindableSubObject = (value as INotifyPropertyChanged);
            if (bindableSubObject != null)
            {
                bindableSubObject.PropertyChanged += PropagatorListener;

                propagationProperties.RemoveAll(p => p.Key == property.PropertyName);
                propagationProperties.Add(new KeyValuePair<string, object>(property.PropertyName, value));
            }
        }

        propagationRootObject.SetValue(property, value);
    }

    private void PropagatorListener(object sender, PropertyChangedEventArgs e)
    {
        if (listenedProperties?.Contains(e.PropertyName) ?? true)
            PropagationThrower(sender);
    }

    private void PropagationThrower(object sender)
    {
        if (propagationProperties.Any(p => p.Value == sender))
        {
            var prop = propagationProperties.FirstOrDefault(p => p.Value == sender);
            changesNotifyer?.Invoke(prop.Key);
        }
    }
}

【讨论】：