Question

我有一个带有CoerceValueCallback的DependencyProperty的控件。此属性绑定到模型对象上的属性。

当将控件属性设置为导致强制的值时，Binding会将 uncoerced 值推送到模型对象。控件上的属性值被正确强制。

如何让Binding将强制值推送到模型对象？

void Initialize()
{
    UIObject ui = new UIObject();
    ModelObject m = new ModelObject();
    m.P = 4;

    Binding b = new Binding("P");
    b.Source = m;
    b.Mode = BindingMode.TwoWay;
    Debug.WriteLine("SetBinding");
    // setting the binding will push the model value to the UI
    ui.SetBinding(UIObject.PProperty, b);

    // Setting the UI value will result in coercion but only in the UI.
    // The value pushed to the model through the binding is not coerced.
    Debug.WriteLine("Set to -4");
    ui.P = -4;

    Debug.Assert(ui.P == 0);
    // The binding is TwoWay, the DP value is coerced to 0.
    Debug.Assert(m.P == 0); // Not true. This will be -4. Why???
}

class UIObject : FrameworkElement
{
    public static readonly DependencyProperty PProperty =
        DependencyProperty.Register("P", typeof(int), typeof(UIObject), 
        new FrameworkPropertyMetadata(
            new PropertyChangedCallback(OnPChanged), 
            new CoerceValueCallback(CoerceP)));

    public int P
    {
        get { return (int)GetValue(PProperty); }
        set { SetValue(PProperty, value); }
    }

    private static void OnPChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Debug.WriteLine(typeof(UIObject) + ".P changed from " + e.OldValue + " to " + e.NewValue);
    }

    private static object CoerceP(DependencyObject sender, object value)
    {
        int p = (int)value;
        if (p < 0)
        {
            Debug.WriteLine(typeof(UIObject) + ".P coerced from " + p + " to 0");
            p = 0;
        }
        return p;
    }
}

class ModelObject
{
    private int p;
    public int P
    {
        get
        {
            Debug.WriteLine(this + ".P returned " + this.p);
            return this.p;
        }
        set
        {
            Debug.WriteLine(this + ".P changed from +" + this.p + " to " + value);
            this.p = value;
        }
    }
}

Answer 1

我认为这就是强制的整个想法 - 动态正确的价值，而不会触发任何其他依赖的修改。您可以使用以下代码而不是本机强制机制：

OnPChanged(/* ... */)
{
    // ...
    var coercedP = CoerceP(P);
    if (P != coercedP)
        P = coercedP;
    // ...
}

HTH。

Answer 2

我认为强制回调并不是双向的。一种解决方法是在强制回调中更新模型的值。

Answer 3

这是您在目标对象上设置值的扩展方法

public static void SetTargetValue<T>(this FrameworkElement element, DependencyProperty dp, T value)
    {
        var binding = BindingOperations.GetBinding(element, dp);
        if (binding == null) return;
        var name = binding.Path.Path;
        var splits = name.Split('.');
        var target = element.DataContext;
        for (var i = 0; i < splits.Length; i++)
        {
            PropertyInfo property;
            if (i == splits.Length - 1)
            {
                property = target.GetType().GetProperty(splits[i]);
                property.SetValue(target, value);
            }
            else
            {
                property = target.GetType().GetProperty(splits[i]);
                target = property.GetValue(target);
            }
        }
    }

因此，在此方法中，使用绑定，您可以将值设置为source。源码源路径可以有很多名称 - Property1.Property2.Property3等。在强制方法中，您只需要调用此方法：

private static object CoerceProperty(DependencyObject d, object baseValue)
    {
        if (!Check)
        {
            var sender = (FrameworkElement)d;
            sender.SetTargetValue(MyPropertyProperty, myValue);
            return needValue;
        }
        return baseValue;
    }

如何使Binding尊重DependencyProperty值强制？

3 个答案: