知道是否尚未在XAML中设置DependencyProperty

问题描述

我有一个控件，它公开了一个称为PlacementTarget的DP（绑定到一个子弹出窗口FWIW）。

I have a control, which exposes a DP called PlacementTarget (it's bound to a child Popup FWIW).

我想做的是，如果未在XAML中设置PlacementTarget，则（该控件将）将其设置为该控件所在的Window。当我说'not设置，我并不是说简单地是为空（这允许用户开发人员将PlacementTarget设置为null，并且控件不会将其自动设置为Window）。

What I want to do is if PlacementTarget is not set in XAML, then (the control will) set it to the Window the control is in. When I say 'not set' I don't mean simply 'is null' (this allows the user dev to set PlacementTarget to null, and the control won't auto set it to the Window).

我有一个名为placementTargetIsSet的字段，我在Change事件处理程序中将其设置为true

I have a field called placementTargetIsSet, which I set to true in the Change event handler

public readonly static DependencyProperty PlacementTargetProperty =
    DependencyProperty.Register(
        "PlacementTarget", typeof(UIElement), typeof(MyControl),
        new PropertyMetadata(new PropertyChangedCallback(PlacementTargetChanged)));

static void PlacementTargetChanged(
    DependencyObject sender, DependencyPropertyChangedEventArgs e)
{
    MyControl ctrl = (sender as MyControl);
    ctrl.placementTargetIsSet = true;
}

public UIElement PlacementTarget
{
    get { return (UIElement)GetValue(PlacementTargetProperty); }
    set { SetValue(PlacementTargetProperty, value); }
}

但是我发现更改的事件在OnApplyTemplate和加载的事件。例如，当发生OnApplyTemplate或Loaded时，无论是否已设置PlacementTarget（在XAML中），placementTargetIsSet == false。

However I'm finding that the changed event is happening AFTER OnApplyTemplate and the Loaded event. i.e. when OnApplyTemplate or Loaded happen, placementTargetIsSet==false, regardless of whether PlacementTarget been set or not (in XAML).

因此，我何时可以安全地假定PlacementTarget尚未

So when can I safely assume that PlacementTarget has not been set?

推荐答案

您不需要额外的 placementTargetIsSet 字段因此没有PropertyChangedCallback。

You don't need an extra placementTargetIsSet field and hence no PropertyChangedCallback.

为了找出是否已为 PlacementTarget 属性设置了一个值，您可以可以只调用方法，并测试它是否返回：

In order to find out if a value has been set for the PlacementTarget property, you could just call the ReadLocalValue method and test if it returns DependencyProperty.UnsetValue:

bool placementTargetIsSet =
    ReadLocalValue(PlacementTargetProperty) != DependencyProperty.UnsetValue;

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