WPF backs up some properties of a control, does some magic operations, and then restores these properties. What a common operation! However, how to back up is a question worth studying. Direct assignment? That must be because you didn't step in some pits.
Content of this article
- Scenarios and issues
- Solutions and principles
- extending
Scenarios and issues
Now, let's imagine a scenario (for the convenience of coding):
- There is a window with some style properties set
- Now you need to set this window to full-screen, which requires modification of some original properties (there is a bug in the setting that comes with WPF, so I will write a separate blog explanation)
- After canceling the setting window full screen, those previously modified properties must be restored "perfectly"
Generally, you might write like this:
private Window _window;
private WindowStyle _oldStyle;
private void OnEnterFullScreen()
{
_oldStyle = _window.WindowStyle;
_window.WindowStyle = WindowStyle.None;
}
private void OnExitFullScreen()
{
_window.WindowStyle = _oldStyle;
}
However:
- 如果某人在
WindowStyle上设了个动态的样式怎么办？——那当然是不再动态了呀（因为覆盖了样式值） - 如果某人在
WindowStyle上设置了绑定怎么办？——那当然也是不再生效了呀（因为绑定被你覆盖了）
Solutions and principles
因为各大 WPF 入门书籍都说到了 WPF 依赖项属性的优先级机制，所以大家应该基本都知道这个。不了解的，可以立刻去这里看看：依赖项属性值优先级 - WPF Microsoft Docs。
The priority is as follows: Mandatory> Animation> Local Values> Templates> Implicit Styles> Style Triggers> Template Triggers> Styles> Default Styles> Property Inheritance> Metadata Default Values.
Instead, we set the "local value" by assigning values directly in XAML or C#code. Therefore, if a local value is set, all lower-priority styles will of course be invalid.
那么绑定呢？绑定在依赖项属性优先级中并不存在。绑定实际上是通过“本地值”来实现的，将一个绑定表达式设置到“本地值”中，然后在需要值的时候，会 ProvideValue 提供值。所以，如果再设置了本地值，那么绑定的设置就被覆盖掉了。
但是，SetCurrentValue 就是干这件事的！
SetCurrentValue 设计为在不改变依赖项属性任何已有值的情况下，设置属性当前的值。
_window.SetCurrentValue(Window.WindowStyleProperty, WindowStyle.None);
那么，只需要还原 SetCurrentValue 所做的修改，就还原了此依赖项属性的一切设置的值：
_window.InvalidateProperty(Window.WindowStyleProperty);
注意不是 ClearValue，那会清除本地值。
然而还差一点，绑定如果在你应用 SetCurrentValue 期间有改变，那么这次的赋值并不会让绑定立即生效，所以我们还需要手工再让绑定重新更新值：
BindingOperations.GetBindingExpression(_window, Window.WindowStyleProperty)?.UpdateTarget();
So, in summary, the code at the beginning of this article will be updated to the following form:
private Window _window;
private void OnEnterFullScreen()
{
_window.SetCurrentValue(Window.WindowStyleProperty, WindowStyle.None);
}
private void OnExitFullScreen()
{
_window.InvalidateProperty(Window.WindowStyleProperty);
BindingOperations.GetBindingExpression(_window, Window.WindowStyleProperty)?.UpdateTarget();
}
extending
Make the code more generic:
static void ApplyTempProperty(DependencyObject d, DependencyProperty dp, object tempValue)
{
d?.SetCurrentValue(dp, tempValue);
}
static void RestoreProperty(DependencyObject d, DependencyProperty dp)
{
d.InvalidateProperty(dp);
BindingOperations.GetBindingExpression(d, dp)?.UpdateTarget();
}
This article will be updated frequently. Please read the original: https://blog.walterlv.com/post/change-and-restore-wpf-dependency-value-without-disabling-the-declared-use-of-the-property.html to avoid misleading by old erroneous knowledge and have a better reading experience.
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