TextChanged 事件 - 为什么这不会导致无限循环？

发布于 2024-09-03 06:29:41 字数 389 浏览 3 评论 0原文

在尝试做一些更复杂的事情时，我遇到了一种我不太理解的行为。

假设下面的代码处理 textChanged 事件。

 private void textChanged(object sender, TextChangedEventArgs e)
    {
        TextBox current = sender as TextBox;
        current.Text = current.Text + "+";
    }

现在，在文本框中键入一个字符（例如 A）将导致事件触发两次（添加两个“+”），最终显示的文本仅为 A+。

我的两个问题是，为什么该事件只发生两次？为什么只有第一次运行事件才真正设置文本框的文本？

提前致谢！

原文

While trying to do something a bit more complicated, I ran across a behavior I don't quite understand.

Assume the following code below handling the textChanged event.

 private void textChanged(object sender, TextChangedEventArgs e)
    {
        TextBox current = sender as TextBox;
        current.Text = current.Text + "+";
    }

Now, typing a character in the textbox (say, A) will result in the event getting tripped twice (adding two '+'s) with the final text displayed being just A+.

My two questions are, why is the event hit just twice? And why does only the first run through the event actually set the text of the textbox?

Thanks in advance!

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吃颗糖壮壮胆 2024-09-10 06:29:41

嗯 - 在更改/刚刚更改时设置 Text 属性似乎由 TextBox 类显式捕获：

只需使用 Reflector 来查看 TextBox.OnTextPropertyChanged 内部>（缩短）：

TextBox box = (TextBox) d;
if (!box._isInsideTextContentChange)
{
    string newValue = (string) e.NewValue;
    //...
    box._isInsideTextContentChange = true;
    try
    {
        using (box.TextSelectionInternal.DeclareChangeBlock())
        {
           //...
        } //Probably raises TextChanged here
    }
    finally
    {
        box._isInsideTextContentChange = false;
    }
    //...
}

在引发 TextChanged 事件之前，字段 _isInsideTextContentChange 设置为 true。当再次更改 Text 属性时，TextChanged 事件不会再次引发。

因此：特点;-)

Well - setting the Text property while it is being changed / while it has just changed seems to be caught by the TextBox class explicitly:

Just use the Reflector to look inside TextBox.OnTextPropertyChanged (shortened):

TextBox box = (TextBox) d;
if (!box._isInsideTextContentChange)
{
    string newValue = (string) e.NewValue;
    //...
    box._isInsideTextContentChange = true;
    try
    {
        using (box.TextSelectionInternal.DeclareChangeBlock())
        {
           //...
        } //Probably raises TextChanged here
    }
    finally
    {
        box._isInsideTextContentChange = false;
    }
    //...
}

The field _isInsideTextContentChange is set to true before the TextChanged event gets raised. When changing the Text property again, the TextChanged event thus is not raised again.

Therefore: Feature ;-)

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