WPF - MVVM - 文本框不同步与视图模型属性 [英] WPF - MVVM - Textbox getting out of sync with viewmodel property

问题描述

我有一个WPF视图一个文本框，文本字段绑定到视图模型与UpdateSourceTrigger设置的PropertyChanged。在属性二传手在视图模型，我有一个简单的检查，以prevent不超过10个字符的文本：

I have a WPF view with a TextBox, binding the Text field to a ViewModel with UpdateSourceTrigger set to PropertyChanged. In the property setter in the ViewModel, I have a simple check to prevent the text from exceeding 10 characters:

<TextBox Text="{Binding Name, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" />

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        this.DataContext = new MainViewModel();
    }
}


public string Name
{
    get { return _Name; }
    set
    {
        if (_Name != value)
        {
            if (value.Length <= 10)
            {
                _Name = value;
            }
            RaisePropertyChanged("Name");
        }
    }
}

如果该值未设置，我还是RaisePropertyChanged（它只是触发的PropertyChanged）。

If the value isn't set, I still RaisePropertyChanged (which simply fires PropertyChanged).

现在的问题是，当我输入用户界面中的11字，我不更新_Name。我火的PropertyChanged，我可以看到get访问被调用，它返回字符串只有10个字符。然而，我的文本框没有反映这一点;它仍然显示的字符串11个字符。

The problem is that when I type in the 11th character in the UI, I don't update _Name. I fire PropertyChanged, and I can see the get accessor get called and it returns the string with only 10 characters. However, my TextBox doesn't reflect this; it still shows the string with 11 characters.

在最重要的是，是，如果11号角色，我改变文本的制定者为错误，而火属性变化，文本框会更新以显示修改的内容。

On top of that, is that if on the 11th character I change the text in the setter to "ERROR", and fire property changed, the TextBox DOES update to show the altered text.

那么，为什么，如果我改变了二传手回到previous值的文本，用户界面​​没有反映这一点？

So why is it that if I alter the text in the setter back to the previous value, the UI doesn't reflect this?

我知道有处理的最大字符替代方式，但为什么不这项工作？

I know there are alternative ways of handling max characters, but why won't this work?

推荐答案

这是什么，但在框架中的一个错误。该文本属性在文本框并获得新的价值，但GUI现在不同步自己的 TextProperty 。这也happends任何的ItemsControl 如果您想取消的SelectedItem 从视图模型的变化，它是真烦人。

This is nothing but a bug in the framework. The Text property in the TextBox does get your new value but the GUI is now out of sync with its own TextProperty. This also happends for any ItemsControl when you want to cancel a change of SelectedItem from the ViewModel and it's really annoying.

当你使用明确的绑定虽然因此这可以作为一种解决方法。

This bug doesn't happend when you use explicit Binding though so this can be used as a workaround.

的XAML 的

<TextBox Text="{Binding Path=MyName,
                        UpdateSourceTrigger=Explicit}"
         TextChanged="TextBox_TextChanged"/>

$ C $后面ç的

private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
{
    TextBox textBox = sender as TextBox;
    textBox.GetBindingExpression(TextBox.TextProperty).UpdateSource();
}

要验证文本框界面的确是不同步的，只是观察 TextBox.Text 的价值。该文本框会说123456789___0例如，而的TextBlock 说：123456789。

To verify that the TextBox GUI indeed is out of sync, just observe the value of TextBox.Text. The TextBox will say "123456789___0" for example while TextBlock says "123456789".

<StackPanel>
    <TextBox Name="myTextBox"
             Text="{Binding Path=MyName,
                            UpdateSourceTrigger=PropertyChanged}"/>
    <TextBlock Text="{Binding ElementName=myTextBox, Path=Text}"/>
</StackPanel>

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