通过绑定处理MaxLength [英] Handling MaxLength via Binding

问题描述

我试图通过绑定处理TextBox的MaxLength。我正在使用一个名为MaxLengthConverter的助手类（请参阅此处 http://mariabrinas.com/?p=89 ）。 TextBox看起来像这样：

 < TextBox MaxLength ={Binding TestValue，Mode = TwoWay，Converter = {StaticResource MaxLengthConverter }，ConverterParameter ='7'}Text ={Binding TestValue，Mode = TwoWay}InputScope =Number/> 
  

MaxLengthValueConvert如下所示：

  public class MaxLengthConverter：IValueConverter 
 {
 public object Convert（object value，Type targetType，object parameter，CultureInfo culture）
 
 if（ value.ToString（）。Contains（'。'））
 {
 string [] len = value.ToString（）Split（'。'）; 
 parameter = len [0] .Length + 2; 
} 
 
 
 public object ConvertBack（object value，Type targetType，object parameter，CultureInfo culture）
 {
 return parameter; 
} 
} 
  

参数是值的长度。在这个例子中，它是7.最大。 TextBox的长度将为7，但是如果用户键入'。'（小数点），则maxlength将为当前长度+ 2，因此只能写入23.45而不是23.456。
问题是当我离开TextBox（LostFocus）时，ValueConvert将被调用。每当用户在（KeyDown）中键入一些东西时，如何调用ValueConverter？

解决方案

定义一个显式的 UpdateSourceTrigger as PropertyChanged 由于文本框默认为LostFocus

例如：

 < TextBox> 
< TextBox.Text> 
< Binding Source ={StaticResource myDataSource}Path =Name
 UpdateSourceTrigger =PropertyChanged/> 
< /TextBox.Text> 
< / TextBox> 
  

I'm trying to handle the MaxLength of a TextBox via Binding. I'm using a Helper Class called 'MaxLengthConverter' (see here http://mariabrinas.com/?p=89). The TextBox looks currently like this:

<TextBox MaxLength="{Binding TestValue, Mode=TwoWay, Converter={StaticResource MaxLengthConverter}, ConverterParameter='7'}" Text="{Binding TestValue, Mode=TwoWay}" InputScope="Number" />

And the MaxLengthValueConvert looks like this:

public class MaxLengthConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)

        if (value.ToString().Contains('.'))
        {
            string[] len = value.ToString().Split('.');
            parameter = len[0].Length + 2;
        }


    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return parameter;
    }
}

The parameter is the length of the value. In this example it's 7. The max. length of the TextBox will be 7, but if the user types a '.' (decimal point), the maxlength will be the current length + 2, so he can only write 23.45 and not 23.456. The problem is that the ValueConvert will only be called when I'm leaving the TextBox (LostFocus). How can I call the ValueConverter every time the user types something in (KeyDown) ?

解决方案

Define an explicit UpdateSourceTrigger as PropertyChanged since textbox default is LostFocus

Eg:

  <TextBox>
  <TextBox.Text>
    <Binding Source="{StaticResource myDataSource}" Path="Name"
             UpdateSourceTrigger="PropertyChanged"/>
  </TextBox.Text>
</TextBox>

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