为什么绑定不起作用？

Z时代
2024-01-10
分类：问答

所以我创建了一个带有“ShowCode”属性的userControl来学习如何使用属性。我想要这个属性隐藏网格中的第二行。为什么绑定不起作用？

查看：

<UserControl x:Class="Test.UserControl1" 
     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
     xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
     mc:Ignorable="d" 
     d:DesignHeight="300" d:DesignWidth="300"> 
    <Grid Margin="0,0,0,0" Name="outergrid" DataContext="{Binding}"> 
     <Grid.RowDefinitions> 
     <RowDefinition Height="3*" /> 
     <RowDefinition Height="1*" /> 
    </Grid.RowDefinitions> 
    <ContentControl Name="XAMLView" Grid.Row="0"/> 
    <GridSplitter ResizeDirection="Rows" 
       Grid.Row="0" 
       VerticalAlignment="Bottom" 
       HorizontalAlignment="Stretch" /> 
    <Border Width="11" Grid.Row="1" Background="Black" /> 
</Grid>

代码：

public partial class UserControl1 : UserControl 
{ 
    public UserControl1() 
    { 
     InitializeComponent(); 
     outergrid.RowDefinitions[1].SetBinding(RowDefinition.HeightProperty, new Binding() { Path = new PropertyPath("ShowCode"), Source = this, Converter = new BoolToHeightConverter(), ConverterParameter = "True" }); 
    } 
    public bool ShowCode 
    { 
     get { return (bool)GetValue(ShowCodeProperty); } 
     set { SetValue(ShowCodeProperty, value); } 
    } 
    public static readonly DependencyProperty ShowCodeProperty = 
     DependencyProperty.Register("ShowCode", 
     typeof(bool), 
     typeof(UserControl1), 
     new PropertyMetadata(true, new PropertyChangedCallback(OnShowCodeChanged))); 
    static void OnShowCodeChanged(object sender, DependencyPropertyChangedEventArgs args) 
    { 
     UserControl1 source = (UserControl1)sender; 
     //source.outergrid.RowDefinitions[1].Height = 
    } 
    public class BoolToHeightConverter : IValueConverter 
    { 
     public object Convert(object value, Type targetType, 
      object parameter, System.Globalization.CultureInfo culture) 
     { 
      if ((string)parameter == "False") return "0"; 
      return "1*"; 
     } 
     public object ConvertBack(object value, Type targetType, 
      object parameter, System.Globalization.CultureInfo culture) 
     { 
      if ((int)parameter == 0) return false; 
      return true; 
     } 
    } 
}

问题：当我使用它是这样的：

<xamlviewer:UserControl1 ShowCode="False"/>

转换（...）被调用2次和两次“参数”呃“是”真“，否则如果ShowCode =”True“Convert（）只被调用一次而”参数“又是”True“

为什么总是这样？

回答：

这里至少有两件事是错误的。

您的转换器返回一个字符串，它应该返回GridLength。

要转换的值被传递给转换器的value参数，而不是parameter，它是bool。

所以，你应该这样写：

public object Convert(object value, Type targetType, object parameter, CultureInfo culture) 
{ 
    if (value is bool && (bool)value) 
    { 
     return new GridLength(1, GridUnitType.Star); 
    } 
    return new GridLength(0); 
}

需要在结合无转换器参数：

outergrid.RowDefinitions[1].SetBinding(
    RowDefinition.HeightProperty, 
    new Binding() 
    { 
     Path = new PropertyPath("ShowCode"), 
     Source = this, 
     Converter = new BoolToHeightConverter() 
    });

回答：

参数始终是正确的，因为你做它有一个值：

outergrid.RowDefinitions[1].SetBinding(
    RowDefinition.HeightProperty, 
    new Binding() 
    { 
     Path = new PropertyPath("ShowCode"), 
     Source = this, 
     Converter = new BoolToHeightConverter(), 
     ConverterParameter = "True" // <---- You are enforcing its value here. 
    });

你的转换器是搞砸反正：你必须检查value不parameter正确地转换数据来回：

// NOTE: Your property is BOOL not STRING. 
public class BoolToHeightConverter : IValueConverter 
{ 
    public object Convert(object value, Type targetType, 
     object parameter, System.Globalization.CultureInfo culture) 
    { 
     if(value is bool) 
      if((bool)value) return new GridLength(1, DataGridLengthUnitType.Star); 
      else return new GridLength(0); 
     throw new ArgumentException("Not a bool"); 
    } 
    public object ConvertBack(object value, Type targetType, 
     object parameter, System.Globalization.CultureInfo culture) 
    { 
     // You may not need this method at all! 
     if(value is GridLength) 
      return ((GridLength)value).Value == 0); 
     throw new ArgumentException("Not a GridLength"); 
    } 
}