C# 带MarkupExtension的IValueConverter
最近我读到一个C# 带MarkupExtension的IValueConverter,c#,wpf,converter,ivalueconverter,markup-extensions,C#,Wpf,Converter,Ivalueconverter,Markup Extensions,最近我读到一个IValueConverter,它也继承了MarkupExtension。有点像: internal class BoolToVisibilityConverter : MarkupExtension, IValueConverter { private static BoolToVisibilityConverter converter; public BoolToVisibilityConverter() { } public objec
IValueConverter
,它也继承了MarkupExtension
。有点像:
internal class BoolToVisibilityConverter : MarkupExtension, IValueConverter
{
private static BoolToVisibilityConverter converter;
public BoolToVisibilityConverter()
{
}
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value is bool)
{
if ((bool)value)
{
return Visibility.Visible;
}
}
return Visibility.Collapsed;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value is Visibility)
{
Visibility visibility = (Visibility)value;
if (visibility == Visibility.Collapsed)
{
return false;
}
}
return true;
}
public override object ProvideValue(IServiceProvider serviceProvider)
{
return converter ?? (converter = new BoolToVisibilityConverter());
}
}
用法如下所示:
<Button Content="Delete" Visibility="{Binding CanDelete, UpdateSourceTrigger=PropertyChanged, Converter={local:BoolToVisibilityConverter}"/>
internal class BoolToVisibilityConverter : MarkupExtension, IValueConverter
{
public BoolToVisibilityConverter()
{
}
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value is bool)
{
if ((bool)value)
{
return Visibility.Visible;
}
}
return Visibility.Collapsed;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value is Visibility)
{
Visibility visibility = (Visibility)value;
if (visibility == Visibility.Collapsed)
{
return false;
}
}
return true;
}
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this;
}
}
如果我理解正确,第一个解决方案只创建此转换器的一个实例。第二个为每个XAML创建一个新的转换器实例,对吗?在本例中,标记扩展提供的唯一(轻微)优势是更简洁的XAML语法
与此相反:
<BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
...
{Binding SomeBooleanProperty, Converter={StaticResource BooleanToVisibilityConverter}}
在我看来,这真的不值得。如果您为保存击键而烦恼,您可以缩短用于引用转换器的键:
<BooleanToVisibilityConverter x:Key="btvc" />
...
{Binding SomeBooleanProperty, Converter={StaticResource my:btvc}}
...
{Binding SomeBooleanProperty,Converter={StaticResource my:btvc}
由于标记扩展的
ProvideValue
方法是一个实例方法,因此只能在创建类的实例后调用它。由于该类既是标记扩展又是转换器,因此每次代码的两个变体都将创建一个转换器。唯一的区别是第一个变量总是返回相同的转换器:但是它不会,停止创建另一个转换器。使用MarkupExtension
的一个巨大优势是它可以允许您将值传递给转换器,这些值可以用作参数或返回值,例如:
public class CustomNullToVisibilityConverter : MarkupExtension, IValueConverter
{
public object NullValue { get; set; }
public object NotNullValue { get; set; }
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this;
}
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value == null) return NullValue;
return NotNullValue;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}
用法:
...
Visibility="{Binding Property,
Converter={cnv:CustomNullToVisibilityConverter
NotNullValue=Visible, NullValue=Collapsed}}" />
...
确保在.xaml
中引用转换器的名称空间
编辑: 我忘记提到的一件事是,你是正确的,这个方法每次使用时都会创建一个新的转换器实例,这是一个缺点 但是,没有什么可以阻止您将带有
MarkupExtension
的转换器添加到资源字典中-这样它将只实例化一次。像这样:
<cnv:CustomNullToVisibilityConverter x:Key="NullToVisibilityConverter"
NotNullValue=Visible, NullValue=Collapsed />
...
Visibility="{Binding Property, Converter={StaticResource NullToVisibilityConverter}" />
...
...
可见性=“{Binding Property,Converter={StaticResource NullToVisibilityConverter}”/>
...
...
Visibility="{Binding Property,
Converter={cnv:CustomNullToVisibilityConverter
NotNullValue=Visible, NullValue=Collapsed}}" />
...
<cnv:CustomNullToVisibilityConverter x:Key="NullToVisibilityConverter"
NotNullValue=Visible, NullValue=Collapsed />
...
Visibility="{Binding Property, Converter={StaticResource NullToVisibilityConverter}" />
...