💖 💢 🕤 XAML美学：价值转换器 🗒️ 🏄 👲

本文介绍了编写XAML-code时使用的通用方法。

IValueConverter Data Binding XAML WPF UWP Xamarin Forms UI SwitchConverter KeyToValueConverter InlineConverter AggregateConverter ResourceDictionary

数据绑定引擎与数据绑定引擎一起是基于。NET的用户界面开发的重要组成部分 XAML。值转换器暗含在实现接口的单独类中的逻辑 IValueConverter。通常，类名称反映了功能目的，并且实例在标记中声明。

开关转换器和键值转换器

在实践中，许多转换器具有在结构上类似的琐碎的逻辑值 (?:) 或设计 if-else， switch-case-default。但是，有通用的模板 KeyToValueConverter ， SwitchConverter通过声明布尔值并直接在标记中分支，可以避免在项目中向结构中添加相同类型的类。

概念

<KeyToValueConverter
	Key="KeyForMatching"
	Value="ValueIfKeyMatched"
	ByDefault="ValueIfKeyNotMatched" />

<SwitchConverter
	ByDefault="ValueZ">
	<Case
		Key="KeyA"
		Value="ValueA" />
	<Case
		Key="KeyB"
		Value="ValueB" />
	<Case
		Key="KeyC"
		Value="ValueC" />
</SwitchConverter>

应用

<KeyToValueConverter
	x:Key="TrueToVisibleConverter"
	Key="True"
	Value="Visible"
	ByDefault="Collapsed" />
	
<ProgressBar
	Visibility="{Binding IsBusy, Converter={StaticResource TrueToVisibleConverter}}" />

<SwitchConverter
	x:Key="CodeToBackgroundConverter"
	ByDefault="White">
	<Case
		Key="R"
		Value="Red" />
	<Case
		Key="G"
		Value="Green" />
	<Case
		Key="B"
		Value="Blue" />
</SwitchConverter>
	
<Control
	Background="{Binding Code, Converter={StaticResource CodeToBackgroundConverter}}" />

KeyToValueConverter - Key, , Value, ByDefault.

SwitchConverter - Case Key, Case , ¨C31C, ¨C90C¨C32C, .

Value ByDefault , , .

KeyToValueConverter ConverterParameter KeySource

<KeyToValueConverter
	x:Key="EqualsToHiddenConverter"
	KeySource="ConverterParameter"
	Value="Collapsed"
	ByDefault="Visible" />
	
<Control
	Visiblity="{Binding Items.Count, ConverterParameter=0, Converter={StaticResource EqualsToHiddenConverter}}" />

<TextBlock
	Visiblity="{Binding Text, ConverterParameter='Hide Me', Converter={StaticResource EqualsToHiddenConverter}}" />

KeySource :

Manual (by default) - Key ,

ConverterParameter - ConverterParameter ,

PreferManual - manual Key , ConverterParameter

PreferConverterParameter - ConverterParameter , manual Key

, SwitchConverter Case TypedCase,

<SwitchConverter
	ByDefault="Undefined value">
	<TypedCase
		Key="system:String"
		Value="String value" />
	<Case
		Key="0"
		Value="Zero" />
	<Case
		Key="1"
		Value="One" />
	<TypedCase
		Key="system:Int32"
		Value="Int32 value" />
</SwitchConverter>

. SwitchConverter DiagnosticKey, , Trace

var diagnosticMessage = matchedCase.Is()
	? $"{DiagnosticKey}: '{matchedValue}' matched by key '{matchedCase.Key}' for '{value}' and converted to '{convertedValue}'"
	: $"{DiagnosticKey}: The default value '{matchedValue}' matched for '{value}' and converted to '{convertedValue}'";

Trace.WriteLine(diagnosticMessage);

<SwitchConverter
	DiagnosticKey="UniqDiagnosticKey"
	x:Key="CodeToBackgroundConverter"
	ByDefault="White">
	...
</SwitchConverter>

Dependency Value Converter

Key, Value ByDefault (Dependency Properties), Cases DependencyObject. , , Binding,

<KeyToValueConverter
	Key="AnyKey"
	Value="{Binding MatchedValue, Source={StaticResource AnyResource}}"
	ByDefault="{Binding DefaultValue, Source={StaticResource AnyResource}}" />
		
<KeyToValueConverter
	Key="AnyKey"
	Value="{Localizing MatchedTitle}"
	ByDefault="{Localizing DefaultTitle}" />

Inline Converter

, IValueConverter, code-behind .

, .

, code-behind Converting ConvertingBack

<Grid>
	<Grid.Resources>
		<InlineConverter
			x:Key="ComplexInlineConverter"
			Converting="InlineConverter_OnConverting"
			ConvertingBack="InlineConverter_OnConverting" />
	</Grid.Resources>
	
	<TextBlock
		Text="{Binding Number, Converter={StaticResource InlineConverter}}"/>
</Grid>

private void InlineConverter_OnConverting(object sender, ConverterEventArgs e)
{
	// e.Value - access to input value
	// this.DataContext - access to Data Context or another properties of the view
	// access to child visual elements of this root view
	e.ConvertedValue = // set output value
		$"DataContext: {DataContext}, Converter Value: {e.Value}";
}

private void InlineConverter_OnConvertingBack(object sender, ConverterEventArgs e)
{
	// ...
}

Aggregate Converter

, .

<AggregateConverter>
	<StepAConverter />
	<StepBConverter />
	<StepCConverter />
</AggregateConverter>

App.xaml

将通用值转换器放在单独的资源字典中，然后将它们合并为App.xaml文件中的全局资源，这很有用。这使您可以在不同的表示形式中重用值转换器，而无需重新声明它们。

<Application
	xmlns="http://xamarin.com/schemas/2014/forms"
	xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
	x:Class="Any.App">
	<Application.Resources>
		<ResourceDictionary>
			<ResourceDictionary.MergedDictionaries>
				<ResourceDictionary
						Source="AppConverters.xaml" />
				...
			</ResourceDictionary.MergedDictionaries>
		</ResourceDictionary>
	</Application.Resources>
</Application>

Ace框架

可以在Ace Framework gitlab bitbucket库中找到所提供的转换器的实现示例

感谢您的关注和兴趣！

XAML美学：价值转换器

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