[WPF Custom Control Library] use TextBlockHighlightSource strengthen highlighting function, and the use of simplified call TypeConverter

1. Strengthen the highlighted function

The article describes the use of additional attributes to achieve highlighting the TextBlock, but also left a question: can not define the highlight (or low light) color. To solve this problem, I created TextBlockHighlightSourcethis class, more information than a simple string storage, this class definition is as follows:

Correspondingly, also changes the type of additional attributes for the class and the attribute value change into this event:

private static void OnHighlightTextChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
    var oldValue = (TextBlockHighlightSource)args.OldValue;
    var newValue = (TextBlockHighlightSource)args.NewValue;
    if (oldValue == newValue)
        return;

    void OnPropertyChanged(object sender,EventArgs e)
    {
        if (obj is TextBlock target)
        {
            MarkHighlight(target, newValue);
        }
    };

    if(oldValue!=null)
        newValue.PropertyChanged -= OnPropertyChanged;

    if (newValue != null)
        newValue.PropertyChanged += OnPropertyChanged;

    OnPropertyChanged(null, null);
}

MarkHighlightThe key code is modified to this:

if (highlightSource.LowlightForeground != null)
    run.Foreground = highlightSource.LowlightForeground;

if (highlightSource.HighlightForeground != null)
    run.Foreground = highlightSource.HighlightForeground;

if (highlightSource.HighlightBackground != null)
    run.Background = highlightSource.HighlightBackground;

Use is this:

<TextBlock Text="Git hub"
           TextWrapping="Wrap">
    <kino:TextBlockService.HighlightText>
        <kino:TextBlockHighlightSource Text="hub"
                                       LowlightForeground="Black"
                                       HighlightBackground="#FFF37D33" />
    </kino:TextBlockService.HighlightText>
</TextBlock>

2. Use TypeConverter simplify call

TextBlockHighlightSourceIt offers a lot of features, but the string and used directly compared, to create a TextBlockHighlightSourcefar more complicated. In order to simplify it can be used to call custom TypeConverter.

First, let's look at TypeConverterthe concept. XAML is the XML nature, where property values are all strings. If the type of the corresponding attribute is XAML built-in type (i.e. Boolea, Char, String, Decimal, Single, Double, Int16, Int32, Int64, TimeSpan, Uri, Byte, Array type, etc.), XAML parser directly convert the string into a corresponding assigned to the property value; for other types, XAML parser need to do more work.

<Grid.RowDefinitions>
    <RowDefinition Height="Auto"/>
    <RowDefinition Height="*"/>
</Grid.RowDefinitions>

As in the above paragraph XAML "Auto" and "*", XAML parser to parse respectively GridLength.Auto and new GridLength (1, GridUnitType.Star) again assigned to Height, which corresponds to the code:

grid.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
grid.RowDefinitions.Add(new RowDefinition { Height = new GridLength(1, GridUnitType.Star) });

为了完成这个工作，XAML解析器需要TypeConverter的协助。XAML解析器通过两个步骤查找TypeConverter：
1. 检查属性声明上的TypeConverterAttribute。
2. 如果属性声明中没有TypeConverterAttribute，检查类型声明中的TypeConverterAttribute。

属性声明上TypeConverterAttribute的优先级高于类型声明。如果以上两步都找不到类型对应的TypeConverterAttribute，XAML解析器将会报错：属性"*"的值无效。找到TypeConverterAttribute指定的TypeConverter后，XAML解析器调用它的object ConvertFromString(string text)函数将字符串转换成属性的值。

WPF内置的TypeConverter十分十分多，但有时还是需要自定义TypeConverter，自定义TypeConverter的基本步骤如下：

• 创建一个继承自TypeConverter的类；
• 重写virtual bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType);
• 重写virtual bool CanConvertTo(ITypeDescriptorContext context, Type destinationType);
• 重写virtual object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value);
• 重写virtual object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType);
• 使用TypeConverterAttribute 指示XAML解析器可用的TypeConverter；

到这里我想TypeConverter的概念已经介绍得够详细了。回到本来话题，要简化TextBlockHighlightSource的调用我创建了TextBlockHighlightSourceConverter这个类，它继承自TypeConverter，里面的关键代码如下：

public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
{
    if (sourceType == typeof(string))
    {
        return true;
    }

    return base.CanConvertFrom(context, sourceType);
}

public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
{
    switch (value)
    {
        case null:
            throw GetConvertFromException(null);
        case string source:
            return new TextBlockHighlightSource { Text = value.ToString() };
    }

    return base.ConvertFrom(context, culture, value);
}

然后在TextBlockHighlightSource上使用TypeConverterAttribute:

[TypeConverter(typeof(TextBlockHighlightSourceConverter))]
public class TextBlockHighlightSource : FrameworkElement

这样在XAML中TextBlockHighlightSource的调用方式就可以和使用字符串一样简单了。

<TextBlock Text="Github"
           kino:TextBlockService.HighlightText="hub" />

3. 使用Style

有没有发现TextBlockHighlightSource继承自FrameworkElement？这种奇特的写法是为了让TextBlockHighlightSource可以使用全局的Style。毕竟要在应用程序里统一Highlight的颜色还是全局样式最好使，但作为附加属性，TextBlockHighlightSource并不是VisualTree的一部分，它拿不到VisualTree上的Resources。最简单的解决方案是让TextBlockHighlightSource继承自FrameworkElement，把它放到VisualTree里，用法如下：

<StackPanel>
    <FrameworkElement.Resources>
        <Style TargetType="kino:TextBlockHighlightSource">
            <Setter Property="LowlightForeground" Value="Blue"/>
        </Style>
    </FrameworkElement.Resources>
    <TextBox x:Name="FilterElement3"/>
    <kino:TextBlockHighlightSource Text="{Binding ElementName=FilterElement3,Path=Text}" 
                                   HighlightForeground="DarkBlue"
                                   HighlightBackground="Yellow"
                                   x:Name="TextBlockHighlightSource2"/>
    <TextBlock Text="A very powerful projector with special features for Internet usability, USB" 
               kino:TextBlockService.HighlightText="{Binding ElementName=TextBlockHighlightSource2}"
               TextWrapping="Wrap"/>
</StackPanel>

也许你会觉得这种写法有些奇怪，毕竟我也觉得在View上放一个隐藏的元素真的很怪。其实在一万二千年前微软就已经有这种写法，在DomainDataSource的文档里就有用到:

<Grid x:Name="LayoutRoot" Background="White">  
    <Grid.RowDefinitions>
        <RowDefinition Height="25" />
        <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <riaControls:DomainDataSource x:Name="source" QueryName="GetProducts" AutoLoad="true">
        <riaControls:DomainDataSource.DomainContext>
            <domain:ProductDomainContext />
        </riaControls:DomainDataSource.DomainContext>   
        <riaControls:DomainDataSource.FilterDescriptors>
            <riaData:FilterDescriptorCollection LogicalOperator="And">
              <riaData:FilterDescriptor PropertyPath="Color" Operator="IsEqualTo" Value="Blue" />
              <riaData:FilterDescriptor PropertyPath="ListPrice" Operator="IsLessThanOrEqualTo">
                  <riaControls:ControlParameter 
                      ControlName="MaxPrice" 
                      PropertyName="SelectedItem.Content" 
                      RefreshEventName="SelectionChanged" />
              </riaData:FilterDescriptor>
            </riaData:FilterDescriptorCollection>
        </riaControls:DomainDataSource.FilterDescriptors>
    </riaControls:DomainDataSource>
    <ComboBox x:Name="MaxPrice" Grid.Row="0" Width="60" SelectedIndex="0">
        <ComboBoxItem Content="100" />
        <ComboBoxItem Content="500" />
        <ComboBoxItem Content="1000" />
    </ComboBox>
    <data:DataGrid Grid.Row="1" ItemsSource="{Binding Data, ElementName=source}" />
</Grid>

把DataSource放到View上这种做法可能是WinForm的祖传家训，结构可耻但有用。

4. 结语

写这篇博客的时候我才发觉这个附加属性还叫HighlightText好像不太好，但也懒得改了。

这篇文章介绍了使用TypeConverter简化调用，以及继承自FrameworkElement以便使用Style。

5. 参考

TypeConverter 类
TypeConverters 和 XAML
Type Converters for XAML Overview
TypeConverterAttribute Class
如何：实现类型转换器

6. 源码

TextBlock at master · DinoChan_Kino.Toolkit.Wpf