1.1 由属性值触发的 Trigger

最基本的触发器，Property 是关注的属性名称，value 是触发条件，一旦触发条件满足，就会应用 Trigger 的 Setters，触发条件不再满足时，各属性值会被还原。

<StackPanel>
    <StackPanel.Resources>
        <Style TargetType="CheckBox" >
            <Setter Property="Height" Value="25"/>
            <Setter Property="Margin" Value="5 0"/>
            <Setter Property="FontSize" Value="14"/>
            <Setter Property="VerticalAlignment" Value="Center"/>
            <Setter Property="VerticalContentAlignment" Value="Center"/>
            <Style.Triggers>
                <Trigger Property="IsChecked" Value="True">
                    <Setter Property="FontSize" Value="15"/>
                    <Setter Property="Foreground" Value="LightSkyBlue"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </StackPanel.Resources>
    <CheckBox Content="篮球" />
    <CheckBox Content="香蕉" />
    <CheckBox Content="单车" />
    <CheckBox Content="自驾" />
</StackPanel>

1.2 MultiTrigger

MultiTrigger 指多个条件下同时成立时触发，MultiTrigger 比 Trigger 多了一个 Conditions 属性，需要同时成立的条件就在这个集合里。

<StackPanel>
    <StackPanel.Resources>
        <Style TargetType="CheckBox" >
            <Setter Property="Height" Value="25"/>
            <Setter Property="Margin" Value="5 0"/>
            <Setter Property="FontSize" Value="14"/>
            <Setter Property="VerticalAlignment" Value="Center"/>
            <Setter Property="VerticalContentAlignment" Value="Center"/>
            <Style.Triggers>
                <Trigger Property="IsChecked" Value="True">
                    <Setter Property="FontSize" Value="15"/>
                    <Setter Property="Foreground" Value="LightSkyBlue"/>
                </Trigger>
                <MultiTrigger>
                    <MultiTrigger.Conditions>
                        <Condition Property="IsChecked" Value="True"/>
                        <Condition Property="Content" Value="香蕉"/>
                    </MultiTrigger.Conditions>
                    <Setter Property="FontSize" Value="17"/>
                    <Setter Property="Foreground" Value="Red"/>
                </MultiTrigger>
            </Style.Triggers>
        </Style>
    </StackPanel.Resources>

    <CheckBox Content="篮球" />
    <CheckBox Content="香蕉" />
    <CheckBox Content="单车" />
    <CheckBox Content="自驾" />
</StackPanel>

多个 Trigger 或 MultiTrigger 时，会按代码的顺序触发，比如点击香蕉，FontSize 最终是 17，如果把 MultiTrigger 实例写在 Trigger 实例前面，点击香蕉后，FontSize 的值是 Trigger 的 15 。

1.3 由数据触发的 DataTrigger

<StackPanel>
    <local:LongToBoolConverter x:Key="cvtltb"/>

    <StackPanel.Resources>
        <Style TargetType="TextBox">
            <Style.Triggers>
                <DataTrigger Binding="{Binding RelativeSource={RelativeSource Mode=Self},
                                               Path=Text.Length,
                                               Converter={StaticResource cvtltb}}"
                             Value="True">
                    <Setter Property="Foreground" Value="Red"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </StackPanel.Resources>
    <TextBox />
</StackPanel>

public class LongToBoolConverter : System.Windows.Data.IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return (int)value > 6 ? true : false;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

1.4 MultiDataTrigger

public class Student
{
    public string Id { get; set; }

    public string Name { get; set; }

    public bool LoveBasketball { get; set; }
}

...

<Window.Resources>
    <coll:ArrayList x:Key="stuList">
        <local:Student Id="1" Name="D1" LoveBasketball="True"></local:Student>
        <local:Student Id="2" Name="D2" LoveBasketball="False"></local:Student>
        <local:Student Id="3" Name="D3" LoveBasketball="True"></local:Student>
        <local:Student Id="3" Name="D4" LoveBasketball="False"></local:Student>
        <local:Student Id="5" Name="D5" LoveBasketball="True"></local:Student>
        <local:Student Id="6" Name="D6" LoveBasketball="False"></local:Student>
    </coll:ArrayList>
</Window.Resources>

<StackPanel MenuItem.Click="StackPanel_Click">
    <StackPanel.Resources>
        <Style TargetType="ListBoxItem">
            <Setter Property="ContentTemplate" Value="{StaticResource it}"/>
            <Style.Triggers>
                <MultiDataTrigger>
                    <MultiDataTrigger.Conditions>
                        <Condition Binding="{Binding Id}" Value="3"/>
                        <Condition Binding="{Binding Name}" Value="D3"/>
                    </MultiDataTrigger.Conditions>
                    <Setter Property="Background" Value="Red"></Setter>
                </MultiDataTrigger>
            </Style.Triggers>
        </Style>
    </StackPanel.Resources>

    <ListBox ItemsSource="{StaticResource stuList}"/>
</StackPanel>

1.5 由事件触发的 EventTrigger

EventTrigger 触发后并非应用一组 Setter，而是执行一段动画，因此 UI 的动画效果往往跟 EventTrigger 相关联。

<Button Content="确认">
    <Button.Style>
        <Style TargetType="Button">
            <Setter Property="Width" Value="200"/>
            <Setter Property="Height" Value="30"/>
            <Style.Triggers>
                <EventTrigger RoutedEvent="MouseEnter">
                    <EventTrigger.Actions>
                        <BeginStoryboard>
                            <Storyboard>
                                <DoubleAnimation To="400" Duration="0:0:0.2" Storyboard.TargetProperty="Width"/>
                                <DoubleAnimation To="60" Duration="0:0:0.2" Storyboard.TargetProperty="Height"/>
                            </Storyboard>
                        </BeginStoryboard>
                    </EventTrigger.Actions>
                </EventTrigger>
                <EventTrigger RoutedEvent="MouseLeave">
                    <EventTrigger.Actions>
                        <BeginStoryboard>
                            <Storyboard>
                                <DoubleAnimation Duration="0:0:0.2" Storyboard.TargetProperty="Width"/>
                                <DoubleAnimation Duration="0:0:0.2" Storyboard.TargetProperty="Height"/>
                            </Storyboard>
                        </BeginStoryboard>
                    </EventTrigger.Actions>
                </EventTrigger>
            </Style.Triggers>
        </Style>
    </Button.Style>
</Button>

微软官方文档中对 Trigger 的介绍很简洁明了：

https://docs.microsoft.com/zh-cn/dotnet/desktop/wpf/fundamentals/styles-templates-overview?view=netdesktop-5.0#triggers