Triggers

Introduction#

Discussion on the various types of Triggers available in WPF, including Trigger, DataTrigger, MultiTrigger, MultiDataTrigger, and EventTrigger.

Triggers allow any class that derives from FrameworkElement or FrameworkContentElement to set or change their properties based on certain conditions defined in the trigger. Basically, if an element can be styled, it can be triggered as well.

Remarks#

All triggers, except for EventTrigger must be defined within a <Style> element. An EventTrigger may be defined in either a <Style> element, or a control’s Triggers property.
<Trigger> elements may contain any number of <Setter> elements. These elements are responsible for setting properties on the containing element when the <Trigger> element’s condition is met.
If a property is defined in the root element markup, the property change defined in the <Setter> element will not take effect, even if the trigger condition has been met. Consider the markup <TextBlock Text="Sample">. The Text property of the proceeding code will never change based on a trigger because root property definitions take precidence over properties defined in styles.
Like bindings, once a trigger has been used, it cannot be modified.

Trigger

The simplest of the five trigger types, the Trigger is responsible for setting properties based on other properties within the same control.

<TextBlock>
    <TextBlock.Style>
        <Style TargetType="{x:Type TextBlock}">
            <Style.Triggers>
                <Trigger Property="Text" Value="Pass">
                    <Setter Property="Foreground" Value="Green"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>

In this example, the foreground color of the TextBlock will turn green when it’s Text property is equal to the string "Pass".

MultiTrigger

A MultiTrigger is similar to a standard Trigger in that it only applies to properties within the same control. The difference is that a MultiTrigger has multiple conditions which must be satisfied before the trigger will operate. Conditions are defined using the <Condition> tag.

<TextBlock x:Name="_txtBlock" IsEnabled="False">
    <TextBlock.Style>
        <Style TargetType="{x:Type TextBlock}">
            <Style.Triggers>
                <MultiTrigger>
                    <MultiTrigger.Conditions>
                        <Condition Property="Text" Value="Pass"/>
                        <Condition Property="IsEnabled" Value="True"/>
                    </MultiTrigger.Conditions>
                    <Setter Property="Foreground" Value="Green"/>
                </MultiTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>

Notice the MultiTrigger will not activate until both conditions are met.

DataTrigger

A DataTrigger can be attached to any property, be it on it’s own control, another control, or even a property in a non UI class. Consider the following simple class.

public class Cheese
{
    public string Name { get; set; }
    public double Age { get; set; }
    public int StinkLevel { get; set; }
}

Which we will attach as the DataContext in the following TextBlock.

<TextBlock Text="{Binding Name}">
    <TextBlock.DataContext>
        <local:Cheese Age="12" StinkLevel="100" Name="Limburger"/>
    </TextBlock.DataContext>
    <TextBlock.Style>
        <Style TargetType="{x:Type TextBlock}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding StinkLevel}" Value="100">
                    <Setter Property="Foreground" Value="Green"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>

In the preceeding code, the TextBlock.Foreground property will be Green. If we change the StinkLevel property in our XAML to anything other than 100, the Text.Foreground property will revert to it’s default value.