WPF Basics - Triggers

Triggers are implemented as part of styles, and indicate a trigger condition and a new set of setters to apply when the trigger condition is met. Mouseover effects are a very common application of triggers. Triggers can fully be expressed using XAML, eliminating the need to write code for many simple effects.

There are three types of triggers in WPF:

1. Property Triggers - run when the value of a dependency property changes
2. Data Triggers - run when the value of any .NET property changes, using data binding
3. Event Triggers - run when a routed event occurs

Triggers are implement as part of a style using the <Style.Triggers> property element, and a series of <Trigger>,<DataTrigger>, or <EventTrigger> elements.

Property Triggers

Property triggers are used to monitor a DependencyProperty's value. The following example applies a glow effect to a button when the mouse pointer is over the button (IsMouseOver=true). The WPF framework automatically handles resetting the style back when the mouse pointer is moved off the button.

<Canvas>
    <Canvas.Resources>
        <Style TargetType="{x:Type Button}">
            <Style.Triggers>
                <Trigger Property="IsMouseOver" Value="true">
                    <Setter Property="BitmapEffect">
                        <Setter.Value>
                                <OuterGlowBitmapEffect GlowColor="Gold" GlowSize="10" />
                        </Setter.Value>
                    </Setter>
                </Trigger>
            </Style.Triggers>
        </Style>
    </Canvas.Resources>
    <Button Canvas.Left="19" Canvas.Top="19" Width="75" Height="22">Button 1</Button>
</Canvas>