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Triggers

  • Artikkeli

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.NET Multi-platform App UI (.NET MAUI) triggers allow you to express actions declaratively in XAML that change the appearance of controls based on events or data changes. In addition, state triggers, which are a specialized group of triggers, define when a VisualState should be applied.

You can assign a trigger directly to a control's Triggers collection, or add it to a page-level or app-level resource dictionary to be applied to multiple controls.

Property triggers

A Trigger represents a trigger that applies property values, or performs actions, when the specified property meets a specified condition.

The following example shows a Trigger that changes an Entry background color when it receives focus:

<Entry Placeholder="Enter name">
    <Entry.Triggers>
        <Trigger TargetType="Entry"
                 Property="IsFocused"
                 Value="True">
            <Setter Property="BackgroundColor"
                    Value="Yellow" />
            <!-- Multiple Setter elements are allowed -->
        </Trigger>
    </Entry.Triggers>
</Entry>

The trigger's declaration specifies the following:

  • TargetType - the control type that the trigger applies to.
  • Property - the property on the control that is monitored.
  • Value - the value, when it occurs for the monitored property, that causes the trigger to activate.
  • Setter - a collection of Setter elements that are applied when the trigger condition is met.

In addition, optional EnterActions and ExitActions collections can be specified. For more information, see EnterActions and ExitActions.

Apply a trigger using a style

Triggers can also be added to a Style declaration on a control, in a page, or an application ResourceDictionary. The following example declares an implicit style for all Entry controls on the page:

<ContentPage.Resources>
    <Style TargetType="Entry">
        <Style.Triggers>
            <Trigger TargetType="Entry"
                     Property="IsFocused"
                     Value="True">
                <Setter Property="BackgroundColor"
                        Value="Yellow" />
                <!-- Multiple Setter elements are allowed -->
            </Trigger>
        </Style.Triggers>
    </Style>
</ContentPage.Resources>

Data triggers

A DataTrigger represents a trigger that applies property values, or performs actions, when the bound data meets a specified condition. The Binding markup extension is used to monitor for the specified condition.

The following example shows a DataTrigger that disables a Button when the Entry is empty:

<Entry x:Name="entry"
       Text=""
       Placeholder="Enter text" />
<Button Text="Save">
    <Button.Triggers>
        <DataTrigger TargetType="Button"
                     Binding="{Binding Source={x:Reference entry},
                                       Path=Text.Length}"
                     Value="0">
            <Setter Property="IsEnabled"
                    Value="False" />
            <!-- Multiple Setter elements are allowed -->
        </DataTrigger>
    </Button.Triggers>
</Button>

In this example, when the length of the Entry is zero, the trigger is activated.

Tip

When evaluating Path=Text.Length always provide a default value for the target property (eg. Text="") because otherwise it will be null and the trigger won't work like you expect.

In addition, optional EnterActions and ExitActions collections can be specified. For more information, see EnterActions and ExitActions.

Event triggers

An EventTrigger represents a trigger that applies a set of actions in response to an event. Unlike Trigger, EventTrigger has no concept of termination of state, so the actions will not be undone once the condition that raised the event is no longer true.

An EventTrigger only requires an Event property to be set:

<EventTrigger Event="TextChanged">
    <local:NumericValidationTriggerAction />
</EventTrigger>

In this example, there are no Setter elements. Instead, there's a NumericalValidationTriggerAction object.

Note

Event triggers don't support EnterActions and ExitActions.

A trigger action implementation must:

  • Implement the generic TriggerAction<T> class, with the generic parameter corresponding with the type of control the trigger will be applied to. You can use classes such as VisualElement to write trigger actions that work with a variety of controls, or specify a control type like Entry.
  • Override the Invoke method. This method is called whenever the trigger event occurs.
  • Optionally expose properties that can be set in XAML when the trigger is declared.

The following example shows the NumericValidationTriggerAction class:

public class NumericValidationTriggerAction : TriggerAction<Entry>
{
    protected override void Invoke(Entry entry)
    {
        double result;
        bool isValid = Double.TryParse(entry.Text, out result);
        entry.TextColor = isValid ? Colors.Black : Colors.Red;
    }
}

Warning

Be careful when sharing triggers in a ResourceDictionary. One instance will be shared among controls so any state that is configured once will apply to them all.

Multi-triggers

A MultiTrigger represents a trigger that applies property values, or performs actions, when a set of conditions are satisfied. All the conditions must be true before the Setter objects are applied.

The following example shows a MultiTrigger that binds to two Entry objects:

<Entry x:Name="email"
       Text="" />
<Entry x:Name="phone"
       Text="" />
<Button Text="Save">
    <Button.Triggers>
        <MultiTrigger TargetType="Button">
            <MultiTrigger.Conditions>
                <BindingCondition Binding="{Binding Source={x:Reference email},
                                            Path=Text.Length}"
                                  Value="0" />
                <BindingCondition Binding="{Binding Source={x:Reference phone},
                                            Path=Text.Length}"
                                  Value="0" />
            </MultiTrigger.Conditions>
            <Setter Property="IsEnabled" Value="False" />
            <!-- multiple Setter elements are allowed -->
        </MultiTrigger>
    </Button.Triggers>
</Button>

In addition, the MultiTrigger.Conditions collection can also contain PropertyCondition objects:

<PropertyCondition Property="Text"
                   Value="OK" />

EnterActions and ExitActions

An alternative approach to implementing changes when a trigger occurs is by specifying EnterActions and ExitActions collections, and creating TriggerAction<T> implementations.

The EnterActions collection, of type IList<TriggerAction>, defines a collection that will be invoked when the trigger condition is met. The ExitActions collection, of type IList<TriggerAction>, defines a collection that will be invoked after the trigger condition is no longer met.

Note

The TriggerAction objects defined in the EnterActions and ExitActions collections are ignored by the EventTrigger class.

The following example shows a property trigger that specifies an EnterAction and an ExitAction:

<Entry Placeholder="Enter job title">
    <Entry.Triggers>
        <Trigger TargetType="Entry"
                 Property="Entry.IsFocused"
                 Value="True">
            <Trigger.EnterActions>
                <local:FadeTriggerAction StartsFrom="0" />
            </Trigger.EnterActions>

            <Trigger.ExitActions>
                <local:FadeTriggerAction StartsFrom="1" />
            </Trigger.ExitActions>
        </Trigger>
    </Entry.Triggers>
</Entry>

A trigger action implementation must:

  • Implement the generic TriggerAction<T> class, with the generic parameter corresponding with the type of control the trigger will be applied to. You can use classes such as VisualElement to write trigger actions that work with a variety of controls, or specify a control type like Entry.
  • Override the Invoke method. This method is called whenever the trigger event occurs.
  • Optionally expose properties that can be set in XAML when the trigger is declared.

The following example shows the FadeTriggerAction class:

public class FadeTriggerAction : TriggerAction<VisualElement>
{
    public int StartsFrom { get; set; }

    protected override void Invoke(VisualElement sender)
    {
        sender.Animate("FadeTriggerAction", new Animation((d) =>
        {
            var val = StartsFrom == 1 ? d : 1 - d;
            sender.BackgroundColor = Color.FromRgb(1, val, 1);
        }),
        length: 1000, // milliseconds
        easing: Easing.Linear);
    }
}

Note

You can provide EnterActions and ExitActions as well as Setter objects in a trigger, but be aware that the Setter objects are called immediately (they do not wait for the EnterAction or ExitAction to complete).

State triggers

State triggers are a specialized group of triggers that define the conditions under which a VisualState should be applied.

State triggers are added to the StateTriggers collection of a VisualState. This collection can contain a single state trigger, or multiple state triggers. A VisualState will be applied when any state triggers in the collection are active.

When using state triggers to control visual states, .NET MAUI uses the following precedence rules to determine which trigger (and corresponding VisualState) will be active:

  1. Any trigger that derives from StateTriggerBase.
  2. An AdaptiveTrigger activated due to the MinWindowWidth condition being met.
  3. An AdaptiveTrigger activated due to the MinWindowHeight condition being met.

If multiple triggers are simultaneously active (for example, two custom triggers) then the first trigger declared in the markup takes precedence.

Note

State triggers can be set in a Style, or directly on elements.

For more information about visual states, see Visual states.

State trigger

The StateTrigger class, which derives from the StateTriggerBase class, has an IsActive bindable property. A StateTrigger triggers a VisualState change when the IsActive property changes value.

The StateTriggerBase class, which is the base class for all state triggers, has an IsActive property and an IsActiveChanged event. This event fires whenever a VisualState change occurs. In addition, the StateTriggerBase class has overridable OnAttached and OnDetached methods.

Important

The StateTrigger.IsActive bindable property hides the inherited StateTriggerBase.IsActive property.

The following XAML example shows a Style that includes StateTrigger objects:

<Style TargetType="Grid">
    <Setter Property="VisualStateManager.VisualStateGroups">
        <VisualStateGroupList>
            <VisualStateGroup>
                <VisualState x:Name="Checked">
                    <VisualState.StateTriggers>
                        <StateTrigger IsActive="{Binding IsToggled}"
                                      IsActiveChanged="OnCheckedStateIsActiveChanged" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Property="BackgroundColor"
                                Value="Black" />
                    </VisualState.Setters>
                </VisualState>
                <VisualState x:Name="Unchecked">
                    <VisualState.StateTriggers>
                        <StateTrigger IsActive="{Binding IsToggled, Converter={StaticResource inverseBooleanConverter}}"
                                      IsActiveChanged="OnUncheckedStateIsActiveChanged" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Property="BackgroundColor"
                                Value="White" />
                    </VisualState.Setters>
                </VisualState>
            </VisualStateGroup>
        </VisualStateGroupList>
    </Setter>
</Style>

In this example, the implicit Style targets Grid objects. When the IsToggled property of the bound object is true, the background color of the Grid is set to black. When the IsToggled property of the bound object becomes false, a VisualState change is triggered, and the background color of the Grid becomes white.

In addition, every time a VisualState change occurs, the IsActiveChanged event for the VisualState is raised. Each VisualState registers an event handler for this event:

void OnCheckedStateIsActiveChanged(object sender, EventArgs e)
{
    StateTriggerBase stateTrigger = sender as StateTriggerBase;
    Console.WriteLine($"Checked state active: {stateTrigger.IsActive}");
}

void OnUncheckedStateIsActiveChanged(object sender, EventArgs e)
{
    StateTriggerBase stateTrigger = sender as StateTriggerBase;
    Console.WriteLine($"Unchecked state active: {stateTrigger.IsActive}");
}

In this example, when a handler for the IsActiveChanged event is raised, the handler outputs whether the VisualState is active or not. For example, the following messages are output to the console window when changing from the Checked visual state to the Unchecked visual state:

Checked state active: False
Unchecked state active: True

Note

Custom state triggers can be created by deriving from the StateTriggerBase class, and overriding the OnAttached and OnDetached methods to perform any required registrations and cleanup.

Adaptive trigger

An AdaptiveTrigger triggers a VisualState change when the window is a specified height or width. This trigger has two bindable properties:

Note

The AdaptiveTrigger derives from the StateTriggerBase class and can therefore attach an event handler to the IsActiveChanged event.

The following XAML example shows a Style that includes AdaptiveTrigger objects:

<Style TargetType="StackLayout">
    <Setter Property="VisualStateManager.VisualStateGroups">
        <VisualStateGroupList>
            <VisualStateGroup>
                <VisualState x:Name="Vertical">
                    <VisualState.StateTriggers>
                        <AdaptiveTrigger MinWindowWidth="0" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Property="Orientation"
                                Value="Vertical" />
                    </VisualState.Setters>
                </VisualState>
                <VisualState x:Name="Horizontal">
                    <VisualState.StateTriggers>
                        <AdaptiveTrigger MinWindowWidth="800" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Property="Orientation"
                                Value="Horizontal" />
                    </VisualState.Setters>
                </VisualState>
            </VisualStateGroup>
        </VisualStateGroupList>
    </Setter>
</Style>

In this example, the implicit Style targets StackLayout objects. When the window width is between 0 and 800 device-independent units, StackLayout objects to which the Style is applied will have a vertical orientation. When the window width is >= 800 device-independent units, the VisualState change is triggered, and the StackLayout orientation changes to horizontal.

The MinWindowHeight and MinWindowWidth properties can be used independently or in conjunction with each other. The following XAML shows an example of setting both properties:

<AdaptiveTrigger MinWindowWidth="800"
                 MinWindowHeight="1200"/>

In this example, the AdaptiveTrigger indicates that the corresponding VisualState will be applied when the current window width is >= 800 device-independent units and the current window height is >= 1200 device-independent units.

Compare state trigger

The CompareStateTrigger triggers a VisualState change when a property is equal to a specific value. This trigger has two bindable properties:

  • Property, of type object, which indicates the property being compared by the trigger.
  • Value, of type object, which indicates the value at which the VisualState should be applied.

Note

The CompareStateTrigger derives from the StateTriggerBase class and can therefore attach an event handler to the IsActiveChanged event.

The following XAML example shows a Style that includes CompareStateTrigger objects:

<Style TargetType="Grid">
    <Setter Property="VisualStateManager.VisualStateGroups">
        <VisualStateGroupList>
            <VisualStateGroup>
                <VisualState x:Name="Checked">
                    <VisualState.StateTriggers>
                        <CompareStateTrigger Property="{Binding Source={x:Reference checkBox}, Path=IsChecked}"
                                             Value="True" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Property="BackgroundColor"
                                Value="Black" />
                    </VisualState.Setters>
                </VisualState>
                <VisualState x:Name="Unchecked">
                    <VisualState.StateTriggers>
                        <CompareStateTrigger Property="{Binding Source={x:Reference checkBox}, Path=IsChecked}"
                                             Value="False" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Property="BackgroundColor"
                                Value="White" />
                    </VisualState.Setters>
                </VisualState>
            </VisualStateGroup>
        </VisualStateGroupList>
    </Setter>
</Style>
...
<Grid>
    <Border BackgroundColor="White"
            StrokeShape="RoundRectangle 12"
            Margin="24"
            Padding="24"
            HorizontalOptions="Center"
            VerticalOptions="Center">
        <StackLayout Orientation="Horizontal">
            <CheckBox x:Name="checkBox"
                      VerticalOptions="Center" />
            <Label Text="Check the CheckBox to modify the Grid background color."
                   VerticalOptions="Center" />
        </StackLayout>
    </Border>
</Grid>

In this example, the implicit Style targets Grid objects. When the IsChecked property of the CheckBox is false, the background color of the Grid is set to white. When the CheckBox.IsChecked property becomes true, a VisualState change is triggered, and the background color of the Grid becomes black.

Device state trigger

The DeviceStateTrigger triggers a VisualState change based on the device platform the app is running on. This trigger has a single bindable property:

  • Device, of type string, which indicates the device platform on which the VisualState should be applied.

Note

The DeviceStateTrigger derives from the StateTriggerBase class and can therefore attach an event handler to the IsActiveChanged event.

The following XAML example shows a Style that includes DeviceStateTrigger objects:

<Style x:Key="DeviceStateTriggerPageStyle"
       TargetType="ContentPage">
    <Setter Property="VisualStateManager.VisualStateGroups">
        <VisualStateGroupList>
            <VisualStateGroup>
                <VisualState x:Name="iOS">
                    <VisualState.StateTriggers>
                        <DeviceStateTrigger Device="iOS" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Property="BackgroundColor"
                                Value="Silver" />
                    </VisualState.Setters>
                </VisualState>
                <VisualState x:Name="Android">
                    <VisualState.StateTriggers>
                        <DeviceStateTrigger Device="Android" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Property="BackgroundColor"
                                Value="#2196F3" />
                    </VisualState.Setters>
                </VisualState>
            </VisualStateGroup>
        </VisualStateGroupList>
    </Setter>
</Style>

In this example, the explicit Style targets ContentPage objects. ContentPage objects that consume the style set their background color to silver on iOS, and to pale blue on Android.

Orientation state trigger

The OrientationStateTrigger triggers a VisualState change when the orientation of the device changes. This trigger has a single bindable property:

Note

The OrientationStateTrigger derives from the StateTriggerBase class and can therefore attach an event handler to the IsActiveChanged event.

The following XAML example shows a Style that includes OrientationStateTrigger objects:

<Style x:Key="OrientationStateTriggerPageStyle"
       TargetType="ContentPage">
    <Setter Property="VisualStateManager.VisualStateGroups">
        <VisualStateGroupList>
            <VisualStateGroup>
                <VisualState x:Name="Portrait">
                    <VisualState.StateTriggers>
                        <OrientationStateTrigger Orientation="Portrait" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Property="BackgroundColor"
                                Value="Silver" />
                    </VisualState.Setters>
                </VisualState>
                <VisualState x:Name="Landscape">
                    <VisualState.StateTriggers>
                        <OrientationStateTrigger Orientation="Landscape" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Property="BackgroundColor"
                                Value="White" />
                    </VisualState.Setters>
                </VisualState>
            </VisualStateGroup>
        </VisualStateGroupList>
    </Setter>
</Style>

In this example, the explicit Style targets ContentPage objects. ContentPage objects that consume the style set their background color to silver when the orientation is portrait, and set their background color to white when the orientation is landscape.