Triggers allow you to express actions declaratively in XAML that change the appearance of controls based on events or property changes.

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

There are four types of trigger:

• Property Trigger - occurs when a property on a control is set to a particular value.

• Data Trigger - uses data binding to trigger based on the properties of another control.

• Event Trigger - occurs when an event occurs on the control.

• Multi Trigger - allows multiple trigger conditions to be set before an action occurs.

Property Triggers

A simple trigger can be expressed purely in XAML, adding a Trigger element to a control's triggers collection. This 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" />
        </Trigger>
    </Entry.Triggers>
</Entry>

The important parts of the trigger's declaration are:

• 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 can be added and when the trigger condition is met. You must specify the Property and Value to set.

• EnterActions and ExitActions (not shown) - are written in code and can be used in addition to (or instead of) Setter elements. They are described below.

Applying a Trigger using a Style

Triggers can also be added to a Style declaration on a control, in a page, or an application ResourceDictionary. This example declares an implicit style (ie. no Key is set) which means it will apply to all Entry controls on the page.

<ContentPage.Resources>
    <ResourceDictionary>
        <Style TargetType="Entry">
                        <Style.Triggers>
                <Trigger TargetType="Entry"
                         Property="IsFocused" Value="True">
                    <Setter Property="BackgroundColor" Value="Yellow" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </ResourceDictionary>
</ContentPage.Resources>

Data Triggers

Data triggers use data binding to monitor another control to cause the Setters to get called. Instead of the Property attribute in a property trigger, set the Binding attribute to monitor for the specified value.

The example below uses the data binding syntax {Binding Source={x:Reference entry}, Path=Text.Length} which is how we refer to another control's properties. When the length of the entry is zero, the trigger is activated. In this sample the trigger disables the button when the input is empty.

<!-- the x:Name is referenced below in DataTrigger-->
<!-- tip: make sure to set the Text="" (or some other default) -->
<Entry x:Name="entry"
       Text=""
       Placeholder="required field" />

<Button x:Name="button" Text="Save"
        FontSize="Large"
        HorizontalOptions="Center">
    <Button.Triggers>
        <DataTrigger TargetType="Button"
                     Binding="{Binding Source={x:Reference entry},
                                       Path=Text.Length}"
                     Value="0">
            <Setter Property="IsEnabled" Value="False" />
        </DataTrigger>
    </Button.Triggers>
</Button>

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 to specifying Setters you can also provide EnterActions and ExitActions.

Event Triggers

The EventTrigger element requires only an Event property, such as "Clicked" in the example below.

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

Notice that there are no Setter elements but rather a reference to a class defined by local:NumericValidationTriggerAction which requires the xmlns:local to be declared in the page's XAML:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:WorkingWithTriggers;assembly=WorkingWithTriggers"

The class itself implements TriggerAction which means it should provide an override for the Invoke method that is called whenever the trigger event occurs.

A trigger action implementation should:

• 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 superclasses 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 is called whenever the trigger criteria are met.

• Optionally expose properties that can be set in the XAML when the trigger is declared (such as Anchor, Scale, and Length in this example).

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

The properties exposed by the trigger action can be set in the XAML declaration as follows:

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

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.

Note that event triggers do not support EnterActions and ExitActions described below.

Multi Triggers

A MultiTrigger looks similar to a Trigger or DataTrigger except there can be more than one condition. All the conditions must be true before the Setters are triggered.

Here's an example of a trigger for a button that binds to two different inputs (email and phone):

<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>

The Conditions collection could also contain PropertyCondition elements like this:

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

Building a "require all" multi trigger

The multi trigger only updates its control when all conditions are true. Testing for "all field lengths are zero" (such as a login page where all inputs must be complete) is tricky because you want a condition "where Text.Length > 0" but this can't be expressed in XAML.

This can be done with an IValueConverter. The converter code below transforms the Text.Length binding into a bool that indicates whether a field is empty or not:

public class MultiTriggerConverter : IValueConverter
{
    public object Convert(object value, Type targetType,
        object parameter, CultureInfo culture)
    {
        if ((int)value > 0) // length > 0 ?
            return true;            // some data has been entered
        else
            return false;           // input is empty
    }

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

To use this converter in a multi trigger, first add it to the page's resource dictionary (along with a custom xmlns:local namespace definition):

<ResourceDictionary>
   <local:MultiTriggerConverter x:Key="dataHasBeenEntered" />
</ResourceDictionary>

The XAML is shown below. Note the following differences from the first multi trigger example:

• The button has IsEnabled="false" set by default.
• The multi trigger conditions use the converter to turn the Text.Length value into a boolean.
• When all the conditions are true, the setter makes the button's IsEnabled property true.

<Entry x:Name="user" Text="" Placeholder="user name" />

<Entry x:Name="pwd" Text="" Placeholder="password" />

<Button x:Name="loginButton" Text="Login"
        FontSize="Large"
        HorizontalOptions="Center"
        IsEnabled="false">
  <Button.Triggers>
    <MultiTrigger TargetType="Button">
      <MultiTrigger.Conditions>
        <BindingCondition Binding="{Binding Source={x:Reference user},
                              Path=Text.Length,
                              Converter={StaticResource dataHasBeenEntered}}"
                          Value="true" />
        <BindingCondition Binding="{Binding Source={x:Reference pwd},
                              Path=Text.Length,
                              Converter={StaticResource dataHasBeenEntered}}"
                          Value="true" />
      </MultiTrigger.Conditions>
      <Setter Property="IsEnabled" Value="True" />
    </MultiTrigger>
  </Button.Triggers>
</Button>

These screenshots show the difference between the two multi trigger examples above. In the top part of the screens, text input in just one Entry is enough to enable the Save button. In the bottom part of the screens, the Login button remains inactive until both fields contain data.

EnterActions and ExitActions

Another way to implement changes when a trigger occurs is by adding EnterActions and ExitActions collections and specifying TriggerAction<T> implementations.

You can provide both EnterActions and ExitActions as well as Setters in a trigger, but be aware that the Setters are called immediately (they do not wait for the EnterAction or ExitAction to complete). Alternatively you can perform everything in the code and not use Setters at all.

<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>
                        <!-- You can use both Enter/Exit and Setter together if required -->
        </Trigger>
    </Entry.Triggers>
</Entry>

As always, when a class is referenced in XAML you should declare a namespace such as xmlns:local as shown here:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:WorkingWithTriggers;assembly=WorkingWithTriggers"

The FadeTriggerAction code is shown below:

public class FadeTriggerAction : TriggerAction<VisualElement>
{
    public FadeTriggerAction() {}

    public int StartsFrom { set; get; }

    protected override void Invoke (VisualElement visual)
    {
            visual.Animate("", new Animation( (d)=>{
                var val = StartsFrom==1 ? d : 1-d;
                visual.BackgroundColor = Color.FromRgb(1, val, 1);

            }),
            length:1000, // milliseconds
            easing: Easing.Linear);
    }
}

Note: EnterActions and ExitActions are ignored on Event Triggers.