Xamarin.Forms Expander

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The Xamarin.Forms Expander control provides an expandable container to host any content. The control has a header and content, and the content is shown or hidden by tapping the Expander header. When only the Expander header is shown, the Expander is collapsed. When the Expander content is visible the Expander is expanded.

The following screenshots show an Expander in its collapsed and expanded states, with red boxes indicating the header and the content:

Screenshot of an Expander in collapsed and expanded states, on iOS and Android

Important

Expander is currently experimental and can only be used by setting the Expander_Experimental flag. For more information, see Experimental Flags.

In addition, the Expander control is fully implemented in the Xamarin.Forms namespace. Therefore, it's available on all platforms supported by Xamarin.Forms.

The Expander control defines the following properties:

  • CollapseAnimationEasing, of type Easing, which represents the easing function to be applied to the Expander content when it's collapsing.
  • CollapseAnimationLength, of type uint, which defines the duration of the animation when the Expander is collapsing. The default value of this property is 250ms.
  • Command, of type ICommand, which is executed when the Expander header is tapped.
  • CommandParameter, of type object, which is the parameter that's passed to the Command.
  • Content, of type View, which defines the content to be displayed when the Expander expands.
  • ContentTemplate, of type DataTemplate, which is the template used to dynamically inflate the content of the Expander.
  • ExpandAnimationEasing, of type Easing, which represents the easing function to be applied to the Expander content during expansion.
  • ExpandAnimationLength, of type uint, which defines the duration of the animation when the Expander expands. The default value of this property is 250ms.
  • ForceUpdateSizeCommand, of type ICommand, which defines the command that's executed when the size of the Expander is force updated. This property uses the OneWayToSource binding mode.
  • Header, of type View, which defines the header content.
  • IsExpanded, of type bool, which determines if the Expander is expanded . This property uses the TwoWay binding mode, and has a default value of false.
  • State, of type ExpanderState, which represents the state of the Expander. This property uses the OneWayToSource binding mode.

These properties are backed by BindableProperty objects, which means that they can be targets of data bindings, and styled.

Note

The Content property is the content property of the Expander class, and therefore does not need to be explicitly set from XAML.

The ExpanderState enumeration defines the following members:

  • Expanding indicates that the Expander is expanding.
  • Expanded indicates that the Expander is expanded.
  • Collapsing indicates that the Expander is collapsing.
  • Collapsed indicates that the Expander is collapsed.

The Expander control also defines a Tapped event that's fired when the Expander header is tapped. In addition, Expander includes a ForceUpdateSize method that can be called to programmatically resize the Expander at runtime.

Create an Expander

The following example shows how to instantiate an Expander in XAML:

<Expander>
    <Expander.Header>
        <Label Text="Baboon"
               FontAttributes="Bold"
               FontSize="Medium" />
    </Expander.Header>
    <Grid Padding="10">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="Auto" />
        </Grid.ColumnDefinitions>
        <Image Source="http://upload.wikimedia.org/wikipedia/commons/thumb/f/fc/Papio_anubis_%28Serengeti%2C_2009%29.jpg/200px-Papio_anubis_%28Serengeti%2C_2009%29.jpg"
               Aspect="AspectFill"
               HeightRequest="120"
               WidthRequest="120" />
        <Label Grid.Column="1"
               Text="Baboons are African and Arabian Old World monkeys belonging to the genus Papio, part of the subfamily Cercopithecinae."
               FontAttributes="Italic" />
    </Grid>
</Expander>

In this example, the Expander is collapsed by default and displays a Label as its header. Tapping on the header results in the Expander expanding to reveal its content, which is a Grid containing child controls. When the Expander is expanded, tapping its header collapses the Expander.

Important

When setting the Expander.Content property, either implicitly or explicitly, the Expander content is created when the page containing it is navigated to, even if the Expander is collapsed. However, the Expander.ContentTemplate property can be set to content that only gets inflated when the Expander expands for the first time. For more information, see Create Expander content on demand.

Alternatively, an Expander can be created in code:

Expander expander = new Expander
{
    Header = new Label
    {
        Text = "Baboon",
        FontAttributes = FontAttributes.Bold,
        FontSize = Device.GetNamedSize(NamedSize.Medium, typeof(Label))
    }
};

Grid grid = new Grid
{
    Padding = new Thickness(10),
    ColumnDefinitions =
    {
        new ColumnDefinition { Width = GridLength.Auto },
        new ColumnDefinition { Width = GridLength.Auto }
    }
};

grid.Children.Add(new Image
{
    Source = "http://upload.wikimedia.org/wikipedia/commons/thumb/f/fc/Papio_anubis_%28Serengeti%2C_2009%29.jpg/200px-Papio_anubis_%28Serengeti%2C_2009%29.jpg",
    Aspect = Aspect.AspectFill,
    HeightRequest = 120,
    WidthRequest = 120
});

grid.Children.Add(new Label
{
    Text = "Baboons are African and Arabian Old World monkeys belonging to the genus Papio, part of the subfamily Cercopithecinae.",
    FontAttributes = FontAttributes.Italic
}, 1, 0);

expander.Content = grid;

Create Expander content on demand

Expander content can be created on demand, in response to the Expander expanding. This can be accomplished by setting the Expander.ContentTemplate property to a DataTemplate that contains the content:

<Expander>
    <Expander.Header>
        <Label Text="{Binding Name}"
               FontAttributes="Bold"
               FontSize="Medium" />
    </Expander.Header>
    <Expander.ContentTemplate>
        <DataTemplate>
            <Grid Padding="10">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="Auto" />
                </Grid.ColumnDefinitions>
                <Image Source="{Binding ImageUrl}"
                       Aspect="AspectFill"
                       HeightRequest="120"
                       WidthRequest="120" />
                <Label Grid.Column="1"
                       Text="{Binding Details}"
                       FontAttributes="Italic" />
            </Grid>
        </DataTemplate>
    </Expander.ContentTemplate>
</Expander>

In this example, the Expander content is only inflated when the Expander expands for the first time.

The advantage of this approach is that when a page contains multiple Expander objects, the content for an Expander is only created when expanded for the first time by the user.

Add an expansion indicator

An Image can be added to an Expander header, to provide a visual indication of expansion state. A DataTrigger can be attached to the Image, that changes the Source property based on the value of the Expander.IsExpanded property:

<Expander>
    <Expander.Header>
        <Grid>
            <Label Text="{Binding Name}"
                   FontAttributes="Bold"
                   FontSize="Medium" />
            <Image Source="expand.png"
                   HorizontalOptions="End"
                   VerticalOptions="Start">
                <Image.Triggers>
                    <DataTrigger TargetType="Image"
                                 Binding="{Binding Source={RelativeSource AncestorType={x:Type Expander}}, Path=IsExpanded}"
                                 Value="True">
                        <Setter Property="Source"
                                Value="collapse.png" />
                    </DataTrigger>
                </Image.Triggers>
            </Image>
        </Grid>
    </Expander.Header>
    <Expander.ContentTemplate>
        <DataTemplate>
            <Grid Padding="10">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="Auto" />
                </Grid.ColumnDefinitions>
                <Image Source="{Binding ImageUrl}"
                       Aspect="AspectFill"
                       HeightRequest="120"
                       WidthRequest="120" />
                <Label Grid.Column="1"
                       Text="{Binding Details}"
                       FontAttributes="Italic" />
            </Grid>
        </DataTemplate>
    </Expander.ContentTemplate>
</Expander>

In this example, the Image displays the expand icon by default:

Screenshot of an Expander icon in collapsed state, on iOS and Android

The IsExpanded property becomes true when the Expander header is tapped, which results in the collapse icon being displayed:

Screenshot of an Expander icon in expand state, on iOS and Android

For more information about triggers, see Xamarin.Forms Triggers.

Embed an Expander in an Expander

The content of an Expander can be set to another Expander control, to enable multiple levels of expansion. The following XAML shows an Expander whose content is another Expander object:

<Expander>
    <Expander.Header>
        <Label Text="{Binding Name}"
               FontAttributes="Bold"
               FontSize="Medium" />
    </Expander.Header>
    <Expander Padding="10">
        <Expander.Header>
            <Label Text="{Binding Location}"
                   FontSize="Medium" />
        </Expander.Header>
            <Expander.ContentTemplate>
                <DataTemplate>
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto" />
                            <ColumnDefinition Width="Auto" />
                        </Grid.ColumnDefinitions>
                        <Image Source="{Binding ImageUrl}"
                               Aspect="AspectFill"
                               HeightRequest="120"
                               WidthRequest="120" />
                        <Label Grid.Column="1"
                               Text="{Binding Details}"
                               FontAttributes="Italic" />
                    </Grid>
                </DataTemplate>
            </Expander.ContentTemplate>
    </Expander>
</Expander>

In this example, tapping the root Expander header reveals the header for the child Expander:

Screenshot of an embedded Expander, on iOS and Android

Tapping the child Expander header results in its content being inflated and displayed:

Screenshot of an embedded Expander, on iOS and Android

Define the expand and collapse animation

The animation that occurs when an Expander expands or collapses can be defined by setting the ExpandAnimationEasing and CollapseAnimationEasing properties to any of the easing functions included in Xamarin.Forms, or custom easing functions. By default, the expand and collapse animations occur over 250ms. However, these durations can be changed by setting the ExpandAnimationLength and CollapseAnimationLength properties to uint values.

The following XAML shows an example of defining the animation that occurs when the Expander is expanded or collapsed by the user:

<Expander ExpandAnimationEasing="{x:Static Easing.CubicIn}"
          ExpandAnimationLength="500"
          CollapseAnimationEasing="{x:Static Easing.CubicOut}"
          CollapseAnimationLength="500">
    <Expander.Header>
        <Label Text="{Binding Name}"
               FontAttributes="Bold"
               FontSize="Medium" />
    </Expander.Header>
    <Expander.ContentTemplate>
        <DataTemplate>
            <Grid Padding="10">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="Auto" />
                </Grid.ColumnDefinitions>
                <Image Source="{Binding ImageUrl}"
                       Aspect="AspectFill"
                       HeightRequest="120"
                       WidthRequest="120" />
                <Label Grid.Column="1"
                       Text="{Binding Details}"
                       FontAttributes="Italic" />
            </Grid>
        </DataTemplate>
    </Expander.ContentTemplate>
</Expander>

In this example, the CubicIn easing function slowly accelerates the expand animation over 500ms, and the CubicOut easing function quickly decelerates the collapse animation over 500ms.

For more information about easing functions, see Xamarin.Forms Easing Functions.

Resize an Expander at runtime

An Expander can be programmatically resized at runtime with the ForceUpdateSize method.

Given an Expander named expander, whose content includes a Label that has a TapGestureRecognizer attached to it, the following code example shows calling the ForceUpdateSize method:

void OnLabelTapped(object sender, EventArgs e)
{
    Label label = sender as Label;
    Expander expander = label.Parent.Parent.Parent as Expander;

    if (label.FontSize == Device.GetNamedSize(NamedSize.Default, label))
    {
        label.FontSize = Device.GetNamedSize(NamedSize.Large, label);
    }
    else
    {
        label.FontSize = Device.GetNamedSize(NamedSize.Default, label);
    }
    expander.ForceUpdateSize();
}

In this example, the FontSize of a Label changes when the Label is tapped. Due to the size of the font changing, it's necessary to update the size of the Expander by calling its ForceUpdateSize method.

Disable an Expander

An application may enter a state where expanding an Expander is not a valid operation. In such cases, the Expander can be disabled by setting its IsEnabled property to false. This will prevent users from expanding or collapsing the Expander.