Partajați prin

Xamarin.Forms CarouselView Scrolling

  • Articol

CarouselView defines the following scroll related properties:

  • HorizontalScrollBarVisibility, of type ScrollBarVisibility, which specifies when the horizontal scroll bar is visible.
  • IsDragging, of type bool, which indicates whether the CarouselView is scrolling. This is a read only property, whose default value is false.
  • IsScrollAnimated, of type bool, which specifies whether an animation will occur when scrolling the CarouselView. The default value is true.
  • ItemsUpdatingScrollMode, of type ItemsUpdatingScrollMode, which represents the scrolling behavior of the CarouselView when new items are added to it.
  • VerticalScrollBarVisibility, of type ScrollBarVisibility, which specifies when the vertical scroll bar is visible.

All of these properties are backed by BindableProperty objects, which means that they can be targets of data bindings.

CarouselView also defines two ScrollTo methods, that scroll items into view. One of the overloads scrolls the item at the specified index into view, while the other scrolls the specified item into view. Both overloads have additional arguments that can be specified to indicate the exact position of the item after the scroll has completed, and whether to animate the scroll.

CarouselView defines a ScrollToRequested event that is fired when one of the ScrollTo methods is invoked. The ScrollToRequestedEventArgs object that accompanies the ScrollToRequested event has many properties, including IsAnimated, Index, Item, and ScrollToPosition. These properties are set from the arguments specified in the ScrollTo method calls.

In addition, CarouselView defines a Scrolled event that is fired to indicate that scrolling occurred. The ItemsViewScrolledEventArgs object that accompanies the Scrolled event has many properties. For more information, see Detect scrolling.

When a user swipes to initiate a scroll, the end position of the scroll can be controlled so that items are fully displayed. This feature is known as snapping, because items snap to position when scrolling stops. For more information, see Snap points.

CarouselView can also load data incrementally as the user scrolls. For more information, see Load data incrementally.

Detect scrolling

The IsDragging property can be examined to determine whether the CarouselView is currently scrolling through items.

In addition, CarouselView defines a Scrolled event which is fired to indicate that scrolling occurred. This event should be consumed when data about the scroll is required.

The following XAML example shows a CarouselView that sets an event handler for the Scrolled event:

<CarouselView Scrolled="OnCollectionViewScrolled">
    ...
</CarouselView>

The equivalent C# code is:

CarouselView carouselView = new CarouselView();
carouselView.Scrolled += OnCarouselViewScrolled;

In this code example, the OnCarouselViewScrolled event handler is executed when the Scrolled event fires:

void OnCarouselViewScrolled(object sender, ItemsViewScrolledEventArgs e)
{
    Debug.WriteLine("HorizontalDelta: " + e.HorizontalDelta);
    Debug.WriteLine("VerticalDelta: " + e.VerticalDelta);
    Debug.WriteLine("HorizontalOffset: " + e.HorizontalOffset);
    Debug.WriteLine("VerticalOffset: " + e.VerticalOffset);
    Debug.WriteLine("FirstVisibleItemIndex: " + e.FirstVisibleItemIndex);
    Debug.WriteLine("CenterItemIndex: " + e.CenterItemIndex);
    Debug.WriteLine("LastVisibleItemIndex: " + e.LastVisibleItemIndex);
}

In this example, the OnCarouselViewScrolled event handler outputs the values of the ItemsViewScrolledEventArgs object that accompanies the event.

Important

The Scrolled event is fired for user initiated scrolls, and for programmatic scrolls.

Scroll an item at an index into view

The first ScrollTo method overload scrolls the item at the specified index into view. Given a CarouselView object named carouselView, the following example shows how to scroll the item at index 6 into view:

carouselView.ScrollTo(6);

Note

The ScrollToRequested event is fired when the ScrollTo method is invoked.

Scroll an item into view

The second ScrollTo method overload scrolls the specified item into view. Given a CarouselView object named carouselView, the following example shows how to scroll the Proboscis Monkey item into view:

MonkeysViewModel viewModel = BindingContext as MonkeysViewModel;
Monkey monkey = viewModel.Monkeys.FirstOrDefault(m => m.Name == "Proboscis Monkey");
carouselView.ScrollTo(monkey);

Note

The ScrollToRequested event is fired when the ScrollTo method is invoked.

Disable scroll animation

A scrolling animation is displayed when moving between items in a CarouselView. This animation occurs both for user initiated scrolls, and for programmatic scrolls. Setting the IsScrollAnimated property to false will disable the animation for both scrolling categories.

Alternatively, the animate argument of the ScrollTo method can be set to false to disable the scrolling animation on programmatic scrolls:

carouselView.ScrollTo(monkey, animate: false);

Control scroll position

When scrolling an item into view, the exact position of the item after the scroll has completed can be specified with the position argument of the ScrollTo methods. This argument accepts a ScrollToPosition enumeration member.

MakeVisible

The ScrollToPosition.MakeVisible member indicates that the item should be scrolled until it's visible in the view:

carouselView.ScrollTo(monkey, position: ScrollToPosition.MakeVisible);

This example code results in the minimal scrolling required to scroll the item into view.

Note

The ScrollToPosition.MakeVisible member is used by default, if the position argument is not specified when calling the ScrollTo method.

Start

The ScrollToPosition.Start member indicates that the item should be scrolled to the start of the view:

carouselView.ScrollTo(monkey, position: ScrollToPosition.Start);

This example code results in the item being scrolled to the start of the view.

Center

The ScrollToPosition.Center member indicates that the item should be scrolled to the center of the view:

carouselViewView.ScrollTo(monkey, position: ScrollToPosition.Center);

This example code results in the item being scrolled to the center of the view.

End

The ScrollToPosition.End member indicates that the item should be scrolled to the end of the view:

carouselViewView.ScrollTo(monkey, position: ScrollToPosition.End);

This example code results in the item being scrolled to the end of the view.

Control scroll position when new items are added

CarouselView defines a ItemsUpdatingScrollMode property, which is backed by a bindable property. This property gets or sets a ItemsUpdatingScrollMode enumeration value that represents the scrolling behavior of the CarouselView when new items are added to it. The ItemsUpdatingScrollMode enumeration defines the following members:

  • KeepItemsInView keeps the first item in the list displayed when new items are added.
  • KeepScrollOffset ensures that the current scroll position is maintained when new items are added.
  • KeepLastItemInView adjusts the scroll offset to keep the last item in the list displayed when new items are added.

The default value of the ItemsUpdatingScrollMode property is KeepItemsInView. Therefore, when new items are added to a CarouselView the first item in the list will remain displayed. To ensure that the last item in the list is displayed when new items are added, set the ItemsUpdatingScrollMode property to KeepLastItemInView:

<CarouselView ItemsUpdatingScrollMode="KeepLastItemInView">
    ...
</CarouselView>

The equivalent C# code is:

CarouselView carouselView = new CarouselView
{
    ItemsUpdatingScrollMode = ItemsUpdatingScrollMode.KeepLastItemInView
};

Scroll bar visibility

CarouselView defines HorizontalScrollBarVisibility and VerticalScrollBarVisibility properties, which are backed by bindable properties. These properties get or set a ScrollBarVisibility enumeration value that represents when the horizontal, or vertical, scroll bar is visible. The ScrollBarVisibility enumeration defines the following members:

  • Default indicates the default scroll bar behavior for the platform, and is the default value for the HorizontalScrollBarVisibility and VerticalScrollBarVisibility properties.
  • Always indicates that scroll bars will be visible, even when the content fits in the view.
  • Never indicates that scroll bars will not be visible, even if the content doesn't fit in the view.

Snap points

When a user swipes to initiate a scroll, the end position of the scroll can be controlled so that items are fully displayed. This feature is known as snapping, because items snap to position when scrolling stops, and is controlled by the following properties from the ItemsLayout class:

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

Note

When snapping occurs, it will occur in the direction that produces the least amount of motion.

Snap points type

The SnapPointsType enumeration defines the following members:

  • None indicates that scrolling does not snap to items.
  • Mandatory indicates that content always snaps to the closest snap point to where scrolling would naturally stop, along the direction of inertia.
  • MandatorySingle indicates the same behavior as Mandatory, but only scrolls one item at a time.

By default on a CarouselView, the SnapPointsType property is set to SnapPointsType.MandatorySingle, which ensures that scrolling only scrolls one item at a time.

The following screenshots show a CarouselView with snapping turned off:

Screenshot of a CarouselView without snap points, on iOS and Android

Snap points alignment

The SnapPointsAlignment enumeration defines Start, Center, and End members.

Important

The value of the SnapPointsAlignment property is only respected when the SnapPointsType property is set to Mandatory, or MandatorySingle.

Start

The SnapPointsAlignment.Start member indicates that snap points are aligned with the leading edge of items. The following XAML example shows how to set this enumeration member:

<CarouselView ItemsSource="{Binding Monkeys}"
              PeekAreaInsets="100">
    <CarouselView.ItemsLayout>
        <LinearItemsLayout Orientation="Horizontal"
                           SnapPointsType="MandatorySingle"
                           SnapPointsAlignment="Start" />
    </CarouselView.ItemsLayout>
    ...
</CarouselView>

The equivalent C# code is:

CarouselView carouselView = new CarouselView
{
    ItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Horizontal)
    {
        SnapPointsType = SnapPointsType.MandatorySingle,
        SnapPointsAlignment = SnapPointsAlignment.Start
    },
    // ...
};

When a user swipes to initiate a scroll in a horizontally scrolling CarouselView, the left item will be aligned with the left of the view:

Screenshot of a CarouselView with start snap points, on iOS and Android

Center

The SnapPointsAlignment.Center member indicates that snap points are aligned with the center of items.

By default on a CarouselView, the SnapPointsAlignment property is set to Center. However, for completeness, the following XAML example shows how to set this enumeration member:

<CarouselView ItemsSource="{Binding Monkeys}"
              PeekAreaInsets="100">
    <CarouselView.ItemsLayout>
        <LinearItemsLayout Orientation="Horizontal"
                           SnapPointsType="MandatorySingle"
                           SnapPointsAlignment="Center" />
    </CarouselView.ItemsLayout>
    ...
</CarouselView>

The equivalent C# code is:

CarouselView carouselView = new CarouselView
{
    ItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Horizontal)
    {
        SnapPointsType = SnapPointsType.MandatorySingle,
        SnapPointsAlignment = SnapPointsAlignment.Center
    },
    // ...
};

When a user swipes to initiate a scroll in a horizontally scrolling CarouselView, the center item will be aligned with the center of the view:

Screenshot of a CarouselView with center snap points, on iOS and Android

End

The SnapPointsAlignment.End member indicates that snap points are aligned with the trailing edge of items. The following XAML example shows how to set this enumeration member:

<CarouselView ItemsSource="{Binding Monkeys}"
              PeekAreaInsets="100">
    <CarouselView.ItemsLayout>
        <LinearItemsLayout Orientation="Horizontal"
                           SnapPointsType="MandatorySingle"
                           SnapPointsAlignment="End" />
    </CarouselView.ItemsLayout>
    ...
</CarouselView>

The equivalent C# code is:

CarouselView carouselView = new CarouselView
{
    ItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Horizontal)
    {
        SnapPointsType = SnapPointsType.MandatorySingle,
        SnapPointsAlignment = SnapPointsAlignment.End
    },
    // ...
};

When a user swipes to initiate a scroll in a horizontally scrolling CarouselView, the right item will be aligned with the right of the view.

Screenshot of a CarouselView with end snap points, on iOS and Android