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Xamarin.Forms CollectionView Scrolling

  • Článok

CollectionView 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 group the item belongs to, the exact position of the item after the scroll has completed, and whether to animate the scroll.

CollectionView 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, CollectionView 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.

CollectionView also defines a ItemsUpdatingScrollMode property that represents the scrolling behavior of the CollectionView when new items are added to it. For more information about this property, see Control scroll position when new items are added.

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.

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

Detect scrolling

CollectionView defines a Scrolled event which is fired to indicate that scrolling occurred. The ItemsViewScrolledEventArgs class, which represents the object that accompanies the Scrolled event, defines the following properties:

  • HorizontalDelta, of type double, represents the change in the amount of horizontal scrolling. This is a negative value when scrolling left, and a positive value when scrolling right.
  • VerticalDelta, of type double, represents the change in the amount of vertical scrolling. This is a negative value when scrolling upwards, and a positive value when scrolling downwards.
  • HorizontalOffset, of type double, defines the amount by which the list is horizontally offset from its origin.
  • VerticalOffset, of type double, defines the amount by which the list is vertically offset from its origin.
  • FirstVisibleItemIndex, of type int, is the index of the first item that's visible in the list.
  • CenterItemIndex, of type int, is the index of the the center item that's visible in the list.
  • LastVisibleItemIndex, of type int, is the index of the last item that's visible in the list.

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

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

The equivalent C# code is:

CollectionView collectionView = new CollectionView();
collectionView.Scrolled += OnCollectionViewScrolled;

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

void OnCollectionViewScrolled(object sender, ItemsViewScrolledEventArgs e)
{
    // Custom logic
}

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 CollectionView object named collectionView, the following example shows how to scroll the item at index 12 into view:

collectionView.ScrollTo(12);

Alternatively, an item in grouped data can be scrolled into view by specifying the item and group indexes. The following example shows how to scroll the third item in the second group into view:

// Items and groups are indexed from zero.
collectionView.ScrollTo(2, 1);

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 CollectionView object named collectionView, 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");
collectionView.ScrollTo(monkey);

Alternatively, an item in grouped data can be scrolled into view by specifying the item and the group. The following example shows how to scroll the Proboscis Monkey item in the Monkeys group into view:

GroupedAnimalsViewModel viewModel = BindingContext as GroupedAnimalsViewModel;
AnimalGroup group = viewModel.Animals.FirstOrDefault(a => a.Name == "Monkeys");
Animal monkey = group.FirstOrDefault(m => m.Name == "Proboscis Monkey");
collectionView.ScrollTo(monkey, group);

Note

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

Disable scroll animation

A scrolling animation is displayed when scrolling an item into view. However, this animation can be disabled by setting the animate argument of the ScrollTo method to false:

collectionView.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:

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

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

Screenshot of a CollectionView vertical list with ScrollToPosition.MakeVisible, on iOS and Android

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:

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

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

Screenshot of a CollectionView vertical list with ScrollToPosition.Start, on iOS and Android

Center

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

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

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

Screenshot of a CollectionView vertical list with ScrollToPosition.Center, on iOS and Android

End

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

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

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

Screenshot of a CollectionView vertical list with ScrollToPosition.End, on iOS and Android

Control scroll position when new items are added

CollectionView 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 CollectionView 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 CollectionView 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:

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

The equivalent C# code is:

CollectionView collectionView = new CollectionView
{
    ItemsUpdatingScrollMode = ItemsUpdatingScrollMode.KeepLastItemInView
};

Scroll bar visibility

CollectionView 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, the SnapPointsType property is set to SnapPointsType.None, which ensures that scrolling does not snap items, as shown in the following screenshots:

Screenshot of a CollectionView vertical list 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.

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

<CollectionView ItemsSource="{Binding Monkeys}">
    <CollectionView.ItemsLayout>
        <LinearItemsLayout Orientation="Vertical"
                           SnapPointsType="MandatorySingle"
                           SnapPointsAlignment="Start" />
    </CollectionView.ItemsLayout>
    ...
</CollectionView>

The equivalent C# code is:

CollectionView collectionView = new CollectionView
{
    ItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Vertical)
    {
        SnapPointsType = SnapPointsType.MandatorySingle,
        SnapPointsAlignment = SnapPointsAlignment.Start
    },
    // ...
};

When a user swipes to initiate a scroll, the top item will be aligned with the top of the view:

Screenshot of a CollectionView vertical list with start snap points, on iOS and Android

Center

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

<CollectionView ItemsSource="{Binding Monkeys}">
    <CollectionView.ItemsLayout>
        <LinearItemsLayout Orientation="Vertical"
                           SnapPointsType="MandatorySingle"
                           SnapPointsAlignment="Center" />
    </CollectionView.ItemsLayout>
    ...
</CollectionView>

The equivalent C# code is:

CollectionView collectionView = new CollectionView
{
    ItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Vertical)
    {
        SnapPointsType = SnapPointsType.MandatorySingle,
        SnapPointsAlignment = SnapPointsAlignment.Center
    },
    // ...
};

When a user swipes to initiate a scroll, the top item will be center aligned at the top of the view:

Screenshot of a CollectionView vertical list 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:

<CollectionView ItemsSource="{Binding Monkeys}">
    <CollectionView.ItemsLayout>
        <LinearItemsLayout Orientation="Vertical"
                           SnapPointsType="MandatorySingle"
                           SnapPointsAlignment="End" />
    </CollectionView.ItemsLayout>
    ...
</CollectionView>

The equivalent C# code is:

CollectionView collectionView = new CollectionView
{
    ItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Vertical)
    {
        SnapPointsType = SnapPointsType.MandatorySingle,
        SnapPointsAlignment = SnapPointsAlignment.End
    },
    // ...
};

When a user swipes to initiate a scroll, the bottom item will be aligned with the bottom of the view:

Screenshot of a CollectionView vertical list with end snap points, on iOS and Android