ItemTouchHelper.Callback

public boolean canDropOver(
    @NonNull RecyclerView recyclerView,
    @NonNull RecyclerView.ViewHolder current,
    @NonNull RecyclerView.ViewHolder target
)

Return true if the current ViewHolder can be dropped over the the target ViewHolder.

This method is used when selecting drop target for the dragged View. After Views are eliminated either via bounds check or via this method, resulting set of views will be passed to chooseDropTarget.

Default implementation returns true.

public RecyclerView.ViewHolder chooseDropTarget(
    @NonNull RecyclerView.ViewHolder selected,
    @NonNull List<RecyclerView.ViewHolder> dropTargets,
    int curX,
    int curY
)

Called by ItemTouchHelper to select a drop target from the list of ViewHolders that are under the dragged View.

Default implementation filters the View with which dragged item have changed position in the drag direction. For instance, if the view is dragged UP, it compares the view.getTop() of the two views before and after drag started. If that value is different, the target view passes the filter.

Among these Views which pass the test, the one closest to the dragged view is chosen.

This method is called on the main thread every time user moves the View. If you want to override it, make sure it does not do any expensive operations.

public void clearView(
    @NonNull RecyclerView recyclerView,
    @NonNull RecyclerView.ViewHolder viewHolder
)

Called by the ItemTouchHelper when the user interaction with an element is over and it also completed its animation.

This is a good place to clear all changes on the View that was done in onSelectedChanged, onChildDraw or onChildDrawOver.

public int convertToAbsoluteDirection(int flags, int layoutDirection)

Converts a given set of flags to absolution direction which means START and END are replaced with LEFT and RIGHT depending on the layout direction.

public static int convertToRelativeDirection(int flags, int layoutDirection)

Replaces a movement direction with its relative version by taking layout direction into account.

public long getAnimationDuration(
    @NonNull RecyclerView recyclerView,
    int animationType,
    float animateDx,
    float animateDy
)

Called by the ItemTouchHelper when user action finished on a ViewHolder and now the View will be animated to its final position.

Default implementation uses ItemAnimator's duration values. If animationType is ANIMATION_TYPE_DRAG, it returns getMoveDuration, otherwise, it returns getRemoveDuration. If RecyclerView does not have any RecyclerView.ItemAnimator attached, this method returns DEFAULT_DRAG_ANIMATION_DURATION or DEFAULT_SWIPE_ANIMATION_DURATION depending on the animation type.

public int getBoundingBoxMargin()

When finding views under a dragged view, by default, ItemTouchHelper searches for views that overlap with the dragged View. By overriding this method, you can extend or shrink the search box.

public static @NonNull ItemTouchUIUtil getDefaultUIUtil()

Returns the ItemTouchUIUtil that is used by the Callback class for visual changes on Views in response to user interactions. ItemTouchUIUtil has different implementations for different platform versions.

By default, Callback applies these changes on itemView.

For example, if you have a use case where you only want the text to move when user swipes over the view, you can do the following:

    public void clearView(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder){
        getDefaultUIUtil().clearView(((ItemTouchViewHolder) viewHolder).textView);
    }
    public void onSelectedChanged(RecyclerView.ViewHolder viewHolder, int actionState) {
        if (viewHolder != null){
            getDefaultUIUtil().onSelected(((ItemTouchViewHolder) viewHolder).textView);
        }
    }
    public void onChildDraw(Canvas c, RecyclerView recyclerView,
            RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState,
            boolean isCurrentlyActive) {
        getDefaultUIUtil().onDraw(c, recyclerView,
                ((ItemTouchViewHolder) viewHolder).textView, dX, dY,
                actionState, isCurrentlyActive);
        return true;
    }
    public void onChildDrawOver(Canvas c, RecyclerView recyclerView,
            RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState,
            boolean isCurrentlyActive) {
        getDefaultUIUtil().onDrawOver(c, recyclerView,
                ((ItemTouchViewHolder) viewHolder).textView, dX, dY,
                actionState, isCurrentlyActive);
        return true;
    }

public float getMoveThreshold(@NonNull RecyclerView.ViewHolder viewHolder)

Returns the fraction that the user should move the View to be considered as it is dragged. After a view is moved this amount, ItemTouchHelper starts checking for Views below it for a possible drop.

public abstract int getMovementFlags(
    @NonNull RecyclerView recyclerView,
    @NonNull RecyclerView.ViewHolder viewHolder
)

Should return a composite flag which defines the enabled move directions in each state (idle, swiping, dragging).

Instead of composing this flag manually, you can use makeMovementFlags or makeFlag.

This flag is composed of 3 sets of 8 bits, where first 8 bits are for IDLE state, next 8 bits are for SWIPE state and third 8 bits are for DRAG state. Each 8 bit sections can be constructed by simply OR'ing direction flags defined in ItemTouchHelper.

For example, if you want it to allow swiping LEFT and RIGHT but only allow starting to swipe by swiping RIGHT, you can return:

     makeFlag(ACTION_STATE_IDLE, RIGHT) | makeFlag(ACTION_STATE_SWIPE, LEFT | RIGHT);

public float getSwipeEscapeVelocity(float defaultValue)

Defines the minimum velocity which will be considered as a swipe action by the user.

You can increase this value to make it harder to swipe or decrease it to make it easier. Keep in mind that ItemTouchHelper also checks the perpendicular velocity and makes sure current direction velocity is larger then the perpendicular one. Otherwise, user's movement is ambiguous. You can change the threshold by overriding getSwipeVelocityThreshold.

The velocity is calculated in pixels per second.

The default framework value is passed as a parameter so that you can modify it with a multiplier.

public float getSwipeThreshold(@NonNull RecyclerView.ViewHolder viewHolder)

Returns the fraction that the user should move the View to be considered as swiped. The fraction is calculated with respect to RecyclerView's bounds.

Default value is .5f, which means, to swipe a View, user must move the View at least half of RecyclerView's width or height, depending on the swipe direction.

public float getSwipeVelocityThreshold(float defaultValue)

Defines the maximum velocity ItemTouchHelper will ever calculate for pointer movements.

To consider a movement as swipe, ItemTouchHelper requires it to be larger than the perpendicular movement. If both directions reach to the max threshold, none of them will be considered as a swipe because it is usually an indication that user rather tried to scroll then swipe.

The velocity is calculated in pixels per second.

You can customize this behavior by changing this method. If you increase the value, it will be easier for the user to swipe diagonally and if you decrease the value, user will need to make a rather straight finger movement to trigger a swipe.

public int interpolateOutOfBoundsScroll(
    @NonNull RecyclerView recyclerView,
    int viewSize,
    int viewSizeOutOfBounds,
    int totalSize,
    long msSinceStartScroll
)

Called by the ItemTouchHelper when user is dragging a view out of bounds.

You can override this method to decide how much RecyclerView should scroll in response to this action. Default implementation calculates a value based on the amount of View out of bounds and the time it spent there. The longer user keeps the View out of bounds, the faster the list will scroll. Similarly, the larger portion of the View is out of bounds, the faster the RecyclerView will scroll.

public boolean isItemViewSwipeEnabled()

Returns whether ItemTouchHelper should start a swipe operation if a pointer is swiped over the View.

Default value returns true but you may want to disable this if you want to start swiping on a custom view touch using startSwipe.

public boolean isLongPressDragEnabled()

Returns whether ItemTouchHelper should start a drag and drop operation if an item is long pressed.

Default value returns true but you may want to disable this if you want to start dragging on a custom view touch using startDrag.

public static int makeFlag(int actionState, int directions)

Shifts the given direction flags to the offset of the given action state.

public static int makeMovementFlags(int dragFlags, int swipeFlags)

Convenience method to create movement flags.

For instance, if you want to let your items be drag &dropped vertically and swiped left to be dismissed, you can call this method with: makeMovementFlags(UP | DOWN, LEFT);

public void onChildDraw(
    @NonNull Canvas c,
    @NonNull RecyclerView recyclerView,
    @NonNull RecyclerView.ViewHolder viewHolder,
    float dX,
    float dY,
    int actionState,
    boolean isCurrentlyActive
)

Called by ItemTouchHelper on RecyclerView's onDraw callback.

If you would like to customize how your View's respond to user interactions, this is a good place to override.

Default implementation translates the child by the given dX, dY. ItemTouchHelper also takes care of drawing the child after other children if it is being dragged. This is done using child re-ordering mechanism. On platforms prior to L, this is achieved via getChildDrawingOrder and on L and after, it changes View's elevation value to be greater than all other children.)

public void onChildDrawOver(
    @NonNull Canvas c,
    @NonNull RecyclerView recyclerView,
    RecyclerView.ViewHolder viewHolder,
    float dX,
    float dY,
    int actionState,
    boolean isCurrentlyActive
)

Called by ItemTouchHelper on RecyclerView's onDraw callback.

If you would like to customize how your View's respond to user interactions, this is a good place to override.

Default implementation translates the child by the given dX, dY. ItemTouchHelper also takes care of drawing the child after other children if it is being dragged. This is done using child re-ordering mechanism. On platforms prior to L, this is achieved via getChildDrawingOrder and on L and after, it changes View's elevation value to be greater than all other children.)

public abstract boolean onMove(
    @NonNull RecyclerView recyclerView,
    @NonNull RecyclerView.ViewHolder viewHolder,
    @NonNull RecyclerView.ViewHolder target
)

Called when ItemTouchHelper wants to move the dragged item from its old position to the new position.

If this method returns true, ItemTouchHelper assumes viewHolder has been moved to the adapter position of target ViewHolder (ViewHolder#getAdapterPositionInRecyclerView()).

If you don't support drag &drop, this method will never be called.

public void onMoved(
    @NonNull RecyclerView recyclerView,
    @NonNull RecyclerView.ViewHolder viewHolder,
    int fromPos,
    @NonNull RecyclerView.ViewHolder target,
    int toPos,
    int x,
    int y
)

Called when onMove returns true.

ItemTouchHelper does not create an extra Bitmap or View while dragging, instead, it modifies the existing View. Because of this reason, it is important that the View is still part of the layout after it is moved. This may not work as intended when swapped Views are close to RecyclerView bounds or there are gaps between them (e.g. other Views which were not eligible for dropping over).

This method is responsible to give necessary hint to the LayoutManager so that it will keep the View in visible area. For example, for LinearLayoutManager, this is as simple as calling scrollToPositionWithOffset. Default implementation calls scrollToPosition if the View's new position is likely to be out of bounds.

It is important to ensure the ViewHolder will stay visible as otherwise, it might be removed by the LayoutManager if the move causes the View to go out of bounds. In that case, drag will end prematurely.

public void onSelectedChanged(
    @Nullable RecyclerView.ViewHolder viewHolder,
    int actionState
)

Called when the ViewHolder swiped or dragged by the ItemTouchHelper is changed.

public abstract void onSwiped(@NonNull RecyclerView.ViewHolder viewHolder, int direction)

Called when a ViewHolder is swiped by the user.

If you are returning relative directions (START , END) from the getMovementFlags method, this method will also use relative directions. Otherwise, it will use absolute directions.

If you don't support swiping, this method will never be called.

ItemTouchHelper will keep a reference to the View until it is detached from RecyclerView. As soon as it is detached, ItemTouchHelper will call clearView.