ItemTouchHelper: Prevent out of bounds dragging

Question

I have a recycler view with ItemTouchHelper. It allows dragging the items.

I want to limit the dragging to the bounds of recycler view - ie you can't just drag the view outside the container, so that it disappears.

I tried checking the absolute coordinates like this:

 @Override
    public void onChildDraw(Canvas c, RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState, boolean isCurrentlyActive) {
        recyclerView.getLocationOnScreen(pos);
        int rvY = pos[1];
        viewHolder.itemView.getLocationOnScreen(pos);
        int vhY = pos[1];


        if (rvY > vhY || rvY + recyclerView.getHeight() < vhY + viewHolder.itemView.getHeight()) {
            return;
        }
super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive);
...
}

but then I run into kinda rendering concurency - if I move the view slowly, it will stop moving when going out of bounds, but if I move faster - then it anyway leaves the recycler view bounds.

Any ideas / approaches?

Answer 1

The dY and dX value must be clipped to the RecyclerView 's bounds:

override fun onChildDraw(c: Canvas, recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder, dX: Float, dY: Float, actionState: Int, isCurrentlyActive: Boolean) {
    val clippedDx  = clip(recyclerView.width, viewHolder.itemView.left, viewHolder.itemView.right, dX)
    val clippedDy  = clip(recyclerView.height, viewHolder.itemView.top, viewHolder.itemView.bottom, dY)
    super.onChildDraw(c, recyclerView, viewHolder, clippedDx, clippedDy, actionState, isCurrentlyActive)
}

private fun clip(size: Int, start: Int, end: Int, delta: Float): Float {
    val newStart = start + delta
    val newEnd = end + delta

    val oobStart = 0 - newStart
    val oobEnd = newEnd - size

    return when {
        oobStart > 0 -> delta + oobStart
        oobEnd > 0 -> delta - oobEnd
        else -> delta
    }
}

Answer 2

I adjusted user1185087's answer so that instead of teleporting your item, it simply won't handle it if the drag does go out.

@Override
public void onChildDraw(@NotNull Canvas c, @NotNull RecyclerView recyclerView,
                        @NotNull RecyclerView.ViewHolder viewHolder, float dX, float dY,
                        int actionState, boolean isCurrentlyActive) {
    float topY = viewHolder.itemView.getTop() + dY;
    float bottomY = topY + viewHolder.itemView.getHeight();

    // Only redraw child if it is inbounds of view
    if (topY > 0 && bottomY < recyclerView.getHeight()){
        super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive);
    }
}

Answer 3

I have the same problem. All answers help me find the better way without lags. :)

If use only dY it will create lags. So I add position determination for itemView inside onChildDraw .

override fun onChildDraw(c: Canvas, recyclerView: RecyclerView,
                         viewHolder: RecyclerView.ViewHolder, dX: Float, dY: Float,
                         actionState: Int, isCurrentlyActive: Boolean) {
    var edgeY = dY

    if (actionState == ItemTouchHelper.ACTION_STATE_DRAG) {
        val lastPosition = (recyclerView.adapter?.itemCount ?: 0) - 1

        val view = viewHolder.itemView
        val pieceHeight = view.height / PIECE_RATIO

        val topEdge = recyclerView.top - pieceHeight
        val bottomEdge = recyclerView.height - view.bottom + pieceHeight

        if (movePosition == 0 && dY < topEdge) {
            edgeY = topEdge
        } else if (movePosition == lastPosition && dY > bottomEdge) {
            edgeY = bottomEdge
        }
    }

    super.onChildDraw(c, recyclerView, viewHolder, dX, edgeY, actionState, isCurrentlyActive)
}

• PIECE_RATIO - how far can view go.
• movePosition - current position for dragging viewHolder . Need for best performance. You need setup it inside onSelectedChanged and onMove . Clear variable inside clearView .

Code for this:

/**
 * Variable need for best performance and more productive, because
 * get [RecyclerView.ViewHolder.getAdapterPosition] inside [onChildDraw]
 * is hard calculating operation
 */
protected var movePosition = RecyclerView.NO_POSITION

override fun onSelectedChanged(viewHolder: RecyclerView.ViewHolder?, actionState: Int) {
    super.onSelectedChanged(viewHolder, actionState)

    /**
     * Get position on drag start
     */
    if (actionState == ItemTouchHelper.ACTION_STATE_DRAG) {
        movePosition = viewHolder?.adapterPosition ?: RecyclerView.NO_POSITION
    }
}

override fun clearView(recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder) {
    super.clearView(recyclerView, viewHolder)

    /**
     * Clear position on drag end
     */
    movePosition = RecyclerView.NO_POSITION
}

override fun onMove(recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder,
                    target: RecyclerView.ViewHolder): Boolean {
    /**
     * Get position on drag
     */
    movePosition = target.adapterPosition
    return false
}

Child classes can use movePosition like this:

override fun onMove(recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder,
                target: RecyclerView.ViewHolder): Boolean {
    super.onMove(recyclerView, viewHolder, target)
    return callback.onTouchMove(viewHolder.adapterPosition, movePosition)
}

All source code and samples you can find in link: github project