RecyclerView ItemTouchHelper.Callback：拖动交换条件

Question

I want to implement dragging the cards in such a way that the rearrangement of the cards starts when the card that I am dragging does not completely overlap the element, but only 50%.

Check out an example:

Now, for the right card to move to the left, I need to completely overlap it with the one I am dragging.

I tried overriding this method from ItemTouchHelper.Callback:

 public float getMoveThreshold(@NonNull ViewHolder viewHolder) {
     return .5f;
 }

But it didn't help.

So how can I make the swap happen at 50% and not at 100% overlap?

Answer 1

After many attempts, I found the solution myself:

You need to override the chooseDropTarget method from ItemTouchHelper.Callback.

This is how I did it:

  //  x ≥ 0.5 (if less than 0.5, it will shake due to constant overlap)
val DRAG_THRESHOLD_PERSENT = 0.5
override fun chooseDropTarget(
    selected: ViewHolder,
    targets: MutableList<ViewHolder>,
    curX: Int, curY: Int
): ViewHolder? {
    val verticalOffset = (selected.itemView.height * DRAG_THRESHOLD_PERSENT).toInt()
    val horizontalOffset = (selected.itemView.width * DRAG_THRESHOLD_PERSENT).toInt()
    val left = curX - horizontalOffset
    val right = curX + selected.itemView.width + horizontalOffset
    val top = curY - verticalOffset
    val bottom = curY + selected.itemView.height + verticalOffset
    var winner: ViewHolder? = null
    var winnerScore = -1
    val dx = curX - selected.itemView.left
    val dy = curY - selected.itemView.top
    val targetsSize = targets.size
    for (i in 0 until targetsSize) {
        val target = targets[i]
        if (dx > 0) {
            val diff = target.itemView.right - right
            if (diff < 0 && target.itemView.right > selected.itemView.right) {
                val score = abs(diff)
                if (score > winnerScore) {
                    winnerScore = score
                    winner = target
                }
            }
        }
        if (dx < 0) {
            val diff = target.itemView.left - left
            if (diff > 0 && target.itemView.left < selected.itemView.left) {
                val score = abs(diff)
                if (score > winnerScore) {
                    winnerScore = score
                    winner = target
                }
            }
        }
        if (dy < 0) {
            val diff = target.itemView.top - top
            if (diff > 0 && target.itemView.top < selected.itemView.top) {
                val score = abs(diff)
                if (score > winnerScore) {
                    winnerScore = score
                    winner = target
                }
            }
        }
        if (dy > 0) {
            val diff = target.itemView.bottom - bottom
            if (diff < 0 && target.itemView.bottom > selected.itemView.bottom) {
                val score = abs(diff)
                if (score > winnerScore) {
                    winnerScore = score
                    winner = target
                }
            }
        }
    }
    return winner
}

How it works:
I am calculating the offset of the selected view using the factor and width/height. After that I create new borders (left, right, top, bottom), add an offset to the borders of the selected view and then use them instead of the original in the method

Important:

• You shouldn't call super
• The coefficient must be greater than 0.5

Answer 2

I have no solution, but still, I would like to share my observation. It seems like whatever value you return from getMoveThreshold() method that is below 1 does not change the default behaviour, however, values above 1 in fact does.