RecyclerView的ItemTouchHelper.Callback：拖动交换条件

Question

我希望实现拖动卡片的功能，这样当我拖动的那张卡片只覆盖了元素的50%时，卡片的重新排列就会开始。

看一个例子：

现在，为了使右边的卡片向左移动，我需要完全将其与我正在拖动的卡片重叠。

我尝试重写ItemTouchHelper.Callback中的此方法:

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

但这并没有起到作用。

那么，我该如何使交换在50%重叠处而不是100%重叠处发生呢？

- vadiole

Answer 1

经过多次尝试，我自己找到了解决方案：

你需要重写 ItemTouchHelper.Callback 的 chooseDropTarget 方法。

这是我如何做的：

  //  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
}

如何工作：

我使用因子和宽度/高度计算所选视图的偏移量。然后，我创建新的边框（左，右，上，下），将偏移添加到所选视图的边框中，然后在方法中使用它们代替原始边框。

重要提示：

• 不应调用super
• 系数必须大于0.5

- vadiole

Answer 2

我没有解决方案，但我想分享我的观察。似乎无论你从getMoveThreshold()方法返回什么值，只要低于1，就不会改变默认行为，但是高于1的值实际上会改变。

- Mieszko Koźma