为什么ItemTouchHelper.Callback的onChildDraw会在clearView之后被调用

问题是我想在 RecyclerView 中拖动该项目时对其执行提升。

根据文档所述，我应该在以下位置自定义拖动视图的行为ItemTouchHelper.Callback#onChildDraw https://developer.android.com/reference/android/support/v7/widget/helper/ItemTouchHelper.Callback.html#onChildDraw(android.graphics.Canvas,%20android.support.v7.widget.RecyclerView,%20android.support.v7.widget.RecyclerView.ViewHolder,%20float,%20float,%20int,%20boolean)因为它说

如果您想自定义视图对用户交互的响应方式，这是一个覆盖的好地方。

然后我应该清除我的自定义ItemTouchHelper.Callback#clearView https://developer.android.com/reference/android/support/v7/widget/helper/ItemTouchHelper.Callback.html#clearView(android.support.v7.widget.RecyclerView,%20android.support.v7.widget.RecyclerView.ViewHolder)因为它说

当用户与元素的交互结束并且它也完成其动画时由 ItemTouchHelper 调用。

但问题是onChildDraw之后总会再被调用一次clearView.

所以即使我清除了自定义行为clearView，仍然会被重新调用onChildDraw。那么目的是什么呢clearView?

这是我的代码：

class MyItemTouchCallback extends ItemTouchHelper.Callback {

    @Override
    public int getMovementFlags(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
        return makeFlag(ItemTouchHelper.ACTION_STATE_DRAG, ItemTouchHelper.UP | ItemTouchHelper.DOWN);
    }

    @Override
    public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
        mAdapter.notifyItemMoved(viewHolder.getAdapterPosition(), target.getAdapterPosition());
        return true;
    }

    @Override
    public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction) {
    }

    @Override
    public void onChildDraw(Canvas c, RecyclerView recyclerView,
                            RecyclerView.ViewHolder viewHolder,
                            float dX, float dY, int actionState, boolean isCurrentlyActive) {
        super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive);
        // Set elevation for dragged itemView
        ViewCompat.setElevation(viewHolder.itemView, getResources().getDimension(R.dimen.common_elevation));
    }

    @Override
    public void clearView(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
        super.clearView(recyclerView, viewHolder);
        // Clear elevation for dragged itemView
        ViewCompat.setElevation(viewHolder.itemView, 0);
    }
}

当然我们可以用一个trick来解决这个问题，很容易理解。记住使用布尔变量viewBeingCleared当拖动的itemView被清除时，我们可以决定调用自定义代码onChildDraw or not.

然而这并不是最好的解决方案。我们最好know why然后我们会know how，不是吗？

绝招解决方案：

class MyItemTouchCallback extends ItemTouchHelper.Callback {

    boolean viewBeingCleared;

    @Override
    public int getMovementFlags(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
        return makeFlag(ItemTouchHelper.ACTION_STATE_DRAG, ItemTouchHelper.UP | ItemTouchHelper.DOWN);
    }

    @Override
    public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
        mAdapter.notifyItemMoved(viewHolder.getAdapterPosition(), target.getAdapterPosition());
        return true;
    }

    @Override
    public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction) {
    }

    @Override
    public void onChildDraw(Canvas c, RecyclerView recyclerView,
                            RecyclerView.ViewHolder viewHolder,
                            float dX, float dY, int actionState, boolean isCurrentlyActive) {
        super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive);
        if (viewBeingCleared) {
            viewBeingCleared = false;
        } else {
            ViewCompat.setElevation(viewHolder.itemView, getResources().getDimension(R.dimen.common_elevation));
        }
    }

    @Override
    public void clearView(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
        super.clearView(recyclerView, viewHolder);
        ViewCompat.setElevation(viewHolder.itemView, 0);
        viewBeingCleared = true;
    }
}