android - 为什么在clearView之后将调用ItemTouchHelper.Callback的onChildDraw

事情是我想在将项目拖到RecyclerView中的同时对其进行标高。

根据文档的说明，我应该在ItemTouchHelper.Callback#onChildDraw中自定义拖动 View 的行为，因为它说



然后，我应该在ItemTouchHelper.Callback#clearView中清除我的自定义设置，因为它说



但是问题是onChildDraw总是在clearView之后再被调用一次。

因此，即使我清除了clearView中的自定义行为，仍然会在onChildDraw中对其进行重新调用。那么clearView的目的是什么？

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);
    }
}

最佳答案

当然，我们可以通过技巧解决此问题，这很容易理解。记住在清除拖动的itemView时使用 bool 变量viewBeingCleared，然后我们可以决定是否调用onChildDraw中的自定义代码。

但是，这不是最佳解决方案。我们最好知道为什么使用，然后我们将知道如何使用，不是吗？

特技解决方案:

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;
    }
}