RecyclerView拖拽view後，itemView發生交換的規則重寫

最近有個需求，要實現列表的itemView拖拽後，交換位置。網上搜索了一番，發現RecyclerView來實現此方案很容易，RecyclerView的預設幾個api很強大，只需幾行代碼就可以實現簡單的拖拽需求。

有一篇文章值得推薦給大家，RecyclerView 擴展(二) - 手把手教你認識ItemTouchHelper; https://juejin.im/post/5d750947f265da03ea5aa2a2  ItemTouchHelper(二)源碼簡析  https://www.jianshu.com/p/b8e45aa3a6ff

• 調用Callback的chooseDropTarget方法來找到符合交換條件的ItemView。這裡符合的條件是指，選中的ItemView的bottom大於目標ItemView的bottom或者ItemView的top大於目標ItemView的top。通常來說，我們可以重寫chooseDropTarget方法，來定義什麼條件下就交換位置。

這裡呢，直接說重點，我的需求是要自己來控制滿足一定的條件，才能交換位置。

其實chooseDropTarget的預設實現已經滿足了，他的規則是選中的ItemView的bottom大於目標itemView的bottom或者是top大於目標itemView的top值。

那麼我這裡想要實現的是，選中的selectedView的左右或者上下邊界，接近targetView的邊界，他們相減，有一個固定的差值，這裡我定義為ScreenWidth() / 列數 / 單個itemView四等分

比如： int offsetWidth = ScreenWidth / 3/ 4;

下麵直接看代碼，可以比較下跟chooseDropTarget的預設實現的差異，其實差不多，就是加了個固定的差值。這樣不需要拖拽到view的邊界才發生交換

@Override
    public RecyclerView.ViewHolder chooseDropTarget(RecyclerView.ViewHolder selected, List<RecyclerView.ViewHolder> dropTargets, int curX, int curY) {
//        return super.chooseDropTarget(selected, dropTargets, curX, curY);
        int right = curX + selected.itemView.getWidth();
        int bottom = curY + selected.itemView.getHeight();
        RecyclerView.ViewHolder winner = null;
        int winnerScore = -1;
        final int dx = curX - selected.itemView.getLeft();
        final int dy = curY - selected.itemView.getTop();

        final int targetsSize = dropTargets.size();
        for (int i = 0; i < targetsSize; i++) {
            final RecyclerView.ViewHolder target = dropTargets.get(i);
            int offsetWidth = (int) (DisplayUtils.getWindowWidth(selected.itemView.getContext()) / 12.0f);
            int offsetHeight = offsetWidth;

            if (dx > 0) {
                int diff = target.itemView.getRight() - offsetWidth - right;
                if (diff < 0 && target.itemView.getRight() > selected.itemView.getRight()) {
                    final int score = Math.abs(diff);
                    if (score > winnerScore) {
                        winnerScore = score;
                        winner = target;
                    }
                }
            }
            if (dx < 0) {
                int diff = target.itemView.getLeft() + offsetWidth - curX;
                if (diff > 0 && target.itemView.getLeft() < selected.itemView.getLeft()) {
                    final int score = Math.abs(diff);
                    if (score > winnerScore) {
                        winnerScore = score;
                        winner = target;
                    }
                }
            }
            if (dy < 0) {
                int diff = target.itemView.getTop() + offsetHeight - curY;
                if (diff > 0 && target.itemView.getTop() < selected.itemView.getTop()) {
                    final int score = Math.abs(diff);
                    if (score > winnerScore) {
                        winnerScore = score;
                        winner = target;
                    }
                }
            }

            if (dy > 0) {
                int diff = target.itemView.getBottom() - offsetHeight - bottom;
                if (diff < 0 && target.itemView.getBottom() > selected.itemView.getBottom()) {
                    final int score = Math.abs(diff);
                    if (score > winnerScore) {
                        winnerScore = score;
                        winner = target;
                    }
                }
            }
        }
        return winner;
    }