本文主要是解決RecyclerView上下移動(dòng)切換位置時(shí)能夠移出RecyclerView控件的范圍，同時(shí)增加了一些開(kāi)始，結(jié)束，限制移動(dòng)范圍的接口，開(kāi)始部分是分析，伸手黨請(qǐng)直接拉到最后

1、基礎(chǔ)使用

新項(xiàng)目做的差不多了，產(chǎn)品也開(kāi)始附加功能了，要求：為了方便客戶要增加移動(dòng)列表位置進(jìn)行排序的功能。沒(méi)問(wèn)題，其實(shí)心里正在暗暗得意，早就防著你呢，這次布局使用了RecyclerView，實(shí)現(xiàn)排序？太簡(jiǎn)單了，直接上代碼

抽象接口用于操作交換

public interface ItemMoveHelperApi { 
   /**
     * Item 切換位置
     *
     * @param fromPosition 開(kāi)始位置
     * @param toPosition   結(jié)束位置
     */
    void onItemMoved(int fromPosition, int toPosition);
}

實(shí)現(xiàn)交換

public class ItemMoveCallBackImpl extends ItemTouchHelper.Callback {

    private ItemMoveHelperApi mHelperApi;

    public ItemMoveCallBackImpl(ItemMoveHelperApi helperApi) {
        this.mHelperApi = helperApi;
    }
    @Override
    public boolean isLongPressDragEnabled() {
        return true;
    }
    @Override
    public boolean isItemViewSwipeEnabled() {
        return false;
    }
    @Override
    public int getMovementFlags(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
        int dragFlags = ItemTouchHelper.UP | ItemTouchHelper.DOWN;
        int swipeFlags = ItemTouchHelper.START | ItemTouchHelper.END;
        return makeMovementFlags(dragFlags, swipeFlags);
    }
    @Override
    public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
        if (mHelperApi != null) {
            mHelperApi.onItemMoved(viewHolder.getAdapterPosition(), target.getAdapterPosition());
        }
        return true;
    }
    @Override
    public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction) {
    }
}

RecyclerView使用

ItemMoveCallBackImpl mMoveCallBack = new ItemMoveCallBackImpl(this);
ItemTouchHelper touchHelper = new ItemTouchHelper(mMoveCallBack);
touchHelper.attachToRecyclerView(recyclerView);

交換實(shí)現(xiàn)

@Override
public void onItemMoved(int fromPosition, int toPosition) {
    mAdapter.notifyItemMoved(fromPosition, toPosition);
    //數(shù)據(jù)交換，如果不寫(xiě)的話，滑動(dòng)后會(huì)變回原來(lái)的順序
    ......
}

2、添加開(kāi)始結(jié)束接口

果然樂(lè)極生悲，雖然這個(gè)代碼一點(diǎn)都沒(méi)有問(wèn)題，但是到了實(shí)際項(xiàng)目中發(fā)現(xiàn)一個(gè)尷尬的問(wèn)題，RecyclerView有下拉刷新，當(dāng)RecyclerView處于頂部的時(shí)候，交換和刷新會(huì)有沖突... 沒(méi)辦法，繼續(xù)查找代碼。在查找API的過(guò)程中發(fā)現(xiàn)了一個(gè)方法

/**
 * Called when the ViewHolder swiped or dragged by the ItemTouchHelper is changed.
 * <p/>
 * If you override this method, you should call super.
 *
 * @param viewHolder  The new ViewHolder that is being swiped or dragged. Might be null if
 *                    it is cleared.
 * @param actionState One of {@link ItemTouchHelper#ACTION_STATE_IDLE},
 *                    {@link ItemTouchHelper#ACTION_STATE_SWIPE} or
 *                    {@link ItemTouchHelper#ACTION_STATE_DRAG}.
 * @see #clearView(RecyclerView, RecyclerView.ViewHolder)
 */
public void onSelectedChanged(ViewHolder viewHolder, int actionState) {
    if (viewHolder != null) {
        sUICallback.onSelected(viewHolder.itemView);
    }
}

通過(guò)注釋和自己代碼試驗(yàn)得出結(jié)論，這個(gè)方法是在移動(dòng)開(kāi)始和結(jié)束的時(shí)候調(diào)用，這樣一來(lái)就簡(jiǎn)單了，首先修改一下ItemMoveHelperApi 接口，新增2個(gè)方法

/**
 * 開(kāi)始移動(dòng)
 */
void onMoveStart();
/**
 * 停止移動(dòng)
 */
void onMoveEnd();

然后在ItemMoveCallBackImpl 中重寫(xiě)onSelectedChanged方法

@Override
public void onSelectedChanged(RecyclerView.ViewHolder viewHolder, int actionState) {
    if (mHelperApi == null) {
        super.onSelectedChanged(viewHolder, actionState);
        return;
    }
    if (viewHolder == null) {
        mHelperApi.onMoveEnd();
    } else {
        mHelperApi.onMoveStart();
    }
    super.onSelectedChanged(viewHolder, actionState);
}

這樣就能在onMoveStart中禁止下拉刷新控件，在onMoveEnd重新啟動(dòng)下拉刷新控件并保存當(dāng)前列表的順序，這樣就不用去每次交換位置都去保存順序了。

3、限制移動(dòng)位置

果然程序員的世界和別人不一樣（或者說(shuō)是產(chǎn)品的世界和別人（程序員）的不一樣？），在我看來(lái)不錯(cuò)的東西人家都能提出缺點(diǎn)(?) RecyclerView的交換移動(dòng)到頂部的時(shí)候是可以移動(dòng)出去的，這個(gè)要改，沒(méi)辦法，改吧，先上代碼

@Override
public void onChildDraw(Canvas c, RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState, boolean isCurrentlyActive) {
    dY = getLimitedDy(recyclerView, viewHolder, dY);
    super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive);
}
/**
 * 根據(jù)方向和條件獲取限制在RecyclerView內(nèi)部的DY值
 *
 * @param recyclerView 列表
 * @param viewHolder   drag的ViewHolder
 * @param dY           限制前的DY值
 * @return 限制后的DY值
 */
private float getLimitedDy(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, float dY) {
    RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
    if (!(layoutManager instanceof LinearLayoutManager)
            || ((LinearLayoutManager) layoutManager).getOrientation() != OrientationHelper.VERTICAL) {
        return dY;
    }
    int position = viewHolder.getLayoutPosition();
    mDragEndPosition = mDragEndPositionFlag ?
            mDragStartPosition : recyclerView.getAdapter().getItemCount() - 1;
    if (position == mDragStartPosition) {
        return dY < 0 ? 0 : dY;
    } else if (position == mDragEndPosition) {
        return dY > 0 ? 0 : dY;
    }
    return dY;
}

方法onChildDraw會(huì)在Drag的ViewHolder移動(dòng)的時(shí)候使用，參數(shù)dX，dY表示當(dāng)前View和相鄰的ViewHolder在X，Y方向上的位移差，根據(jù)Y軸的移動(dòng)方向判斷下個(gè)ViewHolder是否參與交換，如果不參與，則dY返回0，ViewHolder就不會(huì)像下個(gè)位置繪制了。同時(shí)根據(jù)這個(gè)原理加入了限制移動(dòng)的開(kāi)始和結(jié)束位置的接口

private int mDragStartPosition; //能夠拖拽的開(kāi)始位置

/**
 * 設(shè)置拖拽開(kāi)始位置
 *
 * @param dragStartPosition 開(kāi)始位置
 */
public void setDragStartPosition(int dragStartPosition) {
    mDragStartPosition = dragStartPosition;
}

private int mDragEndPosition; //能夠拖拽的結(jié)束位置
private boolean mDragEndPositionFlag; //是否設(shè)置了拖拽結(jié)束位置

/**
 * 設(shè)置拖拽結(jié)束的位置
 *
 * @param dragEndPosition 結(jié)束位置
 */
public void setDragEndPosition(int dragEndPosition) {
    mDragEndPositionFlag = true;
    mDragEndPosition = dragEndPosition;
}

一般HeaderView和FooterView是不能移動(dòng)的，但是在RecyclerView是沒(méi)有這個(gè)概念的，有了上面這個(gè)接口就可以將HeaderView和FooterView限制在可移動(dòng)的位置之外

至此所有問(wèn)題完美解決，最后附上全部代碼

public interface ItemMoveHelperApi { 
   /**
     * Item 切換位置
     *
     * @param fromPosition 開(kāi)始位置
     * @param toPosition   結(jié)束位置
     */
    void onItemMoved(int fromPosition, int toPosition);

   /**
    * 開(kāi)始移動(dòng)
    */
   void onMoveStart();

   /**
    * 停止移動(dòng)
    */
   void onMoveEnd();
}

public class ItemMoveCallBackImpl extends ItemTouchHelper.Callback {

    private ItemMoveHelperApi mHelperApi;//移動(dòng)API的回調(diào)接口

    private boolean mDragEnabled = true; //是否能夠通過(guò)長(zhǎng)按切換位置

    private int mDragStartPosition; //能夠拖拽的開(kāi)始位置

    private int mDragEndPosition; //能夠拖拽的結(jié)束位置

    private boolean mDragEndPositionFlag; //是否設(shè)置了拖拽結(jié)束位置

    public ItemMoveCallBackImpl(ItemMoveHelperApi helperApi) {
        this.mHelperApi = helperApi;
    }

    @Override
    public boolean isLongPressDragEnabled() {
        return mDragEnabled ;
    }

    @Override
    public boolean isItemViewSwipeEnabled() {
        return false;
    }

    @Override
    public int getMovementFlags(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
        int dragFlags = ItemTouchHelper.UP | ItemTouchHelper.DOWN;
        int swipeFlags = ItemTouchHelper.START | ItemTouchHelper.END;
        return makeMovementFlags(dragFlags, swipeFlags);
    }

    @Override
    public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
        if (mHelperApi != null) {
            mHelperApi.onItemMoved(viewHolder.getAdapterPosition(), target.getAdapterPosition());
        }
        return true;
    }

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

   @Override
   public void onSelectedChanged(RecyclerView.ViewHolder viewHolder, int actionState) {
       if (mHelperApi == null) {
           super.onSelectedChanged(viewHolder, actionState);
           return;
       }
       if (viewHolder == null) {
           mHelperApi.onMoveEnd();
       } else {
           mHelperApi.onMoveStart();
       }
       super.onSelectedChanged(viewHolder, actionState);
   }

   @Override
   public void onChildDraw(Canvas c, RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState, boolean isCurrentlyActive) {
       dY = getLimitedDy(recyclerView, viewHolder, dY);
       super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive);
   }
   /**
    * 根據(jù)方向和條件獲取限制在RecyclerView內(nèi)部的DY值
    *
    * @param recyclerView 列表
    * @param viewHolder   drag的ViewHolder
    * @param dY           限制前的DY值
    * @return 限制后的DY值
    */
   private float getLimitedDy(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, float dY) {
    RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
       if (!(layoutManager instanceof LinearLayoutManager)
               || ((LinearLayoutManager) layoutManager).getOrientation() != OrientationHelper.VERTICAL) {
           return dY;
       }
       int position = viewHolder.getLayoutPosition();
       mDragEndPosition = mDragEndPositionFlag ?
               mDragStartPosition : recyclerView.getAdapter().getItemCount() - 1;
       if (position == mDragStartPosition) {
           return dY < 0 ? 0 : dY;
       } else if (position == mDragEndPosition) {
           return dY > 0 ? 0 : dY;
       }
       return dY;
   }
   /**
    * 設(shè)置能否拖拽
    *
    * @param dragEnabled 拖拽標(biāo)記
    */
   public void setDragEnabled(boolean dragEnabled) {
       mDragEnabled = dragEnabled;
   }
   /**
    * 設(shè)置拖拽開(kāi)始位置
    *
    * @param dragStartPosition 開(kāi)始位置
    */
   public void setDragStartPosition(int dragStartPosition) {
       mDragStartPosition = dragStartPosition;
   }
   /**
    * 設(shè)置拖拽結(jié)束的位置
    *
    * @param dragEndPosition 結(jié)束位置
    */
   public void setDragEndPosition(int dragEndPosition) {
       mDragEndPositionFlag = true;
       mDragEndPosition = dragEndPosition;
   }
   /**
    * 清除固定結(jié)束位置的標(biāo)記
    */
   public void clearDragEndPosition() {
       mDragEndPositionFlag = false;
   }
}