Android RecycleView如何实现Item拖拽效果

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基于公司产品的优化需求，其中一个需求涉及到RecycleView的拖拽，以及拖拽后item位置的持久化，目的是可以用户自定义界面偏好，并在用户下次进入本界面后，之前设置的偏好仍然有效。我写了一个小Demo用作演示效果。

先看效果（只看效果，不看颜值）

步骤1、建接口文件ItemTouchHelperViewHolder，该接口文件中描述的是选中和放开当前Item调用的方法。

public interface ItemTouchHelperViewHolder {    void onItemSelected(); //选中item    void onItemCleared();//放开item}

步骤2、写Item得ViewHolder的类，该类需要继承RecyclerView.ViewHolder类，同时要实现步骤中的接口。

public class ItemViewHolder extends RecyclerView.ViewHolder implements ItemTouchHelperViewHolder {     private TextView tvName;     public TextView getTvName() {        return tvName;    }     public void setTvName(TextView tvName) {        this.tvName = tvName;    }     public ItemViewHolder(@NonNull View itemView) {        super(itemView);        tvName = itemView.findViewById(R.id.tv_item_name);    }     @Override    public void onItemSelected() {        tvName.setBackgroundColor(Color.GRAY);    }     @Override    public void onItemCleared() {        tvName.setBackgroundColor(Color.YELLOW);}}

步骤3、建立接口文件ItemTouchHelperAdapter，该文件中描写的是移动RecycleView的Item时会调用的方法。

public interface ItemTouchHelperAdapter {    void onItemMove(int fromPosition,int toPosition);}

步骤4、实现一个适配器，继承RecyclerView.Adapter，同时实现步骤3的接口。

public class RecyclerGridAdapter extends RecyclerView.Adapter implements ItemTouchHelperAdapter {    private ArrayList localDataSet;    private SharedPreferences sp;    private SharedPreferences.Editor spEditor;    final static  String SAVE_KEY = "star_sort";    final static String USER_PREFERENCE = "user_preference";    private Context context;     public RecyclerGridAdapter(ArrayList dataSet,Context context) {        String defaultStr = dataSet.toString();        if(context != null){            this.context = context;            sp = context.getSharedPreferences(USER_PREFERENCE,Context.MODE_PRIVATE);            spEditor = sp.edit();            String saveString = sp.getString(SAVE_KEY,defaultStr);            //考虑，若要更改数据源，需要怎么实现 todo            String[] splitStr = saveString.replace("[","").replace("]","").replace(" ","").split(",");            localDataSet = new ArrayList();            localDataSet.addAll(Arrays.asList(splitStr));        }    }      @NonNull    @Override    public ItemViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_view_holder_layout, parent, false);        return new ItemViewHolder(view);    }     @Override    public void onBindViewHolder(@NonNull ItemViewHolder holder, int position) {        holder.getTvName().setText(localDataSet.get(position));    }     @Override    public int getItemCount() {        return localDataSet.size();    }     @Override    public void onItemMove(int fromPosition, int toPosition) {        String prve = localDataSet.remove(fromPosition);        if((toPosition > fromPosition) && (localDataSet.size() <= toPosition)){            //将当前item移至最后一位            localDataSet.add(prve);        }else{            localDataSet.add(toPosition, prve);        }        notifyItemMoved(fromPosition, toPosition);        spEditor.putString(SAVE_KEY,localDataSet.toString());        spEditor.apply();    }}

步骤5、实现ItemTouchHelper.Callback接口，至于什么是ItemTouchHelper，网上一查很多解释，我这不做阐述了。

public class SimpleItemTouchHelperCallback extends ItemTouchHelper.Callback {     private ItemTouchHelperAdapter adapter;     public SimpleItemTouchHelperCallback(ItemTouchHelperAdapter adapter) {        this.adapter = adapter;    }     @Override    public int getMovementFlags(@NonNull RecyclerView recyclerView, @NonNull RecyclerView.ViewHolder viewHolder) {        final int dragFlags = ItemTouchHelper.UP | ItemTouchHelper.DOWN | ItemTouchHelper.LEFT | ItemTouchHelper.RIGHT;        return makeFlag(ItemTouchHelper.ACTION_STATE_DRAG, dragFlags);    }     @Override    public boolean onMove(@NonNull RecyclerView recyclerView, @NonNull RecyclerView.ViewHolder viewHolder, @NonNull RecyclerView.ViewHolder target) {        if (viewHolder.getItemViewType() != target.getItemViewType()) {            return false;        }        adapter.onItemMove(viewHolder.getAdapterPosition(), target.getAdapterPosition());        return true;    }     @Override    public void onSwiped(@NonNull RecyclerView.ViewHolder viewHolder, int direction) {     }      @Override    public void onSelectedChanged(@Nullable RecyclerView.ViewHolder viewHolder, int actionState) {        if (actionState == ItemTouchHelper.ACTION_STATE_DRAG) {            ItemTouchHelperViewHolder itemTouchHelperViewHolder = (ItemTouchHelperViewHolder) viewHolder;            itemTouchHelperViewHolder.onItemSelected();        }        super.onSelectedChanged(viewHolder, actionState);    }     @Override    public void clearView(@NonNull RecyclerView recyclerView, @NonNull RecyclerView.ViewHolder viewHolder) {        super.clearView(recyclerView, viewHolder);        ItemViewHolder itemViewHolder = (ItemViewHolder) viewHolder;        itemViewHolder.onItemCleared();    }}

步骤6，现在就可以调用啦，基于步骤5实现的ItemToucherHelper.Callback实例构建ItemTouchHelper实例，然后attach给RecycleView就好啦。

class MainActivity : AppCompatActivity() {    override fun onCreate(savedInstanceState: Bundle?) {        super.onCreate(savedInstanceState)        val viewBinding = DataBindingUtil.setContentView(this,R.layout.activity_main)        with(viewBinding){            var data = ArrayList()            var index = 10            while (index-- >0){                data.add(index.toString())            }            var adapter = RecyclerGridAdapter(data,this@MainActivity)            recycleTest.layoutManager = GridLayoutManager(this@MainActivity,4)            recycleTest.adapter = adapter            var callback = SimpleItemTouchHelperCallback(adapter)            var itemTouchHelper = ItemTouchHelper(callback)            itemTouchHelper.attachToRecyclerView(recycleTest)        }    }}

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