LinearLayoutManager prepareForDrop未触发

Question

我试图抓住Recyclerview项目在拖放操作之后 >时抓住这一点，例如读出坐标。

认为prepareForDrop LinearLayoutManager事件对你来说是正确的：

public class OverviewActivity extends AppCompatActivity implements OnStartDragListener {

private RecyclerView recyclerView;
private RecyclerAdapter myAdapter;
private LinearLayoutManager linearLayoutManager;
private ItemTouchHelper mItemTouchHelper;

private dbhelper db;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_overview);

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
    linearLayoutManager = new LinearLayoutManager(this) {
        @Override
        public void prepareForDrop(View view, View target, int x, int y) {
            // do something
            return;
        }
    };
    recyclerView.setLayoutManager(linearLayoutManager);

    myAdapter = new RecyclerAdapter(this, this);
    recyclerView.setAdapter(myAdapter);
    myAdapter.setOnScrollToListener(new OnScrollToListener() {
        @Override
        public void scrollTo(int position) {
            recyclerView.scrollToPosition(position);
        }
    });

    ItemTouchHelper.Callback callback = new SimpleItemTouchHelperCallback(myAdapter);
    mItemTouchHelper = new ItemTouchHelper(callback);
    mItemTouchHelper.attachToRecyclerView(recyclerView);

    db = new dbhelper(this);
    db.open();

    myAdapter.addRootCards(3);
}
.......

创建LinearLayoutManager并将其附加到RecyclerView和Override for prepareForDrop事件已实施。

问题是：从不调用prepareForDrop（），但为什么，错误是什么？

Answer 1

根据源代码，如果LayoutManager instanceOf是ViewDropHandler，则只从onMoved调用prepareForDrop（）。在你的情况下，它是。这让我相信onMoved（）没有被调用。根据它的javadocs onMoved（）仅在onMove（）返回true时被调用。您还没有展示SimpleItemTouchHelperCallback的实现，但我的猜测是它的onMove（）实现返回false。