带滚动条的RecyclerView

Question

我正在尝试向我的RecyclerView添加一个简单的滚动条。我已经找到了两条关于如何做到这一点的建议，但都没有给出结果。这些建议是setting the orientation of the LayoutManager to VERTICAL和/或adding the android:scrollbars="vertical" attribute to the RecyclerView。第一种方法不显示滚动条，而第二崩溃的应用程序，具有NullPointerException：

12-14 16:39:09.392 31768-31768/com.example.app E/AndroidRuntime: FATAL EXCEPTION: main 
     Process: com.example.app, PID: 31768 
     java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.support.v7.widget.RecyclerView$LayoutManager.canScrollVertically()' on a null object reference 
      at android.support.v7.widget.RecyclerView.computeVerticalScrollRange(RecyclerView.java:1654) 
      at android.view.View.onDrawScrollBars(View.java:13924) 
      at android.view.View.onDrawForeground(View.java:17657) 
      at android.view.View.draw(View.java:16189) 
      at android.support.v7.widget.RecyclerView.draw(RecyclerView.java:3097) 
      at android.view.View.updateDisplayListIfDirty(View.java:15174) 
      at android.view.View.draw(View.java:15948) 
      at android.view.ViewGroup.drawChild(ViewGroup.java:3609) 
      at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3399) 
      at android.view.View.updateDisplayListIfDirty(View.java:15169) 
      at android.view.View.draw(View.java:15948) 
      at android.view.ViewGroup.drawChild(ViewGroup.java:3609) 
      at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3399) 
      at android.view.View.updateDisplayListIfDirty(View.java:15169) 
      at android.view.View.draw(View.java:15948) 
      at android.view.ViewGroup.drawChild(ViewGroup.java:3609) 
      at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3399) 
      at android.view.View.updateDisplayListIfDirty(View.java:15169) 
      at android.view.View.draw(View.java:15948) 
      at android.view.ViewGroup.drawChild(ViewGroup.java:3609) 
      at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3399) 
      at android.view.View.updateDisplayListIfDirty(View.java:15169) 
      at android.view.View.draw(View.java:15948) 
      at android.view.ViewGroup.drawChild(ViewGroup.java:3609) 
      at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3399) 
      at android.view.View.updateDisplayListIfDirty(View.java:15169) 
      at android.view.View.draw(View.java:15948) 
      at android.view.ViewGroup.drawChild(ViewGroup.java:3609) 
      at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3399) 
      at android.view.View.draw(View.java:16181) 
      at com.android.internal.policy.PhoneWindow$DecorView.draw(PhoneWindow.java:2690) 
      at android.view.View.updateDisplayListIfDirty(View.java:15174) 
      at android.view.ThreadedRenderer.updateViewTreeDisplayList(ThreadedRenderer.java:281) 
      at android.view.ThreadedRenderer.updateRootDisplayList(ThreadedRenderer.java:287) 
      at android.view.ThreadedRenderer.draw(ThreadedRenderer.java:322) 
      at android.view.ViewRootImpl.draw(ViewRootImpl.java:2615) 
      at android.view.ViewRootImpl.performDraw(ViewRootImpl.java:2434) 
      at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2067) 
      at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107) 
      at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013) 
      at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858) 
      at android.view.Choreographer.doCallbacks(Choreographer.java:670) 
      at android.view.Choreographer.doFrame(Choreographer.java:606) 
      at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844) 
      at android.os.Handler.handleCallback(Handler.java:739) 
      at android.os.Handler.dispatchMessage(Handler.java:95) 
      at android.os.Looper.loop(Looper.java:148) 
      at android.app.ActivityThread.main(ActivityThread.java:5417) 
      at java.lang.reflect.Method.invoke(Native Method) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

这是我初始化RecyclerView代码，只要我没有属性工作正常android:scrollbars="vertical" ，但不显示滚动条：

LinearLayoutManager layoutManager = new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false); 
layoutManager.setSmoothScrollbarEnabled(true); 

RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler); 
recyclerView.setLayoutManager(layoutManager); 
recyclerView.setHasFixedSize(true); 
recyclerView.addItemDecoration(new DividerDecoration(this)); 
recyclerView.setAdapter(new MyAdapter(dataset)); 

我做错了什么？是否有一种更简单的方法来获取RecyclerView的滚动条？

---

这是XML，只是一个简单的RecyclerView在FrameLayout：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    android:layout_width="match_parent" android:layout_height="match_parent"> 

     <android.support.v7.widget.RecyclerView android:id="@+id/recycler" 
      android:layout_width="match_parent" android:layout_height="match_parent"> 
      <!--android:scrollbars="vertical"--> 
</FrameLayout> 

Answer 1

我发现了问题，虽然还是没有我所期待的结果。原因是我在onCreate()之后初始化RecyclerView，因此内容视图会跳过RecyclerView，因为没有连接适配器。然后，当RecyclerView试图调用LayoutManager.canScrollVertically()，因为属性android:scrollbars="vertical"的，它得到了NullPointerException因为没有LayoutManager连接。

的解决方案是使用空Adapter来初始化onCreate()的RecyclerView，然后交换Adapter当我接收数据。事情是这样的：

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_recycler); 

    mRecycler = (RecyclerView) findViewById(R.id.recycler); 

    mRecycler.setLayoutManager(new LinearLayoutManager(this)); 
    mRecycler.setHasFixedSize(true); 
    mRecycler.addItemDecoration(new DividerDecoration(this)); 
    mRecycler.addAdapter(new MyAdapter(null)); 

    new MyTask(this).execute; 
} 

@Override 
protected void onPostExecute(ArrayList<Data> dataset) { 
    mRecycler.swapAdapter(new MyAdapter(dataset), false); 
} 

这使得滚动条与android:scrollbars="vertical"设置，但我希望的是持续的滚动条，用户可以用，而不仅仅是一个视觉指示器交互。