android - 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。我找到了两个关于如何做到这一点的建议，但都没有给我结果。这些建议是setting the orientation of the LayoutManager to VERTICAL和/或 adding the android:scrollbars="vertical" attribute to the RecyclerView 。第一个方法不显示滚动条，第二个方法使应用程序崩溃，并出现 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)

这是我用于初始化RecyclerView的代码，只要我没有属性android:scrollbars="vertical"，它就可以正常工作，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));

我做错了什么吗？有没有更简单的方法来获取 RecyclerView 的滚动条？

---

这是 XML，只是 FrameLayout 中的一个简单的 RecyclerView:

<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，但仍然不是我希望的结果。原来是因为我正在初始化 RecyclerView之后onCreate() ，因此内容 View 将跳过 RecyclerView因为没有连接适配器。然后当 RecyclerView尝试调用 LayoutManager.canScrollVertically() ，因为属性 android:scrollbars="vertical" ，它得到 NullPointerException因为没有LayoutManager附后。

解决方案是初始化RecyclerView在onCreate()为空 Adapter ，然后交换 Adapter当我收到数据时。像这样的事情:

@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);
}

这将启用 android:scrollbars="vertical" 设置的滚动条，但我希望有一个用户可以与之交互的持久滚动条，而不仅仅是一个视觉指示器。