ViewPager 上的 SetCurrentItem 导致异常

Question

我遇到了一个奇怪的问题，在过去三个小时内无法解决。所以我需要帮助。我正在创建一个类似于 Play Music 中的 PlayerUI。 Activity 有一个 ViewPager 并根据 PlayLists 中的曲目准备片段列表，并允许用户使用片段 UI 上的滑动或“下一个”和“上一个”按钮从一个曲目切换到另一个曲目。

滑动功能一切正常。在我的上一个和下一个按钮中，我在 viewPager 上调用 setCurrentItem 并根据来自 viewPager 的 getCurrentItem 的值增加或减少计数。问题是下一个效果很好，但是当我在最后一个项目之前点击下一个项目时会引发异常。与上一个按钮相同。它工作得很好，但是当我在第二个片段上单击上一个时它会引发异常。

这是我的错误日志::

java.lang.NullPointerException: Attempt to read from field 'int android.view.View.mViewFlags' on a null object reference
        at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:3366)
        at android.view.View.updateDisplayListIfDirty(View.java:14127)
        at android.view.View.getDisplayList(View.java:14189)
        at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:3389)
        at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:3368)
        at android.view.View.updateDisplayListIfDirty(View.java:14127)
        at android.view.View.getDisplayList(View.java:14189)
        at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:3389)
        at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:3368)
        at android.view.View.updateDisplayListIfDirty(View.java:14127)
        at android.view.View.getDisplayList(View.java:14189)
        at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:3389)
        at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:3368)
        at android.view.View.updateDisplayListIfDirty(View.java:14127)
        at android.view.View.getDisplayList(View.java:14189)
        at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:3389)
        at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:3368)
        at android.view.View.updateDisplayListIfDirty(View.java:14127)
        at android.view.View.getDisplayList(View.java:14189)
        at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:3389)
        at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:3368)
        at android.view.View.updateDisplayListIfDirty(View.java:14127)
        at android.view.View.getDisplayList(View.java:14189)
        at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:3389)
        at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:3368)
        at android.view.View.updateDisplayListIfDirty(View.java:14127)
        at android.view.View.getDisplayList(View.java:14189)
        at android.view.ThreadedRenderer.updateViewTreeDisplayList(ThreadedRenderer.java:273)
        at android.view.ThreadedRenderer.updateRootDisplayList(ThreadedRenderer.java:279)
        at android.view.ThreadedRenderer.draw(ThreadedRenderer.java:318)
        at android.view.ViewRootImpl.draw(ViewRootImpl.java:2530)
        at android.view.ViewRootImpl.performDraw(ViewRootImpl.java:2352)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1982)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1061)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5885)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
        at android.view.Choreographer.doCallbacks(Choreographer.java:580)
        at android.view.Choreographer.doFrame(Choreographer.java:550)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

这是我的 Activity 的代码，Fragments 是我在视图寻呼机上设置的片段列表

ViewPagerAdapter adapter = new       ViewPagerAdapter(getSupportFragmentManager(),fragments);
adapter.setTrackList(trackList);
viewPager.setAdapter(adapter);
viewPager.setOffscreenPageLimit(0);
viewPager.setCurrentItem(currentTrack.getPosition());

当用户单击 Next 和 Previous 按钮（位于片段中）时，我只需执行以下操作之一

((PlayerActivity)getActivity()).viewPager.setCurrentItem(((PlayerActivity)getActivity()).viewPager.getCurrentItem()+1);

((PlayerActivity)getActivity()).viewPager.setCurrentItem(((PlayerActivity)getActivity()).viewPager.getCurrentItem()-1);

可能是什么问题？

Answer 1

Runnable dirtyHack = new Runnable() {
        @Override
        public void run() {
            pager.setCurrentItem(2, 0);
        }
    };
    Handler handler = new Handler();
    handler.postDelayed(dirtyHack, 100);

...

可能布局还没有膨胀，所以我必须等待或获得NPE。请参考以下链接

https://code.google.com/p/android/issues/detail?id=75309

Android ViewPager setCurrentItem not working after onResume

setCurrentItem after setting adapter on GridViewPager return NPE

Answer 2

您收到此错误是因为您超出了 ViewPager 中的片段数量，当您在最后一个项目中并点击下一步按钮时，您应该设置 currentItem(0)