Android系统.片段getActivity()有时会返回null

Question

在开发者控制台错误报告中,有时我会看到有NPE问题的报告.我不明白我的代码有什么问题.在模拟器上,我的设备应用程序在没有forcecloses的情况下运行良好,但是当调用getActivity()方法时,一些用户在片段类中获得NullPointerException.

活动

pulic class MyActivity extends FragmentActivity{

    private ViewPager pager; 
    private TitlePageIndicator indicator;
    private TabsAdapter adapter;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        pager = (ViewPager) findViewById(R.id.pager);
        indicator = (TitlePageIndicator) findViewById(R.id.indicator);
        adapter = new TabsAdapter(getSupportFragmentManager(), false);

        adapter.addFragment(new FirstFragment());
        adapter.addFragment(new SecondFragment());
        indicator.notifyDataSetChanged();
        adapter.notifyDataSetChanged();

        // push first task
        FirstTask firstTask = new FirstTask(MyActivity.this);
        // set first fragment as listener
        firstTask.setTaskListener((TaskListener) adapter.getItem(0));
        firstTask.execute();
    }

    indicator.setOnPageChangeListener(new ViewPager.OnPageChangeListener()  {
        @Override
        public void onPageSelected(int position) {
            Fragment currentFragment = adapter.getItem(position);
            ((Taskable) currentFragment).executeTask();
        }

        @Override
        public void onPageScrolled(int i, float v, int i1) {}

        @Override
        public void onPageScrollStateChanged(int i) {}
    });
}

AsyncTask类

public class FirstTask extends AsyncTask{

    private TaskListener taskListener;

    ...

    @Override
    protected void onPostExecute(T result) {
        ... 
        taskListener.onTaskComplete(result);
    }   
}

片段类

public class FirstFragment extends Fragment immplements Taskable, TaskListener{

    public FirstFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.first_view, container, false);
    }

    @Override
    public void executeTask() {
        FirstTask firstTask = new FirstTask(MyActivity.this);
        firstTask.setTaskListener(this);
        firstTask.execute();
    }

    @Override
    public void onTaskComplete(T result) {
        // NPE is here 
        Resources res = getActivity().getResources();
        ...
    }
}

当应用程序从后台恢复时,可能会发生此错误.在这种情况下我应该如何妥善处理这种情况？

Answer 1

似乎我找到了解决问题的方法.这里和这里给出了非常好的解释.这是我的例子:

pulic class MyActivity extends FragmentActivity{

private ViewPager pager; 
private TitlePageIndicator indicator;
private TabsAdapter adapter;
private Bundle savedInstanceState;

 @Override
public void onCreate(Bundle savedInstanceState) {

    .... 
    this.savedInstanceState = savedInstanceState;
    pager = (ViewPager) findViewById(R.id.pager);;
    indicator = (TitlePageIndicator) findViewById(R.id.indicator);
    adapter = new TabsAdapter(getSupportFragmentManager(), false);

    if (savedInstanceState == null){    
        adapter.addFragment(new FirstFragment());
        adapter.addFragment(new SecondFragment());
    }else{
        Integer  count  = savedInstanceState.getInt("tabsCount");
        String[] titles = savedInstanceState.getStringArray("titles");
        for (int i = 0; i < count; i++){
            adapter.addFragment(getFragment(i), titles[i]);
        }
    }


    indicator.notifyDataSetChanged();
    adapter.notifyDataSetChanged();

    // push first task
    FirstTask firstTask = new FirstTask(MyActivity.this);
    // set first fragment as listener
    firstTask.setTaskListener((TaskListener) getFragment(0));
    firstTask.execute();

}

private Fragment getFragment(int position){
     return savedInstanceState == null ? adapter.getItem(position) : getSupportFragmentManager().findFragmentByTag(getFragmentTag(position));
}

private String getFragmentTag(int position) {
    return "android:switcher:" + R.id.pager + ":" + position;
}

 @Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt("tabsCount",      adapter.getCount());
    outState.putStringArray("titles", adapter.getTitles().toArray(new String[0]));
}

 indicator.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
        @Override
        public void onPageSelected(int position) {
            Fragment currentFragment = adapter.getItem(position);
            ((Taskable) currentFragment).executeTask();
        }

        @Override
        public void onPageScrolled(int i, float v, int i1) {}

        @Override
        public void onPageScrollStateChanged(int i) {}
 });

此代码中的主要思想是,在正常运行应用程序时,您将创建新片段并将其传递给适配器.当您恢复应用程序时,片段管理器已经拥有此片段的实例,您需要从片段管理器获取它并将其传递给适配器.

UPDATE

此外,在调用getActivity()之前使用片段检查isAdded是一个很好的做法.当片段与活动分离时,这有助于避免空指针异常.例如,活动可能包含推送异步任务的片段.任务完成后,将调用onTaskComplete侦听器.

@Override
public void onTaskComplete(List<Feed> result) {

    progress.setVisibility(View.GONE);
    progress.setIndeterminate(false);
    list.setVisibility(View.VISIBLE);

    if (isAdded()) {

        adapter = new FeedAdapter(getActivity(), R.layout.feed_item, result);
        list.setAdapter(adapter);
        adapter.notifyDataSetChanged();
    }

}

如果我们打开片段,推送任务,然后快速按回以返回上一个活动,当任务完成时,它将尝试通过调用getActivity()方法访问onPostExecute()中的活动.如果活动已经分离并且此检查不存在:

if (isAdded()) 

然后应用程序崩溃.

Answer 2

好的,我知道这个问题实际上已经解决但我决定分享我的解决方案.我为我创建了抽象父类Fragment:

public abstract class ABaseFragment extends Fragment{

    protected IActivityEnabledListener aeListener;

    protected interface IActivityEnabledListener{
        void onActivityEnabled(FragmentActivity activity);
    }

    protected void getAvailableActivity(IActivityEnabledListener listener){
        if (getActivity() == null){
            aeListener = listener;

        } else {
            listener.onActivityEnabled(getActivity());
        }
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

        if (aeListener != null){
            aeListener.onActivityEnabled((FragmentActivity) activity);
            aeListener = null;
        }
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);

        if (aeListener != null){
            aeListener.onActivityEnabled((FragmentActivity) context);
            aeListener = null;
        }
    }
}

正如你所看到的,我已经添加了一个监听器,所以每当我需要获取Fragments Activity而不是标准时getActivity(),我需要调用

 getAvailableActivity(new IActivityEnabledListener() {
        @Override
        public void onActivityEnabled(FragmentActivity activity) {
            // Do manipulations with your activity
        }
    });

Answer 3

最好摆脱这种情况是在onAttach调用时保持活动参考,并在需要的地方使用活动参考,例如

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    mContext = context;
}

@Override
public void onDetach() {
    super.onDetach();
    mContext = null;
}

编辑,因为onAttach(Activity)已折旧,现在onAttach(Context)正在使用

Answer 4

在父Activity中的onStart之前,不要在Fragment中调用需要getActivity()的方法.

private MyFragment myFragment;


public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);

    FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
    myFragment = new MyFragment();

    ft.add(android.R.id.content, youtubeListFragment).commit();

    //Other init calls
    //...
}


@Override
public void onStart()
{
    super.onStart();

    //Call your Fragment functions that uses getActivity()
    myFragment.onPageSelected();
}

Answer 5

我一直在与这种问题作斗争，我想我已经想出了一个可靠的解决方案。

很难确定athis.getActivity()不会返回，特别是如果您正在处理任何类型的网络行为，这使您的代码有足够的时间撤回引用。nullFragmentActivity

在下面的解决方案中，我声明了一个名为ActivityBuffer. 本质上，这class涉及维护对 owning 的可靠引用Activity，并承诺只要有有效引用可用就Runnable在有效Activity上下文中执行s 。如果RunnablesContext可用，s 将立即安排在 UI 线程上执行，否则执行将推迟到Context准备就绪。

/** A class which maintains a list of transactions to occur when Context becomes available. */
public final class ActivityBuffer {

    /** A class which defines operations to execute once there's an available Context. */
    public interface IRunnable {
        /** Executes when there's an available Context. Ideally, will it operate immediately. */
        void run(final Activity pActivity);
    }

    /* Member Variables. */
    private       Activity        mActivity;
    private final List<IRunnable> mRunnables;

    /** Constructor. */
    public ActivityBuffer() {
        // Initialize Member Variables.
        this.mActivity  = null;
        this.mRunnables = new ArrayList<IRunnable>();
    }

    /** Executes the Runnable if there's an available Context. Otherwise, defers execution until it becomes available. */
    public final void safely(final IRunnable pRunnable) {
        // Synchronize along the current instance.
        synchronized(this) {
            // Do we have a context available?
            if(this.isContextAvailable()) {
                // Fetch the Activity.
                final Activity lActivity = this.getActivity();
                // Execute the Runnable along the Activity.
                lActivity.runOnUiThread(new Runnable() { @Override public final void run() { pRunnable.run(lActivity); } });
            }
            else {
                // Buffer the Runnable so that it's ready to receive a valid reference.
                this.getRunnables().add(pRunnable);
            }
        }
    }

    /** Called to inform the ActivityBuffer that there's an available Activity reference. */
    public final void onContextGained(final Activity pActivity) {
        // Synchronize along ourself.
        synchronized(this) {
            // Update the Activity reference.
            this.setActivity(pActivity);
            // Are there any Runnables awaiting execution?
            if(!this.getRunnables().isEmpty()) {
                // Iterate the Runnables.
                for(final IRunnable lRunnable : this.getRunnables()) {
                    // Execute the Runnable on the UI Thread.
                    pActivity.runOnUiThread(new Runnable() { @Override public final void run() {
                        // Execute the Runnable.
                        lRunnable.run(pActivity);
                    } });
                }
                // Empty the Runnables.
                this.getRunnables().clear();
            }
        }
    }

    /** Called to inform the ActivityBuffer that the Context has been lost. */
    public final void onContextLost() {
        // Synchronize along ourself.
        synchronized(this) {
            // Remove the Context reference.
            this.setActivity(null);
        }
    }

    /** Defines whether there's a safe Context available for the ActivityBuffer. */
    public final boolean isContextAvailable() {
        // Synchronize upon ourself.
        synchronized(this) {
            // Return the state of the Activity reference.
            return (this.getActivity() != null);
        }
    }

    /* Getters and Setters. */
    private final void setActivity(final Activity pActivity) {
        this.mActivity = pActivity;
    }

    private final Activity getActivity() {
        return this.mActivity;
    }

    private final List<IRunnable> getRunnables() {
        return this.mRunnables;
    }

}

就其实现而言，我们必须注意应用生命周期方法以与Pawan M上面描述的行为一致：

public class BaseFragment extends Fragment {

    /* Member Variables. */
    private ActivityBuffer mActivityBuffer;

    public BaseFragment() {
        // Implement the Parent.
        super();
        // Allocate the ActivityBuffer.
        this.mActivityBuffer = new ActivityBuffer();
    }

    @Override
    public final void onAttach(final Context pContext) {
        // Handle as usual.
        super.onAttach(pContext);
        // Is the Context an Activity?
        if(pContext instanceof Activity) {
            // Cast Accordingly.
            final Activity lActivity = (Activity)pContext;
            // Inform the ActivityBuffer.
            this.getActivityBuffer().onContextGained(lActivity);
        }
    }

    @Deprecated @Override
    public final void onAttach(final Activity pActivity) {
        // Handle as usual.
        super.onAttach(pActivity);
        // Inform the ActivityBuffer.
        this.getActivityBuffer().onContextGained(pActivity);
    }

    @Override
    public final void onDetach() {
        // Handle as usual.
        super.onDetach();
        // Inform the ActivityBuffer.
        this.getActivityBuffer().onContextLost();
    }

    /* Getters. */
    public final ActivityBuffer getActivityBuffer() {
        return this.mActivityBuffer;
    }

}

最后，在您Fragment扩展BaseFragment到您对调用 不可信的任何区域中getActivity()，只需调用this.getActivityBuffer().safely(...)并ActivityBuffer.IRunnable为任务声明！

void run(final Activity pActivity)然后保证您的内容沿着 UI 线程执行。

的ActivityBuffer则可以使用如下：

this.getActivityBuffer().safely(
  new ActivityBuffer.IRunnable() {
    @Override public final void run(final Activity pActivity) {
       // Do something with guaranteed Context.
    }
  }
);