片段中的 getActivity() 返回 null。父活动中的 FragmentPagerAdapter 问题？答案

【问题标题】：getActivity() in fragment returns null. Issue with FragmentPagerAdapter in parent activity?片段中的 getActivity() 返回 null。父活动中的 FragmentPagerAdapter 问题？
【发布时间】：2015-11-26 17:31:35
【问题描述】：

我有一个包含两个片段的 viewPager 的活动。我遇到了一个问题，在我的片段中调用 getActivity() 在活动被销毁然后重新创建后返回 null（我正在更改系统设置，而我的应用程序在后台暂停，这会导致活动被销毁然后重新创建）。这是我的父活动的样子。

public class MainActivity extends AppCompatActivity {

private CalculatorPagerAdapter mSectionsPagerAdapter;

/**
 * The {@link ViewPager} that will host the section contents.
 */
private ViewPager mViewPager;
private final int HISTORY_PAGE_INDEX = 1;
private final int CALCULATOR_PAGE_INDEX = 0;

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

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

    mSectionsPagerAdapter = new CalculatorPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.container);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    mViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

        }

        /**
         * Refreshes the listView in the history fragment whenever the user moves to it.
         * @param position
         */
        @Override
        public void onPageSelected(int position) {
            CalculatorPagerAdapter fragmentPagerAdapter = (CalculatorPagerAdapter) mViewPager.getAdapter();
            if(position == HISTORY_PAGE_INDEX){
                ((HistoryFragment) fragmentPagerAdapter.getItem(HISTORY_PAGE_INDEX)).refreshHistory();
            }else if(position == CALCULATOR_PAGE_INDEX){
                //((CalculatorFragment) fragmentPagerAdapter.getItem(CALCULATOR_PAGE_INDEX)).refreshCalculator();
            }
        }

        @Override
        public void onPageScrollStateChanged(int state) {

        }
    });

    TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
    tabLayout.setupWithViewPager(mViewPager);

}

@Override
public void onDestroy(){
    super.onDestroy();
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {

    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        Intent intent = new Intent(this, SettingsActivity.class);
        startActivity(intent);
        return true;
    }

    return super.onOptionsItemSelected(item);
}


/**
 * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
 * one of the sections/tabs/pages.
 */
public class CalculatorPagerAdapter extends FragmentPagerAdapter {

    HistoryFragment historyFragment;
    CalculatorFragment calculatorFragment;

    public CalculatorPagerAdapter(FragmentManager fm) {
        super(fm);
        historyFragment = new HistoryFragment();
        calculatorFragment = new CalculatorFragment();
    }


    @Override
    public Fragment getItem(int position) {
        if (position ==0) return calculatorFragment;
        else return historyFragment;
    }

    @Override
    public int getCount() {
        return 2;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        switch (position) {
            case 0:
                return "Calculator";
            case 1:
                return "History";
        }
        return null;
    }
}

}

我注意到活动和片段的生命周期调用按以下顺序发生。

初始创建时（正常）

activity.onCreate
fragment.onAttach
fragment.onCreateView

当我更改触发活动销毁和重新创建的系统设置时（在此处发布）

activity.onDestroy
fragment.onDestroy
fragment.onAttach（不存在父活动？）
activity.onCreate
fragment.onCreateView

发生这种情况后，片段中的 getActivity 将返回 null。我是否在我的父活动中错误地处理了我的片段？

这是由于尝试使用 null getActivity() 而导致的崩溃日志。这是由附加到我的 viewpager 的 OnPageChangeListener 的 onPageSelected 方法触发的。

11-26 12:56:21.739 31769-31769/com.example.keegan.securetipping I/Timeline: Timeline: Activity_idle id: android.os.BinderProxy@d035425 time:1218086194
11-26 12:56:24.864 31769-31769/com.example.keegan.securetipping E/InputEventReceiver: Exception dispatching input event.
11-26 12:56:24.864 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI: Exception in MessageQueue callback: handleReceiveCallback
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.support.v4.app.FragmentActivity.getApplicationContext()' on a null object reference
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at com.example.keegan.securetipping.HistoryFragment.refreshHistory(HistoryFragment.java:88)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at com.example.keegan.securetipping.MainActivity$1.onPageSelected(MainActivity.java:54)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1794)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:569)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:553)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.support.v4.view.ViewPager.onTouchEvent(ViewPager.java:2102)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.View.dispatchTouchEvent(View.java:8547)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2400)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2093)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2372)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1722)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.app.Activity.dispatchTouchEvent(Activity.java:2742)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.support.v7.internal.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:60)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.support.v7.internal.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:60)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2333)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.View.dispatchPointerEvent(View.java:8742)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4136)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4002)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3557)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3610)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3576)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3693)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3584)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3750)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3557)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3610)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3576)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3584)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3557)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:5823)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:5797)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:5768)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:5913)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.os.MessageQueue.nativePollOnce(Native Method)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.os.MessageQueue.next(MessageQueue.java:143)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.os.Looper.loop(Looper.java:195)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at android.app.ActivityThread.main(ActivityThread.java:5373)
11-26 12:56:24.868 31769-31769/com.example.keegan.securetipping E/MessageQueue-JNI:     at java.lang.reflect.Method.invoke(Native Method)
11-26 12:56:24.869 31769-31769/com.example.keegan.securetipping D/AndroidRuntime: Shutting down VM
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime: FATAL EXCEPTION: main
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime: Process: com.example.keegan.securetipping, PID: 31769
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.support.v4.app.FragmentActivity.getApplicationContext()' on a null object reference
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at com.example.keegan.securetipping.HistoryFragment.refreshHistory(HistoryFragment.java:88)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at com.example.keegan.securetipping.MainActivity$1.onPageSelected(MainActivity.java:54)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1794)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:569)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:553)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.support.v4.view.ViewPager.onTouchEvent(ViewPager.java:2102)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.View.dispatchTouchEvent(View.java:8547)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2400)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2093)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2406)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2107)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2372)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1722)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.app.Activity.dispatchTouchEvent(Activity.java:2742)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.support.v7.internal.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:60)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.support.v7.internal.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:60)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2333)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.View.dispatchPointerEvent(View.java:8742)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4136)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4002)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3557)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3610)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3576)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3693)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3584)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3750)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3557)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3610)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3576)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3584)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3557)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:5823)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:5797)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:5768)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:5913)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.os.MessageQueue.nativePollOnce(Native Method)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.os.MessageQueue.next(MessageQueue.java:143)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:195)
11-26 12:56:24.874 31769-31769/com.example.keegan.securetipping E/AndroidRuntime:   at android.app.ActivityThread.main(Act

这是我的 HistoryFragment 类中引发异常的代码位。我只是调用 getApplicationContext 以获取此示例的错误。通常会在我的 FetchHistoryEntries AsyncTask 中抛出错误。

/**
 * Refreshes the listView in the history fragment with data from the history database.
 */
public void refreshHistory(){

    getActivity().getApplicationContext();
    new FetchHistoryEntries(getActivity()).execute();
}

我的历史片段只包含一个 listView。这是它的 XML。

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

<ListView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/history_listview"/>

【问题讨论】：

请发布你的日志
它在第 88 行的历史片段中声明了 NPE，请也发布历史片段和布局。
添加了相关信息。谢谢！
请发布您的信息不足以找出问题所在，这段代码很难预测编译器的流程。
如果我的猜测是正确的，你会在这一行得到 NPE getActivity().getApplicationContext();删除此行后尝试

标签： java android android-fragments

【解决方案1】：

当一个活动被销毁时，您会丢失正在使用的上下文，这就是您的应用程序抛出异常的原因，因为您在销毁抛出空指针异常的活动后调用getActivity()，为避免这种情况，您应该将您的 refreshHistory 更改为以下内容：

public void refreshHistory(){
    if (isAdded()) {
        getActivity().getApplicationContext();
        new FetchHistoryEntries(getActivity()).execute();
    }
}

isAdded()当fragment附加到activity且activity不为null时返回true。

您还应该考虑在您的活动被销毁时取消您的AsyncTask，您可以这样做：首先像这样FetchHistoryEntries task;定义一个与您的AsyncTask相同类型的全局变量第二，像这样执行您的全局任务

task = new FetchHistoryEntries(getActivity()).execute;

最后，在您的 onDestroy 覆盖方法中取消您的任务，如下所示：

if (task != null) {
    task.cancel(true);
}

希望对你有帮助，祝你好运

【讨论】：

也许你的意思是(task = new FetchHistoryEntries(getActivity())).execute();？
我可以避免崩溃，但问题是如果 getActivity() 为空，那么我无法执行我需要执行的操作以使片段功能按预期进行。从 fragmentPagerAdapter 返回的片段未附加到父活动，我不知道为什么。

【解决方案2】：

我实际上是通过切换到一个 FragmentStatePagerAdapter 而不是一个 FragmentPagerAdapter 来解决这个问题的。状态寻呼机适配器可以更轻松地保存和恢复片段状态，这正是我的问题所在。这个要点有一个方便的例子。 https://gist.github.com/nesquena/c715c9b22fb873b1d259

【讨论】：