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仅当我启动此片段（第一次）并同时按下后退按钮时，应用程序才会崩溃。我没有在我的活动中覆盖 onBackPressed() 。代码：
片段类

public class Fragment_View extends Fragment {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View view =  inflater.inflate(R.layout.fragmentview, container, false);
    root_view = view;
    setupExtraViews();
    return view;
}

private void setupExtraViews() {
    if (root_view.findViewById(R.id.someview) != null) {
            Fragment_Top_Bar top_bar = new Fragment_Top_Bar();
        //TODO: app crashes here on the .commit part when this fragment is started and back button is pressed at the same time.
        getChildFragmentManager().beginTransaction().
                add(R.id.someview, top_bar).commit();
    }
}
...

例外是： 

E/InputEventSender﹕ Exception dispatching finished signal.
E/MessageQueue-JNI﹕ Exception in MessageQueue callback: handleReceiveCallback
E/MessageQueue-JNI﹕ java.lang.IllegalStateException: Activity has been destroyed
        at android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:1387)
        at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:636)
        at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:615)
        at    in.com.xxxxxx.main_app.settings.Fragment_Class.setupExtraViews(Fragment_Class.java:45)
        at in.com.xxxxxx.main_app.settings.Fragment_Class.onCreateView(Fragment_Class.java:24)
        at android.support.v4.app.Fragment.performCreateView(Fragment.java:1786)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:947)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1126)
        at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:739)
        at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1489)
        at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:486)
        at android.support.v4.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:501)
        at android.support.v4.app.FragmentActivity.onBackPressed(FragmentActivity.java:176)
        at android.app.Activity.onKeyUp(Activity.java:2274)
        at android.view.KeyEvent.dispatch(KeyEvent.java:2985)
        at android.app.Activity.dispatchKeyEvent(Activity.java:2508)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:2174)
        at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:4505)
        at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4472)
        at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4033)
        at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4087)
        at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4056)
        at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4171)
        at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4064)
        at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4228)
        at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4033)
        at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4087)
        at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4056)
        at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4064)
        at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4033)
        at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4087)
        at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4056)
        at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4204)
        at android.view.ViewRootImpl$ImeInputStage.onFinishedInputEvent(ViewRootImpl.java:4364)
        at     android.view.inputmethod.InputMethodManager$PendingEvent.run(InputMethodManager.java:2257)
        at android.view.inputmethod.InputMethodManager.invokeFinishedInputEventCallback(InputMethodManager.java:1907)
        at android.view.inputmethod.InputMethodManager.finishedInputEvent(InputMethodManager.java:1898)
        at android.view.inputmethod.InputMethodManager$ImeInputEventSender.onInputEventFinished(InputMethodManager.java:2234)
        at android.view.InputEventSender.dispatchInputEventFinished(InputEventSender.java:141)
        at android.os.MessageQueue.nativePollOnce(Native Method)
        at android.os.MessageQueue.next(MessageQueue.java:138)
        at android.os.Looper.loop(Looper.java:123)
        at android.app.ActivityThread.main(ActivityThread.java:5356)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
        at dalvik.system.NativeStart.main(Native Method)

然后这个完全相同的错误再次出现。因此，在创建片段时按下后退按钮时应用程序崩溃，然后此异常出现两次。我认为 .commit() 是异步的。所以我认为，在计划实际提交和计划获得提交之间，活动正在被 onBackPressed 破坏。因此错误。我不确定这是不是真正的原因。所以请帮忙...

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