在后台丢失 MainActivity

Question

每次应用程序进入后台，RAM开始填满，应用程序开始丢失变量（如果我没有错，这是Android的正常行为，它会丢失变量的实例）

当我从后台恢复应用程序时，应用程序崩溃，因为我失去了 MainActivity。

如果根本没有活动，是否有办法重新启动此活动？ 是一个模棱两可的问题，但我发现了这个问题，但我认为这不是最佳选择

ActivityManager mngr = (ActivityManager) getSystemService( ACTIVITY_SERVICE );

List<ActivityManager.RunningTaskInfo> taskList = mngr.getRunningTasks(10);

if(taskList.get(0).numActivities == 1 &&
   taskList.get(0).topActivity.getClassName().equals(this.getClass().getName())) {
    Log.i(TAG, "This is last activity in the stack");
}

我失去了完整的活动，我可以得到活动

@Override
public void onAttach(Activity activity) {
    LocalNotification.registerReceiver(reloadReceiver, LocalNotification.RELOAD_CARTMG);
    try {
        mListener = (OnFragmentInteractionListener) activity;
        MainActivity ma = (MainActivity)activity; //THIS IS LOOSE
        ma.openMyList = false;
        ma.openMyAddress = false;
        ma.hideSearchButton();
    } catch (ClassCastException e) {
        throw new ClassCastException(activity.toString() + " must implement OnFragmentInteractionListener");
    }

    super.onAttach(activity);
}

这是应用内的日志

com.XXXX.mg E/Uncaught Exception detected in thread {}: Unable to start activity ComponentInfo{com.XXXX.mg/com.XXXX.view.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.XXXX.view.MainActivity.showShoppingCartIcon()' on a null object reference
                                                       java.lang.RuntimeException: Unable to start activity ComponentInfo{com.XXXX.mg/com.XXXX.view.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.XXXX.view.MainActivity.showShoppingCartIcon()' on a null object reference
                                                           at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2426)
                                                           at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490)
                                                           at android.app.ActivityThread.-wrap11(ActivityThread.java)
                                                           at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354)
                                                           at android.os.Handler.dispatchMessage(Handler.java:102)
                                                           at android.os.Looper.loop(Looper.java:148)
                                                           at android.app.ActivityThread.main(ActivityThread.java:5443)
                                                           at java.lang.reflect.Method.invoke(Native Method)
                                                           at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
                                                           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
                                                        Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.XXXX.view.MainActivity.showShoppingCartIcon()' on a null object reference
                                                           at com.XXXX.view.shoppingCart.ShoppingCartFragment.onDetach(ShoppingCartFragment.java:411)
                                                           at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1202)
                                                           at android.support.v4.app.FragmentManagerImpl.removeFragment(FragmentManager.java:1349)
                                                           at android.support.v4.app.BackStackRecord.popFromBackStack(BackStackRecord.java:915)
                                                           at android.support.v4.app.FragmentManagerImpl.popBackStackState(FragmentManager.java:1722)
                                                           at android.support.v4.app.FragmentManagerImpl$3.run(FragmentManager.java:593)
                                                           at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1617)
                                                           at android.support.v4.app.FragmentController.execPendingActions(FragmentController.java:339)
                                                           at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:602)
                                                           at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1260)
                                                           at android.app.Activity.performStart(Activity.java:6261)
                                                           at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2389)
                                                           at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490) 
                                                           at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                           at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354) 
                                                           at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                           at android.os.Looper.loop(Looper.java:148) 
                                                           at android.app.ActivityThread.main(ActivityThread.java:5443) 
                                                           at java.lang.reflect.Method.invoke(Native Method) 
                                                           at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728) 
                                                           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) 

Answer 1

我不确定你的应用程序是否因为失去 MainActivity 而崩溃，你应该在编码时期待这种行为，像@pablobu 建议的那样使用 onSaveInstanceState 是正确的方法。

我们可以查看日志吗？如果您丢失实例的状态很好，您应该保存这些状态并恢复它们Recreating an Activity 是因为碎片吗？你也可以保存这些，Handling Fragments Tutorial 这是一本好书。

请您给我们您的 logcat 输出，这可能对我们了解实际情况有很大帮助。

编辑

通过您的 Logcat 编辑，尝试查看片段生命周期，onAttach 比 onActivityCreated() 先执行，因此如果您的 Activity 已经销毁，您的 onAttach 方法将没有您的 Activity，您应该等到您的 Activity 完成重新创建。

此外，你有你的

super.onAttach(activity);

在代码块的末尾，放在代码块之前。

/**
This method was deprecated in API level 23.
Use onAttach(Context) instead.
*/
@Override
public void onAttach(Context context) { 
super.onAttach(context);
MainActivity mainActivity;
if (context instanceof Activity) {
    mainActivity = (MainActivity) context;
    try {
        mListener = (OnFragmentInteractionListener) mainActivity;
        /**
        Since you are getting a reference and accesing attributes you should be careful
        with NullPointerException, check if not null first.
        Or better yet refactor a little your code using an interface to handle this behaviour or use
        the one you already created and just tell the activity what to do.

        */
        /*if(mainActivity != null) {
            ma.openMyList = false;
            ma.openMyAddress = false;
            ma.hideSearchButton();
        }*/

    } catch (ClassCastException e) {
        throw new      ClassCastException(mainActivity.getClass().getSimpleName() + " must implement OnFragmentInteractionListener");
    }
}
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
//since your Listener is global use mListener behaviour that I suggested here.
if (mListener != null){
        mListener.openListOfSomething(false);
        mListener.hideSearch();
    }
}

Answer 2

您可以使用 Bundle savedInstatceState，它在 OnCreate 和类似方法中给出：

private String aVariable;

@Override
public void onCreate (Bundle savedInstantceState) {
   super.onCreate(savedInstanceState);
   // Get the variable from savedInstanceState, if it isn't empty.
   // If it is empty, it could be the first Activity run.
   try { 
      aVariable = savedInstanceState.get("Key");
   } catch (NullPointerException e) {
      aVariable = "Baum";
   }
}

@Override
public void onSaveInstanceState(Bundle outState){
    super.onSaveInstanceState(outState);
    outState.putString(aVariable, "Key");
}

这些方法在第一次运行时将变量“aVariable”设置为“Baum” （NullPointerException，因为“Key”没有字符串）。 稍后，当 Activity 即将被销毁时，会调用 onSaveInstanceState。在那里，您将变量（aVariable）保存在 Key（“Key”）下。