在Activity和Frgment生命週期中對於資料儲存應該是大部分都能儲存的，像按Home返回到後臺，再切換回來後應該不會出大問題的，但一次記憶體過底把儲存的Activity給Destroy後引起了錯誤，具體是Activity裡Fragment A裡有一個Fragment B, Fragment B 按理是在A的onCreate裡進行網路請求後非同步建立，那麼Activity銷燬後走OnCreate應該不會有什麼問題。

但是呢，Fragment B報空指標。這便引出這個文章。

Activity & Fragment

介紹文章直接有官方網站地址如下：

上面的圖是初學者應該瞭解的，但Android開發一段時間後也應該複習一下。具體情景是DetailActivity裡replace一個framelayout後新增的fragment A, 在fragment A裡再使用相同方式新增 fragment B, 但使用的是fragmentManager 是呼叫 A的getChildFragmentManager()

正常情況下看列印的資料

05-24 17:24:18.584 22965-22965/com.yorkyu.fragmentlifedemo D/DetailActivity: onCreate: 
05-24 17:24:18.586 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onAttach: 
05-24 17:24:18.586 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onCreate: 
05-24 17:24:18.587 22965-22965/com.yorkyu
 
.fragmentlifedemo D/AFragment: onCreateView: 
05-24 17:24:18.622 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onViewCreated: 
05-24 17:24:18.622 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onActivityCreated: 
05-24 17:24:18.622 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onViewStateRestored: 
05
 
-24 17:24:18.622 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onStart: 
05-24 17:24:18.622 22965-22965/com.yorkyu.fragmentlifedemo D/DetailActivity: onStart: 
05-24 17:24:18.623 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onResume: 
05-24 17:24:23.593 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onAttach: 
05-24 17:24:23.594 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onCreate: 
05-24 17:24:23.595 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onCreateView: 
05-24 17:24:23.612 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onViewCreated: 
05-24 17:24:23.612 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onActivityCreated: 
05-24 17:24:23.613 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onViewStateRestored: 
05-24 17:24:23.613 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onStart: 
05-24 17:24:23.613 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onResume: 

這裡按home鍵後

 05-24 17:26:50.408 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onPause: 
05-24 17:26:50.409 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onPause: 
05-24 17:26:50.409 22965-22965/com.yorkyu.fragmentlifedemo D/DetailActivity: onPause: 
05-24 17:26:50.726 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onSaveInstanceState: 
05-24 17:26:50.730 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onSaveInstanceState: 
05-24 17:26:50.730 22965-22965/com.yorkyu.fragmentlifedemo D/DetailActivity: onSaveInstanceState: 
05-24 17:26:50.730 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onStop: 
05-24 17:26:50.730 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onStop: 
05-24 17:26:50.730 22965-22965/com.yorkyu.fragmentlifedemo D/DetailActivity: onStop:

也很好對應上面的圖片了。相同之處是在onPause後呼叫了onSaveInstanceState。官方也介紹瞭如下，

不過，即使您什麼都不做，也不實現 onSaveInstanceState()，Activity 類的 onSaveInstanceState() 預設實現也會恢復部分 Activity 狀態。具體地講，預設實現會為佈局中的每個 View 呼叫相應的 onSaveInstanceState() 方法，讓每個檢視都能提供有關自身的應儲存資訊。Android 框架中幾乎每個小部件都會根據需要實現此方法，以便在重建 Activity 時自動儲存和恢復對 UI 所做的任何可見更改。例如，EditText 小部件儲存使用者輸入的任何文字，CheckBox 小部件儲存複選框的選中或未選中狀態。您只需為想要儲存其狀態的每個小部件提供一個唯一的 ID（通過 android:id 屬性）。如果小部件沒有 ID，則系統無法儲存其狀態。

儘管 onSaveInstanceState() 的預設實現會儲存有關您的Activity UI 的有用資訊，您可能仍需替換它以儲存更多資訊。例如，您可能需要儲存在 Activity 生命週期內發生了變化的成員值（它們可能與 UI 中恢復的值有關聯，但預設情況下系統不會恢復儲存這些 UI 值的成員）。

由於 onSaveInstanceState() 的預設實現有助於儲存 UI 的狀態，因此如果您為了儲存更多狀態資訊而替換該方法，應始終先呼叫 onSaveInstanceState() 的超類實現，然後再執行任何操作。 同樣，如果您替換 onRestoreInstanceState() 方法，也應呼叫它的超類實現，以便預設實現能夠恢復檢視狀態。

注：由於無法保證系統會呼叫 onSaveInstanceState()，因此您只應利用它來記錄 Activity 的瞬態（UI 的狀態）— 切勿使用它來儲存永續性資料，而應使用 onPause() 在使用者離開 Activity 後儲存永續性資料（例如應儲存到資料庫的資料）。

再看下按回的列印

05-24 17:31:42.592 22965-22965/com.yorkyu.fragmentlifedemo D/DetailActivity: onRestart: 
05-24 17:31:42.593 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onStart: 
05-24 17:31:42.593 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onStart: 
05-24 17:31:42.593 22965-22965/com.yorkyu.fragmentlifedemo D/DetailActivity: onStart: 
05-24 17:31:42.594 22965-22965/com.yorkyu.fragmentlifedemo D/AFragment: onResume: 
05-24 17:31:42.595 22965-22965/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onResume: 

執行的生命週期不多，不會走onCreate，而且都沒有銷燬，所以看不是問題。這裡如果開啟開發者選項中的不保留活動, 那麼問題就來了，Activity會Destory

05-24 11:13:03.969 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onPause: 
05-24 11:13:03.970 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onPause: 
05-24 11:13:03.970 12289-12289/com.yorkyu.fragmentlifedemo D/DetailActivity: onPause: 
05-24 11:13:04.911 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onSaveInstanceState: 
05-24 11:13:04.912 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onSaveInstanceState: 
05-24 11:13:04.928 12289-12289/com.yorkyu.fragmentlifedemo D/DetailActivity: onSaveInstanceState: 
05-24 11:13:04.929 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onStop: 
05-24 11:13:04.929 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onStop: 
05-24 11:13:04.929 12289-12289/com.yorkyu.fragmentlifedemo D/DetailActivity: onStop: 
05-24 11:13:04.950 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onDestroyView: 
05-24 11:13:04.951 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onDestroyView: 
05-24 11:13:04.952 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onDestroy: 
05-24 11:13:04.952 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onDetach: 
05-24 11:13:04.953 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onDestroy: 
05-24 11:13:04.953 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onDetach: 
05-24 11:13:04.953 12289-12289/com.yorkyu.fragmentlifedemo D/DetailActivity: onDestroy: 

返回列印結果

  05-24 11:13:31.331 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onAttach: 
05-24 11:13:31.331 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onAttach: 
05-24 11:13:31.332 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onCreate: 
05-24 11:13:31.332 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onCreate: 
05-24 11:13:31.358 12289-12289/com.yorkyu.fragmentlifedemo D/DetailActivity: onCreate: 
05-24 11:13:31.360 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onCreateView: 
05-24 11:13:31.392 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onViewCreated: 
05-24 11:13:31.392 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onActivityCreated: 
05-24 11:13:31.392 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onCreateView: 
05-24 11:13:31.396 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onViewCreated: 
05-24 11:13:31.396 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onActivityCreated: 
05-24 11:13:31.396 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onViewStateRestored: 
05-24 11:13:31.396 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onViewStateRestored: 
05-24 11:13:31.396 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onAttach: 
05-24 11:13:31.396 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onCreate: 
05-24 11:13:31.397 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onDestroyView: 
05-24 11:13:31.397 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onDestroyView: 
05-24 11:13:31.397 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onCreateView: 
05-24 11:13:31.400 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onViewCreated: 
05-24 11:13:31.400 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onActivityCreated: 
05-24 11:13:31.400 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onViewStateRestored: 
05-24 11:13:31.400 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onStart: 
05-24 11:13:31.400 12289-12289/com.yorkyu.fragmentlifedemo D/DetailActivity: onStart: 
05-24 11:13:31.400 12289-12289/com.yorkyu.fragmentlifedemo D/DetailActivity: onRestoreInstanceState: 
05-24 11:13:31.401 12289-12289/com.yorkyu.fragmentlifedemo D/AFragment: onResume: 
05-24 11:13:31.405 12289-12289/com.yorkyu.fragmentlifedemo D/ActivityThreadInjector: clearCachedDrawables.
05-24 11:13:36.367 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onAttach: 
05-24 11:13:36.367 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onCreate: 
05-24 11:13:36.399 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onAttach: 
05-24 11:13:36.399 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onCreate: 
05-24 11:13:36.400 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onCreateView: 
05-24 11:13:36.413 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onViewCreated: 
05-24 11:13:36.413 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onActivityCreated: 
05-24 11:13:36.413 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onViewStateRestored: 
05-24 11:13:36.414 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onStart: 
05-24 11:13:36.414 12289-12289/com.yorkyu.fragmentlifedemo D/BFragment - main - 1: onResume: 

看到fragment B會先onCreate再destoryview，然後非同步方法再new instance 一個新的， 因此產生多個例項，造成記憶體洩露。其實fragment A也同的情況。

解決辦法

onSaveInstanceState裡處理

必須在回撥前通過fragmentManager移除當前頁面的fragment，這樣再走onCreate都會new Instance，這樣也有一個問題就是每次跳轉頁面返回這個頁面後一樣會new Instance，效能不是太好。但解決了非同步動態新增Fragment和Activity及Fragment自動恢復機制引起重複建立。我觀察過這種情況大概在我這個應用裡會造成每次1MB左右的記憶體洩露。

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);

        int backStackEntryCount = mChildFragmentManager.getBackStackEntryCount();
        for (int i = 0; i < backStackEntryCount; i++) {
            mChildFragmentManager.popBackStackImmediate();
        }
        Log.d(TAG, "onSaveInstanceState: ");
    }

在OnCreate()或者onRestart()裡判斷

既然無法很好在onDestory去remove fragment,那麼就配合activity 與fragment的自動儲存機制，在onSaveInstanceState()裡保留自動保留外的資料。大意程式碼如下：

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d(TAG, "onCreate: ");

        if (savedInstanceState != null) {
            // do something
        } else {
            // do anothering
        } 

    }

參考