App在後臺久置後，再次從桌面或最近的任務列表喚醒時經常會發生崩潰，這往往是App在後臺被系統殺死，再次恢復的時候遇到了問題，而在使用FragmentActivity+Fragment的時候會更加頻繁。比如，如果Fragment沒有提供預設構造方法，就會在重建的時候因為反射建立Fragment失敗而崩潰，再比如，在onCreate裡面new 一個FragmentDialog，並且show，被後臺殺死後，再次喚醒的時候，就會show兩個對話方塊，這是為什麼？其實這就涉及了後臺殺死及恢復的機制，其中涉及的知識點主要是FragmentActivity、ActivityManagerService、LowMemoryKiller機制、ActivityStack、Binder等一系列知識點。放在一篇文章裡面可能會有些長，因此，Android後臺殺死系列寫了三篇：

本篇是Android後臺殺死系列的第一篇，主要講解在開發過程中，由於後臺殺死涉及的一些崩潰，以及如何避免這些崩潰，還有就是簡單的介紹一下onSaveInstanceState與onRestoreInstanceState執行時機與原理，這兩個函式也是Android面試時常問的兩個點，是比簡單的啟動模式Activity宣告週期稍微更深入細緻一些的地方，也通過這個點引入後臺殺死及恢復原理。

FragmentActivity被後臺殺死後恢復邏輯

當App被後臺異常殺死後，再次點選icon，或者從最近任務列表進入的時候，系統會幫助恢復當時的場景，重新建立Activity，對於FragmentActivity，由於其中有Framgent，邏輯會相對再複雜一些，系統會首先重建被銷燬的Fragment。

舉個栗子

我們建立一個Activity，並且在onCreate函式中新建並show一個DialogFragment，之後通過某種方式將APP異常殺死(RogueKiller模擬後臺殺死工具)，再次從最近的任務喚起App的時候，會發現顯示了兩個DialogFragment，程式碼如下：

public class DialogFragmentActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        DialogFragment dialogFragment = new FragmentDlg();
        dialogFragment.show(getSupportFragmentManager(), "");
    }
 

這不僅讓我們奇怪，為什麼呢？雖然被殺死了，但是onCreate函式在執行的時候還是隻執行了一次啊，為什麼會出現兩個DialogFragment，這裡其實就有一個DialogFragment是通過Android自身的恢復重建機制重建出來，在異常殺死的情況下onCreate(Bundle savedInstanceState)函式的savedInstanceState引數也不是null，而是包含了被殺死時所儲存的場景資訊。再來看個崩潰的例子，新建一個CrashFragment，並且丟棄預設無參構造方法：

public class CrashFragment extends Fragment {

    public CrashFragment(String tag) {
        super();
    }
}

之後再Activity中Add或replace新增這個CrashFragment，在CrashFragment顯示後，通過RogueKiller模擬後臺殺死工具模擬後臺殺死，再次從最近任務列表裡喚起App的時候，就會遇到崩潰，

Caused by: android.support.v4.app.Fragment$InstantiationException: 
  Unable to instantiate fragment xxx.CrashFragment: 
  make sure class name exists, is public, and has an empty constructor that is public
        at android.support.v4.app.Fragment.instantiate(Fragment.java:431)
        at android.support.v4.app.FragmentState.instantiate(Fragment.java:102)
        at android.support.v4.app.FragmentManagerImpl.restoreAllState(FragmentManager.java:1952)
        at android.support.v4.app.FragmentController.restoreAllState(FragmentController.java:144)
        at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:307)
        at android.support.v7.app.AppCompatActivity.onCreate(AppCompatActivity.java:81)

上面的這兩個問題主要涉及後臺殺死後FragmentActivity自身的恢復機制，其實super.onCreate(savedInstanceState)在恢復時做了很多我們沒有看到的事情，先看一下崩潰：

為什麼Fragment沒有無參構造方法會引發崩潰

看一下support-V4中FragmentActivity中onCreate程式碼如下：

protected void onCreate(@Nullable Bundle savedInstanceState) {
    mFragments.attachHost(null /*parent*/);

    super.onCreate(savedInstanceState);
                    ...
    if (savedInstanceState != null) {
        Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
        mFragments.restoreAllState(p, nc != null ? nc.fragments : null);
    }
    mFragments.dispatchCreate();
}

可以看到如果savedInstanceState != null，就會執行mFragments.restoreAllState邏輯，其實這裡就牽扯到恢復時重建邏輯，再被後臺異常殺死前，或者說在Activity的onStop執行前，Activity的現場以及Fragment的現場都是已經被儲存過的，其實是被儲存早ActivityManagerService中，儲存的格式FragmentState，重建的時候，會採用反射機制重新創Fragment

void restoreAllState(Parcelable state, List<Fragment> nonConfig) {

     ...
         for (int i=0; i<fms.mActive.length; i++) {
        FragmentState fs = fms.mActive[i];
        if (fs != null) {
            Fragment f = fs.instantiate(mHost, mParent);
            mActive.add(f);
    ...

其實就是呼叫FragmentState的instantiate，進而呼叫Fragment的instantiate，最後通過反射，構建Fragment，也就是，被加到FragmentActivity的Fragment在恢復的時候，會被自動建立，並且採用Fragment的預設無參構造方法，如果沒喲這個方法，就會丟擲InstantiationException異常，這也是為什麼第二個例子中會出現崩潰的原因。

 */
public static Fragment instantiate(Context context, String fname, @Nullable Bundle args) {
    try {
        Class<?> clazz = sClassMap.get(fname);
        if (clazz == null) {
            // Class not found in the cache, see if it's real, and try to add it
            clazz = context.getClassLoader().loadClass(fname);
            sClassMap.put(fname, clazz);
        }
        Fragment f = (Fragment)clazz.newInstance();
        if (args != null) {
            args.setClassLoader(f.getClass().getClassLoader());
            f.mArguments = args;
        }
        return f;
    } catch (ClassNotFoundException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (java.lang.InstantiationException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (IllegalAccessException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    }
}
q

可以看到場景二提示的errormsg跟丟擲的異常是可以對應上的，其實Fragment原始碼裡面也說得很清楚：

 /**
 * Default constructor.  <strong>Every</strong> fragment must have an
 * empty constructor, so it can be instantiated when restoring its
 * activity's state.  It is strongly recommended that subclasses do not
 * have other constructors with parameters, since these constructors
 * will not be called when the fragment is re-instantiated; instead,
 * arguments can be supplied by the caller with {@link #setArguments}
 * and later retrieved by the Fragment with {@link #getArguments}.
 * 
 * <p>Applications should generally not implement a constructor.  The
 * first place application code an run where the fragment is ready to
 * be used is in {@link #onAttach(Activity)}, the point where the fragment
 * is actually associated with its activity.  Some applications may also
 * want to implement {@link #onInflate} to retrieve attributes from a
 * layout resource, though should take care here because this happens for
 * the fragment is attached to its activity.
 */

public Fragment() {
}

大意就是，Fragment必須有一個空構造方法，這樣才能保證重建流程，並且，Fragment的子類也不推薦有帶引數的構造方法，最好採用setArguments來儲存引數。下面再來看下為什麼會出現兩個DialogFragment。

為什麼出現兩個DialogFragment

Fragment在被建立之後，如果不通過add或者replace新增到Activity的佈局中是不會顯示的，在儲存現場的時候，也是儲存了add的這個狀態的，來看一下Fragment的add邏輯：此時被後臺殺死，或旋轉螢幕，被恢復的DialogFragmentActivity時會出現兩個FragmentDialog，一個被系統恢復的，一個新建的。

@Override
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Fragment fr = Fragment.instance("")
        getSupportFragmentManager().beginTransaction()
        .add(R.id.container,fr).commit();

public class FragmentActivity extends Activity{
    ...
    final FragmentManagerImpl mFragments = new FragmentManagerImpl();
   ...
    final FragmentContainer mContainer = new FragmentContainer() {
        @Override
        @Nullable
        public View findViewById(int id) {
            return FragmentActivity.this.findViewById(id);
        }

        @Override
        public boolean hasView() {
            Window window = FragmentActivity.this.getWindow();
            return (window != null && window.peekDecorView() != null);
        }
    };

@Override
public FragmentTransaction beginTransaction() {
    return new (this);
}

public FragmentTransaction add(Fragment fragment, String tag) {
    doAddOp(0, fragment, tag, OP_ADD);//非同步操作的，跟Hander類似
    return this;
}


private void doAddOp(int containerViewId, Fragment fragment, String tag, int opcmd) {
    fragment.mFragmentManager = mManager;
     ...
    Op op = new Op();
    op.cmd = opcmd;
    op.fragment = fragment;
    addOp(op);
}

public void addFragment(Fragment fragment, boolean moveToStateNow) {

    if (mAdded == null) {
        mAdded = new ArrayList<Fragment>();
    }

    makeActive(fragment);

    if (!fragment.mDetached) {
        if (mAdded.contains(fragment)) {
            throw new IllegalStateException("Fragment already added: " + fragment);
        }
        mAdded.add(fragment);
        fragment.mAdded = true;
        fragment.mRemoving = false;
        if (fragment.mHasMenu && fragment.mMenuVisible) {
            mNeedMenuInvalidate = true;
        }
        if (moveToStateNow) {
            moveToState(fragment);
        }
    }
}    

final class FragmentManagerImpl extends FragmentManager implements LayoutInflaterFactory {
    ...

    ArrayList<Runnable> mPendingActions;
    Runnable[] mTmpActions;
    boolean mExecutingActions;

    ArrayList<Fragment> mActive;
    ArrayList<Fragment> mAdded;
    ArrayList<Integer> mAvailIndices;
    ArrayList<BackStackRecord> mBackStack;

 void moveToState(Fragment f, int newState, int transit, int transitionStyle,
        boolean keepActive) {
        ...        
     if (f.mState < newState) { //低於當前Activity的狀態
        switch (f.mState) {
            case Fragment.INITIALIZING:
                    ...
                f.mActivity = mActivity;
                f.mParentFragment = mParent;
                f.mFragmentManager = mParent != null
                        ? mParent.mChildFragmentManager : mActivity.mFragments;
                f.mCalled = false;
                f.onAttach(mActivity);
               ...
                if (!f.mRetaining) {
                    f.performCreate(f.mSavedFragmentState);
                } 
            case Fragment.CREATED:
                if (newState > Fragment.CREATED) {

                      f.mView = f.performCreateView(f.getLayoutInflater(
                      f.mSavedFragmentState), container, f.mSavedFragmentState);
                      f.onViewCreated(f.mView, f.mSavedFragmentState);

                    f.performActivityCreated(f.mSavedFragmentState);
                    if (f.mView != null) {
                        f.restoreViewState(f.mSavedFragmentState);
                    }
                    f.mSavedFragmentState = null;
                }
            case Fragment.ACTIVITY_CREATED:
            case Fragment.STOPPED:
                    if (newState > Fragment.STOPPED) {
                        f.performStart();
                    }
            case Fragment.STARTED:
                if (newState > Fragment.STARTED) {
                   f.mResumed = true;
                    f.performResume();

void restoreAllState(Parcelable state, ArrayList<Fragment> nonConfig) {

    if (state == null) return;
    FragmentManagerState fms = (FragmentManagerState)state;
    mActive = new ArrayList<Fragment>(fms.mActive.length);
     for (int i=0; i<fms.mActive.length; i++) {
        FragmentState fs = fms.mActive[i];
        if (fs != null) {
            Fragment f = fs.instantiate(mActivity, mParent);

            mActive.add(f);
            fs.mInstance = null;

    // Build the list of currently added fragments.
    if (fms.mAdded != null) {
        mAdded = new ArrayList<Fragment>(fms.mAdded.length);
        for (int i=0; i<fms.mAdded.length; i++) {
            Fragment f = mActive.get(fms.mAdded[i]);
            if (f == null) {
                throwException(new IllegalStateException(
                        "No instantiated fragment for index #" + fms.mAdded[i]));
            }
            f.mAdded = true;
            if (DEBUG) Log.v(TAG, "restoreAllState: added #" + i + ": " + f);
            if (mAdded.contains(f)) {
                throw new IllegalStateException("Already added!");
            }
            mAdded.add(f);
        }

    // Build the back stack.
    if (fms.mBackStack != null) {
        mBackStack = new ArrayList<BackStackRecord>(fms.mBackStack.length);
        for (int i=0; i<fms.mBackStack.length; i++) {
            BackStackRecord bse = fms.mBackStack[i].instantiate(this);

            mBackStack.add(bse);
            if (bse.mIndex >= 0) {
                setBackStackIndex(bse.mIndex, bse);
}

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

    mStopped = false;
    mReallyStopped = false;
    mHandler.removeMessages(MSG_REALLY_STOPPED);

    if (!mCreated) {
        mCreated = true;
        mFragments.dispatchActivityCreated();
    }

    mFragments.noteStateNotSaved();
    mFragments.execPendingActions();

    mFragments.doLoaderStart();

    // NOTE: HC onStart goes here.

    mFragments.dispatchStart();
    mFragments.reportLoaderStart();
}

在2.3之後，onSaveInstanceState的時機都放在了onStop之前，看一下FragmentActivity的onSaveInstanceState原始碼：

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    Parcelable p = mFragments.saveAllState();
    if (p != null) {
        outState.putParcelable(FRAGMENTS_TAG, p);
    }
}

可以看出，首先就是父類的onSaveInstanceState，主要是儲存一些視窗及View的資訊，比如ViewPager當前顯示的是第幾個View等。之後，就是就是通過FragmentManager的saveAllState，來儲存FragmentActivity自身的現場-Fragment的一些狀態，這些資料是FragmentActivity恢復Framgent所必須的資料，處理不好就會出現上面的那種異常。

OnRestoreInstanceState的呼叫時機

之前已經說過，OnRestoreInstanceState雖然與onSaveInstanceState是配對實現的，但是其呼叫卻並非完全成對的，在Activity跳轉或者返回主介面時，onSaveInstanceState是一定會呼叫的，但是OnRestoreInstanceState卻不會，它只有Activity或者App被異常殺死，走恢復流程的時候才會被呼叫。如果沒有被異常殺死，不走Activity的恢復新建流程，也就不會回撥OnRestoreInstanceState，簡單看一下Activity的載入流程圖：

可以看出，OnRestoreInstanceState的呼叫時機是在onStart之後，在onPostCreate之前。那麼正常的建立為什麼沒呼叫呢？看一下ActivityThread中啟動Activity的原始碼：

 private Activity performLaunchActivity(Activi

         ...
          mInstrumentation.callActivityOnCreate(activity, r.state);

               r.activity = activity;
               r.stopped = true;
               if (!r.activity.mFinished) {
                   activity.performStart();
                   r.stopped = false;
               }
               if (!r.activity.mFinished) {
                   if (r.state != null) {
                       mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
                   }
               }
               if (!r.activity.mFinished) {
                   activity.mCalled = false;
                   mInstrumentation.callActivityOnPostCreate(activity, r.state);

               }
     }

可以看出，只有r.state != null的時候，才通過mInstrumentation.callActivityOnRestoreInstanceState回撥OnRestoreInstanceState，r.state就是ActivityManagerService通過Binder傳給ActivityThread資料，主要用來做場景恢復。以上就是onSaveInstanceState與OnRestoreInstance執行時機的一些分析。下面結合具體的系統View控制元件來分析一下這兩個函式的具體應用：比如ViewPager與FragmentTabHost，這兩個空間是主介面最常用的控制元件，內部對後臺殺死做了相容，這也是為什麼被殺死後，Viewpager在恢復後，能自動定位到上次瀏覽的位置。

ViewPager應對後臺殺死做的相容

首先看一下ViewPager做的相容，ViewPager在後臺殺死的情況下，仍然能恢復到上次關閉的位置，這也是對體驗的一種優化，這其中的原理是什麼？之前分析onSaveInstanceState與onRestoreInstanceState的時候，只關注了Fragment的處理，其實還有一些針對Window視窗及Vie的處理，先看一下onSaveInstanceState針對視窗儲存了什麼：

protected void onSaveInstanceState(Bundle outState) {
    outState.putBundle(WINDOW_HIERARCHY_TAG, mWindow.saveHierarchyState());
  }

PhonwWinow.java

@Override
public Bundle saveHierarchyState() {
    Bundle outState = new Bundle();
    if (mContentParent == null) {
        return outState;
    }

    SparseArray<Parcelable> states = new SparseArray<Parcelable>();
    mContentParent.saveHierarchyState(states);
    outState.putSparseParcelableArray(VIEWS_TAG, states);

    // save the focused view id
      View focusedView = mContentParent.findFocus();
      ...
      outState.putInt(FOCUSED_ID_TAG, focusedView.getId());
    // save the panels
    if (panelStates.size() > 0) {
        outState.putSparseParcelableArray(PANELS_TAG, panelStates);
    }
    if (mActionBar != null) {
        outState.putSparseParcelableArray(ACTION_BAR_TAG, actionBarStates);
    }

    return outState;
}

Window其實就是PhonwWinow，saveHierarchyState其實就是針對當前視窗中的View儲存一些場景資訊 ，比如：當前獲取焦點的View的id、ActionBar、View的一些狀態，當然saveHierarchyState遞迴遍歷所有子View，儲存所有需要儲存的狀態：

ViewGroup.java

@Override
protected void dispatchSaveInstanceState(SparseArray<Parcelable> container) {
    super.dispatchSaveInstanceState(container);
    final int count = mChildrenCount;
    final View[] children = mChildren;
    for (int i = 0; i < count; i++) {
        View c = children[i];
        if ((c.mViewFlags & PARENT_SAVE_DISABLED_MASK) != PARENT_SAVE_DISABLED) {
            c.dispatchSaveInstanceState(container);
        }
    }
}

可見，該函式首先通過super.dispatchSaveInstanceState儲存自身的狀態，再遞迴傳遞給子View。onSaveInstanceState主要用於獲取View需要儲存的State，並將自身的ID作為Key，儲存到SparseArray states列表中，其實就PhoneWindow的一個列表，這些資料最後會通過Binder儲存到ActivityManagerService中去

View.java

  protected void dispatchSaveInstanceState(SparseArray<Parcelable> container) {
    if (mID != NO_ID && (mViewFlags & SAVE_DISABLED_MASK) == 0) {
        mPrivateFlags &= ~PFLAG_SAVE_STATE_CALLED;
        Parcelable state = onSaveInstanceState();
        if ((mPrivateFlags & PFLAG_SAVE_STATE_CALLED) == 0) {
            throw new IllegalStateException(
                    "Derived class did not call super.onSaveInstanceState()");
        }
        if (state != null) {
            container.put(mID, state);
        }
    }
}

那麼針對ViewPager到底儲存了什麼資訊？通過下面的程式碼很容易看出，其實就是新建個了一個SavedState場景資料，並且將當前的位置mCurItem存進去。

  @Override
public Parcelable onSaveInstanceState() {
    Parcelable superState = super.onSaveInstanceState();
    SavedState ss = new SavedState(superState);
    ss.position = mCurItem;
    if (mAdapter != null) {
        ss.adapterState = mAdapter.saveState();
    }
    return ss;
}

到這裡儲存的事情基本就完成了。接下來看一下ViewPager的恢復以及onRestoreInstanceState到底做了什麼，

protected void onRestoreInstanceState(Bundle savedInstanceState) {
    if (mWindow != null) {
        Bundle windowState = savedInstanceState.getBundle(WINDOW_HIERARCHY_TAG);
        if (windowState != null) {
            mWindow.restoreHierarchyState(windowState);
        }
    }
}

從程式碼可以看出，其實就是獲取當時儲存的視窗資訊，之後通過mWindow.restoreHierarchyState做資料恢復，

@Override
public void restoreHierarchyState(Bundle savedInstanceState) {
    if (mContentParent == null) {
        return;
    }

    SparseArray<Parcelable> savedStates
            = savedInstanceState.getSparseParcelableArray(VIEWS_TAG);
    if (savedStates != null) {
        mContentParent.restoreHierarchyState(savedStates);
    }
    ...

    if (mActionBar != null) {
        ...
          mActionBar.restoreHierarchyState(actionBarStates);
      }
}

對於ViewPager會發生什麼？從原始碼很容易看出，其實就是取出SavedState，並獲取到異常殺死的時候的位置，以便後續的恢復，

ViewPager.java

@Override
public void onRestoreInstanceState(Parcelable state) {
    if (!(state instanceof SavedState)) {
        super.onRestoreInstanceState(state);
        return;
    }

    SavedState ss = (SavedState)state;
    super.onRestoreInstanceState(ss.getSuperState());

    if (mAdapter != null) {
        mAdapter.restoreState(ss.adapterState, ss.loader);
        setCurrentItemInternal(ss.position, false, true);
    } else {
        mRestoredCurItem = ss.position;
        mRestoredAdapterState = ss.adapterState;
        mRestoredClassLoader = ss.loader;
    }
} 

以上就解釋了ViewPager是如何通過onSaveInstanceState與onRestoreInstanceState儲存、恢復現場的。如果是ViewPager+FragmentAdapter的使用方式，就同時涉及FragmentActivity的恢復、也牽扯到Viewpager的恢復，其實FragmentAdapter也同樣針對後臺殺死做了一些相容，防止重複新建Fragment，看一下FragmentAdapter的原始碼：

FragmentPagerAdapter.java

@Override
public Object instantiateItem(ViewGroup container, int position) {
    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }

    final long itemId = getItemId(position);

    // Do we already have this fragment?
    <!--是否已經新建了Fragment？？-->

    String name = makeFragmentName(container.getId(), itemId);
    Fragment fragment = mFragmentManager.findFragmentByTag(name);

    1 如果Activity中存在相應Tag的Fragment，就不要通過getItem新建

    if (fragment != null) {
        mCurTransaction.attach(fragment);
    } else {
    2 如果Activity中不存在相應Tag的Fragment，就需要通過getItem新建
        fragment = getItem(position);
        mCurTransaction.add(container.getId(), fragment,
                makeFragmentName(container.getId(), itemId));
    }
    if (fragment != mCurrentPrimaryItem) {
        FragmentCompat.setMenuVisibility(fragment, false);
        FragmentCompat.setUserVisibleHint(fragment, false);
    }

    return fragment;
}

從1與2 可以看出，通過後臺恢復，在FragmentActivity的onCreate函式中，會重建Fragment列表，那些被重建的Fragment不會再次通過getItem再次建立，再來看一下相似的控制元件FragmentTabHost，FragmentTabHost也是主頁常用的控制元件，FragmentTabHost也有相應的後臺殺死處理機制，從名字就能看出，這個是專門針對Fragment才創建出來的控制元件。

FragmentTabHost應對後臺殺死做的相容

FragmentTabHost其實跟ViewPager很相似，在onSaveInstanceState執行的時候儲存當前位置，並在onRestoreInstanceState恢復postion，並重新賦值給Tabhost，之後FragmentTabHost在onAttachedToWindow時，就可以根據恢復的postion設定當前位置，程式碼如下：

FragmentTabHost.java

@Override
protected Parcelable onSaveInstanceState() {
    Parcelable superState = super.onSaveInstanceState();
    SavedState ss = new SavedState(superState);
    ss.curTab = getCurrentTabTag();
    return ss;
}

@Override
protected void onRestoreInstanceState(Parcelable state) {
    if (!(state instanceof SavedState)) {
        super.onRestoreInstanceState(state);
        return;
    }
    SavedState ss = (SavedState) state;
    super.onRestoreInstanceState(ss.getSuperState());
    setCurrentTabByTag(ss.curTab);
}

在FragmentTabHost執行onAttachedToWindow時候，會首先getCurrentTabTag ,如果是經歷了後臺殺死，這裡得到的值其實是恢復的SavedState裡的值，之後通過doTabChanged切換到響應的Tab，注意這裡切換的時候，Fragment由於已經重建了，是不會再次新建的。

@Override
protected void onAttachedToWindow() {
    super.onAttachedToWindow();

    String currentTab = getCurrentTabTag();
    ...

    ft = doTabChanged(currentTab, ft);

    if (ft != null) {
        ft.commit();
        mFragmentManager.executePendingTransactions();
    }
}

App開發時針對後臺殺死處理方式

• 最簡單的方式，但是效果一般：取消系統恢復

比如：針對FragmentActivity ，不重建：

protected void onCreate(Bundle savedInstanceState) {
     if (savedInstanceState != null) {
     savedInstanceState.putParcelable(“android:support:fragments”, null);}
     super.onCreate(savedInstanceState);
}  

如果是系統的Actvity改成是“android:fragments”，不過這裡需要注意：對於ViewPager跟FragmentTabHost不需要額外處理，處理了可能反而有反作用。

針對Window，如果不想讓View使用恢復邏輯，在基類的FragmentActivity中覆蓋onRestoreInstanceState函式即可。

protected void onRestoreInstanceState(Bundle savedInstanceState) {
}

當然以上的做法都是比較粗暴的做法，最好還是順著Android的設計，在需要儲存現場的地方儲存，在需要恢復的地方，去除相應的資料進行恢復。以上就是後臺殺死針對FragmentActivity、onSaveInstanceState、onRestoreInstanceState的一些分析，後面會有兩篇針對後臺殺死原理，以及ActivityManagerService如何處理殺死及恢復的文章。

參考文件