Android Chat 190310期 —— onSaveInstanceState到底做了些什麼？
Android Chat是最近我剛建立的一個文集，這篇文章也是 Android Chat 這個文集的第一篇文章（發現自己好久沒寫簡書了），主要目的是想跟大家分享，或者更多的是交流一下在Android開發過程中容易被忽視的一些東西，對於描述的內容有異議的可以留言，大家一起交流和學習。今天的內容是從原始碼角度瞭解Activity中的 onSaveInstanceState 和 onRestoreInstanceState ：
1. onSaveInstanceState到底做了些什麼？
protected void onSaveInstanceState(Bundle outState) { // 儲存視窗檢視狀態 outState.putBundle(WINDOW_HIERARCHY_TAG, mWindow.saveHierarchyState()); outState.putInt(LAST_AUTOFILL_ID, mLastAutofillId); // 儲存Fragment狀態 Parcelable p = mFragments.saveAllState(); if (p != null) { outState.putParcelable(FRAGMENTS_TAG, p); } if (mAutoFillResetNeeded) { outState.putBoolean(AUTOFILL_RESET_NEEDED, true); // 儲存Android自動填充的狀態，非重點 getAutofillManager().onSaveInstanceState(outState); } // 呼叫ActivityLifeCallbacks的onSaveInstanceState方法進行狀態儲存 getApplication().dispatchActivitySaveInstanceState(this, outState); }
從原始碼上看，不難發現onSaveInstanceState主要做了以下三個內容：
- 儲存視窗檢視狀態
- 儲存Fragment狀態
- 呼叫ActivityLifeCallbacks的onSaveInstanceState方法進行狀態儲存
通過呼叫PhoneWindow的saveHierarchyState方法實際上是呼叫mContentParent的saveHierarchyState方法：
@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); .... return outState; }
其中mContentParent是Activity的主檢視，本質是一個View物件，在Activity#setContentView()方法裡進行初始化操作，接下來回到View的saveHierarchyState方法：
public void saveHierarchyState(SparseArray<Parcelable> container) { dispatchSaveInstanceState(container); } 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) { // 以當前View的ID為key值儲存起來 container.put(mID, state); } } }
在View的dispatchSaveInstanceState方法裡，會將View#onSaveInstanceState()方法返回的Parcelable物件以當前View的ID為key值儲存起來，並返回，這也意味著 如果當前View沒有設定ID，那麼將無法進行View的onSaveInstanceState操作 ：
android:id="@+id/tv_save_state"
View的onSaveInstanceState方法預設返回空狀態 BaseSavedState.EMPTY_STATE ，可交由子View重寫，如TextView#onSaveInstanceState()：
@Override public Parcelable onSaveInstanceState() { Parcelable superState = super.onSaveInstanceState(); // Save state if we are forced to final boolean freezesText = getFreezesText(); boolean hasSelection = false; int start = -1; int end = -1; if (mText != null) { start = getSelectionStart(); end = getSelectionEnd(); if (start >= 0 || end >= 0) { // Or save state if there is a selection hasSelection = true; } } if (freezesText || hasSelection) { SavedState ss = new SavedState(superState); if (freezesText) { if (mText instanceof Spanned) { final Spannable sp = new SpannableStringBuilder(mText); if (mEditor != null) { removeMisspelledSpans(sp); sp.removeSpan(mEditor.mSuggestionRangeSpan); } ss.text = sp; } else { ss.text = mText.toString(); } } if (hasSelection) { // XXX Should also save the current scroll position! ss.selStart = start; ss.selEnd = end; } if (isFocused() && start >= 0 && end >= 0) { ss.frozenWithFocus = true; } ss.error = getError(); if (mEditor != null) { ss.editorState = mEditor.saveInstanceState(); } return ss; } return superState; }
接下來進入Fragment狀態的儲存方法，mFragments.saveAllState()，mFragments是一個FragmentController物件，主要用於提供FragmentManager物件，維護Fragments的生命週期，這裡不做過多介紹，所以最終是呼叫FragmentManager#saveAllState()方法，該方法最終會返回一個FragmentManagerState物件：
Parcelable saveAllState() { ... // 第一步：收集所有處於active狀態的Fragment int N = mActive.size(); FragmentState[] active = new FragmentState[N]; boolean haveFragments = false; for (int i=0; i<N; i++) { Fragment f = mActive.valueAt(i); if (f != null) { ... FragmentState fs = new FragmentState(f); active[i] = fs; if (f.mState > Fragment.INITIALIZING && fs.mSavedFragmentState == null) { fs.mSavedFragmentState = saveFragmentBasicState(f); ... } else { fs.mSavedFragmentState = f.mSavedFragmentState; } } } // 第二步：收集已經新增的Fragments的Index int[] added = null; BackStackState[] backStack = null; N = mAdded.size(); if (N > 0) { added = new int[N]; for (int i=0; i<N; i++) { added[i] = mAdded.get(i).mIndex; ... } } // 第三步：儲存回退棧的內容，FragmentManager中的BackStack主要是用來儲存FragmentTransaction，具體可見下篇文章的講解 if (mBackStack != null) { N = mBackStack.size(); if (N > 0) { backStack = new BackStackState[N]; for (int i=0; i<N; i++) { backStack[i] = new BackStackState(this, mBackStack.get(i)); if (DEBUG) Log.v(TAG, "saveAllState: adding back stack #" + i + ": " + mBackStack.get(i)); } } } FragmentManagerState fms = new FragmentManagerState(); fms.mActive = active; fms.mAdded = added; fms.mBackStack = backStack; fms.mNextFragmentIndex = mNextFragmentIndex; if (mPrimaryNav != null) { fms.mPrimaryNavActiveIndex = mPrimaryNav.mIndex; } saveNonConfig(); return fms; }
對於第一步的方法最終會呼叫FragmentManager#saveFragmentBasicState()方法，此方法主要有以下任務：
Bundle saveFragmentBasicState(Fragment f) { Bundle result = null; ... // 呼叫Fragment#onSaveInstanceState()方法（可由子類重寫） f.performSaveInstanceState(mStateBundle); dispatchOnFragmentSaveInstanceState(f, mStateBundle, false); ... // 儲存View的檢視狀態，跟上面內容介紹一致 if (f.mView != null) { saveFragmentViewState(f); } // mSavedViewState為已儲存的檢視狀態 if (f.mSavedViewState != null) { if (result == null) { result = new Bundle(); } result.putSparseParcelableArray( FragmentManagerImpl.VIEW_STATE_TAG, f.mSavedViewState); } // 記錄Fragment的顯示屬性 if (!f.mUserVisibleHint) { if (result == null) { result = new Bundle(); } // Only add this if it's not the default value result.putBoolean(FragmentManagerImpl.USER_VISIBLE_HINT_TAG, f.mUserVisibleHint); } return result; }
Fragment的performSaveInstanceState除了儲存自身狀態還會儲存子Fragment的狀態，實現如下：
void performSaveInstanceState(Bundle outState) { onSaveInstanceState(outState); if (mChildFragmentManager != null) { Parcelable p = mChildFragmentManager.saveAllState(); if (p != null) { outState.putParcelable(Activity.FRAGMENTS_TAG, p); } } }
最後，看一下Activity#onSaveInstanceState的最後一步：
getApplication().dispatchActivitySaveInstanceState(this, outState);
實際上是遍歷已註冊的ActivityLifecycleCallback，並呼叫其onActivitySaveInstanceState()方法：
void dispatchActivitySaveInstanceState(Activity activity, Bundle outState) { Object[] callbacks = collectActivityLifecycleCallbacks(); if (callbacks != null) { for (int i=0; i<callbacks.length; i++) { ((ActivityLifecycleCallbacks)callbacks[i]).onActivitySaveInstanceState(activity, outState); } } }
至此，Activity#onSaveInstanceState的工作已經做完了，總結起來，其時序圖如下：
onSaveInstanceState時序圖
講解過程省略了不少程式碼，需要做詳細瞭解的可以進一步檢視原始碼，關於onRestoreInstanceState的方法實現，主要是以上方法的一個逆過程，這裡不再做過多的敘述，接下來內容主要講onSaveInstanceState與onRestoreInstanceState的呼叫時機。
2. onSaveInstanceState與onRestoreInstanceState呼叫時機
onSaveInstanceState會在以下情況下被呼叫：
- 當用戶按下home鍵；
- 長按home鍵切換應用程式；
- 按下電源鍵；
- 啟動一個新的Activity；
- 螢幕方向切換；
- 電話打入等情況發生
而onRestoreInstanceState只有在activity確實是被系統回收，重新建立activity的情況下才會被呼叫，所以主要分為以下兩種情況：
a. activity沒有被銷燬：
onPause -> onSaveInstanceState -> onStop -> onRestart -> onStart -> onResume
b. activity被銷燬：
onPause -> onSaveInstanceState -> onStop -> onDestroy -> onCreate -> onStart -> onRestoreInstanceState -> onResume
OK，今天的內容就到這裡，下篇文章見 ~