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 ~