Android Frameworks 190310期 —— onSaveInstanceState到底做了些什么？

Android Frameworks是最近我刚创建的一个文集，这篇文章也是Android Frameworks这个文集的第一篇文章（发现自己好久没写简书了），主要目的是想跟大家从源码的角度分享，或者更多的是交流一下Android Frameworks里的一些关键的机制，对于描述的内容有异议的可以留言，大家一起交流和学习。今天的内容是从源码角度了解Activity中的 onSaveInstanceState 和 onRestoreInstanceState：1. onSaveInstanceState到底做了些什么？protected void onSaveInstanceState(Bundle outState) { // 存储窗口视图状态 outState.putBundle(WINDOW_HIERARCHY_TAG, mWindow.saveHierarchyState()); outState.putInt(LAST_AUTOFILL_ID, mLastAutofillId); // 存储Fragment状态 Parcelable p = mFragments.saveAllState(); if (p != null) { outState.putParcelable(FRAGMENTS_TAG, p); } if (mAutoFillResetNeeded) { outState.putBoolean(AUTOFILL_RESET_NEEDED, true); // 存储Android自动填充的状态，非重点 getAutofillManager().onSaveInstanceState(outState); } // 调用ActivityLifeCallbacks的onSaveInstanceState方法进行状态存储 getApplication().dispatchActivitySaveInstanceState(this, outState);}从源码上看，不难发现onSaveInstanceState主要做了以下三个内容：存储窗口视图状态存储Fragment状态调用ActivityLifeCallbacks的onSaveInstanceState方法进行状态存储通过调用PhoneWindow的saveHierarchyState方法实际上是调用mContentParent的saveHierarchyState方法：@Overridepublic 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的主视图，本质是一个View对象，在Activity#setContentView()方法里进行初始化操作，接下来回到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) { // 以当前View的ID为key值存储起来 container.put(mID, state); } }}在View的dispatchSaveInstanceState方法里，会将View#onSaveInstanceState()方法返回的Parcelable对象以当前View的ID为key值存储起来，并返回，这也意味着如果当前View没有设置ID，那么将无法进行View的onSaveInstanceState操作：android:id="@+id/tv_save_state"View的onSaveInstanceState方法默认返回空状态 BaseSavedState.EMPTY_STATE，可交由子View重写，如TextView#onSaveInstanceState()：@Overridepublic 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;}接下来进入Fragment状态的保存方法，mFragments.saveAllState()，mFragments是一个FragmentController对象，主要用于提供FragmentManager对象，维护Fragments的生命周期，这里不做过多介绍，所以最终是调用FragmentManager#saveAllState()方法，该方法最终会返回一个FragmentManagerState对象：Parcelable saveAllState() { … // 第一步：收集所有处于active状态的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：收集已经添加的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：保存回退栈的内容，FragmentManager中的BackStack主要是用来存储FragmentTransaction，具体可见下篇文章的讲解 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;}对于第一步的方法最终会调用FragmentManager#saveFragmentBasicState()方法，此方法主要有以下任务：Bundle saveFragmentBasicState(Fragment f) { Bundle result = null; … // 调用Fragment#onSaveInstanceState()方法（可由子类重写） f.performSaveInstanceState(mStateBundle); dispatchOnFragmentSaveInstanceState(f, mStateBundle, false); … // 保存View的视图状态，跟上面内容介绍一致 if (f.mView != null) { saveFragmentViewState(f); } // mSavedViewState为已保存的视图状态 if (f.mSavedViewState != null) { if (result == null) { result = new Bundle(); } result.putSparseParcelableArray( FragmentManagerImpl.VIEW_STATE_TAG, f.mSavedViewState); } // 记录Fragment的显示属性 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除了保存自身状态还会保存子Fragment的状态，实现如下：void performSaveInstanceState(Bundle outState) { onSaveInstanceState(outState); if (mChildFragmentManager != null) { Parcelable p = mChildFragmentManager.saveAllState(); if (p != null) { outState.putParcelable(Activity.FRAGMENTS_TAG, p); } }}最后，看一下Activity#onSaveInstanceState的最后一步：getApplication().dispatchActivitySaveInstanceState(this, outState);实际上是遍历已注册的ActivityLifecycleCallback，并调用其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的工作已经做完了，总结起来，其时序图如下：讲解过程省略了不少代码，需要做详细了解的可以进一步查看源码，关于onRestoreInstanceState的方法实现，主要是以上方法的一个逆过程，这里不再做过多的叙述，接下来内容主要讲onSaveInstanceState与onRestoreInstanceState的调用时机。2. onSaveInstanceState与onRestoreInstanceState调用时机关于onSaveInstanceState调用时机的追踪，我们先来简单看一下一个Activity的启动过程：Activity的启动过程与Binder进程间通信机制息息相关，对于Binder进程间通信机制以及Activity的启动过程，我们在后期的文章里会仔细说明，这里就不在做过多说明。Activity的启动过程主要的步骤可以概括如下：MainActivity向ActivityManagerService发送一个启动SecondActivity的进程间通信请求；ActivityManagerService保存SecondActivity的相关信息，并向MainActivity发送一个进入中止状态（pause）的进程间通信请求；MainActivity进入中止状态后，向ActivityManagerService发送一个已进入中止状态的进程间通信请求；ActivityManagerService发现SecondActivity所在进程不存在，则会发起启动新进程的请求；当新进程启动完毕后，会向SecondActivity发送一个启动完成的进程间通信请求；ActivityManagerService将保存的SecondActivity信息发送给新创建的进程，以便其将SecondActivity启动起来我们主要看第三步，Activity接收ActivityManagerService的进程间通信请求最终都会由该Activity所在的进程对象ActivityThread里的Handler对象去处理：// ActivityThread.javaprivate class H extends Handler { public void handleMessage(Message msg) { if (DEBUG_MESSAGES) Slog.v(TAG, “>>> handling: " + codeToString(msg.what)); switch (msg.what) { … case PAUSE_ACTIVITY: { // 通知Activity进入中止状态 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, “activityPause”); SomeArgs args = (SomeArgs) msg.obj; handlePauseActivity((IBinder) args.arg1, false, (args.argi1 & USER_LEAVING) != 0, args.argi2, (args.argi1 & DONT_REPORT) != 0, args.argi3); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break; … }}核心方法为handlePauseActivity：private void handlePauseActivity(IBinder token, boolean finished, boolean userLeaving, int configChanges, boolean dontReport, int seq) { ActivityClientRecord r = mActivities.get(token); if (DEBUG_ORDER) Slog.d(TAG, “handlePauseActivity " + r + “, seq: " + seq); if (!checkAndUpdateLifecycleSeq(seq, r, “pauseActivity”)) { return; } if (r != null) { … r.activity.mConfigChangeFlags |= configChanges; performPauseActivity(token, finished, r.isPreHoneycomb(), “handlePauseActivity”); … }}接着在往下看performPauseActivity：final Bundle performPauseActivity(IBinder token, boolean finished, boolean saveState, String reason) { ActivityClientRecord r = mActivities.get(token); return r != null ? performPauseActivity(r, finished, saveState, reason) : null;}final Bundle performPauseActivity(ActivityClientRecord r, boolean finished, boolean saveState, String reason) { … if (finished) { r.activity.mFinished = true; } // Next have the activity save its current state and managed dialogs… if (!r.activity.mFinished && saveState) { callCallActivityOnSaveInstanceState(r); } …}这时候关键方法已经显而易见了：private void callCallActivityOnSaveInstanceState(ActivityClientRecord r) { r.state = new Bundle(); r.state.setAllowFds(false); if (r.isPersistable()) { r.persistentState = new PersistableBundle(); mInstrumentation.callActivityOnSaveInstanceState(r.activity, r.state, r.persistentState); } else { mInstrumentation.callActivityOnSaveInstanceState(r.activity, r.state); }}public void callActivityOnSaveInstanceState(Activity activity, Bundle outState, PersistableBundle outPersistentState) { activity.performSaveInstanceState(outState, outPersistentState);}final void performSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) { // 这就是我们希望看到的方法 onSaveInstanceState(outState, outPersistentState); saveManagedDialogs(outState); storeHasCurrentPermissionRequest(outState); if (DEBUG_LIFECYCLE) Slog.v(TAG, “onSaveInstanceState " + this + “: " + outState + “, " + outPersistentState);}总结起来，onSaveInstanceState会在以下情况下被调用：当用户按下home键；长按home键切换应用程序；按下电源键；启动一个新的Activity；屏幕方向切换；电话打入等情况发生而onRestoreInstanceState只有在activity确实是被系统回收，重新创建activity的情况下才会被调用，所以主要分为以下两种情况：a. activity没有被销毁：onPause -> onSaveInstanceState -> onStop -> onRestart -> onStart -> onResumeb. activity被销毁：onPause -> onSaveInstanceState -> onStop -> onDestroy -> onCreate -> onStart -> onRestoreInstanceState -> onResumeOK，今天的内容就到这里，下篇文章见 ~