前言
Activity是Android四大組件的老大，我們對它的生命周期方法調用順序都爛熟於心了，可是這些生命周期方法到底是怎么調用的呢？在啟動它的時候會用到startActivty這個方法，但是這個方法的背后是怎樣來實現的呢，來看看源碼一探究竟(API23，無關代碼省略)
應用進程啟動activity流程
首先來到startActivity(Intent intent):
@Override public void startActivity(Intent intent) { this.startActivity(intent, null); } @Override public void startActivity(Intent intent, @Nullable Bundle options) { if (options != null) { startActivityForResult(intent, -1, options); } else { startActivityForResult(intent, -1); } } 內部都是調用的startActivityForResult，並且請求碼為-1（即沒有回調）
public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) { if (mParent == null) { Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity( this, mMainThread.getApplicationThread(), mToken, this, intent, requestCode, options); if (ar != null) { mMainThread.sendActivityResult( mToken, mEmbeddedID, requestCode, ar.getResultCode(), ar.getResultData()); } if (requestCode >= 0) { mStartedActivity = true; } cancelInputsAndStartExitTransition(options); } else { if (options != null) { mParent.startActivityFromChild(this, intent, requestCode, options); } else { mParent.startActivityFromChild(this, intent, requestCode); } } } 首次啟動肯定mParent為空，執行Instrumentation(這個類用來做一些Activity的實際操作，並且Instrumentation將在任何應用程序運行前初始化，可以通過它監測系統與應用程序之間的交互)的execStartActivity方法：
public ActivityResult execStartActivity( Context who, IBinder contextThread, IBinder token, Activity target, Intent intent, int requestCode, Bundle options) { IApplicationThread whoThread = (IApplicationThread) contextThread; Uri referrer = target != null ? target.onProvideReferrer() : null; if (referrer != null) { intent.putExtra(Intent.EXTRA_REFERRER, referrer); } if (mActivityMonitors != null) { synchronized (mSync) { final int N = mActivityMonitors.size(); for (int i=0; i<N; i++) { final ActivityMonitor am = mActivityMonitors.get(i); if (am.match(who, null, intent)) { am.mHits++; if (am.isBlocking()) { return requestCode >= 0 ? am.getResult() : null; } break; } } } } try { intent.migrateExtraStreamToClipData(); intent.prepareToLeaveProcess(); int result = ActivityManagerNative.getDefault() .startActivity(whoThread, who.getBasePackageName(), intent, intent.resolveTypeIfNeeded(who.getContentResolver()), token, target != null ? target.mEmbeddedID : null, requestCode, 0, null, options); checkStartActivityResult(result, intent); } catch (RemoteException e) { throw new RuntimeException("Failure from system", e); } return null; } 參數：
- Context who：activity啟動的上下文
- IBinder contextThread：上下文所在的主線程
- IBinder token：啟動的activity的標識
- Activity target：啟動的activity
- Intent intent：傳入的intent
- int requestCode：請求碼
- Bundle options：額外參數
ActivityMonitor用來監視應用啟動activity的操作，最后調用ActivityManagerNative.getDefault().startActivity，先看看ActivityManagerNative.getDefault():
static public IActivityManager getDefault() { return gDefault.get(); } private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() { protected IActivityManager create() { IBinder b = ServiceManager.getService("activity"); if (false) { Log.v("ActivityManager", "default service binder = " + b); } IActivityManager am = asInterface(b); if (false) { Log.v("ActivityManager", "default service = " + am); } return am; } }; 一個單例用於返回IActivityManager對象，調用asInterface：
static public IActivityManager asInterface(IBinder obj) { if (obj == null) { return null; } IActivityManager in = (IActivityManager)obj.queryLocalInterface(descriptor); if (in != null) { return in; } return new ActivityManagerProxy(obj); } 最后返回ActivityManagerProxy類，這個類是ActivityManagerNative的內部類
class ActivityManagerProxy implements IActivityManager { public ActivityManagerProxy(IBinder remote) { mRemote = remote; } public IBinder asBinder() { return mRemote; } public int startActivity(IApplicationThread caller, String callingPackage, Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo, Bundle options) throws RemoteException { Parcel data = Parcel.obtain(); Parcel reply = Parcel.obtain(); data.writeInterfaceToken(IActivityManager.descriptor); data.writeStrongBinder(caller != null ? caller.asBinder() : null); data.writeString(callingPackage); intent.writeToParcel(data, 0); data.writeString(resolvedType); data.writeStrongBinder(resultTo); data.writeString(resultWho); data.writeInt(requestCode); data.writeInt(startFlags); if (profilerInfo != null) { data.writeInt(1); profilerInfo.writeToParcel(data, Parcelable.PARCELABLE_WRITE_RETURN_VALUE); } else { data.writeInt(0); } if (options != null) { data.writeInt(1); options.writeToParcel(data, 0); } else { data.writeInt(0); } mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0); reply.readException(); int result = reply.readInt(); reply.recycle(); data.recycle(); return result; } ...... } 這里就涉及到了android的Binder機制，android的進程通信是使用Binder來進行進程間的通信（例外：Zygote和SystemService是使用socket，《Android應用進程啟動流程（Zygote進程與SystemServer進程）》），簡單來說就是這里的ActivityManagerProxy作為客戶端調用了startActivity方法，其實具體的實現是在服務端（也有一個對應的startActivity方法），這里對應的類為ActivityManagerService（同樣繼承自ActivityManagerNative）。
系統進程啟動activity流程
首先來到ActivityManagerService的
startActivity
@Override public final int startActivity(IApplicationThread caller, String callingPackage, Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo, Bundle options) { return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo, resultWho, requestCode, startFlags, profilerInfo, options, UserHandle.getCallingUserId()); } startActivityAsUser：
@Override public final int startActivityAsUser(IApplicationThread caller, String callingPackage, Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo, Bundle options, int userId) { enforceNotIsolatedCaller("startActivity"); userId = handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(), userId, false, ALLOW_FULL_ONLY, "startActivity", null); // TODO: Switch to user app stacks here. return mStackSupervisor.startActivityMayWait(caller, -1, callingPackage, intent, resolvedType, null, null, resultTo, resultWho, requestCode, startFlags, profilerInfo, null, null, options, false, userId, null, null); } 設置了userId，並且再傳入mStackSupervisor的startActivityMayWait，來到ActivityStackSupervisor類（所有activity的棧操作都通過這個類）
startActivityMayWait：
final int startActivityMayWait(IApplicationThread caller, int callingUid, String callingPackage, Intent intent, String resolvedType, IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor, IBinder resultTo, String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo, WaitResult outResult, Configuration config, Bundle options, boolean ignoreTargetSecurity, int userId, IActivityContainer iContainer, TaskRecord inTask) { ...... int res = startActivityLocked(caller, intent, resolvedType, aInfo, voiceSession, voiceInteractor, resultTo, resultWho, requestCode, callingPid, callingUid, callingPackage, realCallingPid, realCallingUid, startFlags, options, ignoreTargetSecurity, componentSpecified, null, container, inTask); ...... } 在這個函數里主要進行了Intent的檢查，Activity信息的獲取還有重量級進程的判斷，然后調用
startActivityLocked：
final int startActivityLocked(IApplicationThread caller, Intent intent, String resolvedType, ActivityInfo aInfo, IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor, IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid, String callingPackage, int realCallingPid, int realCallingUid, int startFlags, Bundle options, boolean ignoreTargetSecurity, boolean componentSpecified, ActivityRecord[] outActivity, ActivityContainer container, TaskRecord inTask) { ...... ActivityRecord r = new ActivityRecord(mService, callerApp, callingUid, callingPackage, intent, resolvedType, aInfo, mService.mConfiguration, resultRecord, resultWho, requestCode, componentSpecified, voiceSession != null, this, container, options); ...... err = startActivityUncheckedLocked(r, sourceRecord, voiceSession, voiceInteractor, startFlags, true, options, inTask); ...... } ActivityRecord是server端Activity的對應類，包含了Activity的信息，每一個進程也對應一個ProcessRecord類來描述其信息，再繼續調用：
startActivityUncheckedLocked：
......
final boolean launchSingleTop = r.launchMode == ActivityInfo.LAUNCH_SINGLE_TOP; final boolean launchSingleInstance = r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE; final boolean launchSingleTask = r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK; ...... ActivityStack.logStartActivity(EventLogTags.AM_CREATE_ACTIVITY, r, r.task); targetStack.mLastPausedActivity = null; targetStack.startActivityLocked(r, newTask, doResume, keepCurTransition, options); ...... 首先判斷了啟動模式，然后再調用了startActivityLocked（不同於剛才的ActivityStackSupervisor.startActivityLocked，這里是ActivityStack類），上面的步驟完成了Activity的目標棧的判斷(targetStack)
ActivityStack.startActivityLocked:
final void startActivityLocked(ActivityRecord r, boolean newTask, boolean doResume, boolean keepCurTransition, Bundle options) { ...... task = r.task; if (DEBUG_ADD_REMOVE) Slog.i(TAG, "Adding activity " + r + " to stack to task " + task, new RuntimeException("here").fillInStackTrace()); task.addActivityToTop(r); task.setFrontOfTask(); r.putInHistory(); if (!isHomeStack() || numActivities() > 0) { boolean showStartingIcon = newTask; ProcessRecord proc = r.app; if (proc == null) { proc = mService.mProcessNames.get(r.processName, r.info.applicationInfo.uid); } if (proc == null || proc.thread == null) { showStartingIcon = true; } if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION, "Prepare open transition: starting " + r); if ((r.intent.getFlags() & Intent.FLAG_ACTIVITY_NO_ANIMATION) != 0) { mWindowManager.prepareAppTransition(AppTransition.TRANSIT_NONE, keepCurTransition); mNoAnimActivities.add(r); } else { mWindowManager.prepareAppTransition(newTask ? r.mLaunchTaskBehind ? AppTransition.TRANSIT_TASK_OPEN_BEHIND : AppTransition.TRANSIT_TASK_OPEN : AppTransition.TRANSIT_ACTIVITY_OPEN, keepCurTransition); mNoAnimActivities.remove(r); } mWindowManager.addAppToken(task.mActivities.indexOf(r), r.appToken, r.task.taskId, mStackId, r.info.screenOrientation, r.fullscreen, (r.info.flags & ActivityInfo.FLAG_SHOW_FOR_ALL_USERS) != 0, r.userId, r.info.configChanges, task.voiceSession != null, r.mLaunchTaskBehind); boolean doShow = true; if (newTask) { if ((r.intent.getFlags() & Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED) != 0) { resetTaskIfNeededLocked(r, r); doShow = topRunningNonDelayedActivityLocked(null) == r; } } else if (options != null && new ActivityOptions(options).getAnimationType() == ActivityOptions.ANIM_SCENE_TRANSITION) { doShow = false; } if (r.mLaunchTaskBehind) { mWindowManager.setAppVisibility(r.appToken, true); ensureActivitiesVisibleLocked(null, 0); } else if (SHOW_APP_STARTING_PREVIEW && doShow) { ActivityRecord prev = mResumedActivity; if (prev != null) { // 滿足判斷則不需要復用之前已經啟動的任務 // (1) 當前activity在不同的task if (prev.task != r.task) { prev = null; } // 當前activity不可見 else if (prev.nowVisible) { prev = null; } } mWindowManager.setAppStartingWindow( r.appToken, r.packageName, r.theme, mService.compatibilityInfoForPackageLocked( r.info.applicationInfo), r.nonLocalizedLabel, r.labelRes, r.icon, r.logo, r.windowFlags, prev != null ? prev.appToken : null, showStartingIcon); r.mStartingWindowShown = true; } } else { // 如果是第一個activity，不需要動畫 mWindowManager.addAppToken(task.mActivities.indexOf(r), r.appToken, r.task.taskId, mStackId, r.info.screenOrientation, r.fullscreen, (r.info.flags & ActivityInfo.FLAG_SHOW_FOR_ALL_USERS) != 0, r.userId, r.info.configChanges, task.voiceSession != null, r.mLaunchTaskBehind); ActivityOptions.abort(options); options = null; } if (VALIDATE_TOKENS) { validateAppTokensLocked(); } if (doResume) { mStackSupervisor.resumeTopActivitiesLocked(this, r, options); } } 將Activity添加到了棧頂，初始化了WindowManager，並且又回到了StackSupervisor，調用
StackSupervisor.resumeTopActivitiesLocked：
boolean resumeTopActivitiesLocked(ActivityStack targetStack, ActivityRecord target, Bundle targetOptions) { if (targetStack == null) { targetStack = mFocusedStack; } // Do targetStack first. boolean result = false; if (isFrontStack(targetStack)) { result = targetStack.resumeTopActivityLocked(target, targetOptions); } for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) { final ArrayList<ActivityStack> stacks = mActivityDisplays.valueAt(displayNdx).mStacks; for (int stackNdx = stacks.size() - 1; stackNdx >= 0; --stackNdx) { final ActivityStack stack = stacks.get(stackNdx); if (stack == targetStack) { // Already started above. continue; } if (isFrontStack(stack)) { stack.resumeTopActivityLocked(null); } } } return result; } 判斷目標任務棧是否在前面，還是調用ActivityStack類的
ActivityStack.resumeTopActivityLocked：
final boolean resumeTopActivityLocked(ActivityRecord prev, Bundle options) { if (mStackSupervisor.inResumeTopActivity) { return false; } boolean result = false; try { mStackSupervisor.inResumeTopActivity = true; //鎖屏相關 if (mService.mLockScreenShown == ActivityManagerService.LOCK_SCREEN_LEAVING) { mService.mLockScreenShown = ActivityManagerService.LOCK_SCREEN_HIDDEN; mService.updateSleepIfNeededLocked(); } result = resumeTopActivityInnerLocked(prev, options); } finally { mStackSupervisor.inResumeTopActivity = false; } return result; } 確保當前棧頂的Activity是否resumed，然后調用
ActivityStack.resumeTopActivityInnerLocked（前方代碼過長預警）：
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, Bundle options) { if (!mService.mBooting && !mService.mBooted) { // AMS未初始化 return false; } ActivityRecord parent = mActivityContainer.mParentActivity; if ((parent != null && parent.state != ActivityState.RESUMED) || !mActivityContainer.isAttachedLocked()) { //如果parent沒有resume，直接返回 return false; } cancelInitializingActivities(); // 尋找第一個棧頂沒有結束的Activity final ActivityRecord next = topRunningActivityLocked(null); //用戶離開標志 final boolean userLeaving = mStackSupervisor.mUserLeaving; mStackSupervisor.mUserLeaving = false; final TaskRecord prevTask = prev != null ? prev.task : null; //如果next為null，啟動Launcher界面 if (next == null) { // There are no more activities! final String reason = "noMoreActivities"; if (!mFullscreen) { //如果當前棧沒有遮蓋整個屏幕，將焦點移到下一個可見的任務棧的正在運行的Activity final ActivityStack stack = getNextVisibleStackLocked(); if (adjustFocusToNextVisibleStackLocked(stack, reason)) { return mStackSupervisor.resumeTopActivitiesLocked(stack, prev, null); } } // Let's just start up the Launcher... ActivityOptions.abort(options); if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: No more activities go home"); if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); // Only resume home if on home display final int returnTaskType = prevTask == null || !prevTask.isOverHomeStack() ? HOME_ACTIVITY_TYPE : prevTask.getTaskToReturnTo(); return isOnHomeDisplay() && mStackSupervisor.resumeHomeStackTask(returnTaskType, prev, reason); } next.delayedResume = false; /** mResumedActivity--系統當前激活的Activity組件 * mLastPausedActivity--上一次被中止的Activity組件 * mPausingActivity--正在被中止的Activity組件 **/ //如果棧頂的Activity已經是resumed，不做任何事 if (mResumedActivity == next && next.state == ActivityState.RESUMED && mStackSupervisor.allResumedActivitiesComplete()) { mWindowManager.executeAppTransition(); mNoAnimActivities.clear(); ActivityOptions.abort(options); if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Top activity resumed " + next); if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); return false; } //判斷下個任務，上一個任務，決定是否調轉到home final TaskRecord nextTask = next.task; if (prevTask != null && prevTask.stack == this && prevTask.isOverHomeStack() && prev.finishing && prev.frontOfTask) { if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); if (prevTask == nextTask) { prevTask.setFrontOfTask(); } else if (prevTask != topTask()) { final int taskNdx = mTaskHistory.indexOf(prevTask) + 1; mTaskHistory.get(taskNdx).setTaskToReturnTo(HOME_ACTIVITY_TYPE); } else if (!isOnHomeDisplay()) { return false; } else if (!isHomeStack()){ if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Launching home next"); final int returnTaskType = prevTask == null || !prevTask.isOverHomeStack() ? HOME_ACTIVITY_TYPE : prevTask.getTaskToReturnTo(); return isOnHomeDisplay() && mStackSupervisor.resumeHomeStackTask(returnTaskType, prev, "prevFinished"); } } //如果系統要進入關機或者休眠狀態，並且下一個Activity是上一次已經終止的Activity，直接返回 if (mService.isSleepingOrShuttingDown() && mLastPausedActivity == next && mStackSupervisor.allPausedActivitiesComplete()) { mWindowManager.executeAppTransition(); mNoAnimActivities.clear(); ActivityOptions.abort(options); if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Going to sleep and all paused"); if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); return false; } //如果不確定是哪個用戶打開的Activity，直接返回 if (mService.mStartedUsers.get(next.userId) == null) { Slog.w(TAG, "Skipping resume of top activity " + next + ": user " + next.userId + " is stopped"); if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); return false; } //如果當前啟動的Activity正在准備stop，終止這個操作 mStackSupervisor.mStoppingActivities.remove(next); mStackSupervisor.mGoingToSleepActivities.remove(next); next.sleeping = false; mStackSupervisor.mWaitingVisibleActivities.remove(next); ...... //如果有正正在pause的Activity，直接返回 if (!mStackSupervisor.allPausedActivitiesComplete()) { if (DEBUG_SWITCH || DEBUG_PAUSE || DEBUG_STATES) Slog.v(TAG_PAUSE, "resumeTopActivityLocked: Skip resume: some activity pausing."); if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); return false; } ...... boolean dontWaitForPause = (next.info.flags&ActivityInfo.FLAG_RESUME_WHILE_PAUSING) != 0; boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, true, dontWaitForPause); //表示當前的Activity正在運行 if (mResumedActivity != null) { if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Pausing " + mResumedActivity); //*********************讓當前運行的Activity進入pause狀態********************* pausing |= startPausingLocked(userLeaving, false, true, dontWaitForPause); } if (pausing) { if (DEBUG_SWITCH || DEBUG_STATES) Slog.v(TAG_STATES, "resumeTopActivityLocked: Skip resume: need to start pausing"); //將即將啟動的Activity放到LRU list，因為直接殺死的話會造成資源浪費 if (next.app != null && next.app.thread != null) { mService.updateLruProcessLocked(next.app, true, null); } if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); return true; } // If the most recent activity was noHistory but was only stopped rather // than stopped+finished because the device went to sleep, we need to make // sure to finish it as we're making a new activity topmost. if (mService.isSleeping() && mLastNoHistoryActivity != null && !mLastNoHistoryActivity.finishing) { if (DEBUG_STATES) Slog.d(TAG_STATES, "no-history finish of " + mLastNoHistoryActivity + " on new resume"); requestFinishActivityLocked(mLastNoHistoryActivity.appToken, Activity.RESULT_CANCELED, null, "resume-no-history", false); mLastNoHistoryActivity = null; } // if (prev != null && prev != next) { if (!mStackSupervisor.mWaitingVisibleActivities.contains(prev) && next != null && !next.nowVisible) { mStackSupervisor.mWaitingVisibleActivities.add(prev); if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Resuming top, waiting visible to hide: " + prev); } else { //隱藏上一個Activity的界面，必須在finish狀態，不然報錯 if (prev.finishing) { mWindowManager.setAppVisibility(prev.appToken, false); if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Not waiting for visible to hide: " + prev + ", waitingVisible=" + mStackSupervisor.mWaitingVisibleActivities.contains(prev) + ", nowVisible=" + next.nowVisible); } else { if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Previous already visible but still waiting to hide: " + prev + ", waitingVisible=" + mStackSupervisor.mWaitingVisibleActivities.contains(prev) + ", nowVisible=" + next.nowVisible); } } } //加載Activity，核實用戶id是否正確 try { AppGlobals.getPackageManager().setPackageStoppedState( next.packageName, false, next.userId); } catch (RemoteException e1) { } catch (IllegalArgumentException e) { Slog.w(TAG, "Failed trying to unstop package " + next.packageName + ": " + e); } //打開動畫，並且通知WindowManager關閉上一個Activity boolean anim = true; if (prev != null) { if (prev.finishing) { if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION, "Prepare close transition: prev=" + prev); if (mNoAnimActivities.contains(prev)) { anim = false; mWindowManager.prepareAppTransition(AppTransition.TRANSIT_NONE, false); } else { mWindowManager.prepareAppTransition(prev.task == next.task ? AppTransition.TRANSIT_ACTIVITY_CLOSE : AppTransition.TRANSIT_TASK_CLOSE, false); } mWindowManager.setAppWillBeHidden(prev.appToken); mWindowManager.setAppVisibility(prev.appToken, false); } else { if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION, "Prepare open transition: prev=" + prev); if (mNoAnimActivities.contains(next)) { anim = false; mWindowManager.prepareAppTransition(AppTransition.TRANSIT_NONE, false); } else { mWindowManager.prepareAppTransition(prev.task == next.task ? AppTransition.TRANSIT_ACTIVITY_OPEN : next.mLaunchTaskBehind ? AppTransition.TRANSIT_TASK_OPEN_BEHIND : AppTransition.TRANSIT_TASK_OPEN, false); } } if (false) { mWindowManager.setAppWillBeHidden(prev.appToken); mWindowManager.setAppVisibility(prev.appToken, false); } } else { if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION, "Prepare open transition: no previous"); if (mNoAnimActivities.contains(next)) { anim = false; mWindowManager.prepareAppTransition(AppTransition.TRANSIT_NONE, false); } else { mWindowManager.prepareAppTransition(AppTransition.TRANSIT_ACTIVITY_OPEN, false); } } ...... ActivityStack lastStack = mStackSupervisor.getLastStack(); //如果Activity所在的進程已經存在 if (next.app != null && next.app.thread != null) { if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Resume running: " + next); // 顯示Activity mWindowManager.setAppVisibility(next.appToken, true); // 開始計時 next.startLaunchTickingLocked(); ActivityRecord lastResumedActivity = lastStack == null ? null :lastStack.mResumedActivity; ActivityState lastState = next.state; mService.updateCpuStats(); //設置Activity狀態 if (DEBUG_STATES) Slog.v(TAG_STATES, "Moving to RESUMED: " + next + " (in existing)"); next.state = ActivityState.RESUMED; mResumedActivity = next; next.task.touchActiveTime(); mRecentTasks.addLocked(next.task); mService.updateLruProcessLocked(next.app, true, null); updateLRUListLocked(next); mService.updateOomAdjLocked(); //根據Activity設置屏幕方向 boolean notUpdated = true; if (mStackSupervisor.isFrontStack(this)) { Configuration config = mWindowManager.updateOrientationFromAppTokens( mService.mConfiguration, next.mayFreezeScreenLocked(next.app) ? next.appToken : null); if (config != null) { next.frozenBeforeDestroy = true; } notUpdated = !mService.updateConfigurationLocked(config, next, false, false); } //確保Activity保持在棧頂 if (notUpdated) { ActivityRecord nextNext = topRunningActivityLocked(null); if (DEBUG_SWITCH || DEBUG_STATES) Slog.i(TAG_STATES, "Activity config changed during resume: " + next + ", new next: " + nextNext); if (nextNext != next) { // Do over! mStackSupervisor.scheduleResumeTopActivities(); } if (mStackSupervisor.reportResumedActivityLocked(next)) { mNoAnimActivities.clear(); if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); return true; } if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); return false; } try { ...... if (next.newIntents != null) { next.app.thread.scheduleNewIntent(next.newIntents, next.appToken); } ...... //是否睡眠 next.sleeping = false; //兼容模式提示框 mService.showAskCompatModeDialogLocked(next); //清理UI資源 next.app.pendingUiClean = true; //設置處理狀態 next.app.forceProcessStateUpTo(mService.mTopProcessState); //清理最近的選擇 next.clearOptionsLocked(); //*********************resume目標Activity********************* next.app.thread.scheduleResumeActivity(next.appToken, next.app.repProcState, mService.isNextTransitionForward(), resumeAnimOptions); mStackSupervisor.checkReadyForSleepLocked(); } catch (Exception e) { ...... } //設置可見 try { next.visible = true; completeResumeLocked(next); } catch (Exception e) { //出現意外，放棄操作，嘗試下一個Activity Slog.w(TAG, "Exception thrown during resume of " + next, e); requestFinishActivityLocked(next.appToken, Activity.RESULT_CANCELED, null, "resume-exception", true); if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); return true; } next.stopped = false; }else {//Activity所在的進程不存在的情況 // 重啟Activity if (!next.hasBeenLaunched) { next.hasBeenLaunched = true; } else { if (SHOW_APP_STARTING_PREVIEW) { mWindowManager.setAppStartingWindow( next.appToken, next.packageName, next.theme, mService.compatibilityInfoForPackageLocked( next.info.applicationInfo), next.nonLocalizedLabel, next.labelRes, next.icon, next.logo, next.windowFlags, null, true); } if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Restarting: " + next); } if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Restarting " + next); //*********************啟動Activity********************* mStackSupervisor.startSpecificActivityLocked(next, true, true); } if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked(); return true; 總的來說就是：
- 首先讓現在正在運行的Activity調用startPausingLocked進入pause狀態
- 如果要啟動的Activity不為空且所在的進程存在的話，所在的進程執行scheduleResumeActivity啟動Activity
- 如果Activity為空，所在的進程不存在，執行ActivityStackSupervisor.startSpecificActivityLocked
接下來繼續分析
++Android Activity啟動流程源碼全解析（2）++
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