简述Activity启动流程：activity启动流程实际上实现了一次跨进程通信，在此我们将activity启动流程划分为三个阶段1，调用发起者（客户端）；2，AMS(ATMS)处理过程；3，被调用方（服务端）处理请求。在简述之前先来了解一下这整个流程当中涉及的几个关键成员：
-Instrumentation
用于实现应用程序测试代码的基类。当在打开仪器的情况下运行时，这个类将在任何应用程序代码之前为您实例化，允许您监视系统与应用程序的所有交互。可以通过AndroidManifest.xml的<Instrumentation>标签描述该类的实现。
-ActivityManager
该类提供与Activity、Service和Process相关的信息以及交互方法， 可以被看作是ActivityManagerService的辅助类。也可以理解为ActivityManager服务是对Activity管理、运行时功能管理和运行时数据结构的封装，进程(Process)、应用程序/包、服务(Service)、任务(Task)信息等
-ServiceManager
ServiceManager底层最终调用的还是c++层的ServiceManager，它是Binder的守护服务，通过它能够获取在Android系统启动时注册的系统服务，这其中就包含这里提到的AMS
-ActivityManagerService
主要负责系统中四大组件的启动、切换、调度及应用进程的管理和调度等工作，其职责与操作系统中的进程管理和调度模块相类似。当发起进程启动或者组件启动时，都会通过Binder通信机制将请求传递给AMS，AMS再做统一处理
-ActivityStarter
此类收集了用于确定将意图和标志如何转换为活动以及相关联的任务和堆栈的所有逻辑。
-ActivityThread
管理应用程序进程中主线程的执行，根据Activity管理者的请求调度和执行activities、broadcasts及其相关的操作。严格来说，UI主线程不是ActivityThread。ActivityThread类是Android APP进程的初始类，它的main函数是这个APP进程的入口。APP进程中UI事件的执行代码段都是由ActivityThread提供的。也就是说，Main Thread实例是存在的，只是创建它的代码我们不可见。ActivityThread的main函数就是在这个Main Thread里被执行的。
-ApplicationThread
ApplicationThread是ActivityThread的内部类，也是一个Binder对象。在此处它是作为Activitythread和AMS通信的桥梁.
-ActivityStack
并不是一个Activity栈，真正意义上的Activity栈是TaskRecord，这个类是负责管理各个Activity栈，内部维护一个ArrayList<TaskRecord>。ActivityStack应该算是任务栈的描述，它管理者一个应用的所有TaskRecord和他们的状态
-ActivityStackSupervisor
负责所有Activity栈的管理。内部管理了mHomeStack、mFocusedStack和mLastFocusedStack三个Activity栈。其中，mHomeStack管理的是Launcher相关的Activity栈；mFocusedStack管理的是当前显示在前台Activity的Activity栈；mLastFocusedStack管理的是上一次显示在前台Activity的Activity栈。
-ClientLifecycleManager
用来管理多个客户端生命周期执行请求的管理类。
另外几个比较重要的概念及之间的关系
ActivityRecord
TaskRecord
ActivityStack
ActivityStackSupervisor
Activity在AMS 中存在的形式为ActivityRecord；
AMS以Task的方式管理Activity，Task在AMS存在的形式为TaskRecord；TaskRecord中的mActivities用栈的方式管理ActivityRecord；
TaskRecord在AMS中依靠ActivityStack去管理，从命名来看，ActivityStack更像Activity栈，但是ActivityStack并不是Activity 栈，而是负责管理TaskRecord的类，android系统中有三种ActivityStack（mHomeStack，mFocusedStack，mLastFocusedStack）；
ActivityStack同样有管理者，ActivityStackSupervisor负责管理ActivityStack；
每一个Activity和TaskRecord 都是属于某个进程，所以进程还需要在AMS有存在形式，进程在AMS中存在的形式就是ProcessRecord；
image.png
正文
一、客户端逻辑
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
@Nullable Bundle options) {
if (mParent == null) {
options = transferSpringboardActivityOptions(options);
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);
// TODO Consider clearing/flushing other event sources and events for child windows.
} else {
if (options != null) {
mParent.startActivityFromChild(this, intent, requestCode, options);
} else {
// Note we want to go through this method for compatibility with
// existing applications that may have overridden it.
mParent.startActivityFromChild(this, intent, requestCode);
}
}
}
启动一个Activity通常都是通过Activity的startActivity方法启动。之后Activity的startActivity方法会调用startActivityForResult方法来传递启动请求，在startActivityForResult方法中如果是第一次启动，mParent为空则会去调用Instrumentation.execStartActivity方法，否则调动Activity.startActivityFromChild方法。而Activity.startActivityFromChild最终也会通过调用Instrumentation.execStartActivity去执行启动逻辑
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
...
try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess(who);
int result = ActivityManager.getService()
.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;
}
public static IActivityTaskManager getService() {
return IActivityTaskManagerSingleton.get();
}
@UnsupportedAppUsage(trackingBug = 129726065)
private static final Singleton<IActivityTaskManager> IActivityTaskManagerSingleton =
new Singleton<IActivityTaskManager>() {
@Override
protected IActivityTaskManager create() {
final IBinder b = ServiceManager.getService(Context.ACTIVITY_TASK_SERVICE);//1
return IActivityTaskManager.Stub.asInterface(b); //2
}
};
在Instrumentation.execStartActivity中通过ActivityManager.getService获取了AMS的代理对象，并最终调用了AMS服务中的startActivity方法
至此，在客户端中的调用逻辑结束
二、AMS逻辑
1.AMS构造ActivityStarter
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, bOptions,
UserHandle.getCallingUserId());
}
@Override
public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
true /*validateIncomingUser*/);
}
public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId,
boolean validateIncomingUser) {
enforceNotIsolatedCaller("startActivity");
userId = mActivityStartController.checkTargetUser(userId, validateIncomingUser,
Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");
// TODO: Switch to user app stacks here.
return mActivityStartController.obtainStarter(intent, "startActivityAsUser")
.setCaller(caller)
.setCallingPackage(callingPackage)
.setResolvedType(resolvedType)
.setResultTo(resultTo)
.setResultWho(resultWho)
.setRequestCode(requestCode)
.setStartFlags(startFlags)
.setProfilerInfo(profilerInfo)
.setActivityOptions(bOptions)
.setMayWait(userId)
.execute();
}
ActivityManagerService.startActivity方法经过多个方法调用会去执行startActivityAsUser方法，在startActivityAsUser方法最后会通过ActivityStartController.obtainStarter方法获得一个包含所有启动信息的ActivityStarter对象并调用execute方法执行。
2.ActivityStarter中的逻辑
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java
int execute() {
try {
// TODO(b/64750076): Look into passing request directly to these methods to allow
// for transactional diffs and preprocessing.
if (mRequest.mayWait) { //1
return startActivityMayWait(mRequest.caller, mRequest.callingUid,
mRequest.callingPackage, mRequest.realCallingPid, mRequest.realCallingUid,
mRequest.intent, mRequest.resolvedType,
mRequest.voiceSession, mRequest.voiceInteractor, mRequest.resultTo,
mRequest.resultWho, mRequest.requestCode, mRequest.startFlags,
mRequest.profilerInfo, mRequest.waitResult, mRequest.globalConfig,
mRequest.activityOptions, mRequest.ignoreTargetSecurity, mRequest.userId,
mRequest.inTask, mRequest.reason,
mRequest.allowPendingRemoteAnimationRegistryLookup,
mRequest.originatingPendingIntent, mRequest.allowBackgroundActivityStart);//2
}
......
}
.......
}
因为在ActivityManagerService.startActivityAsUser中调用了ActivityStarter.setMayWait方法，所以这里 mRequest.mayWait值为true，会去调用startActivityMayWait方法。
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java
private 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 globalConfig, SafeActivityOptions options, boolean ignoreTargetSecurity,
int userId, TaskRecord inTask, String reason,
boolean allowPendingRemoteAnimationRegistryLookup) {
...
int res = startActivity(caller, intent, ephemeralIntent, resolvedType, aInfo, rInfo,
voiceSession, voiceInteractor, resultTo, resultWho, requestCode, callingPid,
callingUid, callingPackage, realCallingPid, realCallingUid, startFlags, options,
ignoreTargetSecurity, componentSpecified, outRecord, inTask, reason,
allowPendingRemoteAnimationRegistryLookup);
...
return res;
}
}
private int startActivity(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,
String callingPackage, int realCallingPid, int realCallingUid, int startFlags,
SafeActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
ActivityRecord[] outActivity, TaskRecord inTask, String reason,
boolean allowPendingRemoteAnimationRegistryLookup) {
...
mLastStartActivityResult = startActivity(caller, intent, ephemeralIntent, resolvedType,
aInfo, rInfo, voiceSession, voiceInteractor, resultTo, resultWho, requestCode,
callingPid, callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
options, ignoreTargetSecurity, componentSpecified, mLastStartActivityRecord,
inTask, allowPendingRemoteAnimationRegistryLookup);
...
return getExternalResult(mLastStartActivityResult);
}
private int startActivity(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,
String callingPackage, int realCallingPid, int realCallingUid, int startFlags,
SafeActivityOptions options,
boolean ignoreTargetSecurity, boolean componentSpecified, ActivityRecord[] outActivity,
TaskRecord inTask, boolean allowPendingRemoteAnimationRegistryLookup) {
...
return startActivity(r, sourceRecord, voiceSession, voiceInteractor, startFlags,
true /* doResume */, checkedOptions, inTask, outActivity);
}
private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
ActivityRecord[] outActivity) {
...
result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
startFlags, doResume, options, inTask, outActivity);
...
postStartActivityProcessing(r, result, mTargetStack);
return result;
}
ActivityStarter.startActivityMayWait方法中调用多个startActivity方法后会调用到一个比较重要的方法startActivityUnchecked。说它重要是因为这个方法里会根据启动标志位和Activity启动模式来决定如何启动一个Activity以及是否要调用deliverNewIntent方法通知Activity有一个Intent试图重新启动它。
private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
ActivityRecord[] outActivity) {
...
ActivityRecord reusedActivity = getReusableIntentActivity();
...
if (reusedActivity != null) {
// When the flags NEW_TASK and CLEAR_TASK are set, then the task gets reused but
// still needs to be a lock task mode violation since the task gets cleared out and
// the device would otherwise leave the locked task.
if (mService.getLockTaskController().isLockTaskModeViolation(reusedActivity.getTask(),
(mLaunchFlags & (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK))
== (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK))) {
Slog.e(TAG, "startActivityUnchecked: Attempt to violate Lock Task Mode");
return START_RETURN_LOCK_TASK_MODE_VIOLATION;
}
// True if we are clearing top and resetting of a standard (default) launch mode
// ({@code LAUNCH_MULTIPLE}) activity. The existing activity will be finished.
final boolean clearTopAndResetStandardLaunchMode =
(mLaunchFlags & (FLAG_ACTIVITY_CLEAR_TOP | FLAG_ACTIVITY_RESET_TASK_IF_NEEDED))
== (FLAG_ACTIVITY_CLEAR_TOP | FLAG_ACTIVITY_RESET_TASK_IF_NEEDED)
&& mLaunchMode == LAUNCH_MULTIPLE;
// If mStartActivity does not have a task associated with it, associate it with the
// reused activity's task. Do not do so if we're clearing top and resetting for a
// standard launchMode activity.
if (mStartActivity.getTask() == null && !clearTopAndResetStandardLaunchMode) {
mStartActivity.setTask(reusedActivity.getTask());
}
if (reusedActivity.getTask().intent == null) {
// This task was started because of movement of the activity based on affinity...
// Now that we are actually launching it, we can assign the base intent.
reusedActivity.getTask().setIntent(mStartActivity);
}
// This code path leads to delivering a new intent, we want to make sure we schedule it
// as the first operation, in case the activity will be resumed as a result of later
// operations.
if ((mLaunchFlags & FLAG_ACTIVITY_CLEAR_TOP) != 0
|| isDocumentLaunchesIntoExisting(mLaunchFlags)
|| isLaunchModeOneOf(LAUNCH_SINGLE_INSTANCE, LAUNCH_SINGLE_TASK)) {
final TaskRecord task = reusedActivity.getTask();
// In this situation we want to remove all activities from the task up to the one
// being started. In most cases this means we are resetting the task to its initial
// state.
final ActivityRecord top = task.performClearTaskForReuseLocked(mStartActivity,
mLaunchFlags);
// The above code can remove {@code reusedActivity} from the task, leading to the
// the {@code ActivityRecord} removing its reference to the {@code TaskRecord}. The
// task reference is needed in the call below to
// {@link setTargetStackAndMoveToFrontIfNeeded}.
if (reusedActivity.getTask() == null) {
reusedActivity.setTask(task);
}
if (top != null) {
if (top.frontOfTask) {
// Activity aliases may mean we use different intents for the top activity,
// so make sure the task now has the identity of the new intent.
top.getTask().setIntent(mStartActivity);
}
deliverNewIntent(top);
}
}
mSupervisor.sendPowerHintForLaunchStartIfNeeded(false /* forceSend */, reusedActivity);
reusedActivity = setTargetStackAndMoveToFrontIfNeeded(reusedActivity);
final ActivityRecord outResult =
outActivity != null && outActivity.length > 0 ? outActivity[0] : null;
// When there is a reused activity and the current result is a trampoline activity,
// set the reused activity as the result.
if (outResult != null && (outResult.finishing || outResult.noDisplay)) {
outActivity[0] = reusedActivity;
}
if ((mStartFlags & START_FLAG_ONLY_IF_NEEDED) != 0) {
// We don't need to start a new activity, and the client said not to do anything
// if that is the case, so this is it! And for paranoia, make sure we have
// correctly resumed the top activity.
resumeTargetStackIfNeeded();
return START_RETURN_INTENT_TO_CALLER;
}
if (reusedActivity != null) {
setTaskFromIntentActivity(reusedActivity);
if (!mAddingToTask && mReuseTask == null) {
// We didn't do anything... but it was needed (a.k.a., client don't use that
// intent!) And for paranoia, make sure we have correctly resumed the top activity.
resumeTargetStackIfNeeded();
if (outActivity != null && outActivity.length > 0) {
outActivity[0] = reusedActivity;
}
return mMovedToFront ? START_TASK_TO_FRONT : START_DELIVERED_TO_TOP;
}
}
}
if (mStartActivity.packageName == null) {
final ActivityStack sourceStack = mStartActivity.resultTo != null
? mStartActivity.resultTo.getStack() : null;
if (sourceStack != null) {
sourceStack.sendActivityResultLocked(-1 /* callingUid */, mStartActivity.resultTo,
mStartActivity.resultWho, mStartActivity.requestCode, RESULT_CANCELED,
null /* data */);
}
ActivityOptions.abort(mOptions);
return START_CLASS_NOT_FOUND;
}
// If the activity being launched is the same as the one currently at the top, then
// we need to check if it should only be launched once.
final ActivityStack topStack = mSupervisor.mFocusedStack;
final ActivityRecord topFocused = topStack.getTopActivity();
final ActivityRecord top = topStack.topRunningNonDelayedActivityLocked(mNotTop);
final boolean dontStart = top != null && mStartActivity.resultTo == null
&& top.realActivity.equals(mStartActivity.realActivity)
&& top.userId == mStartActivity.userId
&& top.app != null && top.app.thread != null
&& ((mLaunchFlags & FLAG_ACTIVITY_SINGLE_TOP) != 0
|| isLaunchModeOneOf(LAUNCH_SINGLE_TOP, LAUNCH_SINGLE_TASK));
if (dontStart) {
// For paranoia, make sure we have correctly resumed the top activity.
topStack.mLastPausedActivity = null;
if (mDoResume) {
mSupervisor.resumeFocusedStackTopActivityLocked();
}
ActivityOptions.abort(mOptions);
if ((mStartFlags & START_FLAG_ONLY_IF_NEEDED) != 0) {
// We don't need to start a new activity, and the client said not to do
// anything if that is the case, so this is it!
return START_RETURN_INTENT_TO_CALLER;
}
deliverNewIntent(top);
// Don't use mStartActivity.task to show the toast. We're not starting a new activity
// but reusing 'top'. Fields in mStartActivity may not be fully initialized.
mSupervisor.handleNonResizableTaskIfNeeded(top.getTask(), preferredWindowingMode,
preferredLaunchDisplayId, topStack);
return START_DELIVERED_TO_TOP;
}
boolean newTask = false;
final TaskRecord taskToAffiliate = (mLaunchTaskBehind && mSourceRecord != null)
? mSourceRecord.getTask() : null;
// Should this be considered a new task?
int result = START_SUCCESS;
if (mStartActivity.resultTo == null && mInTask == null && !mAddingToTask
&& (mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) != 0) {
newTask = true;
result = setTaskFromReuseOrCreateNewTask(taskToAffiliate, topStack);
} else if (mSourceRecord != null) {
result = setTaskFromSourceRecord();
} else if (mInTask != null) {
result = setTaskFromInTask();
} else {
// This not being started from an existing activity, and not part of a new task...
// just put it in the top task, though these days this case should never happen.
setTaskToCurrentTopOrCreateNewTask();
}
if (result != START_SUCCESS) {
return result;
}
mService.grantUriPermissionFromIntentLocked(mCallingUid, mStartActivity.packageName,
mIntent, mStartActivity.getUriPermissionsLocked(), mStartActivity.userId);
mService.grantEphemeralAccessLocked(mStartActivity.userId, mIntent,
mStartActivity.appInfo.uid, UserHandle.getAppId(mCallingUid));
if (newTask) {
EventLog.writeEvent(EventLogTags.AM_CREATE_TASK, mStartActivity.userId,
mStartActivity.getTask().taskId);
}
ActivityStack.logStartActivity(
EventLogTags.AM_CREATE_ACTIVITY, mStartActivity, mStartActivity.getTask());
mTargetStack.mLastPausedActivity = null;
mSupervisor.sendPowerHintForLaunchStartIfNeeded(false /* forceSend */, mStartActivity);
mTargetStack.startActivityLocked(mStartActivity, topFocused, newTask, mKeepCurTransition,
mOptions);
if (mDoResume) {
final ActivityRecord topTaskActivity =
mStartActivity.getTask().topRunningActivityLocked();
if (!mTargetStack.isFocusable()
|| (topTaskActivity != null && topTaskActivity.mTaskOverlay
&& mStartActivity != topTaskActivity)) {
// If the activity is not focusable, we can't resume it, but still would like to
// make sure it becomes visible as it starts (this will also trigger entry
// animation). An example of this are PIP activities.
// Also, we don't want to resume activities in a task that currently has an overlay
// as the starting activity just needs to be in the visible paused state until the
// over is removed.
mTargetStack.ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
// Go ahead and tell window manager to execute app transition for this activity
// since the app transition will not be triggered through the resume channel.
mService.mWindowManager.executeAppTransition();
} else {
// If the target stack was not previously focusable (previous top running activity
// on that stack was not visible) then any prior calls to move the stack to the
// will not update the focused stack. If starting the new activity now allows the
// task stack to be focusable, then ensure that we now update the focused stack
// accordingly.
if (mTargetStack.isFocusable() && !mSupervisor.isFocusedStack(mTargetStack)) {
mTargetStack.moveToFront("startActivityUnchecked");
}
mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack, mStartActivity,
mOptions);
}
} else if (mStartActivity != null) {
mSupervisor.mRecentTasks.add(mStartActivity.getTask());
}
mSupervisor.updateUserStackLocked(mStartActivity.userId, mTargetStack);
mSupervisor.handleNonResizableTaskIfNeeded(mStartActivity.getTask(), preferredWindowingMode,
preferredLaunchDisplayId, mTargetStack);
return START_SUCCESS;
}
这里判断了Activity的启动模式，启动者和被启动者是否为同一个activity等等逻辑，但最终都调用了ActivityStackSupervisor .resumeFocusedStackTopActivityLocked方法
3.ActivityStackSupervisor和ActivityStack中的处理逻辑
frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java
boolean resumeFocusedStackTopActivityLocked(
ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
...
if (targetStack != null && isFocusedStack(targetStack)) {
return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}
final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
if (r == null || !r.isState(RESUMED)) {
mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
} else if (r.isState(RESUMED)) {
// Kick off any lingering app transitions form the MoveTaskToFront operation.
mFocusedStack.executeAppTransition(targetOptions);
}
...
return false;
}
frameworks/base/services/core/java/com/android/server/am/ActivityStack.java
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
...
result = resumeTopActivityInnerLocked(prev, options);
...
return result;
}
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
...
boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, next, false);
if (mResumedActivity != null) {
pausing |= startPausingLocked(userLeaving, false, next, false);
}
...
mStackSupervisor.startSpecificActivityLocked(next, true, true);
...
return true;
}
在ActivityStackSupervisor. resumeFocusedStackTopActivityLocked调用了ActivityStack中的resumeTopActivityUncheckedLocked方法，在ActivityStack.resumeTopActivityInnerLocked方法中会去判断是否有Activity处于Resume状态，如果有的话会先让这个Activity执行Pausing过程，然后再执行startSpecificActivityLocked方法启动要启动Activity。此处分开两段，先看一下栈顶Activity是如何退出的。
①栈顶activity退出
frameworks/base/services/core/java/com/android/server/am/ActivityStack.java
final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping,
ActivityRecord resuming, boolean pauseImmediately) {
...
if (prev.app != null && prev.app.thread != null) {
try {
EventLogTags.writeAmPauseActivity(prev.userId, System.identityHashCode(prev),
prev.shortComponentName, "userLeaving=" + userLeaving);
mService.updateUsageStats(prev, false);
// Android 9.0在这里引入了ClientLifecycleManager和
// ClientTransactionHandler来辅助管理Activity生命周期，
// 他会发送EXECUTE_TRANSACTION消息到ActivityThread.H里面继续处理。
mService.getLifecycleManager().scheduleTransaction(prev.app.thread, prev.appToken,
PauseActivityItem.obtain(prev.finishing, userLeaving,
prev.configChangeFlags, pauseImmediately));
} catch (Exception e) {
// Ignore exception, if process died other code will cleanup.
Slog.w(TAG, "Exception thrown during pause", e);
mPausingActivity = null;
mLastPausedActivity = null;
mLastNoHistoryActivity = null;
}
} else {
mPausingActivity = null;
mLastPausedActivity = null;
mLastNoHistoryActivity = null;
}
...
}
在ActivityStack.startPausingLocked方法中通过ClientLifecycleManager的scheduleTransaction方法把PauseActivityItem事件加入到执行计划中，开始栈顶的pausing过程。
void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
final IApplicationThread client = transaction.getClient();
transaction.schedule();
...
}
frameworks/base/core/java/android/app/servertransaction/ClientTransaction.java
public void schedule() throws RemoteException {
mClient.scheduleTransaction(this);
}
ClientTransaction.schedule方法的mClient是一个IApplicationThread类型，ActivityThread的内部类ApplicationThread派生这个接口类并实现了对应的方法。所以直接跳转到ApplicationThread中的scheduleTransaction方法。ActivityThread类中并没有定义scheduleTransaction方法，所以调用的是他父类ClientTransactionHandler的scheduleTransaction方法。
frameworks/base/core/java/android/app/ActivityThread.java
private class ApplicationThread extends IApplicationThread.Stub {
...
@Override
public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
ActivityThread.this.scheduleTransaction(transaction);
}
}
frameworks/base/core/java/android/app/ClientTransactionHandler.java
void scheduleTransaction(ClientTransaction transaction) {
transaction.preExecute(this);
sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
}
在ClientTransactionHandler.scheduleTransaction方法中调用了sendMessage方法，这个方法是一个抽象方法，其实现在ClientTransactionHandler派生类的ActivityThread中，ActivityThread.sendMessage方法会把消息发送给内部名字叫H的Handler。
frameworks/base/core/java/android/app/ActivityThread.java
void sendMessage(int what, Object obj) {
sendMessage(what, obj, 0, 0, false);
}
private void sendMessage(int what, Object obj, int arg1) {
sendMessage(what, obj, arg1, 0, false);
}
private void sendMessage(int what, Object obj, int arg1, int arg2) {
sendMessage(what, obj, arg1, arg2, false);
}
private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
if (DEBUG_MESSAGES) Slog.v(
TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
+ ": " + arg1 + " / " + obj);
Message msg = Message.obtain();
msg.what = what;
msg.obj = obj;
msg.arg1 = arg1;
msg.arg2 = arg2;
if (async) {
msg.setAsynchronous(true);
}
mH.sendMessage(msg);
}
public void handleMessage(Message msg) {
...
case EXECUTE_TRANSACTION:
final ClientTransaction transaction = (ClientTransaction) msg.obj;
mTransactionExecutor.execute(transaction);
...
break;
...
}
Handler H的实例接收到EXECUTE_TRANSACTION消息后调用TransactionExecutor.execute方法切换Activity状态。TransactionExecutor.execute方法里面先执行Callbacks，然后改变Activity当前的生命周期状态。此处由于没有Callback所以直接跳转executeLifecycleState方法。
frameworks/base/core/java/android/app/servertransaction/TransactionExecutor.java
public void execute(ClientTransaction transaction) {
...
executeCallbacks(transaction);
executeLifecycleState(transaction);
...
}
public void executeCallbacks(ClientTransaction transaction) {
if (callbacks == null) {
// No callbacks to execute, return early.
return;
}
...
for (int i = 0; i < size; ++i) {
final ClientTransactionItem item = callbacks.get(i);
...
item.execute(mTransactionHandler, token, mPendingActions);
item.postExecute(mTransactionHandler, token, mPendingActions);
...
}
}
/** Transition to the final state if requested by the transaction. */
private void executeLifecycleState(ClientTransaction transaction) {
...
// Cycle to the state right before the final requested state.
cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */);
// Execute the final transition with proper parameters.
lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
}
在executeLifecycleState方法里面，会先去调用TransactionExecutor.cycleToPath执行当前生命周期状态之前的状态，然后执行ActivityLifecycleItem.execute方法。由于是从ON_RESUME状态到ON_PAUSE状态切换，中间没有其他状态，cycleToPath这个情况下没有做什么实质性的事情，直接执行execute方法。前面在ActivityStack.startPausingLocked方法里面scheduleTransaction传递的是PauseActivityItem对象，所以executeLifecycleState方法里调用的execute方法其实是PauseActivityItem.execute方法。
frameworks/base/core/java/android/app/servertransaction/PauseActivityItem.java
@Override
public void execute(ClientTransactionHandler client, IBinder token,
PendingTransactionActions pendingActions) {
...
client.handlePauseActivity(token, mFinished, mUserLeaving, mConfigChanges, pendingActions,
"PAUSE_ACTIVITY_ITEM");
...
}
在PauseActivityItem.execute方法中调用ActivityThread.handlePauseActivity方法，经过一步步调用来到performPauseActivity方法，在这个方法中会先去判断是否需要调用callActivityOnSaveInstanceState方法来保存临时数据，然后执行Instrumentation.callActivityOnPause方法继续执行pasue流程。
frameworks/base/core/java/android/app/ActivityThread.java
@Override
public void handlePauseActivity(IBinder token, boolean finished, boolean userLeaving,
int configChanges, PendingTransactionActions pendingActions, String reason) {
...
performPauseActivity(r, finished, reason, pendingActions);
...
}
}
final Bundle performPauseActivity(IBinder token, boolean finished, String reason,
PendingTransactionActions pendingActions) {
ActivityClientRecord r = mActivities.get(token);
return r != null ? performPauseActivity(r, finished, reason, pendingActions) : null;
}
/**
* Pause the activity.
* @return Saved instance state for pre-Honeycomb apps if it was saved, {@code null} otherwise.
*/
private Bundle performPauseActivity(ActivityClientRecord r, boolean finished, String reason,
PendingTransactionActions pendingActions) {
...
if (shouldSaveState) {
callActivityOnSaveInstanceState(r);
}
performPauseActivityIfNeeded(r, reason);
...
return shouldSaveState ? r.state : null;
}
private void performPauseActivityIfNeeded(ActivityClientRecord r, String reason) {
...
try {
r.activity.mCalled = false;
mInstrumentation.callActivityOnPause(r.activity);
...
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
if (!mInstrumentation.onException(r.activity, e)) {
throw new RuntimeException("Unable to pause activity "
+ safeToComponentShortString(r.intent) + ": " + e.toString(), e);
}
}
r.setState(ON_PAUSE);
}
Instrumentation.callActivityOnPause方法中直接调用Activity.performPause，在performPause方法中我们终于看到了熟悉的身影Activity生命周期的onPause方法，至此栈顶Activity的Pausing流程全部完毕。
frameworks/base/core/java/android/app/Instrumentation.java
public void callActivityOnPause(Activity activity) {
activity.performPause();
}
frameworks/base/core/java/android/app/Activity.java
final void performPause() {
...
onPause();
...
}
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