1，跳转页面startActivity的启动流程
分析源码
Intent intent = new Intent(this, TestActivity.class);
this.startActivity(intent);
//下面我们就来对Activity的工作流程进行梳理，达到对Activity整体流程的掌握。从startActivity方法开始，会走到
@Override
public void startActivity(Intent intent, @Nullable Bundle options) {
if (mIntent != null && mIntent.hasExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN)
&& mIntent.hasExtra(AutofillManager.EXTRA_RESTORE_CROSS_ACTIVITY)) {
if (TextUtils.equals(getPackageName(),
intent.resolveActivity(getPackageManager()).getPackageName())) {
// Apply Autofill restore mechanism on the started activity by startActivity()
final IBinder token =
mIntent.getIBinderExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN);
// Remove restore ability from current activity
mIntent.removeExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN);
mIntent.removeExtra(AutofillManager.EXTRA_RESTORE_CROSS_ACTIVITY);
// Put restore token
intent.putExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN, token);
intent.putExtra(AutofillManager.EXTRA_RESTORE_CROSS_ACTIVITY, true);
}
}
if (options != null) {
startActivityForResult(intent, -1, options);
} else {
// Note we want to go through this call for compatibility with
// applications that may have overridden the method.
startActivityForResult(intent, -1);
}
}
startActivityForResult方法：
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);
} else {
...
}
}
看到里面调用了mInstrumentation.execStartActivity方法，其中一个参数mMainThread.getApplicationThread()，它的类型是ApplicationThread，ApplicationThread是ActivityThread的内部类，继承IApplicationThread.Stub，也是个Binder对象，在Activity工作流程中有重要作用。而Instrumentation具有跟踪application及activity生命周期的功能，用于android 应用测试框架中代码检测。接着看下mInstrumentation.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);
}
...
try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess(who);
int result = ActivityTaskManager.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;
}
这里看到Activity的启动又交给了ActivityTaskManager.getService()，这是啥？跟进去看看：
//ActivityTaskManager
public static IActivityTaskManager getService() {
return IActivityTaskManagerSingleton.get();
}
private static final Singleton<IActivityTaskManager> IActivityTaskManagerSingleton =
new Singleton<IActivityTaskManager>() {
@Override
protected IActivityTaskManager create() {
final IBinder b = ServiceManager.getService(Context.ACTIVITY_TASK_SERVICE);
return IActivityTaskManager.Stub.asInterface(b);
}
};
看到IBinder这个标志，这里你应该明白了：这里是获取一个跨进程的服务。获取的什么服务呢？是ActivityTaskManagerService（ATMS），它继承于IActivityTaskManager.Stub，是个Binder对象，并且是通过单例提供服务的。 ATMS是用于管理Activity及其容器（任务、堆栈、显示等）的系统服务，运行在系统服务进程（system_server）之中。
值得说明的是，ATMS是在Android10中新增的，分担了之前ActivityManagerService（AMS）的一部分功能（activity task相关）。
在Android10 之前 ，这个地方获取的是服务是AMS。查看Android10的AMS，你会发现startActivity方法内也是调用了ATMS的startActivity方法。所以在理解上，ATMS就隶属于AMS。
接着看，ActivityTaskManager.getService().startActivity有个返回值result，且调用了checkStartActivityResult(result, intent)：
public static void checkStartActivityResult(int res, Object intent) {
if (!ActivityManager.isStartResultFatalError(res)) {
return;
}
switch (res) {
case ActivityManager.START_INTENT_NOT_RESOLVED:
case ActivityManager.START_CLASS_NOT_FOUND:
if (intent instanceof Intent && ((Intent)intent).getComponent() != null)
throw new ActivityNotFoundException(
"Unable to find explicit activity class "
+ ((Intent)intent).getComponent().toShortString()
+ "; have you declared this activity in your AndroidManifest.xml?");
throw new ActivityNotFoundException(
"No Activity found to handle " + intent);
case ActivityManager.START_PERMISSION_DENIED:
throw new SecurityException("Not allowed to start activity "
+ intent);
...
case ActivityManager.START_CANCELED:
throw new AndroidRuntimeException("Activity could not be started for "
+ intent);
default:
throw new AndroidRuntimeException("Unknown error code "
+ res + " when starting " + intent);
}
}
Activity的管理——AMS
好了，到这里，Activity的启动就跨进程（IPC）的转移到系统进程提供的服务ATMS中了，接着看ATMS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 bOptions) {
return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, bOptions,
UserHandle.getCallingUserId());
}
@Override
public 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*/);
}
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("startActivityAsUser");
userId = getActivityStartController().checkTargetUser(userId, validateIncomingUser,
Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");
// TODO: Switch to user app stacks here.
return getActivityStartController().obtainStarter(intent, "startActivityAsUser")
.setCaller(caller)
.setCallingPackage(callingPackage)
.setResolvedType(resolvedType)
.setResultTo(resultTo)
.setResultWho(resultWho)
.setRequestCode(requestCode)
.setStartFlags(startFlags)
.setProfilerInfo(profilerInfo)
.setActivityOptions(bOptions)
.setMayWait(userId)
.execute();
}
跟到startActivityAsUser中，通过getActivityStartController().obtainStarter方法获取ActivityStarter实例 然后调用一系列方法，最后的execute()方法是开始启动activity：
int execute() {
try {
// TODO(b/64750076): Look into passing request directly to these methods to allow
// for transactional diffs and preprocessing.
if (mRequest.mayWait) {
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);
} else {
return startActivity(mRequest.caller, mRequest.intent, mRequest.ephemeralIntent,
mRequest.resolvedType, mRequest.activityInfo, mRequest.resolveInfo,
mRequest.voiceSession, mRequest.voiceInteractor, mRequest.resultTo,
mRequest.resultWho, mRequest.requestCode, mRequest.callingPid,
mRequest.callingUid, mRequest.callingPackage, mRequest.realCallingPid,
mRequest.realCallingUid, mRequest.startFlags, mRequest.activityOptions,
mRequest.ignoreTargetSecurity, mRequest.componentSpecified,
mRequest.outActivity, mRequest.inTask, mRequest.reason,
mRequest.allowPendingRemoteAnimationRegistryLookup,
mRequest.originatingPendingIntent, mRequest.allowBackgroundActivityStart);
}
} finally {
onExecutionComplete();
}
}
分了两种情况，不过 不论startActivityMayWait还是startActivity最终都是走到下面这个startActivity方法：
private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
ActivityRecord[] outActivity, boolean restrictedBgActivity) {
int result = START_CANCELED;
final ActivityStack startedActivityStack;
try {
mService.mWindowManager.deferSurfaceLayout();
result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
startFlags, doResume, options, inTask, outActivity, restrictedBgActivity);
} finally {
final ActivityStack currentStack = r.getActivityStack();
startedActivityStack = currentStack != null ? currentStack : mTargetStack;
...
}
postStartActivityProcessing(r, result, startedActivityStack);
return result;
}
里面有调用了startActivityUnchecked方法，之后调用RootActivityContainer的resumeFocusedStacksTopActivities方法。RootActivityContainer是Android10新增的类，分担了之前ActivityStackSupervisor的部分功能。接着跳转到ActivityStack的resumeTopActivityUncheckedLocked方法：
//ActivityStack
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
if (mInResumeTopActivity) {
// Don't even start recursing.
return false;
}
boolean result = false;
try {
mInResumeTopActivity = true;
result = resumeTopActivityInnerLocked(prev, options);
final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
if (next == null || !next.canTurnScreenOn()) {
checkReadyForSleep();
}
} finally {
mInResumeTopActivity = false;
}
return result;
}
跟进resumeTopActivityInnerLocked方法：
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
...
boolean pausing = getDisplay().pauseBackStacks(userLeaving, next, false);
if (mResumedActivity != null) {
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityLocked: Pausing " + mResumedActivity);
// 暂停上一个Activity
pausing |= startPausingLocked(userLeaving, false, next, false);
}
...
//这里next.attachedToProcess()，只有启动了的Activity才会返回true
if (next.attachedToProcess()) {
...
try {
final ClientTransaction transaction =
ClientTransaction.obtain(next.app.getThread(), next.appToken);
...
//启动了的Activity就发送ResumeActivityItem事务给客户端了，后面会讲到
transaction.setLifecycleStateRequest(
ResumeActivityItem.obtain(next.app.getReportedProcState(),
getDisplay().mDisplayContent.isNextTransitionForward()));
mService.getLifecycleManager().scheduleTransaction(transaction);
....
} catch (Exception e) {
....
mStackSupervisor.startSpecificActivityLocked(next, true, false);
return true;
}
....
} else {
....
if (SHOW_APP_STARTING_PREVIEW) {
//这里就是 冷启动时 出现白屏 的原因了：取根activity的主题背景 展示StartingWindow
next.showStartingWindow(null , false ,false);
}
// 继续当前Activity，普通activity的正常启动 关注这里即可
mStackSupervisor.startSpecificActivityLocked(next, true, true);
}
return true;
}
先对上一个Activity执行pause操作，再执行当前创建操作，代码最终进入到了ActivityStackSupervisor.startSpecificActivityLocked方法中。这里有个点注意下，启动activity前调用了next.showStartingWindow方法来展示一个window，这就是冷启动时 出现白屏 的原因了。我们继续看ActivityStackSupervisor.startSpecificActivityLocked方法：
void startSpecificActivityLocked(ActivityRecord r, boolean andResume, boolean checkConfig) {
// Is this activity's application already running?
final WindowProcessController wpc =
mService.getProcessController(r.processName, r.info.applicationInfo.uid);
boolean knownToBeDead = false;
if (wpc != null && wpc.hasThread()) {
try {
realStartActivityLocked(r, wpc, andResume, checkConfig);
return;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting activity "
+ r.intent.getComponent().flattenToShortString(), e);
}
knownToBeDead = true;
}
...
try {
if (Trace.isTagEnabled(TRACE_TAG_ACTIVITY_MANAGER)) {
Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "dispatchingStartProcess:"
+ r.processName);
}
// 上面的wpc != null && wpc.hasThread()不满足的话，说明没有进程，就会取创建进程
final Message msg = PooledLambda.obtainMessage(
ActivityManagerInternal::startProcess, mService.mAmInternal, r.processName,
r.info.applicationInfo, knownToBeDead, "activity", r.intent.getComponent());
mService.mH.sendMessage(msg);
} finally {
Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}
}
有个判断条件if (wpc != null && wpc.hasThread())，意思是是否启动了应用进程，内部是通过IApplicationThread是否为空来判断。这里我们只看已启动应用进程的情况，及调用了realStartActivityLocked方法，，：
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
boolean andResume, boolean checkConfig) throws RemoteException {
...
// Create activity launch transaction.
final ClientTransaction clientTransaction = ClientTransaction.obtain(
proc.getThread(), r.appToken);
final DisplayContent dc = r.getDisplay().mDisplayContent;
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
System.identityHashCode(r), r.info,
// TODO: Have this take the merged configuration instead of separate global
// and override configs.
mergedConfiguration.getGlobalConfiguration(),
mergedConfiguration.getOverrideConfiguration(), r.compat,
r.launchedFromPackage, task.voiceInteractor, proc.getReportedProcState(),
r.icicle, r.persistentState, results, newIntents,
dc.isNextTransitionForward(), proc.createProfilerInfoIfNeeded(),
r.assistToken));
// Set desired final state.
final ActivityLifecycleItem lifecycleItem;
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// Schedule transaction.
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
...
return true;
}
中间有段代码如上，通过 ClientTransaction.obtain( proc.getThread(), r.appToken)获取了clientTransaction，其中参数proc.getThread()是IApplicationThread，就是前面提到的ApplicationThread在系统进程的代理。
ClientTransaction是包含一系列的待客户端处理的事务的容器，客户端接收后取出事务并执行。
接着看，使用clientTransaction.addCallback添加了LaunchActivityItem实例：
//都是用来发送到客户端的
private List<ClientTransactionItem> mActivityCallbacks;
public void addCallback(ClientTransactionItem activityCallback) {
if (mActivityCallbacks == null) {
mActivityCallbacks = new ArrayList<>();
}
mActivityCallbacks.add(activityCallback);
}
看下LaunchActivityItem实例的获取：
/** Obtain an instance initialized with provided params. */
public static LaunchActivityItem obtain(Intent intent, int ident, ActivityInfo info,
Configuration curConfig, Configuration overrideConfig, CompatibilityInfo compatInfo,
String referrer, IVoiceInteractor voiceInteractor, int procState, Bundle state,
PersistableBundle persistentState, List<ResultInfo> pendingResults,
List<ReferrerIntent> pendingNewIntents, boolean isForward, ProfilerInfo profilerInfo,
IBinder assistToken) {
LaunchActivityItem instance = ObjectPool.obtain(LaunchActivityItem.class);
if (instance == null) {
instance = new LaunchActivityItem();
}
setValues(instance, intent, ident, info, curConfig, overrideConfig, compatInfo, referrer,
voiceInteractor, procState, state, persistentState, pendingResults,
pendingNewIntents, isForward, profilerInfo, assistToken);
return instance;
}
new了一个LaunchActivityItem然后设置各种值。我们从名字就能看出，它就是用来启动activity的。它是怎么发挥作用的呢？接着看：
回到realStartActivityLocked方法，接着调用了
mService.getLifecycleManager().scheduleTransaction(clientTransaction)，mService是ActivityTaskManagerService，getLifecycleManager()方法获取的是ClientLifecycleManager实例，它的scheduleTransaction方法如下：
void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
final IApplicationThread client = transaction.getClient();
transaction.schedule();
if (!(client instanceof Binder)) {
transaction.recycle();
}
}
很简单，就是调用IApplicationThread的scheduleTransaction方法。由于IApplicationThread是ApplicationThread在系统进程的代理，所以真正执行的地方就是 客户端的ApplicationThread中了。也就是说，Activity启动的操作又跨进程的还给了客户端。
好了，到这里我们稍稍梳理下：启动Activity的操作从客户端 跨进程 转移到 ATMS，ATMS通过ActivityStarter、ActivityStack、ActivityStackSupervisor 对 Activity任务、activity栈、Activity记录 管理后，又用过跨进程把正在启动过程又转移到了客户端。
2.3 线程切换及消息处理——mH
接着上面的分析，我们找到ApplicationThread的scheduleTransaction方法：
@Override
public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
ActivityThread.this.scheduleTransaction(transaction);
}
那就再看ActivityThread的scheduleTransaction方法，实际在其父类ClientTransactionHandler中：
void scheduleTransaction(ClientTransaction transaction) {
transaction.preExecute(this);
sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
}
使用sendMessage发送消息，参数是ActivityThread.H.EXECUTE_TRANSACTION和transaction，接着看
void sendMessage(int what, Object obj) {
sendMessage(what, obj, 0, 0, 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);
}
最后调用了mH.sendMessage(msg)，mH是个啥？我们看看：
//ActivityThread
final H mH = new H();
class H extends Handler {
public static final int BIND_APPLICATION = 110;
@UnsupportedAppUsage
public static final int EXIT_APPLICATION = 111;
@UnsupportedAppUsage
public static final int RECEIVER = 113;
@UnsupportedAppUsage
public static final int CREATE_SERVICE = 114;
@UnsupportedAppUsage
public static final int SERVICE_ARGS = 115;
...
public static final int EXECUTE_TRANSACTION = 159;
public static final int RELAUNCH_ACTIVITY = 160;
...
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
case BIND_APPLICATION:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
AppBindData data = (AppBindData)msg.obj;
handleBindApplication(data);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
case EXIT_APPLICATION:
if (mInitialApplication != null) {
mInitialApplication.onTerminate();
}
Looper.myLooper().quit();
break;
case RECEIVER:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastReceiveComp");
handleReceiver((ReceiverData)msg.obj);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
case CREATE_SERVICE:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceCreate: " + String.valueOf(msg.obj)));
handleCreateService((CreateServiceData)msg.obj);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
case BIND_SERVICE:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
handleBindService((BindServiceData)msg.obj);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
...
case EXECUTE_TRANSACTION:
final ClientTransaction transaction = (ClientTransaction) msg.obj;
mTransactionExecutor.execute(transaction);
if (isSystem()) {
// Client transactions inside system process are recycled on the client side
// instead of ClientLifecycleManager to avoid being cleared before this
// message is handled.
transaction.recycle();
}
break;
case RELAUNCH_ACTIVITY:
handleRelaunchActivityLocally((IBinder) msg.obj);
break;
...
}
Object obj = msg.obj;
if (obj instanceof SomeArgs) {
((SomeArgs) obj).recycle();
}
if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));
}
}
mH是在创建ActivityThread实例时赋值的，是自定义Handler子类H的实例，也就是在ActivityThread的main方法中，并且初始化是已经主线程已经有了mainLooper，所以，使用这个mH来sendMessage就把消息发送到了主线程。
那么是从哪个线程发送的呢？那就要看看ApplicationThread的scheduleTransaction方法是执行在哪个线程了。根据IPC知识，我们知道，服务器的Binder方法运行在Binder的线程池中，也就是说系统进行跨进程调用ApplicationThread的scheduleTransaction就是执行在Binder的线程池中的了。
到这里，消息就在主线程处理了，那么是怎么处理Activity的启动的呢？接着看。我们找到ActivityThread.H.EXECUTE_TRANSACTION这个消息的处理，就在handleMessage方法的倒数第三个case（就在上面代码）：取出ClientTransaction实例，调用TransactionExecutor的execute方法，那就看看：
public void execute(ClientTransaction transaction) {
if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Start resolving transaction");
final IBinder token = transaction.getActivityToken();
...
executeCallbacks(transaction);
executeLifecycleState(transaction);
...
}
继续跟进executeCallbacks方法：
public void executeCallbacks(ClientTransaction transaction) {
final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
if (callbacks == null || callbacks.isEmpty()) {
// No callbacks to execute, return early.
return;
}
if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Resolving callbacks in transaction");
final IBinder token = transaction.getActivityToken();
ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
// In case when post-execution state of the last callback matches the final state requested
// for the activity in this transaction, we won't do the last transition here and do it when
// moving to final state instead (because it may contain additional parameters from server).
final ActivityLifecycleItem finalStateRequest = transaction.getLifecycleStateRequest();
final int finalState = finalStateRequest != null ? finalStateRequest.getTargetState()
: UNDEFINED;
// Index of the last callback that requests some post-execution state.
final int lastCallbackRequestingState = lastCallbackRequestingState(transaction);
final int size = callbacks.size();
for (int i = 0; i < size; ++i) {
final ClientTransactionItem item = callbacks.get(i);
...
item.execute(mTransactionHandler, token, mPendingActions);
item.postExecute(mTransactionHandler, token, mPendingActions);
...
}
}
遍历callbacks，调用ClientTransactionItem的execute方法，而我们这里要关注的是ClientTransactionItem的子类LaunchActivityItem，看下它的execute方法：
public void execute(ClientTransactionHandler client, IBinder token,
PendingTransactionActions pendingActions) {
Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
mPendingResults, mPendingNewIntents, mIsForward,
mProfilerInfo, client, mAssistToken);
client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}
里面调用了client.handleLaunchActivity方法，client是ClientTransactionHandler的实例，是在TransactionExecutor构造方法传入的，TransactionExecutor创建是在ActivityThread中：
//ActivityThread
private final TransactionExecutor mTransactionExecutor = new TransactionExecutor(this);
所以，client.handleLaunchActivity方法就是ActivityThread的handleLaunchActivity方法。
好了，到这里 ApplicationThread把启动Activity的操作，通过mH切到了主线程，走到了ActivityThread的handleLaunchActivity方法。
2.4 Activity启动核心实现——初始化及生命周期
ActivityThread 类中的方法
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent) {
...
final Activity a = performLaunchActivity(r, customIntent);
...
return a;
}
继续跟performLaunchActivity方法，这里就是activity 启动的核心实现了
/** activity 启动的核心实现. */
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
//1、从ActivityClientRecord获取待启动的Activity的组件信息
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);
}
ComponentName component = r.intent.getComponent();
if (component == null) {
component = r.intent.resolveActivity(
mInitialApplication.getPackageManager());
r.intent.setComponent(component);
}
if (r.activityInfo.targetActivity != null) {
component = new ComponentName(r.activityInfo.packageName,
r.activityInfo.targetActivity);
}
//创建ContextImpl对象
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
//2、创建activity实例
java.lang.ClassLoader cl = appContext.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
..
}
try {
//3、创建Application对象（如果没有的话）
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
...
if (activity != null) {
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
if (r.overrideConfig != null) {
config.updateFrom(r.overrideConfig);
}
Window window = null;
if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
window = r.mPendingRemoveWindow;
r.mPendingRemoveWindow = null;
r.mPendingRemoveWindowManager = null;
}
appContext.setOuterContext(activity);
//4、attach方法为activity关联上下文环境
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window, r.configCallback,
r.assistToken);
if (customIntent != null) {
activity.mIntent = customIntent;
}
r.lastNonConfigurationInstances = null;
checkAndBlockForNetworkAccess();
activity.mStartedActivity = false;
int theme = r.activityInfo.getThemeResource();
if (theme != 0) {
activity.setTheme(theme);
}
activity.mCalled = false;
//5、调用生命周期onCreate
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
if (!activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onCreate()");
}
r.activity = activity;
}
r.setState(ON_CREATE);
synchronized (mResourcesManager) {
mActivities.put(r.token, r);
}
}
...
return activity;
}
performLaunchActivity主要完成以下事情：
从ActivityClientRecord获取待启动的Activity的组件信息
通过mInstrumentation.newActivity方法使用类加载器创建activity实例
通过LoadedApk的makeApplication方法创建Application对象，内部也是通过mInstrumentation使用类加载器，创建后就调用了instrumentation.callApplicationOnCreate方法，也就是Application的onCreate方法。
创建ContextImpl对象并通过activity.attach方法对重要数据初始化，关联了Context的具体实现ContextImpl，attach方法内部还完成了window创建，这样Window接收到外部事件后就能传递给Activity了。
调用Activity的onCreate方法，是通过 mInstrumentation.callActivityOnCreate方法完成。
二、Activity的onCreate方法执行完，那么onStart、onResume呢？
LaunchActivityItem
上面看到LaunchActivityItem，是用来启动Activity的，也就是走到Activity的onCreate，那么是不是有 "XXXActivityItem"呢? 有的：
LaunchActivityItem 远程App端的onCreate生命周期事务
ResumeActivityItem 远程App端的onResume生命周期事务
PauseActivityItem 远程App端的onPause生命周期事务
StopActivityItem 远程App端的onStop生命周期事务
DestroyActivityItem 远程App端onDestroy生命周期事务
另外梳理过程中涉及的几个类：
ClientTransaction 客户端事务控制者
ClientLifecycleManager 客户端的生命周期事务控制者
TransactionExecutor 远程通信事务执行者
那么我们再来看看ResumeActivityItem吧。
Activity的onStart、onResume流程分析
我们再来重新看看在ActivityStackSupervisor的realStartActivityLocked方法：
...
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
boolean andResume, boolean checkConfig) throws RemoteException {
...
// Create activity launch transaction.
final ClientTransaction clientTransaction = ClientTransaction.obtain(
proc.getThread(), r.appToken);
final DisplayContent dc = r.getDisplay().mDisplayContent;
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
System.identityHashCode(r), r.info,
mergedConfiguration.getGlobalConfiguration(),
mergedConfiguration.getOverrideConfiguration(), r.compat,
r.launchedFromPackage, task.voiceInteractor, proc.getReportedProcState(),
r.icicle, r.persistentState, results, newIntents,
dc.isNextTransitionForward(), proc.createProfilerInfoIfNeeded(),
r.assistToken));
// Set desired final state.
final ActivityLifecycleItem lifecycleItem;
//这里ResumeActivityItem
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// Schedule transaction.
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
...
return true;
}
...
前面只说了通过clientTransaction.addCallback添加LaunchActivityItem实例，在注意下面接着调用了clientTransaction.setLifecycleStateRequest(lifecycleItem)方法，lifecycleItem是ResumeActivityItem或PauseActivityItem实例，这里我们关注ResumeActivityItem，先看下setLifecycleStateRequest方法：
...
/**
* Final lifecycle state in which the client activity should be after the transaction is
* executed.
*/
private ActivityLifecycleItem mLifecycleStateRequest;
public void setLifecycleStateRequest(ActivityLifecycleItem stateRequest) {
mLifecycleStateRequest = stateRequest;
}
...
mLifecycleStateRequest表示执行transaction后的最终的生命周期状态。
继续看处理ActivityThread.H.EXECUTE_TRANSACTION这个消息的处理，即TransactionExecutor的execute方法：
...
public void execute(ClientTransaction transaction) {
if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Start resolving transaction");
final IBinder token = transaction.getActivityToken();
...
executeCallbacks(transaction);
executeLifecycleState(transaction);
...
}
...
前面我们关注的是executeCallbacks方法，现在看看executeLifecycleState方法：
...
/** Transition to the final state if requested by the transaction. */
private void executeLifecycleState(ClientTransaction transaction) {
final ActivityLifecycleItem lifecycleItem = transaction.getLifecycleStateRequest();
if (lifecycleItem == null) {
// No lifecycle request, return early.
return;
}
final IBinder token = transaction.getActivityToken();
final ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
...
// Execute the final transition with proper parameters.
lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
}
...
这里取出了ActivityLifecycleItem并且调用了它的execute方法，实际就是ResumeActivityItem的方法：
...
@Override
public void execute(ClientTransactionHandler client, IBinder token,
PendingTransactionActions pendingActions) {
Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
client.handleResumeActivity(token, true /* finalStateRequest */, mIsForward,
"RESUME_ACTIVITY");
Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}
...
经过上面分析实际是走到ActivityThread的handleResumeActivity方法：
...
@Override
public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
String reason) {
...
// performResumeActivity内部会走onStart、onResume
final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
if (r == null) {
// We didn't actually resume the activity, so skipping any follow-up actions.
return;
}
...
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (r.mPreserveWindow) {
a.mWindowAdded = true;
r.mPreserveWindow = false;
ViewRootImpl impl = decor.getViewRootImpl();
if (impl != null) {
impl.notifyChildRebuilt();
}
}
...
if (!r.activity.mFinished && willBeVisible && r.activity.mDecor != null && !r.hideForNow) {
if (r.newConfig != null) {
performConfigurationChangedForActivity(r, r.newConfig);
if (DEBUG_CONFIGURATION) {
Slog.v(TAG, "Resuming activity " + r.activityInfo.name + " with newConfig "
+ r.activity.mCurrentConfig);
}
r.newConfig = null;
}
if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward=" + isForward);
WindowManager.LayoutParams l = r.window.getAttributes();
if ((l.softInputMode
& WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
!= forwardBit) {
l.softInputMode = (l.softInputMode
& (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))
| forwardBit;
if (r.activity.mVisibleFromClient) {
ViewManager wm = a.getWindowManager();
View decor = r.window.getDecorView();
wm.updateViewLayout(decor, l);
}
}
r.activity.mVisibleFromServer = true;
mNumVisibleActivities++;
if (r.activity.mVisibleFromClient) {
//添加window、设置可见
r.activity.makeVisible();
}
}
r.nextIdle = mNewActivities;
mNewActivities = r;
if (localLOGV) Slog.v(TAG, "Scheduling idle handler for " + r);
Looper.myQueue().addIdleHandler(new Idler());
}
...
三、handleResumeActivity主要做了以下事情：
调用生命周期：通过performResumeActivity方法，内部调用生命周期onStart、onResume方法
设置视图可见：通过activity.makeVisible方法，添加window、设置可见。（所以视图的真正可见是在onResume方法之后）
先来看第一点，生命周期的调用。即performResumeActivity方法：
...
public ActivityClientRecord performResumeActivity(IBinder token, boolean finalStateRequest,
String reason) {
final ActivityClientRecord r = mActivities.get(token);
...
try {
...
r.activity.performResume(r.startsNotResumed, reason);
...
}
...
return r;
}
...
调用了activity.performResume方法：
...
final void performResume(boolean followedByPause, String reason) {
dispatchActivityPreResumed();
//内部会走onStart
performRestart(true /* start */, reason);
...
// 走onResume
mInstrumentation.callActivityOnResume(this);
...
//这里是走fragment的onResume
mFragments.dispatchResume();
mFragments.execPendingActions();
...
}
...
先调用了performRestart()，performRestart()又会调用performStart()，performStart()内部调用了mInstrumentation.callActivityOnStart(this)，也就是Activity的onStart()方法了。
然后是mInstrumentation.callActivityOnResume，也就是Activity的onResume()方法了。到这里启动后的生命周期走完了。
再看第二点，设置视图可见。即activity.makeVisible()方法：
...
//Activity
void makeVisible() {
if (!mWindowAdded) {
ViewManager wm = getWindowManager();
wm.addView(mDecor, getWindow().getAttributes());
mWindowAdded = true;
}
mDecor.setVisibility(View.VISIBLE);
}
...
这里把activity的顶级布局mDecor通过windowManager.addView()方法，把视图添加到window，并设置mDecor可见。到这里视图是真正可见了。值得注意的是，视图的真正可见是在onResume方法之后的。
Activity视图渲染到Window后，会设置window焦点变化，先走到DecorView的onWindowFocusChanged方法，最后是到Activity的onWindowFocusChanged方法，表示首帧绘制完成，此时Activity可交互。
好了，到这里就是真正创建完成且可见可交互了。
梳理成关系图如下：
image.png
3，setConmentView()方法做了什么事情
//Activity
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);//调用到PhoneWindow的setContentView
initWindowDecorActionBar();
}
PhoneWindow
//
@Override
public void setContentView(int layoutResID) {
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
private void installDecor() {
mForceDecorInstall = false;
if (mDecor == null) {
//创建DecorView
mDecor = generateDecor(-1);
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
} else {
//DecorView 绑定到setWindow
mDecor.setWindow(this);
}
创建DecorView
protected DecorView generateDecor(int featureId) {
// System process doesn't have application context and in that case we need to directly use
// the context we have. Otherwise we want the application context, so we don't cling to the
// activity.
Context context;
if (mUseDecorContext) {
Context applicationContext = getContext().getApplicationContext();
if (applicationContext == null) {
context = getContext();
} else {
context = new DecorContext(applicationContext, this);
if (mTheme != -1) {
context.setTheme(mTheme);
}
}
} else {
context = getContext();
}
return new DecorView(context, featureId, this, getAttributes());
}
说明:解析文件布局、创建DecorView 、关联DecorView到PhoneWindow
4，生命周期调用流程OnCreate、OnResume
通过源码分析
//ActivityThread类中调用
performLaunchActivity
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
//1 创建activity对象
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
//2、创建activity实例
java.lang.ClassLoader cl = appContext.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess(isProtectedComponent(r.activityInfo),
appContext.getAttributionSource());
//3、创建Application对象(如果没有的话)
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
//创建PhoneWindow对象
Window window = null;
if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
window = r.mPendingRemoveWindow;
r.mPendingRemoveWindow = null;
r.mPendingRemoveWindowManager = null;
}
// Activity resources must be initialized with the same loaders as the
// application context.
appContext.getResources().addLoaders(
app.getResources().getLoaders().toArray(new ResourcesLoader[0]));
appContext.setOuterContext(activity);
//4、attach方法为activity关联上下文环境
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window, r.configCallback,
r.assistToken, r.shareableActivityToken);
//5、调用生命周期onCreate
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
@Override
public <ViewRootImpl> void handleResumeActivity(ActivityClientRecord r, boolean finalStateRequest,
boolean isForward, String reason) {
//先执行生命周期，最后添加绑定updateViewLayout刷新UI
// performResumeActivity内部会走onStart、onResume
if (!performResumeActivity(r, finalStateRequest, reason)) {
return;
}
if (r.window == null && !a.mFinished && willBeVisible) {
//获取phoneWindow对象
r.window = r.activity.getWindow();
//获取DecorView对象
View decor = r.window.getDecorView();
//DecorView设置为不可见
decor.setVisibility(View.INVISIBLE);
//获取WindowManager对象
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
//DecorView 复制给activity
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (r.mPreserveWindow) {
a.mWindowAdded = true;
r.mPreserveWindow = false;
// callbacks may have changed.
ViewRootImpl impl = decor.getViewRootImpl();
if (impl != null) {
impl.notifyChildRebuilt();
}
}
if (r.activity.mVisibleFromClient) {
ViewManager wm = a.getWindowManager();
View decor = r.window.getDecorView();
//UI进行刷新的开始，有 onMeasure onLayout onDraw
wm.updateViewLayout(decor, l);
}
}
if (r.activity.mVisibleFromClient) {
//页面可见，设置
r.activity.makeVisible();
}
}
}
//Activity
void makeVisible() {
if (!mWindowAdded) {
ViewManager wm = getWindowManager();
//这里把activity的顶级布局mDecor通过windowManager.addView()方法
wm.addView(mDecor, getWindow().getAttributes());
mWindowAdded = true;
}
mDecor.setVisibility(View.VISIBLE);
}
这里把activity的顶级布局mDecor通过windowManager.addView()方法，把视图添加到window，并设置mDecor可见。到这里视图是真正可见了。值得注意的是，视图的真正可见是在onResume方法之后的。
6，UI绘制为什么从onReume()方法之后才开始
if (r.activity.mVisibleFromClient) {
ViewManager wm = a.getWindowManager();
View decor = r.window.getDecorView();
//UI进行刷新的开始，有 onMeasure onLayout onDraw
wm.updateViewLayout(decor, l);
}
image.png
这个就说明了，刷新UI是在OnCreate()、OnResume()之后才执行的，你在之前的方法子线程刷新UI，不会进行报错的原理，以为UI都没有开始绘制；
这个流程图的调用方法
