Android启动流程源码解析（二）
在之前的Android启动流程源码解析(一)源码分析中，我们最后遗留下来一个问题。那就是我们的Activity的启动是啥时候处理的啊？上万行的分析，也没看到我们想要的onCreate啥的。其实就是resumeFocusedStacksTopActivities方法，所以我们这篇文章就从这个方法开始。
resumeFocusedStacksTopActivities
boolean resumeFocusedStacksTopActivities() {
return resumeFocusedStacksTopActivities(null, null, null);
}
boolean resumeFocusedStacksTopActivities(ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
boolean result = false;
if (targetStack != null && (targetStack.isTopStackOnDisplay()|| getTopDisplayFocusedStack() == targetStack)) {
//******重点方法******如果当前的activitystack正好处于屏幕的顶部，那么直接调用将target设置到顶部显示
result = targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}
for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
//标记是否已经显示在屏幕上
boolean resumedOnDisplay = false;
final ActivityDisplay display = mActivityDisplays.get(displayNdx);
for (int stackNdx = display.getChildCount() - 1; stackNdx >= 0; --stackNdx) {
final ActivityStack stack = display.getChildAt(stackNdx);
//获取到当前ActivityStack顶部正在运行的Activity
final ActivityRecord topRunningActivity = stack.topRunningActivityLocked();
if (!stack.isFocusableAndVisible() || topRunningActivity == null) {
continue;
}
if (stack == targetStack) {
//上面已经做过resume处理了，所以这里我们就不再做处理了
resumedOnDisplay |= result;
continue;
}
if (display.isTopStack(stack) && topRunningActivity.isState(RESUMED)) {
stack.executeAppTransition(targetOptions);
} else {
resumedOnDisplay |= topRunningActivity.makeActiveIfNeeded(target);
}
}
//如果仍然没有显示在屏幕上，那么就获取到屏幕当前持有焦点的ActivityStack，然后将activity显示在上面
if (!resumedOnDisplay) {
final ActivityStack focusedStack = display.getFocusedStack();
if (focusedStack != null) {
focusedStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}
}
}
return result;
}
这里我们只需要关注一个方法 resumeTopActivityUncheckedLocked ，这个方法也特别长，我们就拆分开，只关注重点方法即可
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
if (mInResumeTopActivity) {
// Don't even start recursing.
return false;
}
boolean result = false;
try {
// Protect against recursion.
mInResumeTopActivity = true;
//***重点关注******
result = resumeTopActivityInnerLocked(prev, options);
final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
if (next == null || !next.canTurnScreenOn()) {
checkReadyForSleep();
}
} finally {
mInResumeTopActivity = false;
}
return result;
}
这里我们也只关注resumeTopActivityInnerLocked方法
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
...
if (mResumedActivity != null) {
//****重点方法 ****** 调用acitivity的pause方法
pausing |= startPausingLocked(userLeaving, false, next, false);
}
...
//进行activity的创建处理
mStackSupervisor.startSpecificActivityLocked(next, true, false);
...
}
代码也很长，我们只提取了两个比较重要的函数，一个是调用onPause生命周期函数，另一个是调用onCreate生命周期函数的。
onPause的暂停过程
我们首先来看一下是如何一步步通过调度来执行onPause的生命周期调度的
//ActivityStack.java
final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping, ActivityRecord resuming, boolean pauseImmediately) {
...
//当前正在显示的Activity
ActivityRecord prev = mResumedActivity;
//当前正在显示的Activity需要执行暂停操作了。
//将其赋值给mPausingActivity成员变量。
mPausingActivity = prev;
mLastPausedActivity = prev;
//Activity绑定了对应的APP？难道有不绑定的情况么？
if (prev.attachedToProcess()) {
...
//******重点方法****
mService.getLifecycleManager().scheduleTransaction(prev.app.getThread(),prev.appToken, PauseActivityItem.obtain(prev.finishing, userLeaving,prev.configChangeFlags, pauseImmediately));
...
重点方法已经标注出来了。我们先看看它的参数的创建过程。
PauseActivityItem.obtain
//PauseActivityItem.java
//从池中取出一个PauseActivityItem类
public static PauseActivityItem obtain(boolean finished, boolean userLeaving, int configChanges, boolean dontReport) {
PauseActivityItem instance = ObjectPool.obtain(PauseActivityItem.class);
if (instance == null) {
instance = new PauseActivityItem();
}
instance.mFinished = finished;
instance.mUserLeaving = userLeaving;
instance.mConfigChanges = configChanges;
instance.mDontReport = dontReport;
return instance;
}
//ObjectPool.java
public static T obtain(Class itemClass) {
synchronized (sPoolSync) {
@SuppressWarnings("unchecked")
final ArrayList itemPool = (ArrayList) sPoolMap.get(itemClass);
if (itemPool != null && !itemPool.isEmpty()) {
return itemPool.remove(itemPool.size() - 1);
}
return null;
}
}
可以看到，对于PauseActivityItem的获取，是通过享元模式 来进行处理的。
回到主干。
这的mService是ActivityTaskManagerService，getLifecycleManager方法返回的是ClientLifecycleManager对象。
private final ClientLifecycleManager mLifecycleManager;
mLifecycleManager = new ClientLifecycleManager();
ClientLifecycleManager getLifecycleManager() {
return mLifecycleManager;
}
scheduleTransaction
//ClientLifecycleManager.java
//调用一次生命周期的调度请求
void scheduleTransaction(@NonNull IApplicationThread client, @NonNull IBinder activityToken,@NonNull ActivityLifecycleItem stateRequest) throws RemoteException {
final ClientTransaction clientTransaction = transactionWithState(client, activityToken,stateRequest);
scheduleTransaction(clientTransaction);
}
//实际的调度方法
void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
final IApplicationThread client = transaction.getClient();
transaction.schedule();
if (!(client instanceof Binder)) {
//回收
transaction.recycle();
}
}
//创建一个持有状态的事务类
private static ClientTransaction transactionWithState(@NonNull IApplicationThread client,
@NonNull IBinder activityToken, @NonNull ActivityLifecycleItem stateRequest) {
final ClientTransaction clientTransaction = ClientTransaction.obtain(client, activityToken);
clientTransaction.setLifecycleStateRequest(stateRequest);
return clientTransaction;
}
这里最终会调用ClientTransaction对象的schedule方法。
//ClientTransaction.java
public void schedule() throws RemoteException {
mClient.scheduleTransaction(this);
}
这里的mClient是一个IApplicationThread，其Server端是ActivityThread#ApplicationThread。所以最终调用的是ApplicationThread的scheduleTransaction方法
//ActivityThread.java
public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
//会调用ActivityThread.scheduleTransaction方法->该方法位于ActivityThread的父类中
ActivityThread.this.scheduleTransaction(transaction);
}
//ClientTransactionHandler.java
void scheduleTransaction(ClientTransaction transaction) {
//执行预处理
transaction.preExecute(this);
//通过Handler机制发送事务请求
sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
}
Handler消息机制就不贴出来了，直接看其是怎么处理的。
//ActivityThread.java
private final TransactionExecutor mTransactionExecutor = new TransactionExecutor(this);
case EXECUTE_TRANSACTION://执行生命周期的调度工作
final ClientTransaction transaction = (ClientTransaction) msg.obj;
mTransactionExecutor.execute(transaction);
当接收到Handler以后，会调用TransactionExecutor的execute()方法。
//TransactionExecutor.java
public void execute(ClientTransaction transaction) {
final IBinder token = transaction.getActivityToken();
...
//循环遍历回调请求的所有状态，并在适当的时间执行它们
executeCallbacks(transaction);
//执行生命周期的改变
executeLifecycleState(transaction);
}
ClientTransaction存在两种事务,
- 一种是通过setLifecycleStateRequest设置一个对象的事务类型，用于表示事务执行以后，客户端应该处于的生命周期状态
- 一种是addCallback，增加对客户端的事务类型回调，对客户端一系列的回调。
这两个不同的类型，在这里就会存在不同的处理方法。对于第一种会在executeCallbacks中进行处理，第二种则会在executeLifecycleState中进行处理。
我们这儿的暂停，是通过第二种来进行设置的，所以我们直接看executeLifecycleState这个方法。
//TransactionExecutor.java
//如果事务请求，则转换到最终状态
private void executeLifecycleState(ClientTransaction transaction) {
// ActivityStackSupervisor.java中进行了这个设置
// final ActivityLifecycleItem lifecycleItem;
// if (andResume) {
// lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
// } else {
// lifecycleItem = PauseActivityItem.obtain();
// }
// clientTransaction.setLifecycleStateRequest(lifecycleItem);
//所以这里的lifecycleItem可能是ResumeActivityItem或者PauseActivityItem或者其他的生命周期相关类
final ActivityLifecycleItem lifecycleItem = transaction.getLifecycleStateRequest();
if (lifecycleItem == null) {
//如果不是通过setLifecycleStateRequest设置的，那么该方法不需要处理，直接返回即可
return;
}
//使用适当的参数执行最后的转换
lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
}
我们这里的lifecycleItem是我们刚才创建的PauseActivityItem，这里会执行其execute方法。
//PauseActivityItem.java
@Override
public void execute(ClientTransactionHandler client, IBinder token, PendingTransactionActions pendingActions) {
Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
//这里的client，是Activity独享
client.handlePauseActivity(token, mFinished, mUserLeaving, mConfigChanges, pendingActions,"PAUSE_ACTIVITY_ITEM");
Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}
这里的client对象我们需要回退去跟踪一下
//ActivityThread.java
private final TransactionExecutor mTransactionExecutor = new TransactionExecutor(this);
//TransactionExecutor.java
//在ActivityThread类中中调用了mTransactionExecutor = new TransactionExecutor(this)这个方法，其中的mTransactionHandler是ActivityThread本身
public TransactionExecutor(ClientTransactionHandler clientTransactionHandler) {
mTransactionHandler = clientTransactionHandler;
}
所以说，最终调用的是ActivityThread的handlePauseActivity方法
//ActivityThread.java
public void handlePauseActivity(IBinder token, boolean finished, boolean userLeaving,int configChanges, PendingTransactionActions pendingActions, String reason) {
//获取对应的ActivityClientRecord对象
ActivityClientRecord r = mActivities.get(token);
if (r != null) {
...
//***重点方法***，执行pause方法
performPauseActivity(r, finished, reason, pendingActions);
...
}
}
private Bundle performPauseActivity(ActivityClientRecord r, boolean finished, String reason,
PendingTransactionActions pendingActions) {
...
//****重点方法****
performPauseActivityIfNeeded(r, reason);
...
return shouldSaveState ? r.state : null;
}
private void performPauseActivityIfNeeded(ActivityClientRecord r, String reason) {
...
r.activity.mCalled = false;
//重点方法，通过Instrumentation调用onPause生命周期
mInstrumentation.callActivityOnPause(r.activity);
...
}
最终会通过Instrumentation调用callActivityOnPause方法。
//Instrumentation.java
public void callActivityOnPause(Activity activity) {
activity.performPause();
}
//Activity.java
final void performPause() {
dispatchActivityPrePaused();
mDoReportFullyDrawn = false;
//管理的Fragment的处理
mFragments.dispatchPause();
mCalled = false;
//调用了onPause生命周期方法
onPause();
writeEventLog(LOG_AM_ON_PAUSE_CALLED, "performPause");
//设置mResumed为false，表示当前activity没有展示
mResumed = false;
//调用一些回调函数
dispatchActivityPostPaused();
}
到这里为止，原来在我们面前展示的那个Activity调用了其onPause方法。
Activity的创建过程
回到主线的resumeTopActivityInnerLocked方法中，当执行完startPausingLocked方法后，会调用mStackSupervisor.startSpecificActivityLocked方法
//ActivityStackSupervisor.java
void startSpecificActivityLocked(ActivityRecord r, boolean andResume, boolean checkConfig) {
//根据uid和pid，获取activity对应的进行和线程信息
final WindowProcessController wpc =mService.getProcessController(r.processName, r.info.applicationInfo.uid);
boolean knownToBeDead = false;
if (wpc != null && wpc.hasThread()) {
//如果进程和线程都存在，执行后面的代码
try {
realStartActivityLocked(r, wpc, andResume, checkConfig);
return;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting activity "
+ r.intent.getComponent().flattenToShortString(), e);
}
knownToBeDead = true;
}
//通过message进行进程的启动。
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);
}
}
如果要启动的activity所在的进程和线程都存在，那么直接调用realStartActivityLocked方法进行启动，否则的话，就会调用Handler机制进行进程的创建。
realStartActivityLocked
//真正执行Activity启动的方法
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc, boolean andResume, boolean checkConfig) throws RemoteException {
//如果还有activity没有暂停，这里会直接返回false
if (!mRootActivityContainer.allPausedActivitiesComplete()) {
return false;
}
final TaskRecord task = r.getTaskRecord();
final ActivityStack stack = task.getStack();
//设置标志位，不再接收其他activity的resume的操作
beginDeferResume();
try {
...
//创建了一个Activity事务
final ClientTransaction clientTransaction = ClientTransaction.obtain(proc.getThread(), r.appToken);
final DisplayContent dc = r.getDisplay().mDisplayContent;
//增加一个要执行的事务LaunchActivityItem。
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));
final ActivityLifecycleItem lifecycleItem;
//设置其生命周期LifecycleStateRequest
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
clientTransaction.setLifecycleStateRequest(lifecycleItem);
//执行事务的调度
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
...
proc.onStartActivity(mService.mTopProcessState, r.info);
return true;
}
在这个方法里面，创建了一个事务，在事务中增加了一个callback回调，然后通过setLifecycleStateRequest设置了一个生命周期，最后通过scheduleTransaction执行了调度。在前面的onPause中，我们梳理了整个调度的流程，最后会调用到LaunchActivityItem的execute，然后会调用生命周期所对应的ResumeActivityItem的execute。
我们挨个看，先看LaunchActivityItem的调用
//LaunchActivityItem.java
//一般会在TransactionExecutor中调用这个方法
//ClientTransactionHandler实际是ActivityThread对象，所以这里会执行activitythread类中的handleLaunchActivity方法
@Override
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);
//调用activitythread类中的handleLaunchActivity方法
client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}
其本质调用的是ActivityThread中的handleLaunchActivity方法。
这部分的功能，我们在Android布局窗口绘制分析中进行过解析。这里不再往下进行解析了。
我们直接跳过这部分，来看看如果启动的activity所在的进程和线程都存在。会进行进程的创建工作。这部分我们后面专门再进行一篇关于进程创建的解析工作。
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