疑问: 1.Android程序是如何启动,Activity生命周期如何调用？我也想知道，参考很多博客，当然鄙人实在是才疏学浅，通过系统源码跟着博客一步一步的查看程序启动流程。以下是记录查看Android SDK 27版本源码流程。
1、找到ActivityThread类的main()方法
//创建ActivityThread,并且依附于Application的Thread
ActivityThread thread = new ActivityThread();
thread.attach(false);
-->进入attach(false)方法
//这里是Bind接口，说明APP与系统是通过Bind跨进程通信的
RuntimeInit.setApplicationObject(mAppThread.asBinder());
final IActivityManager mgr = ActivityManager.getService();
try {
//这里是将Bind接口与应用的线程进行关联
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
2、找到ApplicationThread，看这个线程主要做了什么事？
在这里面你会发现很多和生命周期相关方法，那么我们找到scheduleLaunchActivity()方法，这个方法由系统服务调用。
updateProcessState(procState, false);
ActivityClientRecord r = new ActivityClientRecord();
r.token = token;
r.ident = ident;
r.intent = intent;
r.referrer = referrer;
r.voiceInteractor = voiceInteractor;
r.activityInfo = info;
r.compatInfo = compatInfo;
r.state = state;
r.persistentState = persistentState;
r.pendingResults = pendingResults;
r.pendingIntents = pendingNewIntents;
r.startsNotResumed = notResumed;
r.isForward = isForward;
r.profilerInfo = profilerInfo;
r.overrideConfig = overrideConfig;
updatePendingConfiguration(curConfig);
//关键来了:这里是通过Handler消息机制改变Activity状态了
sendMessage(H.LAUNCH_ACTIVITY, r);
3、找到H类，对应找到H.LAUNCH_ACTIVITY事件处理逻辑代码
case LAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
}
break;
上述代码对应handleLaunchActivity(r, null, "LAUNCH_ACTIVITY")处理启动Activity
进入到handleLaunchActivity(r, null, "LAUNCH_ACTIVITY")方法里面，会发现这么一句代码Activity a = performLaunchActivity(r, customIntent)执行启动Activity。
进入performLaunchActivity(r, customIntent) 方法里面：通过反射创建一个Activity
Activity activity = null;
try {
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) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to instantiate activity " + component
+ ": " + e.toString(), e);
}
}
接着你会发现这么一段代码:callActivityOnCreate(activity, r.state, r.persistentState);实际上层层调用到Activity的onCreate() 方法
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
performLaunchActivity()执行完成之后，返回一个Activity的实例。这里Activity的onCreate()方法执行完毕。
接着根据performLaunchActivity()返回的Activity实例，执行 handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
Bundle oldState = r.state;
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
if (!r.activity.mFinished && r.startsNotResumed) {
// The activity manager actually wants this one to start out paused, because it
// needs to be visible but isn't in the foreground. We accomplish this by going
// through the normal startup (because activities expect to go through onResume()
// the first time they run, before their window is displayed), and then pausing it.
// However, in this case we do -not- need to do the full pause cycle (of freezing
// and such) because the activity manager assumes it can just retain the current
// state it has.
performPauseActivityIfNeeded(r, reason);
// We need to keep around the original state, in case we need to be created again.
// But we only do this for pre-Honeycomb apps, which always save their state when
// pausing, so we can not have them save their state when restarting from a paused
// state. For HC and later, we want to (and can) let the state be saved as the
// normal part of stopping the activity.
if (r.isPreHoneycomb()) {
r.state = oldState;
}
}
} else {
// If there was an error, for any reason, tell the activity manager to stop us.
try {
ActivityManager.getService()
.finishActivity(r.token, Activity.RESULT_CANCELED, null,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
进入到handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);里面
r = performResumeActivity(token, clearHide, reason);
再进入到performResumeActivity(token, clearHide, reason);里面你会发现这么一句代码
r.activity.performResume();
进入到performResume()里面，你会发现
// mResumed is set by the instrumentation
mInstrumentation.callActivityOnResume(this);
上述代码经过层层调用，最终调用到activity.onResume();这里就开始执行Activity的onResume()方法。从APP程序启动到Activity的生命周期函数调用顺序流程就一幕了然了。
