Activity 的启动过程分析
最近开始阅读 Android 源码，从 Activity 的启动过程开始入手。
打开 Activity 源码可以发现，startActivity(Intent intent) 方法实际上是调用 startActivityForResult(Intent intent, int requestCode)：1
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public void startActivity(Intent intent) {
startActivityForResult(intent, -1);
}
继续跟进查看 startActivityForResult(Intent intent, int requestCode)：1
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27public void startActivityForResult(Intent intent, int requestCode) {
if (mParent == null) {
// 启动新的 Activity，核心功能在 ApplicationThread 类中完成
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(this,
mMainThread.getApplicationThread(),
mToken, this, intent, requestCode);
if (ar != null) {
// 发送结果
mMainThread.sendActivityResult(mToken, mEmbeddedID,
requestCode, ar.getResultCode(), ar.getResultData());
}
if (requestCode >= 0) {
// If this start is requesting a result, we can avoid making
// the activity visible until the result is received. Setting
// this code during onCreate(Bundle savedInstanceState)
// or onResume() will keep the
// activity hidden during this time, to avoid flickering.
// This can only be done when a result is requested because
// that guarantees we will get information back when the
// activity is finished, no matter what happens to it.
mStartedActivity = true;
}
} else {
mParent.startActivityFromChild(this, intent, requestCode);
}
}
打开 Instrumentation 类，查看 execStartActivity() 方法：1
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19public ActivityResult execStartActivity(Context who, IBinder contextThread,
IBinder token, Activity target, Intent intent, int requestCode) {
IApplicationThread whoThread = (IApplicationThread) contextThread;
. . .
try {
intent.setAllowFds(false);
// 打开 Activity ，具体实现在 ApplicationThread 中
int result = ActivityManagerNative.getDefault().startActivity(
whoThread, intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
null, 0, token, target != null ? target.mEmbeddedID : null,
requestCode, false, false, null, null, false);
// 检查结果，如果 result < 0，则会抛出各种异常，如 ActivityNotFound 等
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
}
return null;
}
打开 ApplicationThread (在 ActivityThread 的中，为其内部类)，会发现 schedule**Activity(..) 类似的方法，直接查看 scheduleLaunchActivity 方法：1
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29public final void scheduleLaunchActivity(Intent intent, IBinder token,
int ident, ActivityInfo info, Configuration curConfig,
CompatibilityInfo compatInfo, Bundle state, List<ResultInfo> pendingResults,
List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,
String profileName, ParcelFileDescriptor profileFd, boolean autoStopProfiler) {
ActivityClientRecord r = new ActivityClientRecord();
r.token = token;
r.ident = ident;
r.intent = intent;
r.activityInfo = info;
r.compatInfo = compatInfo;
r.state = state;
r.pendingResults = pendingResults;
r.pendingIntents = pendingNewIntents;
r.startsNotResumed = notResumed;
r.isForward = isForward;
r.profileFile = profileName;
r.profileFd = profileFd;
r.autoStopProfiler = autoStopProfiler;
updatePendingConfiguration(curConfig);
queueOrSendMessage(H.LAUNCH_ACTIVITY, r);
}
构造一个 ActivityClientRecord 后，调用 queueOrSendMessage(..) 方法，交由一个继承 Handler 的 H 类处理。
查看 H 类的 handleMessage(Message msg)，可以发现处理各种 Activity 的状态事件。1
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13public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + msg.what);
switch (msg.what) {
case LAUNCH_ACTIVITY: {
ActivityClientRecord r = (ActivityClientRecord)msg.obj;
r.packageInfo =
getPackageInfoNoCheck(r.activityInfo.applicationInfo, r.compatInfo);
// 真正启动 Activity 的地方
handleLaunchActivity(r, null);
} break;
. . .
}
}
在 handleLaunchActivity(ActivityClientRecord r, Intent customIntent) 中，会通过 performLaunchActivity(ActivityClientRecord r, Intent customIntent) 创建一个 Activity，查看方法实现：1
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123private Activity performLaunchActivity(ActivityClientRecord r,
Intent customIntent) {
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);
}
// 从 Intent 中解析目标 Activity 的启动参数
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);
}
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
// 通过 ClassLoader 实例化 Activity 对象
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
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);
}
}
try {
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
if (localLOGV) Slog.v(
TAG, r + ": app=" + app
+ ", appName=" + app.getPackageName()
+ ", pkg=" + r.packageInfo.getPackageName()
+ ", comp=" + r.intent.getComponent().toShortString()
+ ", dir=" + r.packageInfo.getAppDir());
if (activity != null) {
ContextImpl appContext = new ContextImpl();
appContext.init(r.packageInfo, r.token, this);
appContext.setOuterContext(activity);
CharSequence title =
r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
+ r.activityInfo.name + " with config " + config);
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config);
if (customIntent != null) {
activity.mIntent = customIntent;
}
r.lastNonConfigurationInstances = null;
activity.mStartedActivity = false;
int theme = r.activityInfo.getThemeResource();
if (theme != 0) {
activity.setTheme(theme);
}
activity.mCalled = false;
// 调用 Activity 的 onCreate 方法，开始进入 Activity 启动的生命周期
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.stopped = true;
if (!r.activity.mFinished) {
// 调用 Activity 的 onStart 方法
activity.performStart();
r.stopped = false;
}
if (!r.activity.mFinished) {
if (r.state != null) {
mInstrumentation.callActivityOnRestoreInstanceState(activity,
r.state);
}
}
if (!r.activity.mFinished) {
activity.mCalled = false;
mInstrumentation.callActivityOnPostCreate(activity, r.state);
if (!activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onPostCreate()");
}
}
}
r.paused = true;
mActivities.put(r.token, r);
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to start activity " + component
+ ": " + e.toString(), e);
}
}
return activity;
}
至此，Activity 已经完成创建和启动，在 performLaunchActivity 执行完毕后，会继续执行 Activity 的 onResume 等方法。