Activity Startup过程分析

最新推荐文章于 2024-09-11 17:13:45 发布

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最新推荐文章于 2024-09-11 17:13:45 发布 · 3.7k 阅读

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本文详细分析了Android中Activity的启动过程，从点击桌面图标开始，涉及Activity.startActivity、Activity.startActivityForResult、Instrumentation.execStartActivity等多个步骤，直到Activity的创建和显示。通过时序图和代码解析，阐述了Activity启动时涉及到的关键组件和方法，包括Launcher、Intent、ActivityManagerService等的交互细节。

Activity Startup

Contents

SequenceDiagram..4

Startfrom click shortcut on Launcher.4

Activity.startActivity.6

Activity.startActivityForResult.7

Instrumentation.execStartActivity.8

Step7: ActivityManagerService.startActivity.9

Step8: ActivityStack.startActivityMayWait.10

Step9: ActivityStack. startActivityLocked.12

Step10: ActivityStack.startActivityUncheckedLocked.17

Step11: ActivityStack. startActivityLocked.25

Step12: ActivityStack.resumeTopActivityLocked.28

Step13: ActivityStack.startPausingLocked.36

Step14: ApplicationThread. schedulePauseActivity.39

Step15: ActivityThread.queueOrSendMessage.39

Step16: ActivityThread.Handler.handleMessage.39

Step17: ActivityThread.handlePauseActivity.40

Step18: ActivityThread.performPauseActivity.41

Step19: ActivityManagerService.activityPaused().42

Step20: ActivityStack.activityPaused().42

Step21: ActivityStack.completePauseLocked().43

Step22: ActivityStack.resumeTopActivityLocked().44

Step23: ActivityStack.startSpecificActivityLocked ().44

Step24: ActivityManagerService. startProcessLocked().45

Step25: ActivityManagerService.startProcessLocked().47

Step27: ActivityThread.main.49

Step28: ActivityThread.attach(false).50

Step 29: ActivityManagerService.attachApplication.50

Step 30: ActivityManagerService.attachApplicationLocked.51

Step31: ActivityStack. realStartActivityLocked.56

Step32: ActivityThread.scheduleLaunchActivity.58

Step33: ActivityThread. queueOrSendMessage(H.LAUNCH_ACTIVITY, r);58

Step34: ActivityThread.Handler.handleMessage.58

ActivityThread.handleLaunchActivity(r, null).58

启动一个新Activity可以通过2种方式，通过点击桌面图标；或者调用startActivity()/StartActivityForResult()。这个文档里主要讨论通过点击桌面应用图标的方式启动Activity。

为了讲解方便，把要启动的Activity命名为MyActivity 。在AndroidManifest.xml文件中做如下配置：

<activity android:name=".MyActivity"

android:label="@string/app_name">

<intent-filter>

<actionandroid:name="android.intent.action.MAIN" />

<categoryandroid:name="android.intent.category.LAUNCHER" />

</intent-filter>

</activity>

先看看时序图有个大概的印象，下面就具体详细分析每一步是如何实现的。

在Android系统中，应用程序是由Launcher启动起来的，其实，Launcher本身也是一个应用程序，就是我们所熟知的Home。其它的应用程序安装后，就会在Launcher的界面上出现一个相应的图标，点击这个图标时，Launcher就会把对应的应用程序启动起来。

在onClick()方法中不仅需要处理某个应用的启动，还包括针对点击folder和AllAppsButton的处理。

http://opengrok.sonyericsson.net/jellybean/xref/jb-lagan/packages/apps/Launcher2/src/com/android/launcher2/Launcher.java

1845 /**

1846 * Launches the intent referred by theclicked shortcut.

1847 *

1848 * @param vThe view representing the clicked shortcut.

1849 */

1850 public voidonClick(View v) {

...

//Tag:

Unlike IDs, tags are not used toidentify views. Tags are essentially an extra piece of information that can beassociated with a view. They are most often used as a convenience to store datarelated to views in the views themselves rather than by putting them in aseparate structure.

1861 Objecttag = v.getTag();

1862 if (taginstanceof ShortcutInfo) {

1863 //Open shortcut

1864 finalIntent intent = ((ShortcutInfo) tag).intent;

1865 int[]pos = new int[2];

1866 v.getLocationOnScreen(pos);

1867 intent.setSourceBounds(newRect(pos[0], pos[1],

1868 pos[0] + v.getWidth(), pos[1] + v.getHeight()));

1869 //这个函数调用是Activity启动的入口

1870 boolean success =startActivitySafely(v, intent, tag);

1871 //设置图标被点击、选中的状态

1872 if(success && v instanceof BubbleTextView) {

1873 mWaitingForResume = (BubbleTextView) v;

1874 mWaitingForResume.setStayPressed(true);

1875 }

1876 } else if(tag instanceof FolderInfo) {

1877 if (vinstanceof FolderIcon) {

1878 FolderIcon fi = (FolderIcon) v;

1879 handleFolderClick(fi);

1880 }

1881 } else if(v == mAllAppsButton) {

1882 if(isAllAppsVisible()) {

1883 showWorkspace(true);

1884 }else {

1885 onClickAllAppsButton(v);

1886 }

1887 }

1888 }

2009 boolean startActivitySafely(View v, Intentintent, Object tag) {

2010 booleansuccess = false;

2011 try {

2012 success = startActivity(v, intent,tag);

2013 } catch(ActivityNotFoundException e) {

2014 Toast.makeText(this,R.string.activity_not_found, Toast.LENGTH_SHORT).show();

2015 Log.e(TAG, "Unable to launch. tag=" + tag + "intent=" + intent, e);

2016 }

2017 returnsuccess;

2018 }

下面继续看Launcher.startActivity()

1982 boolean startActivity(View v, Intentintent, Object tag) {

1983 intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

1984

1985 try {

1986 //Only launch using the new animation if the shortcut has not opted out (this isa

1987 //private contract between launcher and may be ignored in the future).

1988 boolean useLaunchAnimation = (v != null) &&

1989 !intent.hasExtra(INTENT_EXTRA_IGNORE_LAUNCH_ANIMATION);

//下面这段绿色部分是插入的注释

// useLaunchAnimation should betrue, because “INTENT_EXTRA_IGNORE_LAUNCH_ANIMATION” is NOT set inpackages/apps/Launcher2/src/com/android/launcher2/LauncherProvider.java,

483 if (version < 12) {

484 // Contact shortcuts need adifferent set of flags to be launched now

485 // The updateContactsShortcutschange is idempotent, so we can keep using it like

486 // back in the Donut days

487 updateContactsShortcuts(db);

488 version = 12;

489 }

1990 if(useLaunchAnimation) {

1991 ActivityOptions opts = ActivityOptions.makeScaleUpAnimation(v, 0, 0,

1992 v.getMeasuredWidth(), v.getMeasuredHeight());

1993

1994 startActivity(intent, opts.toBundle());

1995 }else {

1996 startActivity(intent);

1997 }

1998 return true;

1999 } catch(SecurityException e) {

2000 Toast.makeText(this, R.string.activity_not_found,Toast.LENGTH_SHORT).show();

2001 Log.e(TAG, "Launcher does not have the permission to launch "+ intent +

2002 ". Make sure to create a MAIN intent-filter for the correspondingactivity " +

2003 "or use the exportedattribute for this activity. "

2004 + "tag="+ tag + " intent=" + intent, e);

2005 }

2006 returnfalse;

2007 }

Activity.startActivity

这个函数里面直接调用startActivityForResult()，其中requestCode=-1表示不需要返回结果。

Activity.java

public void startActivity(Intentintent, Bundle options) {

if (options!= null) {

startActivityForResult(intent, -1, options);

} else {

// Notewe want to go through this call for compatibility with

//applications that may have overridden the method.

startActivityForResult(intent, -1);

}

Activity.startActivityForResult

这里的mMainThread是ActivityThread类型的Activity类成员变量，它代表的是应用程序的主线程，也就是Launcher应用程序运行的进程。这里通过mMainThread.getApplicationThread获得它里面的ApplicationThread成员变量，它是一个 Binder对象，后面我们会看到，ActivityManagerService会使用它来和ActivityThread来进行进程间通信。

这里的mToken也是Activity类的成员变量，它是一个Binder对象的远程接口。它在attch()方法中被赋值。

public void startActivityForResult(Intent intent, intrequestCode, Bundle options) {

if (mParent== null) {// mParent == null表示启动的不是sub-activity

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) {

// Ifthis 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 keepthe

//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 {

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);

}

Instrumentation.execStartActivity

在Instrumentation.java中相同名字的函数一共有2个，一个是为了启动Activity，另外一个是在Fragment中启动Activty。

检查mActivityMonitors列表中是否已经有该Activity对象了，如果存在而且该对象处于block状态，则不启动该Activity

调用AMS.execStartActivity()把启动新Activity的任务通过IPC交给AMS来完成。至此程序的运行已经脱离了Launcher当前运行的进程，进入到AMS的线程中去了。

调用checkStartActivityResult检查异常返回结果。

Instrumentation.java

public ActivityResult execStartActivity(

Contextwho, IBinder contextThread, IBinder token,Activity target,

Intentintent, int requestCode, Bundle options) {

IApplicationThread whoThread = (IApplicationThread) contextThread;

//首先检查启动队列中是否已经包含了相同的启动任务，如果已经存在就不需要再启动新的了

if(mActivityMonitors != null) {

synchronized (mSync) {

finalint N = mActivityMonitors.size();

for(int i=0; i<N; i++) {

final ActivityMonitor am = mActivityMonitors.get(i);

if (am.match(who, null, intent)) {

am.mHits++;

if (am.isBlocking()) {

return requestCode >= 0 ? am.getResult() : null;

}

break;

}

try {

intent.setAllowFds(false);

intent.migrateExtraStreamToClipData();

int result =ActivityManagerNative.getDefault()

.startActivity(whoThread,intent,

intent.resolveTypeIfNeeded(who.getContentResolver()),

token, target != null ?target.mEmbeddedID : null,

requestCode, 0, null,null, options);

checkStartActivityResult(result, intent);

} catch(RemoteException e) {

}

return null;

}

Step 7: ActivityManagerService.startActivity

此处传入的参数列表：

IApplicationThread caller:	ApplicationThread类型的Binder实体
int callingUid:	-1
Intent intent:
String resolvedType:	这里的intent.resolveTypeIfNeeded返回这个intent的MIME类型，在这个例子中，没有AndroidManifest.xml设置MainActivity的MIME类型，因此，这里返回null。
IBinder resultTo:	一个代表发出启动新Activity请求的Activity token
String resultWho:	target != null ? target.mEmbeddedID : null 这里的target不为null，但是target.mEmbddedID为null
int requestCode:	-1
int startFlags:	0
String profileFile:	null
ParcelFileDescriptor profileFd:	null
WaitResult outResult:	null
Configuration config:	null
Bundle options:

这个函数所做的主要工作：

通过调用enforceNotIsolatedCaller()判断当前的请求者Binder.getCallingUid()是否允许启动Activity

ActivityManagerService.java

public final int startActivity(IApplicationThread caller,

Intent intent, String resolvedType, IBinderresultTo,

StringresultWho, int requestCode, int startFlags,

StringprofileFile, ParcelFileDescriptor profileFd, Bundle options) {

enforceNotIsolatedCaller("startActivity");//安全检查，用户是否已经被隔离，这里的用户id是通过Binder.getCallingUid()取得的

int userId =0;

if(intent.getCategories() != null &&intent.getCategories().contains(Intent.CATEGORY_HOME)) {

//Requesting home, set the identity to the current user

// HACK!

userId =mCurrentUserId;

} else {

// TODO:Fix this in a better way - calls coming from SystemUI should probably carry

// thecurrent user's userId

if(Binder.getCallingUid() < Process.FIRST_APPLICATION_UID) {

userId = 0;

} else {

userId = Binder.getOrigCallingUser();

}

returnmMainStack.startActivityMayWait(caller, -1, intent, resolvedType,

resultTo, resultWho,requestCode, startFlags, profileFile, profileFd,

null, null, options, userId);

}

Step 8: ActivityStack.startActivityMayWait

此处传入的参数列表：

IApplicationThread caller:	ApplicationThread类型的Binder实体
int callingUid:	-1
Intent intent:
String resolvedType:	这里的intent.resolveTypeIfNeeded返回这个intent的MIME类型，在这个例子中，没有AndroidManifest.xml设置MainActivity的MIME类型，因此，这里返回null。
IBinder resultTo:	一个代表发出启动新Activity请求的Activity token
String resultWho:	target != null ? target.mEmbeddedID : null 这里的target不为null，但是target.mEmbddedID为null
int requestCode:	-1
int startFlags:	0
String profileFile:	null
ParcelFileDescriptor profileFd:	null
WaitResult outResult:	null
Configuration config:	null
Bundle options:
int userId:	Binder.getOrigCallingUser()

这个函数所做的主要工作：

调用resolveActivity()从PackageMangerService那里获取activity信息aInfo

通过调用ActivityManagementService.startActivityMayWait()获取于此Activity相关的Application信息，并且保存在aInfo.applicationInfo中

final int startActivityMayWait(IApplicationThread caller,int callingUid,

Intentintent, String resolvedType, IBinder resultTo,

StringresultWho, int requestCode, int startFlags, String profileFile,

ParcelFileDescriptor profileFd, WaitResult outResult, Configurationconfig,

Bundle options, int userId) {

// Refusepossible leaked file descriptors

if (intent !=null && intent.hasFileDescriptors()) {

throw newIllegalArgumentException("File descriptors passed in Intent");

}

booleancomponentSpecified = intent.getComponent() != null; // componentSpecified=true

// Don'tmodify the client's object!

intent = newIntent(intent);

// Collectinformation about the target of the Intent.

ActivityInfoaInfo = resolveActivity(intent, resolvedType, startFlags,

profileFile, profileFd, userId);

if (aInfo !=null && mService.isSingleton(aInfo.processName, aInfo.applicationInfo)){

userId =0;

}

aInfo =mService.getActivityInfoForUser(aInfo, userId);

synchronized(mService) {

intcallingPid;

//传入的参数callingUid=-1

if(callingUid >= 0) {

callingPid = -1;

} else if(caller == null) {

callingPid = Binder.getCallingPid();

callingUid = Binder.getCallingUid();

} else {

callingPid = callingUid = -1;

}

//传入参数config=null，所以mConfigWillChange=false

mConfigWillChange = config != null

&& mService.mConfiguration.diff(config) != 0;

if(DEBUG_CONFIGURATION) Slog.v(TAG,

"Starting activity when config will change = " +mConfigWillChange);

//http://blog.chinaunix.net/uid-12469487-id-3050585.html

finallong origId = Binder.clearCallingIdentity();

//对于这个判断(aInfo.applicationInfo.flags&ApplicationInfo.FLAG_CANT_SAVE_STATE) != 0，因为aInfo.applicationInfo.flags中不包含ApplicationInfo.FLAG_CANT_SAVE_STATE标志，所以它的值=false

//下面这段不会执行

if(mMainStack && aInfo != null &&

(aInfo.applicationInfo.flags&ApplicationInfo.FLAG_CANT_SAVE_STATE)!= 0) {

…

}

int res =startActivityLocked(caller, intent, resolvedType,

aInfo, resultTo, resultWho,requestCode, callingPid, callingUid,

startFlags, options,componentSpecified, null);

if(mConfigWillChange && mMainStack) { // mConfigWillChange=false

// Ifthe caller also wants to switch to a new configuration,

// do so now. This allows a clean switch, as we are waiting

//for the current activity to pause (so we will not destroy

//it), and have not yet started the next activity.

mService.enforceCallingPermission(android.Manifest.permission.CHANGE_CONFIGURATION,

"updateConfiguration()");

mConfigWillChange = false;

if(DEBUG_CONFIGURATION) Slog.v(TAG,

"Updating to new configuration after starting activity.");

mService.updateConfigurationLocked(config, null, false, false);

}

Binder.restoreCallingIdentity(origId);

if(outResult != null) { //outResult=null

outResult.result = res;

if(res == ActivityManager.START_SUCCESS) {

mWaitingActivityLaunched.add(outResult);

do {

try {

mService.wait();

} catch (InterruptedException e) {

}

}while (!outResult.timeout && outResult.who == null);

}else if (res == ActivityManager.START_TASK_TO_FRONT) {

ActivityRecordr = this.topRunningActivityLocked(null);

if (r.nowVisible) {

outResult.timeout = false;

outResult.who = new ComponentName(r.info.packageName, r.info.name);

outResult.totalTime = 0;

outResult.thisTime = 0;

}else {

outResult.thisTime = SystemClock.uptimeMillis();

mWaitingActivityVisible.add(outResult);

do {

try {

mService.wait();

} catch (InterruptedException e) {

}

} while (!outResult.timeout && outResult.who == null);

}

returnres;

}

Step 9: ActivityStack. startActivityLocked

这个函数所做的主要工作：

从传进来的参数caller得到调用者的进程信息，并保存在callerApp变量中，这里就是Launcher应用程序的进程信息了。

前面说过，参数resultTo是Launcher这个Activity里面的一个Binder对象，通过它可以获得Launcher这个启动者Activity的相关信息，保存在sourceRecord变量中。

通过launchFlags = intent.getFlags()获取launchFlags信息，对launchFlags中的各种flag进行判断和处理。针对当前场景下，我们只设置了NEW_TASK标志，所以这里的判断和检查都不适用。

判断callingPid和callingUid是否有权限启动新的Activity。

创建即将要启动的Activity的相关信息，并保存在(ActivityRecord)r变量中

如果有pending activities需要先处理,调用mService.doPendingActivityLaunchesLocked()，在doPendingActivityLaunchesLocked() 中会调用startActivityUncheckedLocked()进一步处理，也就是先启动pending acitivies。

上面的事情做完后，才执行startActivityUncheckedLocked()启动我们要启动的Activity。

2403 final intstartActivityLocked(IApplicationThread caller,

2404 Intent intent, String resolvedType,ActivityInfo aInfo, IBinder resultTo,

2405 String resultWho, int requestCode,

2406 int callingPid, int callingUid, intstartFlags, Bundle options,

2407 boolean componentSpecified,ActivityRecord[] outActivity) {

2408

2409 int err =ActivityManager.START_SUCCESS;

2410

2411 ProcessRecord callerApp = null;

//从传进来的参数caller得到调用者的进程信息，并保存在callerApp变量中，这里就是Launcher应用程序的进程信息了。

2412 if(caller != null) {

2413 callerApp = mService.getRecordForAppLocked(caller);

2414 if(callerApp != null) {

2415 callingPid = callerApp.pid;

2416 callingUid= callerApp.info.uid;

2417 }else {

2418 Slog.w(TAG, "Unable to find app for caller " + caller

2419 + " (pid=" + callingPid + ") when starting: "

2420 + intent.toString());

2421 err =ActivityManager.START_PERMISSION_DENIED;

2422 }

2423 }

2424

2425 if (err== ActivityManager.START_SUCCESS) {

2426 finalint userId = aInfo != null ? UserId.getUserId(aInfo.applicationInfo.uid) : 0;

2427 Slog.i(TAG, "START {" + intent.toShortString(true, true, true,false)

2428 + " u=" + userId + "} from pid " + (callerApp !=null ? callerApp.pid : callingPid));

//更新env环境变量信息env->ReleaseStringUTFChars(activity, actStr);

2429 mActivityTrigger.activityStartTrigger(intent);

2430 }

2431

2432 ActivityRecord sourceRecord = null;

2433 ActivityRecord resultRecord = null;

//前面说过，参数resultTo是Launcher这个Activity里面的一个Binder对象，通过它可以获得Launcher这个Activity的相关信息，保存在sourceRecord变量中。

2434 if(resultTo != null) { //resultTo !=null

2435 intindex = indexOfTokenLocked(resultTo);

2436 if(DEBUG_RESULTS) Slog.v(

2437 TAG, "Will send result to " + resultTo + " (index "+ index + ")");

2438 if(index >= 0) {

2439 sourceRecord = mHistory.get(index);

//resultCode=-1

2440 if (requestCode >= 0 && !sourceRecord.finishing) {

2441 resultRecord = sourceRecord;

2442 }

2443 }

2444 }

2445

2446 intlaunchFlags = intent.getFlags();

2447 //条件不满足

2448 if((launchFlags&Intent.FLAG_ACTIVITY_FORWARD_RESULT) != 0

2449 && sourceRecord != null) {

…

2464 }

2465 //条件不满足

2466 if (err== ActivityManager.START_SUCCESS && intent.getComponent() == null) {

2467 // We couldn't find a class that canhandle the given Intent.

2468 //That's the end of that!

2469 err =ActivityManager.START_INTENT_NOT_RESOLVED;

2470 }

2471 //ainfo != null

2472 if (err== ActivityManager.START_SUCCESS && aInfo == null) {

2473 // Wecouldn't find the specific class specified in the Intent.

2474 //Also the end of the line.

2475 err =ActivityManager.START_CLASS_NOT_FOUND;

2476 }

2477 //条件不满足

2478 if (err!= ActivityManager.START_SUCCESS) {

2479 if(resultRecord != null) {

2480 sendActivityResultLocked(-1,

2481 resultRecord, resultWho, requestCode,

2482 Activity.RESULT_CANCELED, null);

2483 }

2484 mDismissKeyguardOnNextActivity = false;

2485 ActivityOptions.abort(options);

2486 return err;

2487 }

2488 //此处的启动当然有权限

2489 final intstartAnyPerm = mService.checkPermission(

2490 START_ANY_ACTIVITY, callingPid, callingUid);

2491 final intcomponentPerm = mService.checkComponentPermission(aInfo.permission, callingPid,

2492 callingUid, aInfo.applicationInfo.uid, aInfo.exported);

2493 if (startAnyPerm != PERMISSION_GRANTED&& componentPerm != PERMISSION_GRANTED) {

2494 if(resultRecord != null) {

2495 sendActivityResultLocked(-1,

2496 resultRecord, resultWho, requestCode,

2497 Activity.RESULT_CANCELED, null);

2498 }

2499 mDismissKeyguardOnNextActivity = false;

2500 String msg;

2501 if(!aInfo.exported) {

2502 msg = "Permission Denial: starting " + intent.toString()

2503 + " from " + callerApp + " (pid=" + callingPid

2504 + ", uid=" + callingUid + ")"

2505 + " not exported from uid " + aInfo.applicationInfo.uid;

2506 }else {

2507 msg = "Permission Denial:starting " + intent.toString()

2508 + " from " + callerApp + " (pid=" + callingPid

2509 + ", uid=" + callingUid + ")"

2510 + " requires " + aInfo.permission;

2511 }

2512 Slog.w(TAG, msg);

2513 thrownew SecurityException(msg);

2514 }

2515

2516 if(mMainStack) {

2517 if(mService.mController != null) {

2518 boolean abort = false;

2519 try {

2520 // The Intent we give to the watcher has the extra data

2521 // stripped off, since it can contain private information.

2522 Intent watchIntent = intent.cloneFilter();

//mService.mController.activityStarting() always return true

2523 abort = !mService.mController.activityStarting(watchIntent,

2524 aInfo.applicationInfo.packageName);

2525 }catch (RemoteException e) {

2526 mService.mController = null;

2527 }

2528

2529 if (abort) {

2530 if (resultRecord != null) {

2531 sendActivityResultLocked(-1,

2532 resultRecord, resultWho,requestCode,

2533 Activity.RESULT_CANCELED, null);

2534 }

2535 // We pretend to the caller that it was really started, but

2536 // they will just get a cancel result.

2537 mDismissKeyguardOnNextActivity = false;

2538 ActivityOptions.abort(options);

2539 return ActivityManager.START_SUCCESS;

2540 }

2541 }

2542 }

2543 //创建即将要启动的Activity的相关信息，并保存在r变量中

2544 ActivityRecord r = new ActivityRecord(mService, this, callerApp,callingUid,

2545 intent, resolvedType, aInfo, mService.mConfiguration,

2546 resultRecord, resultWho, requestCode, componentSpecified);

2547 if(outActivity != null) {

2548 outActivity[0] = r;

2549 }

2550 // 此场景下条件不满足，因为mService.checkAppSwitchAllowedLocked() should return true

2551 if(mMainStack) {

2552 if(mResumedActivity == null

2553 || mResumedActivity.info.applicationInfo.uid != callingUid) {

2554 if (!mService.checkAppSwitchAllowedLocked(callingPid, callingUid,"Activity start")) {

2555 PendingActivityLaunch pal = new PendingActivityLaunch();

2556 pal.r = r;

2557 pal.sourceRecord = sourceRecord;

2558 pal.startFlags = startFlags;

2559 mService.mPendingActivityLaunches.add(pal);

2560 mDismissKeyguardOnNextActivity = false;

2561 ActivityOptions.abort(options);

2562 return ActivityManager.START_SWITCHES_CANCELED;

2563 }

2564 }

2565//是否允许Activity switch,这个变量在ActivityManagerService.java.stopAppSwitches()中被设置成false，与之个的一个变量是APP_SWITCH_DELAY_TIME，关于它的解释如下:

// Amount of time after a call to stopAppSwitches() duringwhich we will prevent further untrusted switches from happening.

2566 if(mService.mDidAppSwitch) {

2567 // This is the second allowed switch since we stopped switches,

2568 // so now just generally allow switches. Use case: user presses

2569 // home (switches disabled, switch to home, mDidAppSwitch now true);

2570 // user taps a home icon (coming from home so allowed, we hit here

2571 // and now allow anyone to switch again).

2572 mService.mAppSwitchesAllowedTime = 0;

2573 }else {

2574 mService.mDidAppSwitch = true;

2575 }

2576 //如果有pending activities需要先处理

2577 mService.doPendingActivityLaunchesLocked(false);

2578 }

2579

2580 err = startActivityUncheckedLocked(r,sourceRecord,

2581 startFlags, true, options);

2582 if(mDismissKeyguardOnNextActivity && mPausingActivity == null) {

2583 //Someone asked to have the keyguard dismissed on the next

2584 //activity start, but we are not actually doing an activity

2585 //switch... just dismiss the keyguard now,because we

2586 //probably want to see whatever is behind it.

2587 mDismissKeyguardOnNextActivity = false;

2588 mService.mWindowManager.dismissKeyguard();

2589 }

2590 returnerr;

2591 }

Step 10: ActivityStack.startActivityUncheckedLocked

这个函数所做的主要工作：

这个函数的最重要工作就是确定Activity运行的Task，各种情况判断比较多，但是对于本文涉及的场景就简单些，因为我们的Activity在开始阶段的launcher.startActivity中就已经为intent设置了FLAG_ACTIVITY_NEW_TASK标志，所以最终就会创建一个新的Task。

首先获得intent的标志值，保存在launchFlags变量中。

根据Activity.launchMode来判断是否需要给当前Activity. launchFlags添加Intent.FLAG_ACTIVITY_NEW_TASK标志。为(target activity)r.launchFlags添加FLAG_ACTIVITY_NEW_TASK标志的条件包括：

1)sourceRecord == null

2)sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE

3)r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE ||r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK

满足(r.resultTo != null &&(launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0)条件，立刻返回结果给调用者。因为startActivityForResult()是不能跨越不同task的。

调用findTaskLocked()搜索当前有没有Task可以用来执行这个Activity。由于r.launchMode的值不为ActivityInfo.LAUNCH_SINGLE_INSTANCE，因此，它通过findTaskLocked函数来查找存不存这样的Task。如果是第一次运行，返回的结果是null，（只有当目标Activity存在而且在task顶端才满足查找条件）即taskTop为null，因此，需要创建一个新的Task来启动这个Activity。

检查需要启动的Activity与现在的topactivity是否相同？不同，因此不需要执行resume top activtity，进一步分析：resume top activity还必须满足其他条件：launchMode中还必须设置了FLAG_ACTIVITY_SINGLE_TOP或者.LAUNCH_SINGLE_TOP或者LAUNCH_SINGLE_TASK。

如果没有可以可用的task，给当前的Activity创建和赋值一个task.

r.setTask(new TaskRecord(mService.mCurTask, r.info, intent),null, true);

调用startActivityLocked(r, newTask,doResume, keepCurTransition, options)继续执行。

2603 final intstartActivityUncheckedLocked(ActivityRecord r,

2604 ActivityRecord sourceRecord, intstartFlags, boolean doResume,

2605 Bundle options) {

//doReume=true

2606 finalIntent intent = r.intent;

2607 final intcallingUid = r.launchedFromUid;

2608 final intuserId = r.userId;

2609 //函数首先获得intent的标志值，保存在launchFlags变量中。

2610 intlaunchFlags = intent.getFlags();

2611 //这个intent的标志值的位Intent.FLAG_ACTIVITY_NO_USER_ACTION没有置位，因此，成员变量mUserLeaving的值为true。

2612 // We'llinvoke onUserLeaving before onPause only if the launching

2613 //activity did not explicitly state that this is an automated launch.

2614 mUserLeaving = (launchFlags&Intent.FLAG_ACTIVITY_NO_USER_ACTION) ==0; // mUserLeaving=true

2615 if(DEBUG_USER_LEAVING) Slog.v(TAG,

2616 "startActivity() => mUserLeaving=" + mUserLeaving);

2617

2618 // If thecaller has asked not to resume at this point, we make note

2619 // ofthis in the record so that we can skip it when trying to find

2620 // thetop running activity.

2621 if(!doResume) {

2622 r.delayedResume = true;

2623 }

2624 //没有设置这个标志，所以notTop=null

2625 ActivityRecord notTop =(launchFlags&Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP)

2626 != 0 ? r : null;

2627 //startFlags = 0 这个参数是从上面一路传下来的

2628 // If theonlyIfNeeded flag is set, then we can do this if the activity

2629 // beinglaunched is the same as the one making the call... or, as

2630 // aspecial case, if we do not know the caller then we count the

2631 //current top activity as the caller.

2632 if ((startFlags&ActivityManager.START_FLAG_ONLY_IF_NEEDED)!= 0) {

2633 ActivityRecord checkedCaller = sourceRecord;

2634 if(checkedCaller == null) {

2635 checkedCaller = topRunningNonDelayedActivityLocked(notTop);

2636 }

2637 if(!checkedCaller.realActivity.equals(r.realActivity)) {

2638 // Caller is not the same as launcher, so always needed.

2639 startFlags &= ~ActivityManager.START_FLAG_ONLY_IF_NEEDED;

2640 }

2641 }

2642

2643 if(sourceRecord == null) {

2644 //This activity is not being started from another... in this

2645 //case we -always- start a new task.

2646 if((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {

2647 Slog.w(TAG, "startActivity called from non-Activity context;forcing Intent.FLAG_ACTIVITY_NEW_TASK for: "

2648 + intent);

2649 launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;

2650 }

2651 } else if(sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {

2652 //The original activity who is starting us is running as a single

2653 //instance... this new activity it isstarting must go on its

2654 // own task.

2655 launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;

2656 } else if(r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE

2657 || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK) {

2658 //The activity being started is a single instance... it always

2659 //gets launched into its own task.

2660 launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;

2661 }

2662//我们刚好满足了下面这个条件，所以launcher就会通过onActivityResult()收到启动新activity结果的回调

2663 if(r.resultTo != null && (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK)!= 0) {

2664 //For whatever reason this activity is being launched into a new

2665 // task... yet the caller has requested a resultback. Well, that

2666 // ispretty messed up, so instead immediately send back a cancel

2667 //and let the new task continue launched as normal without a

2668 //dependency on its originator.

2669 Slog.w(TAG, "Activity is launching as a new task, so cancellingactivity result.");

2670 sendActivityResultLocked(-1,

2671 r.resultTo, r.resultWho, r.requestCode,

2672 Activity.RESULT_CANCELED, null);

2673 r.resultTo = null;

2674 }

2675

2676 booleanaddingToTask = false;

2677 booleanmovedHome = false;

2678 TaskRecord reuseTask = null;

//在要启动new Activity之初的Launcher.startActivity中，Intent.FLAG_ACTIVITY_NEW_TASK就已经被设置了，因此下面的语句将被执行

// 这段代码的逻辑是，当前有没有Task可以用来执行这个Activity。由于r.launchMode的值不为 ActivityInfo.LAUNCH_SINGLE_INSTANCE，因此，它通过findTaskLocked函数来查找存不存这样的Task，这里返回的结果是null，即taskTop为null，因此，需要创建一个新的Task来启动这个Activity。

2679 if(((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0 &&

2680 (launchFlags&Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)

2681 || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK

2682 || r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {

2683 // If bring to front is requested,and no result is requested, and

2684 // we can find a task that wasstarted with this same

2685 // component, then instead oflaunching bring that one to the front.

2686 if (r.resultTo == null) {

2687 // See if there is a task tobring to the front. If this is

2688 // a SINGLE_INSTANCE activity,there can be one and only one

2689 // instance of it in thehistory, and it is always in its own

2690 // unique task, so we do aspecial search.

2691 ActivityRecord taskTop = r.launchMode !=ActivityInfo.LAUNCH_SINGLE_INSTANCE

2692 ? findTaskLocked(intent, r.info)//条件是只有当目标Activity存在而且在task顶端。请牢记这个条件，只有这样后续的moveTaskToFrontLocked()才会成立

2693 : findActivityLocked(intent, r.info);

2694 if (taskTop != null) { //当前场景中，我们假设这是第一次启动，所以taskTop=null

2695 if (taskTop.task.intent == null) {

2696 // This task was started because of movement of

2697 // the activity based on affinity... now that we

2698 // are actually launching it, we can assign the

2699 // base intent.

2700 taskTop.task.setIntent(intent, r.info);

2701 }

2702 // If the target task is not in the front, then we need

2703 // to bring it to the front... except... well, with

2704 // SINGLE_TASK_LAUNCH it's not entirely clear. We'd like

2705 // to have the same behavior as if a new instance was

2706 // being started, which means not bringing it to the front

2707 // if the caller is not itself in the front.

2708 ActivityRecord curTop = topRunningNonDelayedActivityLocked(notTop);

2709 if (curTop != null && curTop.task != taskTop.task) {//如果当前最顶端运行的activity.task不是我们要恢复的task

2710 r.intent.addFlags(Intent.FLAG_ACTIVITY_BROUGHT_TO_FRONT);

2711 boolean callerAtFront = sourceRecord == null

2712 || curTop.task== sourceRecord.task;

2713 if (callerAtFront) {//如果是调用者在最顶端

2714 // We really dowant to push this one into the

2715 // user's face,right now.

2716 movedHome = true;

2717 moveHomeToFrontFromLaunchLocked(launchFlags);

2718 moveTaskToFrontLocked(taskTop.task, r, options);

2719 options = null;

2720 }

2721 }

2722 // If the caller has requested that the target task be

2723 // reset, then do so.

2724 if ((launchFlags&Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED) != 0) {

2725 taskTop =resetTaskIfNeededLocked(taskTop, r);

2726 }

2727 if ((startFlags&ActivityManager.START_FLAG_ONLY_IF_NEEDED) != 0) {

2728 // We don't need to start a new activity, and

2729 // the client said not to doanything if that

2730 // is the case, so this is it! And for paranoia, make

2731 // sure we have correctly resumed the top activity.

2732 if (doResume) {

2733 resumeTopActivityLocked(null, options);

2734 } else {

2735 ActivityOptions.abort(options);

2736 }

2737 return ActivityManager.START_RETURN_INTENT_TO_CALLER;

2738 }

2739 if ((launchFlags &

2740 (Intent.FLAG_ACTIVITY_NEW_TASK|Intent.FLAG_ACTIVITY_CLEAR_TASK))

2741 == (Intent.FLAG_ACTIVITY_NEW_TASK|Intent.FLAG_ACTIVITY_CLEAR_TASK)){

2742 // The caller has requested to completely replace any

2743 // existing task with its new activity. Well that should

2744 // not be too hard...

2745 reuseTask = taskTop.task;

2746 performClearTaskLocked(taskTop.task.taskId);

2747 reuseTask.setIntent(r.intent, r.info);

2748 } else if ((launchFlags&Intent.FLAG_ACTIVITY_CLEAR_TOP) != 0

2749 || r.launchMode ==ActivityInfo.LAUNCH_SINGLE_TASK

2750 || r.launchMode ==ActivityInfo.LAUNCH_SINGLE_INSTANCE) {

2751 // In this situation we want to remove all activities

2752 // from the task up to the one being started. In most

2753 // cases this means we are resetting the task to its

2754 // initial state.

2755 ActivityRecord top =performClearTaskLocked(

2756 taskTop.task.taskId, r, launchFlags);

2757 if (top != null) {

2758 if(top.frontOfTask) {

2759 // Activity aliases may meanwe use different

2760 // intents forthe top activity, so make sure

2761 // the task nowhas the identity of the new

2762 // intent.

2763 top.task.setIntent(r.intent, r.info);

2764 }

2765 logStartActivity(EventLogTags.AM_NEW_INTENT, r, top.task);

2766 top.deliverNewIntentLocked(callingUid,r.intent);

2767 } else {

2768 // A special case:we need to

2769 // start theactivity because it is not currently

2770 // running, and thecaller has asked to clear the

2771 // current task tohave this activity at the top.

2772 addingToTask =true;

2773 // Now pretend likethis activity is being started

2774 // by the top ofits task, so it is put in the

2775 // right place.

2776 sourceRecord =taskTop;

2777 }

2778 } else if (r.realActivity.equals(taskTop.task.realActivity)) {

2779 // In this case the top activity on the task is the

2780 // same as the one being launched, so we take that

2781 // as a request to bring the task to the foreground.

2782 // If the top activity in the task is the root

2783 // activity, deliver this new intent to it if it

2784 // desires.

2785 if((launchFlags&Intent.FLAG_ACTIVITY_SINGLE_TOP) != 0

2786 &&taskTop.realActivity.equals(r.realActivity)) {

2787 logStartActivity(EventLogTags.AM_NEW_INTENT, r, taskTop.task);

2788 if (taskTop.frontOfTask) {

2789 taskTop.task.setIntent(r.intent, r.info);

2790 }

2791 taskTop.deliverNewIntentLocked(callingUid, r.intent);

2792 } else if(!r.intent.filterEquals(taskTop.task.intent)) {

2793 // In this case weare launching the root activity

2794 // of the task, butwith a different intent. We

2795 // should start a newinstance on top.

2796 addingToTask =true;

2797 sourceRecord =taskTop;

2798 }

2799 } else if ((launchFlags&Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED)== 0) {

2800

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