拋出問題：
1. Android系統(tǒng)桌面是什么
2. 點(diǎn)擊應(yīng)用圖標(biāo)后Android系統(tǒng)執(zhí)行了什么操作
用文字總結(jié)App啟動流程可以分為以下步驟：
1. Launcher通過Binder建立Launcher所在進(jìn)程與system_server進(jìn)程（ActivityManagerService所在進(jìn)程）的通信，通知ActivityManagerService即將要啟動一個Activity
2. ActivityManagerService通過Binder讓Launcher進(jìn)入pause狀態(tài)
3. Launcher進(jìn)入pause狀態(tài)后，通過Binder告知ActivityManagerService挪蹭，隨后ActivityManagerService創(chuàng)建一個進(jìn)程（將要打開的應(yīng)用進(jìn)程）并啟動ActivityThread（應(yīng)用的UI線程）
4. ActivityThread通過Binder將ApplicationThread類型的Binder對象傳遞給ActivityManagerService障簿，方便ActivityManagerService后續(xù)與其的通信
5. 準(zhǔn)備工作完成后轮蜕，ActivityManagerService通知ActivityThread啟動Activity
6. ActivityThread調(diào)度執(zhí)行Activity的生命周期方法，完成啟動Activity的工作
Activity是視圖存在的根本，那么我們可以通過命令adb shell dumpsys activity activities判斷是哪個Activity為我們呈現(xiàn)桌面視圖的
點(diǎn)擊應(yīng)用圖標(biāo)后Android系統(tǒng)執(zhí)行了什么操作
呈現(xiàn)Android桌面視圖（View）->點(diǎn)擊View上某個應(yīng)用圖標(biāo)->產(chǎn)生點(diǎn)擊事件->點(diǎn)擊事件被響應(yīng)->通知Android系統(tǒng)的某個/某些進(jìn)程->Android系統(tǒng)執(zhí)行某些操作->啟動App
Launcher如何響應(yīng)由我們產(chǎn)生的點(diǎn)擊事件
產(chǎn)生點(diǎn)擊事件后，如果產(chǎn)生點(diǎn)擊事件的View的Tag是ShortcutInfo（即啟動應(yīng)用的快捷方式）喂很，就會取得ShortcutInfo中保存的Intent（這個Intent指向我們要啟動的App），然后執(zhí)行startActivitySafely(v, intent, tag)方法皆刺，而startActivitySafely方法只是對startActivity方法的簡單封裝少辣。
所以，Launcher響應(yīng)我們產(chǎn)生的點(diǎn)擊事件后羡蛾，實(shí)際上就是啟動一個新的Activity漓帅。請看代碼：
/**
* Launches the intent referred by the clicked shortcut.
*
* @param v The view representing the clicked shortcut.
*/
public void onClick(View v) {
// Make sure that rogue clicks don't get through while allapps is launching, or after the
// view has detached (it's possible for this to happen if the view is removed mid touch).
if (v.getWindowToken() == null) {
return;
}
if (!mWorkspace.isFinishedSwitchingState()) {
return;
}
Object tag = v.getTag();
if (tag instanceof ShortcutInfo) {
// Open shortcut
final Intent intent = ((ShortcutInfo) tag).intent;
int[] pos = new int[2];
v.getLocationOnScreen(pos);
intent.setSourceBounds(new Rect(pos[0], pos[1],
pos[0] + v.getWidth(), pos[1] + v.getHeight()));
boolean success = startActivitySafely(v, intent, tag);
if (success && v instanceof BubbleTextView) {
mWaitingForResume = (BubbleTextView) v;
mWaitingForResume.setStayPressed(true);
}
} else if (tag instanceof FolderInfo) {
if (v instanceof FolderIcon) {
FolderIcon fi = (FolderIcon) v;
handleFolderClick(fi);
}
} else if (v == mAllAppsButton) {
if (isAllAppsVisible()) {
showWorkspace(true);
} else {
onClickAllAppsButton(v);
}
}
}
boolean startActivitySafely(View v, Intent intent, Object tag) {
boolean success = false;
try {
success = startActivity(v, intent, tag);
} catch (ActivityNotFoundException e) {
Toast.makeText(this, R.string.activity_not_found, Toast.LENGTH_SHORT).show();
Log.e(TAG, "Unable to launch. tag=" + tag + " intent=" + intent, e);
}
return success;
}
現(xiàn)在回想下App開發(fā)時，每個App都需要有一個“MainActivity”痴怨，這個Activity必須在AndroidManifest.xml文件中有以下配置：
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
在配置AndroidManifest.xml文件時煎殷，將Activity的Action指定為android.intent.action.MAIN，會使Activity在一個新的Task中啟動（Task是一個Activity棧）腿箩。將category指定為android.intent.category.LAUNCHER，表示通過Intent啟動此Activity時劣摇，只接受category為LAUNCHER的Intent珠移。
所以，Launcher將會通過App的快捷方式（ShortcutInfo）得到應(yīng)用的Intent，并通過這個Intent啟動應(yīng)用的“MainActivity”钧惧，從而啟動應(yīng)用暇韧。
所以我們研究的問題就從“App啟動流程”變?yōu)椤癆ctivity啟動流程”。
Launcher通過Binder通知ActivityManagerService啟動Activity
浓瞪，將Intent的Flag設(shè)為Intent.FLAG_ACTIVITY_NEW_TASK懈玻，使得Android系統(tǒng)將創(chuàng)建一個新的Task來放置即將被打開的新Activity（應(yīng)用的“MainActivity）。然后獲取一個布爾值以用于后續(xù)判斷是否顯示啟動App的動畫乾颁。
然后獲取Intent中是否傳輸了Parcelable格式的用戶句柄涂乌，并通過Context.LAUNCHER_APPS_SERVICE獲取用于在多用戶情境下啟動App的系統(tǒng)服務(wù)。
不管是否顯示啟動App的動畫英岭，最終都會執(zhí)行startActivity(intent)或launcherApps.startMainActivity方法以啟動應(yīng)用的“MainActivity”湾盒。
代碼如下：boolean startActivity(View v, Intent intent, Object tag) {
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
try {
// Only launch using the new animation if the shortcut has not opted out (this is a
// private contract between launcher and may be ignored in the future).
boolean useLaunchAnimation = (v != null) &&
!intent.hasExtra(INTENT_EXTRA_IGNORE_LAUNCH_ANIMATION);
UserHandle user = (UserHandle) intent.getParcelableExtra(ApplicationInfo.EXTRA_PROFILE);
LauncherApps launcherApps = (LauncherApps)
this.getSystemService(Context.LAUNCHER_APPS_SERVICE);
if (useLaunchAnimation) {
ActivityOptions opts = ActivityOptions.makeScaleUpAnimation(v, 0, 0,
v.getMeasuredWidth(), v.getMeasuredHeight());
if (user == null || user.equals(android.os.Process.myUserHandle())) {
// Could be launching some bookkeeping activity
startActivity(intent, opts.toBundle());
} else {
launcherApps.startMainActivity(intent.getComponent(), user,
intent.getSourceBounds(),
opts.toBundle());
}
} else {
if (user == null || user.equals(android.os.Process.myUserHandle())) {
startActivity(intent);
} else {
launcherApps.startMainActivity(intent.getComponent(), user,
intent.getSourceBounds(), null);
}
}
return true;
} catch (SecurityException e) {
Toast.makeText(this, R.string.activity_not_found, Toast.LENGTH_SHORT).show();
Log.e(TAG, "Launcher does not have the permission to launch " + intent +
". Make sure to create a MAIN intent-filter for the corresponding activity " +
"or use the exported attribute for this activity. "
+ "tag="+ tag + " intent=" + intent, e);
}
return false;
}
launcherApps.startMainActivity只在用戶句柄不為空且用戶句柄不等于當(dāng)前進(jìn)程句柄（handle）時（其他用戶的句柄）調(diào)用，
為什么用戶句柄會影響Activity的啟動方式诅妹？
我們需要取得用戶A的句柄（和用戶A相關(guān)的數(shù)據(jù)）罚勾，將我們想啟動的用戶B的App的Intent、用戶A的句柄交給內(nèi)核吭狡，讓擁有權(quán)限的
startActivity(intent)如何啟動Activity
進(jìn)入Activity類后層層深入就可以看到最終調(diào)用的是startActivityForResult方法：
從代碼上看尖殃，如果Launcher有mParent Activity，就會執(zhí)行mParent.startActivityFromChild划煮；如果沒有送丰，就會執(zhí)行mInstrumentation.execStartActivity。進(jìn)入mParent.startActivityFromChild方法會看到最終也是執(zhí)行了mInstrumentation.execStartActivity般此。執(zhí)行完成后蚪战，會取得一個ActivityResult對象，用于給調(diào)用者Activity傳遞一些數(shù)據(jù)铐懊，最后在Activity切換時顯示Transition動畫邀桑。
這里有一點(diǎn)需要指出的是：這里的ParentActivity指的是類似TabActivity、ActivityGroup關(guān)系的嵌套Activity科乎。之所以要強(qiáng)調(diào)parent和child壁畸，是要避免混亂的Activity嵌套關(guān)系。代碼如下：
public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) {
if (mParent == null) {
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) {
// 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;
}
cancelInputsAndStartExitTransition(options);
// TODO Consider clearing/flushing other event sources and events for child windows.
} 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);
}
}
}
我們進(jìn)入Instrumentation（管家婆類）類看看execStartActivity方法吧：
我們通過參數(shù)IBinder contextThread取得一個IApplicationThread類型的對象whoThread茅茂，而contextThread是由mMainThread.getApplicationThread()取得的ApplicationThread對象捏萍，此時mMainThread指的就是Launcher應(yīng)用的主線程，所以whoThread指代的自然是Launcher的ApplicationThread空闲。
因?yàn)锳ctivity的onProvideReferrer()方法默認(rèn)返回null令杈，除非該方法被重寫，而我們使用的Launcher并沒有重寫該方法碴倾，所以不用管referrer逗噩。
然后判斷是否有ActivityMonitor掉丽，如果有，則即將要打開的Activity是否和ActivityMonitor中保存的IntentFilter匹配异雁，如果匹配則增加ActivityMonitor的計數(shù)捶障。大致是用于監(jiān)控符合匹配規(guī)則的Activity的數(shù)量的。
最后調(diào)用ActivityManagerNative.getDefault().startActivity(whoThread, who.getBasePackageName(), intent, intent.resolveTypeIfNeeded(who.getContentResolver()), token, target != null ? target.mEmbeddedID : null, requestCode, 0, null, options);啟動Activity纲刀，并檢查啟動是否成功项炼。換句話說，最終負(fù)責(zé)啟動Activity的是ActivityManager示绊，前面得到的ApplicationThread也是在這里使用的锭部。代碼：
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);
}
if (mActivityMonitors != null) {
synchronized (mSync) {
final int 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.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess();
int result = ActivityManagerNative.getDefault()
.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;
}
