<activityandroid:name="com.loadingUI.LoadingActivity"android:label="@string/Loadinging_activity"><intent-filter><action android:name="android.intent.action.MAIN"/><category android:name="android.intent.category.LAUNCHER"/></intent-filter>
</activity>

 Launcher组件在启动时，会向PackageManagerService查询系统中所有“action”等于"action.MAIN"以及"category"等于"category.LAUNCHER"的Activity组件，为每一个Activity组件创建一个快捷图标。

public final class Launcher extends Activityimplements View.OnClickListener, OnLongClickListener, LauncherModel.Callbacks, View.OnTouchListener

1.2）点击响应函数Launcher#onClick：

/** \packages\apps\Launcher2\src\com\android\launcher2*/
public void onClick(View v) {...Object tag = v.getTag();if (tag instanceof ShortcutInfo) {// 这里组装启动intent的信息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()));/** 看到通过这个函数用来启动一个应用的根Activity*/boolean success = startActivitySafely(v, intent, tag);if (success && v instanceof BubbleTextView) {mWaitingForResume = (BubbleTextView) v;mWaitingForResume.setStayPressed(true);}} .......
}

可以看到Launcher通过函数startActivitySafely来启动应用程序的根Activity;

1.2）Launcher#startActivitySafely方法：

boolean startActivity(View v, Intent intent, Object tag) {/*** 设置启动标志***/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);// 当添加新的启动动画时，使用此启动方式if (useLaunchAnimation) {ActivityOptions opts = ActivityOptions.makeScaleUpAnimation(v, 0, 0,v.getMeasuredWidth(), v.getMeasuredHeight());if (user == null || user.equals(android.os.Process.myUserHandle())) {/** 注意这里将会调用父类的startActivity*/startActivity(intent, opts.toBundle());} else {launcherApps.startMainActivity(intent.getComponent(), user,intent.getSourceBounds(), opts.toBundle());}} else { // 默认的启动方式if (user == null || user.equals(android.os.Process.myUserHandle())) {/** 调用父类Activity的startActivity来启动 **/startActivity(intent);} else {launcherApps.startMainActivity(intent.getComponent(), user,intent.getSourceBounds(), null);}}return true;} catch (SecurityException e) {...}return false;
}

上面代码重要完成两个工作：

    public class Activity extends ContextThemeWrapperimplements LayoutInflater.Factory2,Window.Callback, KeyEvent.Callback,OnCreateContextMenuListener, ComponentCallbacks2,Window.OnWindowDismissedCallback

/** \frameworks\base\core\java\android\app\Activity.java*/
@Override
public void startActivity(Intent intent) {this.startActivity(intent, null);
}@Override
public void startActivity(Intent intent, @Nullable Bundle options) {if (options != null) {startActivityForResult(intent, -1, options);} else {// Note we want to go through this call for compatibility with// applications that may have overridden the method.startActivityForResult(intent, -1);}
}

最终将会调用startActivityForResult来执行函数，第二个参数设为-1表示不需要知道最终的执行结果。

2、来分析Activity#startActivityForResult方法:

public void startActivityForResult(Intent intent, int requestCode) {startActivityForResult(intent, requestCode, null);
}public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) {/** mParent在调用attach函数时传入*/if (mParent == null) {/** Instrumentation类用以监控应用程序与系统之间的交互，*  这里通过其execStartActivity来启动Activity组件**/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());}/** 这个本情况下不需要*/if (requestCode >= 0) {mStartedActivity = true;}/** 有关界面加载SurfaceFlinger*/final View decor = mWindow != null ? mWindow.peekDecorView() : null;if (decor != null) {decor.cancelPendingInputEvents();}} else {...}if (options != null && !isTopOfTask()) {mActivityTransitionState.startExitOutTransition(this, options);}
}

注意execStartActivity中的传参param：

private IBinder mToken;

mToken是一个Binder代理对象，指向了ActivityManagerService中一个类型为ActivityRecord的Binder本地对象。每一个已经启动的Activity组件在AMS（ActivityManagerService）中都存在一个对应的ActivityRecord对象，用来维护对应的Activity组件的运行状态及信息。

Instrumentation是执行application instrumentation代码的基类。当应用程序运行的时候instrumentation处于开启，Instrumentation将在任何应用程序运行前初始化，可以通过它监测系统与应用程序之间的交互。Instrumentation implementation通过的AndroidManifest.xml中的<instrumentation>标签进行描述。

Instrumentation似乎有些类似与window中的“钩子（Hook）函数”，在系统与应用程序之间安装了个“窃听器”。

1、Instrumentation#execStartActivity函数：

Instrumentation.ActivityResult ar =mInstrumentation.execStartActivity(this, mMainThread.getApplicationThread(), mToken, this, intent, requestCode, options);

函数源码：

/** \frameworks\base\core\java\android\app\Instrumentation.java*/
public ActivityResult execStartActivity(Context who, IBinder contextThread, IBinder token, Activity target,Intent intent, int requestCode, Bundle options) {IApplicationThread whoThread = (IApplicationThread) contextThread;/** mActivityMonitors定义：List<ActivityMonitor> mActivityMonitors;*  ActivityMoniter：有关特定的Intent的监视。*  一个ActivityMoniter类的实例通过函数addMonitor*  （Instrumentation.ActivityMonitor）添加到当前*  instrumentation中，一旦添加后，每当启动一个新的Activity，*  ActivityMoniter就会检测，如果匹配，其hit count计数更新*  等其他操作。一个ActivityMonitor也可以用来寻找一个Activity，*  通过waitForActivity（）方法，这个函数将返直到匹配的活动被创建。*/// 这里与主线无关，可以不用关心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;}}}}/** 主要的代码从这里开始**/try {intent.migrateExtraStreamToClipData();intent.prepareToLeaveProcess();// 主要的执行函数在这里，调用了ActivityManagerNative中的startActivityint 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) {}return null;
}

上述代码中最重要的作用是调用了ActivityManagerNative.getDefault().startActivity方法；getDefault用以获取AMS的一个代理对象ActivityManagerNative，接着再调用它的成员函数startActivity来通知AMS将有一个Activity组件启动。

   /** \frameworks\base\core\java\android\app\ActivityManagerNative.java*/public abstract class ActivityManagerNative extends Binder implements IActivityManager

1、ActivityManagerNative#getDefault函数：

    /** \frameworks\base\core\java\android\app\ActivityManagerNative.java*/static public IActivityManager getDefault() {// 单例类Singleton中的函数，get是Singleton类中的方法，见下面附Ireturn gDefault.get();}// Singleton为单例模式的实现，是个抽象类，见附一private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {protected IActivityManager create() {// 前面AMS分析中提到的AMS的主要服务"activity"，通过SM获取IBinder b = ServiceManager.getService("activity");...IActivityManager am = asInterface(b);...return am;}};static public IActivityManager asInterface(IBinder obj) {if (obj == null) {return null;}IActivityManager in = (IActivityManager)obj.queryLocalInterface(descriptor);if (in != null) {return in;}return new ActivityManagerProxy(obj);}

可以看到当调用getDefault函数时，会返回gDefault.get()，返回一个IActivityManager类；

附I、Singleton类

    /*** Singleton helper class for lazily initialization.* Modeled after frameworks/base/include/utils/Singleton.h*/public abstract class Singleton<T> {private T mInstance;protected abstract T create();public final T get() {synchronized (this) {if (mInstance == null) {mInstance = create();}return mInstance;}}}

在定义gDefault时，需要重写abstract函数create()；在create函数中，可以看到它通过ServiceManager获取到一个服务名为"activity"的服务代理对象即一个引用了ActivtyManagerService的代理对象。