Launcher 启动 activity 流程

public class ItemClickHandler {
 private static void onClick(View v) {
     //...
     // Start activities
     startAppShortcutOrInfoActivity(v, shortcut, launcher);     
 } 
}

Launcher. startActivitySafely()//内部调用父类的startActivitySafely()方法

BaseDraggingActivity.startActivitySafely()//调用Activity 的startActivity()方法

Activity.startActivity()
//...
Activity.startActivityForResult()//调用Instrumentation.execStartActivity()方法

public class Instrumentation {

    public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        //...
        try {
            //这儿
            //whoThread :launcher 进程中ActivityThread的ApplicationThread 对象,它是一个IApplicationThread类型的
            //intent : 跳转的信息
            int result = ActivityManager.getService()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, options);
        } catch (RemoteException e) {
        }
        return null;
    }        
}

ActivityManagerService.startActivity()
//...
ActivityManagerService.startActivityAsUser()//execute()

ActivityStarter.execute()
//
ActivityStarter.startActivityMayWait()
//
ActivityStarter.startActivity()
//
ActivityStarter.startActivityUnchecked()//调用resumeFocusedStackTopActivityLocked()
//

ActivityStackSupervisor.resumeFocusedStackTopActivityLocked()
//调用ActivityStack.resumeTopActivityUncheckedLocked()

ActivityStack.resumeTopActivityUncheckedLocked()
//
ActivityStack.resumeTopActivityInnerLocked()//方法内部两个流程
//流程1. pause 当前的 activity
ActivityStack.startPausingLocked
//流程2.创建进程
//调用 ActivityStackSupervisor.startSpecificActivityLocked()

public class ActivityStackSupervisor  {
    //...

    void startSpecificActivityLocked(ActivityRecord r,
            boolean andResume, boolean checkConfig) {
        //从AMS中查询将要启动的Activity的进程是否存在
        // Is this activity's application already running?
        ProcessRecord app = mService.getProcessRecordLocked(r.processName,
                r.info.applicationInfo.uid, true);

        getLaunchTimeTracker().setLaunchTime(r);

         //如果应用进程已经存在
        if (app != null && app.thread != null) {
            try {
                //...
                //如果是 APP 内启动新 Activity 则执行这个分支
                realStartActivityLocked(r, app, andResume, checkConfig);
                //return 下面的不会执行了
                return;
            } catch (RemoteException e) {
            }

        }
        //launcher 启动 APP 执行这个逻辑
        //进程不存在，通过AMS调用startProcessLocked向Zygote进程发送请求
        mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
                "activity", r.intent.getComponent(), false, false, true);
    }        
        }

public final class ActivityThread extends ClientTransactionHandler {
    
    public static void main(String[] args) {
    
        //...
        Looper.prepareMainLooper();

       
        ActivityThread thread = new ActivityThread();
        //ActivityThread的attach()
        thread.attach(false, startSeq);

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }

       //...
        Looper.loop();

    }  
    
    private void attach(boolean system, long startSeq) {
        sCurrentActivityThread = this;
        mSystemThread = system;
        //...
        if (!system) {
            final IActivityManager mgr = ActivityManager.getService();
            try {
                //AMS绑定ApplicationThread对象
                //APP 进程和AMS进程开始通信通信
                mgr.attachApplication(mAppThread, startSeq);
            } catch (RemoteException ex) {
            }
        } else {
        }
       //...
    }    
}

ActivityManagerService.attachApplication()
//
ActivityManagerService.attachApplicationLocked()
//1. thread.bindApplication
//2.mStackSupervisor.attachApplicationLocked(app) 分析这一步

public class ActivityStackSupervisor  {
    
    boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
        final String processName = app.processName;
        boolean didSomething = false;
        for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
            final ActivityDisplay display = mActivityDisplays.valueAt(displayNdx);
            for (int stackNdx = display.getChildCount() - 1; stackNdx >= 0; --stackNdx) {
                if (activity.app == null && app.uid == activity.info.applicationInfo.uid
                        && processName.equals(activity.processName)) {
                    try {
                        //还是到这儿
                        if (realStartActivityLocked(activity, app,
                                top == activity /* andResume */, true /* checkConfig */)) {
                            didSomething = true;
                        }
                    } catch (RemoteException e) {
                    }
                }
            }
        }
        return didSomething;
    }    
}

ActivityThread.handleLaunchActivity()
//
ActivityThread.performLaunchActivity()//通过反射创建 activity
// 依次回调 activity 的 
Activity.attach()
Activity.onCreate()
Activity.start()
Activity.onResume()