首先推荐一个查看源码的Android studio 插件 AndroidSourceViewer，超级好用。

我是跟着《Android进阶解密》这本书来学习源码的，也推荐给大家。
直接上启动流程第一部分的流程图

Activity启动流程1.jpg

Activity的启动一般是从startActivity开始的，

    @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);
        }
    }

startActivity其实也是调用的startActivityForResult，
1、当不设置FLAG_ACTIVITY_NEW_TASK时，新启动的Activity会被添加到调用者的栈中

public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            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);
            }
        }
    }

2、如果requestCode小于0，则和调用startActivity的效果一样，不会有返回值
3、如果在startActivityForResult时使用FLAG_ACTIVITY_NEW_TASK，那会启动一个新的栈，并立马收到CANCEL
可以看到这里根据mParent是否为空进行了不同的处理，
问题1：mParent代表什么，什么时候进行赋值？

首先看一下mParent为null的情况，会执行mInstrumentation.execStartActivity方法。
Instrumentation主要用来监控应用程序和系统的交互。需要在AndroidManifest中通过<instrumentation>声明用于监控应用与系统交互的 Instrumentation 类。Instrumentation 对象会在应用的所有组件之前进行实例化。

    public ActivityResult execStartActivity(
        Context who, IBinder contextThread, IBinder token, String target,
        Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    final ActivityMonitor am = mActivityMonitors.get(i);
                    ActivityResult result = null;
                    if (am.ignoreMatchingSpecificIntents()) {
                        result = am.onStartActivity(intent);
                    }
                    if (result != null) {
                        am.mHits++;
                        return result;
                    } else if (am.match(who, null, intent)) {
                        am.mHits++;
                        if (am.isBlocking()) {
                            return requestCode >= 0 ? am.getResult() : null;
                        }
                        break;
                    }
                }
            }
        }
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            int result = ActivityManager.getService()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target, requestCode, 0, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }

mActivityMonitors部分的代码，我的理解应该是给测试功能使用的，所以看下边ActivityManager.getService().startActivity的调用。
ActivityManager是个工具类，应用开发中很少用到，不过isLowRamDevice()和clearApplicationUserData()可以在应用开发中用来满足特殊的需求。
getService中用到了单例类Singleton

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;
        }
    }
}

持续更新中。。。