从startActivity开始深入

startActivity(new Intent(ActivityOne.this, ActivityTwo.class));

1.首先了解传入的参数Intent

 public Intent(Context packageContext, Class cls) {
        mComponent = new ComponentName(packageContext, cls);
    }

Intent的构造函数里用到了ComponentName对象

 

public ComponentName(Context pkg, Class cls) {
        mPackage = pkg.getPackageName();
        mClass = cls.getName();
    }

这里获取的是当前应用的包名和跳转的Activity的类名

2.回到startActivity(Intent intent)

public void startActivity(Intent intent) {
        this.startActivity(intent, null);
    }

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);
        }
    }

这里判断传入的Bundle是否为空，如果为空，就跑下面

public void startActivityForResult(Intent intent, int requestCode) {
        startActivityForResult(intent, requestCode, null);
    }

 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;
            }

            final View decor = mWindow != null ? mWindow.peekDecorView() : null;
            if (decor != null) {
                decor.cancelPendingInputEvents();
            }
            // 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);
            }
        }
        if (options != null && !isTopOfTask()) {
            mActivityTransitionState.startExitOutTransition(this, options);
        }
    }

先看parent为空吧，parent也是Activity，应该是一些嵌套Activity组

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

 Instrumentation从attach方法那里传进来

final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor) {
 mInstrumentation = instr;