注释1处设置Flag为Intent.FLAG_ACTIVITY_NEW_TASK,这样根Activity会在新的任务栈中启动。
注释2处调用了Activity的startActivity函数。
frameworks/base/core/java/android/app/Activity.java
Activity#startActivity()
@Override
public void startActivity(Intent intent, @Nullable Bundle options) {
if (options != null) {
startActivityForResult(intent, -1, options); //1
} else {
// Note we want to go through this call for compatibility with
// applications that may have overridden the method.
startActivityForResult(intent, -1); //2
}
}
注释1和2处都会调用startActivityForResult函数,其中第二个参数为-1,表示Launcher不需要知道Activity启动的结果。
Activity#startActivityForResult
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode, @Nullable Bundle options) {
if (mParent == null) {
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(this, mMainThread.getApplicationThread(), mToken, this, intent, requestCode, options); //1
if (ar != null) {
mMainThread.sendActivityResult(mToken, mEmbeddedID, requestCode, ar.getResultCode(), ar.getResultData());
}
…
} else {
…
}
}
mParent是Activity类型的,表示当前Activity的父类。因为目前根Activity还没有创建出来,因此,mParent == null成立。
注释1调用Instrumentation的execStartActivity方法,Instrumentation主要用来监控应用程序和系统的交互。
frameworks/base/core/java/android/app/Instrumentation.java
Instrumentation#execStartActivity()
public ActivityResult execStartActivity(Context who, IBinder contextThread, IBinder token, Activity target, Intent intent, int requestCode, Bundle options) {
…
try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess(who);
//1
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;
}
注释1首先调用ActivityManagerNative的getDefault来获取ActivityManageService(以后简称为AMS)的代理对象,接着调用它的startActivity方法。
这里ActivityManagerNative.getDefault()涉及到Binder进程间通信机制,下面进行一个简短的介绍,这不是本文的重点。了解过的同学可以略过下面这段Binder简析,不想了解的也可以直接理解为返回的是AMS。实际调用的是AMS#startActivity()。
AMS中Binder机制简析
frameworks/base/core/java/android/app/ActivityManagerNative.java
ActivityManagerNative.getDefault()
static public IActivityManager getDefault() {
return gDefault.get();
}
private static final Singleton gDefault = new Singleton() {
protected IActivityManager create() {
IBinder b = ServiceManager.getService(“activity”); //1
IActivityManager am = asInterface(b); //2
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);
}
getDefault方法调用了gDefault的get方法,gDefault 是一个Singleton类。注释1处得到名为”activity”的Service代理对象,也就是AMS的代理对象。
注释2处将它封装成ActivityManagerProxy(以后简称为AMP)类型对象,并将它保存到gDefault中,此后调用ActivityManagerNative(以后简称为AMN)的getDefault方法就会直接获得AMS的代理AMP对象。