这篇文章主要解释一下startActivity(intent)方法，基于android的源码。大家都很熟悉这么这2行代码：
Intent intent = new Intent(v.getContext(),ManagePhoneidActivity.class);
startActivity(intent);
这是activity的跳转的基本代码。下面我们就看看它是怎么执行的？
1、startActivity(intent)的源码：
@Override
public void startActivity(Intent intent) {
startActivityForResult(intent, -1);
}
大家都对startActivityForResult这个方法也熟悉吧。这就是父子activity传值的时候常用的方法。
2、 startActivityForResult(intent, -1)的源码：
public void startActivityForResult(Intent intent, int requestCode) {
if (mParent == null) {
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this,
intent, requestCode);
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;
}
} else {
mParent.startActivityFromChild(this, intent, requestCode);
}
}
解释：（1） mInstrumentation 是Activity类的成员变量，它的类型是Intrumentation，它用来监控应用程序和系统的交互。
(2)mMainThread也是 Activity类的成员变量，它的类型是ActivityThread，它代表的是应用程序的主线程。这里通过 mMainThread.getApplicationThread获得它里面的ApplicationThread成员变量，它是一个Binder对象 。
(3)mToken也是 Activity类的成员变量，它是一个Binder对象的远程接口。
3、execStartActivity(this, mMainThread.getApplicationThread(), mToken, this, intent, requestCode)的源码：
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode) {
IApplicationThread whoThread = (IApplicationThread) contextThread;
if (mActivityMonitors != null) {
。。。。
}
try {
intent.setAllowFds(false);
int result = ActivityManagerNative.getDefault()
.startActivity(whoThread, intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
null, 0, token, target != null ? target.mEmbeddedID : null,
requestCode, false, false, null, null, false);
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
}
return null;
}
解释：（1）getDefault()函数返回的是一个 ActivityManagerProxy对象的引用，也就是说，ActivityManager得到了一个本地代理。
ActivityManagerNative.getDefault()的源码：
/**
* Retrieve the system's default/global activity manager.
*/
static public IActivityManager getDefault() {
return gDefault.get();
}
gDefault的源码：
private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
protected IActivityManager create() {
IBinder b = ServiceManager.getService("activity");
if (false) {
Log.v("ActivityManager", "default service binder = " + b);
}
IActivityManager am = asInterface(b);
if (false) {
Log.v("ActivityManager", "default service = " + am);
}
return am;
}
};
asInterface(b)的源码：
/**
* Cast a Binder object into an activity manager interface, generating
* a proxy if needed.
*/
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);
}
上面的代码看到返回什么值了吗？ ActivityManagerProxy
4、startActivity的源码：
class ActivityManagerProxy implements IActivityManager
{
public ActivityManagerProxy(IBinder remote)
{
mRemote = remote;
}
。。。。。。
public int startActivity(IApplicationThread caller, Intent intent,
String resolvedType, Uri[] grantedUriPermissions, int grantedMode,
IBinder resultTo, String resultWho,
int requestCode, boolean onlyIfNeeded,
boolean debug, String profileFile, ParcelFileDescriptor profileFd,
boolean autoStopProfiler) throws RemoteException {
Parcel data = Parcel.obtain();
Parcel reply = Parcel.obtain();
data.writeInterfaceToken(IActivityManager.descriptor);
data.writeStrongBinder(caller != null ? caller.asBinder() : null);
intent.writeToParcel(data, 0);
data.writeString(resolvedType);
data.writeTypedArray(grantedUriPermissions, 0);
data.writeInt(grantedMode);
data.writeStrongBinder(resultTo);
data.writeString(resultWho);
data.writeInt(requestCode);
data.writeInt(onlyIfNeeded ? 1 : 0);
data.writeInt(debug ? 1 : 0);
data.writeString(profileFile);
if (profileFd != null) {
data.writeInt(1);
profileFd.writeToParcel(data, Parcelable.PARCELABLE_WRITE_RETURN_VALUE);
} else {
data.writeInt(0);
}
data.writeInt(autoStopProfiler ? 1 : 0);
mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);
reply.readException();
int result = reply.readInt();
reply.recycle();
data.recycle();
return result;
}
}
这是最重要的代码：
mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);
调用远端代理的transact()函数，而这个mRemote就是 ActivityManagerNative的Binder接口。
