android--startActivity(intent)分析源码

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。

就到这吧。

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