android Activity启动过程(一)从startActivty开始说起

从启动startActivity开始说起

MainActivity.startActivity()
Activity.startActivity()
Activity.startActivityForResult
Instrumentation.execStartActivty
ActivityManagerNative.getDefault().startActivity() 

Activity.java

调用startActivty实际上最后还是调用了startActivityForResult 方法

   Override
public void startActivity(Intent intent) {
        this.startActivity(intent, null);
}
    @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);
        }
}
    @Override
public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) {
    if (mParent == null) {
      //真正执行启动activity的代码逻辑
        Instrumentation.ActivityResult ar =
            mInstrumentation.execStartActivity(
                this, mMainThread.getApplicationThread(), mToken, this,
                intent, requestCode, options);
        //启动activity返回的结果处理
        if (ar != null) {
            mMainThread.sendActivityResult(
                mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                ar.getResultData());
        }
        if (requestCode >= 0) {

             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);
        }
    }
}

实际上调用的启动startActivityForResult方法的最终还是通过Instrumentation类来执行execStartActivity

Instrumentation.java

public ActivityResult execStartActivity(
        Context who, IBinder contextThread, IBinder token, Activity target,
        Intent intent, int requestCode, Bundle options) {
   //这个whoThread执行了启动activity
    IApplicationThread whoThread = (IApplicationThread) contextThread;
    Uri referrer = target != null ? target.onProvideReferrer() : null;
    if (referrer != null) {
        intent.putExtra(Intent.EXTRA_REFERRER, referrer);
    }
    if (mActivityMonitors != null) {
        synchronized (mSync) {
            final int N = mActivityMonitors.size();
          //遍历一遍，查询是否存在这个activity，activity类存放在intent中。
            for (int i = 0; i < N; i++) {
                final ActivityMonitor am = mActivityMonitors.get(i);
                if (am.match(who, null, intent)) {
                    am.mHits++;
                    if (am.isBlocking()) {
                        return requestCode >= 0 ? am.getResult() : null;
                    }
                    break;
                }
            }
        }
    }
    try {
        intent.migrateExtraStreamToClipData();
        intent.prepareToLeaveProcess();
        //这里通过ActivityManager启动了activity
        int result = ActivityManagerNative.getDefault()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, options);
        //检查启动activity返回的结果
        checkStartActivityResult(result, intent);
    } catch (RemoteException e) {
        throw new RuntimeException("Failure from system", e);
    }
    return null;
}

执行activity的启动过程的是ActivityManagerNative.getDefault().startActivity

ActivityManagerNative.java

public abstract class ActivityManagerNative extends Binder implements IActivityManager
{
    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;
        }
    };

    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);
    }  

    static public IActivityManager getDefault() {
        return gDefault.get();
    }

  public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
            throws RemoteException {
        switch (code) {
        case START_ACTIVITY_TRANSACTION:
        {
            data.enforceInterface(IActivityManager.descriptor);
            IBinder b = data.readStrongBinder();
            IApplicationThread app = ApplicationThreadNative.asInterface(b);
            String callingPackage = data.readString();
            Intent intent = Intent.CREATOR.createFromParcel(data);
            String resolvedType = data.readString();
            IBinder resultTo = data.readStrongBinder();
            String resultWho = data.readString();
            int requestCode = data.readInt();
            int startFlags = data.readInt();
            ProfilerInfo profilerInfo = data.readInt() != 0
                    ? ProfilerInfo.CREATOR.createFromParcel(data) : null;
            Bundle options = data.readInt() != 0
                    ? Bundle.CREATOR.createFromParcel(data) : null;
            int result = startActivity(app, callingPackage, intent, resolvedType,
                    resultTo, resultWho, requestCode, startFlags, profilerInfo, options);
            reply.writeNoException();
            reply.writeInt(result);
            return true;
        }
        ...
    }
}

1. gDefault.get()返回的就是一个IActivityManager的单例gDefault
2. IBinder b = ServiceManager.getService("activity")获取一个IBinder这个就是和远程通信的, 也就是ActivityManagerService
3. 可以在asInterface(IBinder obj) 调用就是返回的ActivityManagerProxy就是ActivityManagerService的远程代理类。

ActivityManagerNative.java#ActivityManagerProxy

class ActivityManagerProxy implements IActivityManager
{
    private IBinder mRemote;
    public ActivityManagerProxy(IBinder remote)
    {
        mRemote = remote;
    }

    public IBinder asBinder()
    {
        return mRemote;
    }

    public int startActivity(IApplicationThread caller, String callingPackage, Intent intent,
            String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle options) throws RemoteException {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IActivityManager.descriptor);
        data.writeStrongBinder(caller != null ? caller.asBinder() : null);
        data.writeString(callingPackage);
        intent.writeToParcel(data, 0);
        data.writeString(resolvedType);
        data.writeStrongBinder(resultTo);
        data.writeString(resultWho);
        data.writeInt(requestCode);
        data.writeInt(startFlags);
        if (profilerInfo != null) {
            data.writeInt(1);
            profilerInfo.writeToParcel(data, Parcelable.PARCELABLE_WRITE_RETURN_VALUE);
        } else {
            data.writeInt(0);
        }
        if (options != null) {
            data.writeInt(1);
            options.writeToParcel(data, 0);
        } else {
            data.writeInt(0);
        }
        mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);
        reply.readException();
        int result = reply.readInt();
        reply.recycle();
        data.recycle();
        return result;
    }

...

}

mRemote是一个IBinder，调用了IBinder的transact()方法,这说明了启动Activity实际上也是进行了进程间的通信