11、四大组件之二-Service高级（二）Native Service

一、Service的分类

1.1>Android Service

使用Java编写在JVM中运行的服务

1.2>Native Service

使用C/C++完成的服务，一般在系统开始时完成初始化，如MediaService, Audio Service等。

二、Native Service工作流程分析

int main(int argc, char** argv) {

sp<ProcessState> proc(ProcessState::self());

sp<IServiceManager> sm = defaultServiceManager();

LOGI("ServiceManager: %p", sm.get());

AudioFlinger::instantiate();

MediaPlayerService::instantiate();

CameraService::instantiate();

AudioPolicyService::instantiate();

ProcessState::self()->startThreadPool();

IPCThreadState::self()->joinThreadPool();

}

2.1、创建ProcessState和DefaultServiceManager

sp<ProcessState> proc(ProcessState::self());

sp<IServiceManager> sm = defaultServiceManager();

ProcessState放置在全局变量gProcess中，每个进程只有一个ProcessState对象，负责打开Binder设备驱动，建立线程池等。而IPCThreadState每个线程有一个，IPCThreadState实例登记在Linux线程程的上下文附属数据中，主要负责Binder数据读取，写入和请求处理框架。IPCThreadSate在构造的时候，获取进程的ProcessSate并记录在自己的成员变量mProcess中,通过mProcess可以获取到Binder的句柄。

 2.2、将服务放进ServiceManager

AudioFlinger::instantiate();

===>>

defaultServiceManager()->addService(String16("media.audio_flinger"), new AudioFlinger());

 2.3、开始服务循环

ProcessState::self()->startThreadPool();

IPCThreadState::self()->joinThreadPool();

a读取/写入：talkWithDriver（）@IPCThreadState对ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)进行包装。

b请求处理：executeCommand(...)@ IPCThreadState

c循环结构：joinThreadPool()

joinThreadPool(){

While(1){

talkWithDriver(...)

...

executeCommand(...)

}

}

2.4、透过 IBinder::transact() 函数来与核心服务互传数据。

三、编写自己的Native Service

(1) 编写DemoNativeService 

 int DemoService::instantiate() {
               LOGE("DemoService instantiate");
               int r = defaultServiceManager()->DemoService(
                             String16("alfred.demo"), new DemoService());
               LOGE("DemoService r = %d\n", r);
               return r;
        }

 status_t DemoService::onTransact(
               uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){
    switch(code) {
       case 0: {
                       pid_t pid = data.readInt32();
                       int num = data.readInt32();
                       num = num + 1000;
                       reply->writeInt32(num);
                       return NO_ERROR;
               } break;
       default:
               return BBinder::onTransact(code, data, reply, flags);
    }
 }      

(2) 编写DemoLoader启动DemoNativeService 

 int main(int argc, char** argv)
 {
 sp<ProcessState> proc(ProcessState::self());
 sp<IServiceManager> sm = defaultServiceManager();
 LOGI("ServiceManager: %p", sm.get());
 DemoService::instantiate();
 ProcessState::self()->startThreadPool();
 IPCThreadState::self()->joinThreadPool();
 }

(3) 编写mk文件

(4) 编写Native代码调用DemoNativeService 

 int Demo::setN(int n){
    getDemoService();
    Parcel data, reply;
    data.writeInt32(getpid());
    data.writeInt32(n);

    LOGE("BpDemoService::create remote()->transact()\n");
    binder->transact(0, data, &reply);
    int i = reply.readInt32();
    return i;
 }
 const void Demo::getDemoService(){
    sp<IServiceManager> sm = defaultServiceManager();
    binder = sm->getService(String16("guilh.add"));
    LOGE("Add::getDemoService %p\n",sm.get());
    if (binder == 0) {
            LOGW("DemoService not published, waiting...");
            return;
    }
 }

(5) 使用JNI实现Java和NativeService的衔接

(6) 在Activity中调用DemoNativeService