Camera服务分为Client和Server两部分，本文主要分析Client部分。
1. Camera Client介绍
主要由以下几个文件组成：
Camera.h/ Camera.cpp
ICameraClient.h/ IcameraClient.h
如图中所示，Camera.class继承自IcameraClient.class。Camera.class主要由libandroid_runtime.so中的android_hardware_Camera.cpp调用（frameworks/base/core/jni/android_hardware_Camera.cpp）,也就是说Camera.h是Camera服务框架，对上层的接口。
2. ICameraClient.h分析
class ICameraClient: public IInterface
{
public:
DECLARE_META_INTERFACE(CameraClient);
virtual void notifyCallback(int32_t msgType, int32_t ext1, int32_t ext2) = 0;
virtual void dataCallback(int32_t msgType, const sp<IMemory>& data) = 0;
virtual void dataCallbackTimestamp(nsecs_t timestamp, int32_t msgType, const sp<IMemory>& data) = 0;
};
// ----------------------------------------------------------------------------
class BnCameraClient: public BnInterface<ICameraClient>
{
public:
virtual status_t onTransact( uint32_t code,
const Parcel& data,
Parcel* reply,
uint32_t flags = 0);
};
根据BnInterface类模版的定义BnInterface<ICameraClient>类相当于双继承了BnInterface和ICameraClient。
大家如果熟悉Binder结构的话，看到这个文件可能会迷惑了，这不是一个Client吗？怎么还要继承自IInterface并且要实现BnCameraClient呢？这在Binder结构中是一个service应该做的。因为camera服务和一般的android服务不太一样，一般的android服务，只需要client通过binder调用service的一些功能，并得到返回结果。再说的直接一点儿，就是client只需要单向调用service即可，所以一般的client只需要实现BpXXX，service只需要实现BnXXX。但是Camera服务中，service需要回调Client，所以就是说Camera服务中的client需要双向调用，所以client和service都要实现BnXXX和BpXXX。
service需要回调client的接口就是：
virtual void notifyCallback(int32_t msgType, int32_t ext1, int32_t ext2) = 0;
virtual void dataCallback(int32_t msgType, const sp<IMemory>& data) = 0;
virtual void dataCallbackTimestamp(nsecs_t timestamp, int32_t msgType, const sp<IMemory>& data) = 0;
这几个接口与Camera.h中定义的CameraListener接口中的函数是对应的，从它们的函数名称和参数列表就可以看出。
3.Camera.h
class CameraListener: virtual public RefBase
{
public:
virtual void notify(int32_t msgType, int32_t ext1, int32_t ext2) = 0;
virtual void postData(int32_t msgType, const sp<IMemory>& dataPtr) = 0;
virtual void postDataTimestamp(nsecs_t timestamp, int32_t msgType, const sp<IMemory>& dataPtr) = 0;
};
class Camera : public BnCameraClient, public IBinder::DeathRecipient
{
public:
//对上层的接口
static sp<Camera> create(const sp<ICamera>& camera);
static int32_t getNumberOfCameras();
static status_t getCameraInfo(int cameraId,
struct CameraInfo* cameraInfo);
static sp<Camera> connect(int cameraId);
~Camera();
void init();
status_t reconnect();
void disconnect();
status_t startPreview();
void stopPreview();
//对上层的接口
// ICameraClient interface
virtual void notifyCallback(int32_t msgType, int32_t ext, int32_t ext2);
virtual void dataCallback(int32_t msgType, const sp<IMemory>& dataPtr);
virtual void dataCallbackTimestamp(nsecs_t timestamp, int32_t msgType, const sp<IMemory>& dataPtr);
private:
Camera();
Camera(const Camera&);
Camera& operator=(const Camera);
virtual void binderDied(const wp<IBinder>& who);
class DeathNotifier: public IBinder::DeathRecipient
{
public:
DeathNotifier() {
}
virtual void binderDied(const wp<IBinder>& who);
};
static sp<DeathNotifier> mDeathNotifier;
// helper function to obtain camera service handle
static const sp<ICameraService>& getCameraService();
sp<ICamera> mCamera;
status_t mStatus;
sp<CameraListener> mListener;
friend class DeathNotifier;
static Mutex mLock;
static sp<ICameraService> mCameraService;
};
从接口中可以看出Camera类刚好实现了一个Camera的基本操作，例如播放（startPreview）、停止（stopPreview）、暂停（takePicture）等。在Camera类中connect()是一个静态函数，它用于得到一个Camera的实例。
BnCameraClient继承了BnInterface<ICameraClient>，这是为基于Android的基础类Binder机制实现在进程通讯而构建的。
4.ICameraClient.cpp分析
class BpCameraClient: public BpInterface<ICameraClient>
{
public:
BpCameraClient(const sp<IBinder>& impl)
: BpInterface<ICameraClient>(impl)
{
}
// generic callback from camera service to app
void notifyCallback(int32_t msgType, int32_t ext1, int32_t ext2)
{
remote()->transact(NOTIFY_CALLBACK, data, &reply, IBinder::FLAG_ONEWAY);
}
// generic data callback from camera service to app with image data
void dataCallback(int32_t msgType, const sp<IMemory>& imageData)
{
remote()->transact(DATA_CALLBACK, data, &reply, IBinder::FLAG_ONEWAY);
}
// generic data callback from camera service to app with image data
void dataCallbackTimestamp(nsecs_t timestamp, int32_t msgType, const sp<IMemory>& imageData)
{
remote()->transact(DATA_CALLBACK_TIMESTAMP, data, &reply, IBinder::FLAG_ONEWAY);
}
};
IMPLEMENT_META_INTERFACE(CameraClient, "android.hardware.ICameraClient");
// ----------------------------------------------------------------------
status_t BnCameraClient::onTransact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
switch(code) {
case NOTIFY_CALLBACK: {
notifyCallback(msgType, ext1, ext2);
} break;
case DATA_CALLBACK: {
dataCallback(msgType, imageData);
} break;
case DATA_CALLBACK_TIMESTAMP: {
dataCallbackTimestamp(timestamp, msgType, imageData);
} break;
default:
return BBinder::onTransact(code, data, reply, flags);
}
}
ICameraClient.cpp实现了BnCameraClient和BpCameraClient。BnCameraClient中调用的notifyCallback，dataCallback和dataCallbackTimestamp（ICameraClient.h中定义）是在Camera.cpp中实现的。关于BnCameraClient和BpCameraClient，可以看一下Binder的结构。
5.Camera.cpp分析
首先获取Camera的service：
// establish binder interface to camera service
const sp<ICameraService>& Camera::getCameraService()
{
Mutex::Autolock _l(mLock);
if (mCameraService.get() == 0) {
sp<IServiceManager> sm = defaultServiceManager();
sp<IBinder> binder;
do {
binder = sm->getService(String16("media.camera"));
if (binder != 0)
break;
LOGW("CameraService not published, waiting...");
usleep(500000); // 0.5 s
} while(true);
if (mDeathNotifier == NULL) {
mDeathNotifier = new DeathNotifier();
}
binder->linkToDeath(mDeathNotifier);
mCameraService = interface_cast<ICameraService>(binder);
}
LOGE_IF(mCameraService==0, "no CameraService!?");
return mCameraService;
}
sp<Camera> Camera::connect(int cameraId)
{
LOGV("connect");
sp<Camera> c = new Camera();
const sp<ICameraService>& cs = getCameraService();
if (cs != 0) {
c->mCamera = cs->connect(c, cameraId);
}
if (c->mCamera != 0) {
c->mCamera->asBinder()->linkToDeath(c);
c->mStatus = NO_ERROR;
} else {
c.clear();
}
return c;
}
在getCameraService()中首先通过ServiceManager获取到ICameraService，然后在connect方法中通过调用ICameraService的connect方法，获取到一个ICamera，并赋值给Camera的成员变量mCamera。可以自己去看一下Camera.cpp的剩余代码，几乎所有的函数中，都是对ICameraService或者mCamera的调用。
实现ICameraClient接口：
// callback from camera service
void Camera::notifyCallback(int32_t msgType, int32_t ext1, int32_t ext2)
{
sp<CameraListener> listener;
{
Mutex::Autolock _l(mLock);
listener = mListener;
}
if (listener != NULL) {
listener->notify(msgType, ext1, ext2);
}
}
可以看出，其实就是对CameraListener的调用。
mListener由
void Camera::setListener(const sp<CameraListener>& listener)
{
Mutex::Autolock _l(mLock);
mListener = listener;
}
方法设定。
