一、在android中开发人员可以做那些工作？
应用程序开发：利用android提供的强大的sdk，开发出各种各样新颖的应用。
系统开发：在android中Google实现了与硬件无关的所有代码，但是与硬件密切相关的硬件抽象层却没有也无法提供，对于移动设备不同的设备提供商 底层硬件是千变万化的，不可能提供统一的硬件驱动以及接口实现，只能提供标准的接口，因此硬件提供商需要自个儿开发设备驱动，
并去实现android框架提供的接口。
二、android框架中Camera系统源码分析
在每个android手机中都有一个Camera应用程序用来实现拍照功能，不同硬件提供商可能会对这个应用程序进行改变来适合自己的UI风格，
这里仅仅分析android原生Camera应用以及框架（Android 4.0）
原生Camera应用代码在Camera.java(android4.0\packages\apps\camera\src\com\android\camera)，这个应该算是Camera系统最上层，应用层的实现。
下面是Camera类部分代码
public class Camera extends ActivityBase implements FocusManager.Listener,
View.OnTouchListener, ShutterButton.OnShutterButtonListener,
SurfaceHolder.Callback, ModePicker.OnModeChangeListener,
FaceDetectionListener, CameraPreference.OnPreferenceChangedListener,
LocationManager.Listener, ShutterButton.OnShutterButtonLongPressListener
从上面可以看出，Camera在继承了很多监听接口，用来监听各种事件（对焦事件、用户触摸事件等）。这个应用时继承ActivityBase，
可以重载OnCreate、OnResume等接口，在这些接口中完成相关初始化的工作，基本就是初始化各种监听对象，以及获取相机参数等相关。
比较关键的在 doOnResume这个函数中：
@Override
protected void doOnResume() {
if (mOpenCameraFail || mCameraDisabled) return;
mPausing = false;
mJpegPictureCallbackTime = 0;
mZoomValue = 0;
// Start the preview if it is not started.
if (mCameraState == PREVIEW_STOPPED) {
try {
mCameraDevice = Util.openCamera(this, mCameraId);
initializeCapabilities();
resetExposureCompensation();
startPreview();
if (mFirstTimeInitialized) startFaceDetection();
} catch (CameraHardwareException e) {
Util.showErrorAndFinish(this, R.string.cannot_connect_camera);
return;
} catch (CameraDisabledException e) {
Util.showErrorAndFinish(this, R.string.camera_disabled);
return;
}
}
if (mSurfaceHolder != null) {
// If first time initialization is not finished, put it in the
// message queue.
if (!mFirstTimeInitialized) {
mHandler.sendEmptyMessage(FIRST_TIME_INIT);
} else {
initializeSecondTime();
}
}
keepScreenOnAwhile();
if (mCameraState == IDLE) {
mOnResumeTime = SystemClock.uptimeMillis();
mHandler.sendEmptyMessageDelayed(CHECK_DISPLAY_ROTATION, 100);
}
}
在这个函数中看到通过这个函数获得Camera底层对象
mCameraDevice = Util.openCamera(this, mCameraId)，这里使用Util这个类，这个类的实现在
Util.java (android4.0\packages\apps\camera\src\com\android\camera)中，找到OpenCamera这个函数实现：
public static android.hardware.Camera openCamera(Activity activity, int cameraId)
throws CameraHardwareException, CameraDisabledException {
// Check if device policy has disabled the camera.
DevicePolicyManager dpm = (DevicePolicyManager) activity.getSystemService(
Context.DEVICE_POLICY_SERVICE);
if (dpm.getCameraDisabled(null)) {
throw new CameraDisabledException();
}
try {
return CameraHolder.instance().open(cameraId);
} catch (CameraHardwareException e) {
// In eng build, we throw the exception so that test tool
// can detect it and report it
if ("eng".equals(Build.TYPE)) {
throw new RuntimeException("openCamera failed", e);
} else {
throw e;
}
}
}
从这个函数可以看出，android系统中对下层Camera管理，是通过一个单例模式CameraHolder来管理的，
定位到这个类的实现CameraHolder.java (android4.0\packages\apps\camera\src\com\android\camera)通过调用open函数获取一个Camera硬件设备对象，
因为Camera设备是独享设备，不能同时被两个进程占用，而整个android系统是一个多进程环境，因此需要加入一些进程间互斥同步的方法。
定位到这个类的open函数：
public synchronized android.hardware.Camera open(int cameraId)
throws CameraHardwareException {
Assert(mUsers == 0);
if (mCameraDevice != null && mCameraId != cameraId) {
mCameraDevice.release();
mCameraDevice = null;
mCameraId = -1;
}
if (mCameraDevice == null) {
try {
Log.v(TAG, "open camera " + cameraId);
mCameraDevice = android.hardware.Camera.open(cameraId);
mCameraId = cameraId;
} catch (RuntimeException e) {
Log.e(TAG, "fail to connect Camera", e);
throw new CameraHardwareException(e);
}
mParameters = mCameraDevice.getParameters();
} else {
try {
mCameraDevice.reconnect();
} catch (IOException e) {
Log.e(TAG, "reconnect failed.");
throw new CameraHardwareException(e);
}
mCameraDevice.setParameters(mParameters);
}
++mUsers;
mHandler.removeMessages(RELEASE_CAMERA);
mKeepBeforeTime = 0;
return mCameraDevice;
}
通 过android.hardware.Camera.open(cameraId)调用进入下一层封装，JNI层，这一层是java代码的最下层，对下层 CameraC++代码进行JNI封装，封装实现类在Camera.java (android4.0\frameworks\base\core\java\android\hardware) 下面是这个类的部分实现，里面定义了不少回调函数：
public class Camera {
private static final String TAG = "Camera";
// These match the enums in frameworks/base/include/camera/Camera.h
private static final int CAMERA_MSG_ERROR= 0x001;
private static final int CAMERA_MSG_SHUTTER= 0x002;
private static final int CAMERA_MSG_FOCUS= 0x004;
private static final int CAMERA_MSG_ZOOM= 0x008;
private static final int CAMERA_MSG_PREVIEW_FRAME= 0x010;
private static final int CAMERA_MSG_VIDEO_FRAME= 0x020;
private static final int CAMERA_MSG_POSTVIEW_FRAME= 0x040;
private static final int CAMERA_MSG_RAW_IMAGE= 0x080;
private static final int CAMERA_MSG_COMPRESSED_IMAGE = 0x100;
private static final int CAMERA_MSG_RAW_IMAGE_NOTIFY = 0x200;
private static final int CAMERA_MSG_PREVIEW_METADATA = 0x400;
private static final int CAMERA_MSG_ALL_MSGS= 0x4FF;
private int mNativeContext; // accessed by native methods
private EventHandler mEventHandler;
private ShutterCallback mShutterCallback;
private PictureCallback mRawImageCallback;
private PictureCallback mJpegCallback;
private PreviewCallback mPreviewCallback;
private PictureCallback mPostviewCallback;
private AutoFocusCallback mAutoFocusCallback;
private OnZoomChangeListener mZoomListener;
private FaceDetectionListener mFaceListener;
private ErrorCallback mErrorCallback;
定位到Open函数：
public static Camera open(int cameraId) {
return new Camera(cameraId);
}
Open函数是一个静态方法,构造一个Camera对象：
Camera(int cameraId) {
mShutterCallback = null;
mRawImageCallback = null;
mJpegCallback = null;
mPreviewCallback = null;
mPostviewCallback = null;
mZoomListener = null;
Looper looper;
if ((looper = Looper.myLooper()) != null) {
mEventHandler = new EventHandler(this, looper);
} else if ((looper = Looper.getMainLooper()) != null) {
mEventHandler = new EventHandler(this, looper);
} else {
mEventHandler = null;
}
native_setup(new WeakReference<Camera>(this), cameraId);
}
在构造函数中调用native_setup方法，此方法对应于C++代码的android_hardware_Camera_native_setup方法，
实现在android_hardware_Camera.cpp (android4.0\frameworks\base\core\jni)，具体代码如下：
static void android_hardware_Camera_native_setup(JNIEnv *env, jobject thiz,
jobject weak_this, jint cameraId)
{
sp<Camera> camera = Camera::connect(cameraId);
if (camera == NULL) {
jniThrowRuntimeException(env, "Fail to connect to camera service");
return;
}
// make sure camera hardware is alive
if (camera->getStatus() != NO_ERROR) {
jniThrowRuntimeException(env, "Camera initialization failed");
return;
}
jclass clazz = env->GetObjectClass(thiz);
if (clazz == NULL) {
jniThrowRuntimeException(env, "Can't find android/hardware/Camera");
return;
}
// We use a weak reference so the Camera object can be garbage collected.
// The reference is only used as a proxy for callbacks.
sp<JNICameraContext> context = new JNICameraContext(env, weak_this, clazz, camera);
context->incStrong(thiz);
camera->setListener(context);
// save context in opaque field
env->SetIntField(thiz, fields.context, (int)context.get());
}
在android_hardware_Camera_native_setup方法中调用了Camera对象的connect方法，这个Camera类的 声明在Camera.h (android4.0\frameworks\base\include\camera）
定位到connect方法：
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;
}
这里以下的代码就比较关键了，涉及到Camera框架的实现机制，Camera系统使用的是Server-Client机制，Service和Client位于不同的进程中，进程间使用Binder机制进行通信，
Service端实际实现相机相关的操作，Client端通过Binder接口调用Service对应的操作。
继续分析代码，上面函数调用getCameraService方法，获得CameraService的引用，ICameraService有两个子类，BnCameraService和BpCameraService，这两个子类同时也
继承了IBinder接口，这两个子类分别实现了Binder通信的两端，Bnxxx实现ICameraService的具体功能，Bpxxx利用Binder的通信功能封装ICameraService方法，具体如下：
class ICameraService : public IInterface
{
public:
enum {
GET_NUMBER_OF_CAMERAS = IBinder::FIRST_CALL_TRANSACTION,
GET_CAMERA_INFO,
CONNECT
};
public:
DECLARE_META_INTERFACE(CameraService);
virtual int32_tgetNumberOfCameras() = 0;
virtual status_tgetCameraInfo(int cameraId,
struct CameraInfo* cameraInfo) = 0;
virtual sp<ICamera>connect(const sp<ICameraClient>& cameraClient,
int cameraId) = 0;
};
// ----------------------------------------------------------------------------
class BnCameraService: public BnInterface<ICameraService>
{
public:
virtual status_tonTransact( uint32_t code,
const Parcel& data,
Parcel* reply,
uint32_t flags = 0);
};
}; // na
class BpCameraService: public BpInterface<ICameraService>
{
public:
BpCameraService(const sp<IBinder>& impl)
: BpInterface<ICameraService>(impl)
{
}
// get number of cameras available
virtual int32_t getNumberOfCameras()
{
Parcel data, reply;
data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());
remote()->transact(BnCameraService::GET_NUMBER_OF_CAMERAS, data, &reply);
return reply.readInt32();
}
// get information about a camera
virtual status_t getCameraInfo(int cameraId,
struct CameraInfo* cameraInfo) {
Parcel data, reply;
data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());
data.writeInt32(cameraId);
remote()->transact(BnCameraService::GET_CAMERA_INFO, data, &reply);
cameraInfo->facing = reply.readInt32();
cameraInfo->orientation = reply.readInt32();
return reply.readInt32();
}
// connect to camera service
virtual sp<ICamera> connect(const sp<ICameraClient>& cameraClient, int cameraId)
{
Parcel data, reply;
data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());
data.writeStrongBinder(cameraClient->asBinder());
data.writeInt32(cameraId);
remote()->transact(BnCameraService::CONNECT, data, &reply);
return interface_cast<ICamera>(reply.readStrongBinder());
}
};
IMPLEMENT_META_INTERFACE(CameraService, "android.hardware.ICameraService");
// ----------------------------------------------------------------------
status_t BnCameraService::onTransact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
switch(code) {
case GET_NUMBER_OF_CAMERAS: {
CHECK_INTERFACE(ICameraService, data, reply);
reply->writeInt32(getNumberOfCameras());
return NO_ERROR;
} break;
case GET_CAMERA_INFO: {
CHECK_INTERFACE(ICameraService, data, reply);
CameraInfo cameraInfo;
memset(&cameraInfo, 0, sizeof(cameraInfo));
status_t result = getCameraInfo(data.readInt32(), &cameraInfo);
reply->writeInt32(cameraInfo.facing);
reply->writeInt32(cameraInfo.orientation);
reply->writeInt32(result);
return NO_ERROR;
} break;
case CONNECT: {
CHECK_INTERFACE(ICameraService, data, reply);
sp<ICameraClient> cameraClient = interface_cast<ICameraClient>(data.readStrongBinder());
sp<ICamera> camera = connect(cameraClient, data.readInt32());
reply->writeStrongBinder(camera->asBinder());
return NO_ERROR;
} break;
default:
return BBinder::onTransact(code, data, reply, flags);
}
}
// ----------------------------------------------------------------------------
}; // namespace android
下面继续分析sp<Camera> Camera::connect(int cameraId)这个方法，，定位到getCameraService这个方法
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;
}
定位到mCameraService = interface_cast<ICameraService>(binder); mCameraService是一个ICamerService类型，更加具体具体一点来讲应该是BpCameraService，
因为在这个类中实现了ICameraService的方法。
总结上面Binder机制，仅仅考虑分析Binder用法，对底层实现不进行深究，基本步骤如下：
1.定义进程间通信的接口比如这里的ICameraService；
2.在BnCameraService和BpCamaraService实现这个接口，这两个接口也分别继承于BnInterface和BpInterface；
3.服务端向ServiceManager注册Binder，客户端向ServiceManager获得Binder；
4.然后就可以实现双向进程间通信了；
通过getCameraService得到ICameraService引用后，调用ICameraService的connect方法获得ICamera引用，
c->mCamera = cs->connect(c, cameraId);
进一步跟进connect方法，这里就是BpCameraService类中connect方法的具体实现。
virtual sp<ICamera> connect(const sp<ICameraClient>& cameraClient, int cameraId)
{
Parcel data, reply;
data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());
data.writeStrongBinder(cameraClient->asBinder());
data.writeInt32(cameraId);
remote()->transact(BnCameraService::CONNECT, data, &reply);
return interface_cast<ICamera>(reply.readStrongBinder());
}
在这里返回的ICamera对象，实际上应该是BpCamera对象，这里使用的是匿名Binder，前面获取CameraService的 使用的有名Binder，有名Binder需要借助于ServiceManager获取Binder，而匿名Binder可以通过已经建立后的通信通道 （有名Binder）获得。以上是实现Camera框架部分，具体的实现Camera相关的方法是在ICamera相关的接口，下面是给接口的定义：
class ICamera: public IInterface
{
public:
DECLARE_META_INTERFACE(Camera);
virtual voiddisconnect() = 0;
// connect new client with existing camera remote
virtual status_tconnect(const sp<ICameraClient>& client) = 0;
// prevent other processes from using this ICamera interface
virtual status_tlock() = 0;
// allow other processes to use this ICamera interface
virtual status_tunlock() = 0;
// pass the buffered Surface to the camera service
virtual status_tsetPreviewDisplay(const sp<Surface>& surface) = 0;
// pass the buffered ISurfaceTexture to the camera service
virtual status_tsetPreviewTexture(
const sp<ISurfaceTexture>& surfaceTexture) = 0;
// set the preview callback flag to affect how the received frames from
// preview are handled.
virtual voidsetPreviewCallbackFlag(int flag) = 0;
// start preview mode, must call setPreviewDisplay first
virtual status_tstartPreview() = 0;
// stop preview mode
virtual voidstopPreview() = 0;
// get preview state
virtual boolpreviewEnabled() = 0;
// start recording mode
virtual status_tstartRecording() = 0;
// stop recording mode
virtual voidstopRecording() = 0;
// get recording state
virtual boolrecordingEnabled() = 0;
// release a recording frame
virtual voidreleaseRecordingFrame(const sp<IMemory>& mem) = 0;
// auto focus
virtual status_tautoFocus() = 0;
// cancel auto focus
virtual status_tcancelAutoFocus() = 0;
/*
* take a picture.
* @param msgType the message type an application selectively turn on/off
* on a photo-by-photo basis. The supported message types are:
* CAMERA_MSG_SHUTTER, CAMERA_MSG_RAW_IMAGE, CAMERA_MSG_COMPRESSED_IMAGE,
* and CAMERA_MSG_POSTVIEW_FRAME. Any other message types will be ignored.
*/
virtual status_ttakePicture(int msgType) = 0;
// set preview/capture parameters - key/value pairs
virtual status_tsetParameters(const String8& params) = 0;
// get preview/capture parameters - key/value pairs
virtual String8getParameters() const = 0;
// send command to camera driver
virtual status_tsendCommand(int32_t cmd, int32_t arg1, int32_t arg2) = 0;
// tell the camera hal to store meta data or real YUV data in video buffers.
virtual status_tstoreMetaDataInBuffers(bool enabled) = 0;
};
ICamera接口有两个子类BnCamera和BpCamera，是Binder通信的两端，BpCamera提供客户端调用 接口，BnCamera封装具体的实现，BnCamera也并没有真正实现ICamera相关接口而是在BnCamera子类 CameraService::Client中进行实现。而在CameraService::Client类中会继续调用硬件抽象层中相关方法来具体实现 Camera功能
