Android系统定义GraphicBuffer数据类型来描述一块图形buffer，该对象可以跨进程传输。
IGraphicBufferAlloc是专门用于分配GraphicBuffer对象的工具类，该类是基于Binder进程间通信机制框架设计的，其类继承关系如下：
由于IGraphicBufferAlloc是基于Binder进程间通信框架设计的，因此该类的实现分客户端进程和服务端进程两方面。从上图可以知道，客户端进程通过BpGraphicBufferAlloc对象请求服务端进程中的GraphicBufferAlloc对象来创建GraphicBuffer对象。
那如何获取IGraphicBufferAlloc的远程Binder代理对象呢？IGraphicBufferAlloc被定义为无名Binder对象，并没有注册到ServiceManager进程中，但SurfaceFlinger是有名Binder对象，因此可以通过SurfaceFlinger创建IGraphicBufferAlloc的本地对象GraphicBufferAlloc，并返回其远程代理对象BpGraphicBufferAlloc给客户端进程。那客户端进程又是如何请求SurfaceFlinger创建IGraphicBufferAlloc的本地对象的呢？客户端进程首先从ServiceManager进程中查询SurfaceFlinger的远程代理对象BpSurfaceComposer：
sp<ISurfaceComposer> composer(ComposerService::getComposerService());得到SurfaceFlinger的远程代理对象BpSurfaceComposer后，就可以请求SurfaceFlinger创建IGraphicBufferAlloc的本地对象GraphicBufferAlloc了
mGraphicBufferAlloc = composer->createGraphicBufferAlloc();调用BpSurfaceComposer的createGraphicBufferAlloc()函数发起RPC远程调用
virtual sp<IGraphicBufferAlloc> createGraphicBufferAlloc()
{
uint32_t n;
Parcel data, reply;
data.writeInterfaceToken(ISurfaceComposer::getInterfaceDescriptor());
remote()->transact(BnSurfaceComposer::CREATE_GRAPHIC_BUFFER_ALLOC, data, &reply);
return interface_cast<IGraphicBufferAlloc>(reply.readStrongBinder());
}
服务进程端SurfaceFlinger接收函数调用请求
status_t BnSurfaceComposer::onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
switch(code) {
case CREATE_GRAPHIC_BUFFER_ALLOC: {
CHECK_INTERFACE(ISurfaceComposer, data, reply);
sp<IBinder> b = createGraphicBufferAlloc()->asBinder();
reply->writeStrongBinder(b);
} break;
default:
return BBinder::onTransact(code, data, reply, flags);
}
return NO_ERROR;
}
SurfaceFlinger是BnSurfaceComposer的子类，实现了createGraphicBufferAlloc()函数
sp<IGraphicBufferAlloc> SurfaceFlinger::createGraphicBufferAlloc()
{
sp<GraphicBufferAlloc> gba(new GraphicBufferAlloc());
return gba;
}
关于Binder进程间通信过程请参考Android服务函数远程调用源码分析。这样就在服务进程创建了一个GraphicBufferAlloc本地对象，同时将代理对象BpGraphicBufferAlloc返回到客户进程。客户进程获得IGraphicBufferAlloc的代理对象后，就可以请求服务端GraphicBufferAlloc创建GraphicBuffer对象了。
客户端进程BpGraphicBufferAlloc请求创建GraphicBuffer：
frameworks\native\include\gui\IGraphicBufferAlloc.h
virtual sp<GraphicBuffer> createGraphicBuffer(uint32_t w, uint32_t h,
PixelFormat format, uint32_t usage, status_t* error) {
Parcel data, reply;
data.writeInterfaceToken(IGraphicBufferAlloc::getInterfaceDescriptor());
data.writeInt32(w);
data.writeInt32(h);
data.writeInt32(format);
data.writeInt32(usage);
remote()->transact(CREATE_GRAPHIC_BUFFER, data, &reply);
sp<GraphicBuffer> graphicBuffer;
status_t result = reply.readInt32();
if (result == NO_ERROR) {
graphicBuffer = new GraphicBuffer();
reply.read(*graphicBuffer);
}
*error = result;
return graphicBuffer;
}
该函数通过Binder进程间通信方式发起createGraphicBuffer函数的IPC远程调用，在读取服务端返回的结果前，首先构造一个GraphicBuffer对象，这里使用GraphicBuffer的无参构造函数，然后读取服务端创建的GraphicBuffer的信息。服务端BnGraphicBufferAlloc接收到客户端的请求后的执行过程：
status_t BnGraphicBufferAlloc::onTransact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
switch(code) {
case CREATE_GRAPHIC_BUFFER: {
CHECK_INTERFACE(IGraphicBufferAlloc, data, reply);
uint32_t w = data.readInt32();
uint32_t h = data.readInt32();
PixelFormat format = data.readInt32();
uint32_t usage = data.readInt32();
status_t error;
sp<GraphicBuffer> result =createGraphicBuffer(w, h, format, usage, &error);
reply->writeInt32(error);
if (result != 0) {
reply->write(*result);
reply->writeStrongBinder( new BufferReference(result) );
}
return NO_ERROR;
} break;
default:
return BBinder::onTransact(code, data, reply, flags);
}
}createGraphicBuffer()函数由BnGraphicBufferAlloc的子类GraphicBufferAlloc实现
sp<GraphicBuffer> GraphicBufferAlloc::createGraphicBuffer(uint32_t w, uint32_t h,
PixelFormat format, uint32_t usage, status_t* error) {
sp<GraphicBuffer> graphicBuffer(new GraphicBuffer(w, h, format, usage));
status_t err = graphicBuffer->initCheck();
*error = err;
if (err != 0 || graphicBuffer->handle == 0) {
if (err == NO_MEMORY) {
GraphicBuffer::dumpAllocationsToSystemLog();
}
return 0;
}
return graphicBuffer;
}
该函数根据客户端进程发送过来的宽度，高度，格式等buffer信息来构造GraphicBuffer对象。
GraphicBuffer::GraphicBuffer(uint32_t w, uint32_t h,
PixelFormat reqFormat, uint32_t reqUsage)
: BASE(), mOwner(ownData), mBufferMapper(GraphicBufferMapper::get()),
mInitCheck(NO_ERROR), mIndex(-1)
{
width =
height =
stride =
format =
usage = 0;
handle = NULL;
mInitCheck = initSize(w, h, reqFormat, reqUsage);
}该构造函数首先将当前创建的GraphicBuffer的width，height，stride等成员变量赋值为0，然后调用initSize()函数来分配一块图形buffer
status_t GraphicBuffer::initSize(uint32_t w, uint32_t h, PixelFormat format,
uint32_t reqUsage)
{
GraphicBufferAllocator& allocator = GraphicBufferAllocator::get();
status_t err = allocator.alloc(w, h, format, reqUsage, &handle, &stride);
if (err == NO_ERROR) {
this->width = w;
this->height = h;
this->format = format;
this->usage = reqUsage;
}
return err;
}这里首先通过单例模式获取一个GraphicBufferAllocator对象，GraphicBufferAllocator是分配图形缓冲区的工具类，调用GraphicBufferAllocator的alloc()函数分配图形buffer的过程在前面的
Android图形缓冲区分配过程源码分析中已经详细分析过了。当图形缓冲区分配成功后，使用GraphicBuffer来描述当前分配的图形buffer，并将图形buffer分配结果保存到成员变量mInitCheck中，最后通过initCheck()函数来获取图形buffer分配的结果，并写回到客户进程。将GraphicBuffer对象写回客户进程：
sp<GraphicBuffer> result = createGraphicBuffer(w, h, format, usage, &error);
reply->writeInt32(error);
if (result != 0) {
reply->write(*result);
reply->writeStrongBinder( new BufferReference(result) );
}
由于GraphicBuffer继承于Flattenable类，因此GraphicBuffer对象可以跨进程传输，服务进程端回写GraphicBuffer对象过程如下：
status_t Parcel::write(const Flattenable& val)
{
status_t err;
// size if needed
size_t len = val.getFlattenedSize();
size_t fd_count = val.getFdCount();
//写入数据长度
err = this->writeInt32(len);
if (err) return err;
//写入句柄个数
err = this->writeInt32(fd_count);
if (err) return err;
//写入数据
void* buf = this->writeInplace(PAD_SIZE(len));
if (buf == NULL)
return BAD_VALUE;
int* fds = NULL;
if (fd_count) {
fds = new int[fd_count];
}
//写入句柄值
err = val.flatten(buf, len, fds, fd_count);
for (size_t i=0 ; i<fd_count && err==NO_ERROR ; i++) {
err = this->writeDupFileDescriptor( fds[i] );
}
if (fd_count) {
delete [] fds;
}
return err;
}GraphicBuffer对象的数据长度获取过程：
size_t GraphicBuffer::getFlattenedSize() const {
return (8 + (handle ? handle->numInts : 0))*sizeof(int);
}GraphicBuffer对象中的句柄个数获取过程：
size_t GraphicBuffer::getFdCount() const {
return handle ? handle->numFds : 0;
}由于val指向的是GraphicBuffer对象，因此将调用GraphicBuffer对象的flatten函数来写入native_handle等内容，GraphicBuffer类继承于Flattenable类，并实现了该类的flatten函数：
status_t GraphicBuffer::flatten(void* buffer, size_t size,
int fds[], size_t count) const
{
size_t sizeNeeded = GraphicBuffer::getFlattenedSize();
if (size < sizeNeeded) return NO_MEMORY;
size_t fdCountNeeded = GraphicBuffer::getFdCount();
if (count < fdCountNeeded) return NO_MEMORY;
int* buf = static_cast<int*>(buffer);
buf[0] = 'GBFR';
buf[1] = width;
buf[2] = height;
buf[3] = stride;
buf[4] = format;
buf[5] = usage;
buf[6] = 0;
buf[7] = 0;
if (handle) {
buf[6] = handle->numFds;
buf[7] = handle->numInts;
native_handle_t const* const h = handle;
memcpy(fds, h->data, h->numFds*sizeof(int));
memcpy(&buf[8], h->data + h->numFds, h->numInts*sizeof(int));
}
return NO_ERROR;
}
native_handle_t是上层抽象的可以在进程间传递的数据结构，对private_handle_t的抽象包装。
numFds=1表示有一个文件句柄：fd
numInts= 8表示后面跟了8个INT型的数据：magic,flags,size,offset,base,lockState,writeOwner,pid;
服务进程将创建的GraphicBuffer对象的成员变量handle写回到请求创建图形缓冲区的客户进程，这时客户进程通过以下方式就可以读取服务进程返回的关于创建图形buffer的信息数据。
sp<GraphicBuffer> graphicBuffer;
status_t result = reply.readInt32();
if (result == NO_ERROR) {
graphicBuffer = new GraphicBuffer();
reply.read(*graphicBuffer);
}reply为Parcel类型变量，该对象的read函数实现如下：
status_t Parcel::read(Flattenable& val) const
{
// size
const size_t len = this->readInt32();
const size_t fd_count = this->readInt32();
// payload
void const* buf = this->readInplace(PAD_SIZE(len));
if (buf == NULL)
return BAD_VALUE;
int* fds = NULL;
if (fd_count) {
fds = new int[fd_count];
}
status_t err = NO_ERROR;
for (size_t i=0 ; i<fd_count && err==NO_ERROR ; i++) {
fds[i] = dup(this->readFileDescriptor());
if (fds[i] < 0) err = BAD_VALUE;
}
if (err == NO_ERROR) {
err = val.unflatten(buf, len, fds, fd_count);
}
if (fd_count) {
delete [] fds;
}
return err;
}
val同样指向的是GraphicBuffer对象，因此将调用GraphicBuffer对象的unflatten函数来读取服务进程返回来的数据内容，GraphicBuffer类继承于Flattenable类，并实现了该类的flatten函数：
status_t GraphicBuffer::unflatten(void const* buffer, size_t size,
int fds[], size_t count)
{
if (size < 8*sizeof(int)) return NO_MEMORY;
int const* buf = static_cast<int const*>(buffer);
if (buf[0] != 'GBFR') return BAD_TYPE;
const size_t numFds = buf[6];
const size_t numInts = buf[7];
const size_t sizeNeeded = (8 + numInts) * sizeof(int);
if (size < sizeNeeded) return NO_MEMORY;
size_t fdCountNeeded = 0;
if (count < fdCountNeeded) return NO_MEMORY;
if (handle) {
// free previous handle if any
free_handle();
}
if (numFds || numInts) {
width = buf[1];
height = buf[2];
stride = buf[3];
format = buf[4];
usage = buf[5];
//创建native_handle对象
native_handle* h = native_handle_create(numFds, numInts);
memcpy(h->data, fds, numFds*sizeof(int));
memcpy(h->data + numFds, &buf[8], numInts*sizeof(int));
handle = h;
} else {
width = height = stride = format = usage = 0;
handle = NULL;
}
mOwner = ownHandle;
if (handle != 0) {
//将创建的图形缓冲区映射到客户进程的地址空间来，这样客户端进程就可以直接在图形buffer映射的地址空间绘图
status_t err = mBufferMapper.registerBuffer(handle);
//地址空间映射错误
if (err != NO_ERROR) {
//关闭handle
native_handle_close(handle);
//释放handle
native_handle_delete(const_cast<native_handle*>(handle));
handle = NULL;
return err;
}
}
return NO_ERROR;
}数据读取过程正好和上面的数据写入过程相反，由于服务进程发送了一个native_handle对象的内容到客户端进程，因此需要在客户端进程构造一个native_handle对象：
native_handle_t* native_handle_create(int numFds, int numInts)
{
native_handle_t* h = malloc(sizeof(native_handle_t) + sizeof(int)*(numFds+numInts));
h->version = sizeof(native_handle_t);
h->numFds = numFds;
h->numInts = numInts;
return h;
}
客户端进程读取到服务进程创建的图形buffer的描述信息native_handle后，通过GraphicBufferMapper对象mBufferMapper的registerBuffer函数将创建的图形buffer映射到客户端进程地址空间，关于图形缓冲区的映射过程请参考Android图形缓冲区映射过程源码分析。
这样就将Private_native_t中的数据：magic,flags,size,offset,base,lockState,writeOwner,pid复制到了客户端进程。服务端（SurfaceFlinger）分配了一段内存作为Surface的作图缓冲区，客户端怎样在这个作图缓冲区上绘图呢？两个进程间如何共享内存，这就需要GraphicBufferMapper将分配的图形缓冲区映射到客户端进程地址空间。对于共享缓冲区，他们操作同一物理地址的内存块。
