前文Android匿名共享内存（Ashmem）原理分析了匿名共享内存，它最主要的作用就是View视图绘制，Android视图是按照一帧一帧显示到屏幕的，而每一帧都会占用一定的存储空间，通过Ashmem机制APP与SurfaceFlinger共享绘图数据，提高图形处理性能，本文就看Android是怎么利用Ashmem分配及绘制的：

View视图内存的分配

前文Window添加流程中描述了：在添加窗口的时候，WMS会为APP分配一个WindowState，以标识当前窗口并用于窗口管理，同时向SurfaceFlinger端请求分配Layer抽象图层，在SurfaceFlinger分配Layer的时候创建了两个比较关键的Binder对象，用于填充WMS端Surface，一个是sp handle：是每个窗口标识的句柄，将来WMS同SurfaceFlinger通信的时候方便找到对应的图层。另一个是sp gbp ：共享内存分配的关键对象，同时兼具Binder通信的功能，用来传递指令及共享内存的句柄，注意，这里只是抽象创建了对象，并未真正分配每一帧的内存，内存的分配要等到真正绘制的时候才会申请，首先看一下分配流程：

• 分配的时机：什么时候分配
• 分配的手段：如何分配
• 传递的方式：如何跨进程传递

Surface被抽象成一块画布，只要拥有Surface就可以绘图，其根本原理就是Surface握有可以绘图的一块内存，这块内存是APP端在需要的时候，通过sp gbp向SurfaceFlinger申请的，那么首先看一下APP端如何获得sp gbp这个服务代理的，之后再看如何利用它申请内存，在WMS利用向SurfaceFlinger申请填充Surface的时候，会请求SurfaceFlinger分配这把剑，并将其句柄交给自己

sp<SurfaceControl> SurfaceComposerClient::createSurface(
        const String8& name,  uint32_t w, uint32_t h, PixelFormat format, uint32_t flags){
      sp<SurfaceControl> sur;
      ...
      if (mStatus == NO_ERROR) {
        sp<IBinder> handle;
        sp<IGraphicBufferProducer> gbp;
        <!--关键点1 获取图层的关键信息handle, gbp-->
        status_t err = mClient->createSurface(name, w, h, format, flags,
                &handle, &gbp);
         <!--关键点2 根据返回的图层关键信息 创建SurfaceControl对象-->
        if (err == NO_ERROR) {
            sur = new SurfaceControl(this, handle, gbp);
        }
    }
    return sur;
}

看关键点1，这里其实就是建立了一个sp gbp容器，并请求SurfaceFlinger分配填充内容，SurfaceFlinger收到请求后会为WMS建立与APP端对应的Layer，同时为其分配sp gbp，并填充到Surface中返回给APP，

status_t SurfaceFlinger::createNormalLayer(const sp<Client>& client,
        const String8& name, uint32_t w, uint32_t h, uint32_t flags, PixelFormat& format,
        sp<IBinder>* handle, sp<IGraphicBufferProducer>* gbp, sp<Layer>* outLayer){
    ...
    <!--关键点 1 -->
    *outLayer = new Layer(this, client, name, w, h, flags);
    status_t err = (*outLayer)->setBuffers(w, h, format, flags);
    <!--关键点 2-->
    if (err == NO_ERROR) {
        *handle = (*outLayer)->getHandle();
        *gbp = (*outLayer)->getProducer();
    }
  return err;
}

void Layer::onFirstRef() {
    sp<IGraphicBufferProducer> producer;
    sp<IGraphicBufferConsumer> consumer;
    <!--创建producer与consumer-->
    BufferQueue::createBufferQueue(&producer, &consumer);
    mProducer = new MonitoredProducer(producer, mFlinger);
    mSurfaceFlingerConsumer = new SurfaceFlingerConsumer(consumer, mTextureName,
            this);
   ...
}

void BufferQueue::createBufferQueue(sp<IGraphicBufferProducer>* outProducer,
        sp<IGraphicBufferConsumer>* outConsumer,
        const sp<IGraphicBufferAlloc>& allocator) {
    sp<BufferQueueCore> core(new BufferQueueCore(allocator));
    sp<IGraphicBufferProducer> producer(new BufferQueueProducer(core));
    sp<IGraphicBufferConsumer> consumer(new BufferQueueConsumer(core));
    *outProducer = producer;
    *outConsumer = consumer;
}

从上面两个函数可以很清楚的看到Producer/Consumer的模型原样，也就说每个图层Layer都有自己的producer/ consumer,sp gbp对应的其实是BufferQueueProducer，而BufferQueueProducer是一个Binder通信对象，在服务端是:

class BufferQueueProducer : public BnGraphicBufferProducer,
                            private IBinder::DeathRecipient {}

在APP端是

class BpGraphicBufferProducer : public BpInterface<IGraphicBufferProducer>{}

IGraphicBufferProducer Binder实体在SurfaceFlinger中创建后，打包到Surface对象，并通过binder通信传递给APP端，APP段通过反序列化将其恢复出来，如下：

status_t Surface::readFromParcel(const Parcel* parcel, bool nameAlreadyRead) {
    if (parcel == nullptr) return BAD_VALUE;

    status_t res = OK;
    if (!nameAlreadyRead) {
        name = readMaybeEmptyString16(parcel);
        // Discard this for now
        int isSingleBuffered;
        res = parcel->readInt32(&isSingleBuffered);
        if (res != OK) {
            return res;
        }
    }
    sp<IBinder> binder;
    res = parcel->readStrongBinder(&binder);
    if (res != OK) return res;
   <!--interface_cast会将其转换成BpGraphicBufferProducer-->
    graphicBufferProducer = interface_cast<IGraphicBufferProducer>(binder);
    return OK;
}

自此，APP端就获得了申请内存的句柄BpGraphicBufferProducer，它真正发挥作用是在第一次绘图时，看一下ViewRootImpl中的draw

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
            boolean scalingRequired, Rect dirty) {
            final Canvas canvas;
        try {
            final int left = dirty.left;
            final int top = dirty.top;
            final int right = dirty.right;
            final int bottom = dirty.bottom;
            <!--关键点1 获取绘图内存-->
            canvas = mSurface.lockCanvas(dirty);
        try {
           try {
               <!--关键点2 绘图-->
               mView.draw(canvas);
            }              
        } finally {
            try {
            <!--关键点 3 绘图结束 ，通知surfacefling混排，更新显示界面-->
              surface.unlockCanvasAndPost(canvas);
            } catch (IllegalArgumentException e) {}

先看关键点1，内存的分配时机其实就在这里，直接进入到native层

static jlong nativeLockCanvas(JNIEnv* env, jclass clazz,
        jlong nativeObject, jobject canvasObj, jobject dirtyRectObj) {
    sp<Surface> surface(reinterpret_cast<Surface *>(nativeObject));
    ...
    status_t err = surface->lock(&outBuffer, dirtyRectPtr);
    ...
    sp<Surface> lockedSurface(surface);
    lockedSurface->incStrong(&sRefBaseOwner);
    return (jlong) lockedSurface.get();
}

surface.cpp的lock会进一步调用dequeueBuffer函数来请求分配内存：

int Surface::dequeueBuffer(android_native_buffer_t** buffer, int* fenceFd) {
    ...
    int buf = -1;
    sp<Fence> fence;
    nsecs_t now = systemTime();
    <!--申请buffer,并获得标识符-->
    status_t result = mGraphicBufferProducer->dequeueBuffer(&buf, &fence,
            reqWidth, reqHeight, reqFormat, reqUsage);
    ...
    if ((result & IGraphicBufferProducer::BUFFER_NEEDS_REALLOCATION) || gbuf == 0) {
    <!--申请的内存是在surfaceflinger进程中，Surface通过调用requestBuffer将图形缓冲区映射到Surface所在进程-->        
        result = mGraphicBufferProducer->requestBuffer(buf, &gbuf);
   ...
}

最终会调用BpGraphicBufferProducer的dequeueBuffer向服务端请求分配内存，这里用到了匿名共享内存的知识，在Linux中一切都是文件，共享内存也看成一个文件。分配成功之后，需要跨进程传递tmpfs临时文件的描述符fd。先看下申请的逻辑：

class BpGraphicBufferProducer : public BpInterface<IGraphicBufferProducer>{
    virtual status_t dequeueBuffer(int *buf, sp<Fence>* fence, bool async,
            uint32_t w, uint32_t h, uint32_t format, uint32_t usage) {
        Parcel data, reply;
        data.writeInterfaceToken(IGraphicBufferProducer::getInterfaceDescriptor());
        data.writeInt32(async);
        data.writeInt32(w);
        data.writeInt32(h);
        data.writeInt32(format);
        data.writeInt32(usage);
        //通过BpBinder将要什么的buffer的相关参数保存到data，发送给BBinder
        status_t result = remote()->transact(DEQUEUE_BUFFER, data, &reply);
        if (result != NO_ERROR) {
            return result;
        }
        //BBinder给BpBinder返回了一个int，并不是缓冲区的内存
        *buf = reply.readInt32();
        bool nonNull = reply.readInt32();
        if (nonNull) {
            *fence = new Fence();
            reply.read(**fence);
        }
        result = reply.readInt32();
        return result;
    }
}

在client侧，也就是BpGraphicBufferProducer侧，通过DEQUEUE_BUFFER后核心只返回了一个*buf = reply.readInt32();其实是数组mSlots的下标，在BufferQueue中有个和mSlots对应的数组，也是32个，一一对应，

status_t BnGraphicBufferProducer::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
      case DEQUEUE_BUFFER: {
            CHECK_INTERFACE(IGraphicBufferProducer, data, reply);
            bool async      = data.readInt32();
            uint32_t w      = data.readInt32();
            uint32_t h      = data.readInt32();
            uint32_t format = data.readInt32();
            uint32_t usage  = data.readInt32();
            int buf;
            sp<Fence> fence;
            //调用BufferQueue的dequeueBuffer
            //也返回一个int的buf
            int result = dequeueBuffer(&buf, &fence, async, w, h, format, usage);
            //将buf和fence写入parcel，通过binder传给client
            reply->writeInt32(buf);
            reply->writeInt32(fence != NULL);
            if (fence != NULL) {
                reply->write(*fence);
            }
            reply->writeInt32(result);
            return NO_ERROR;
}

可以看到BnGraphicBufferProducer端获取到长宽及格式，之后利用BufferQueueProducer的dequeueBuffer来申请内存，内存可能已经申请，也可能未申请，未申请，则直接申请新内存，每个surface可以对应32块内存：

status_t BufferQueueProducer::dequeueBuffer(int *outSlot,
        sp<android::Fence> *outFence, uint32_t width, uint32_t height,
        PixelFormat format, uint32_t usage) {
    ...
        sp<GraphicBuffer> graphicBuffer(mCore->mAllocator->createGraphicBuffer(
                width, height, format, usage,
                {mConsumerName.string(), mConsumerName.size()}, &error));

mCore其实就是上面的BufferQueueCore，mCore->mAllocator = new GraphicBufferAlloc()，最终会利用GraphicBufferAlloc对象分配共享内存:

sp<GraphicBuffer> GraphicBufferAlloc::createGraphicBuffer(uint32_t width,
        uint32_t height, PixelFormat format, uint32_t usage,
        std::string requestorName, status_t* error) {

    <!--直接new新建-->
    sp<GraphicBuffer> graphicBuffer(new GraphicBuffer(
            width, height, format, usage, std::move(requestorName)));
    status_t err = graphicBuffer->initCheck();
    return graphicBuffer;
}

从上面看到，直接new GraphicBuffer新建图像内存，

GraphicBuffer::GraphicBuffer(uint32_t inWidth, uint32_t inHeight,
        PixelFormat inFormat, uint32_t inUsage, std::string requestorName)
    : BASE(), mOwner(ownData), mBufferMapper(GraphicBufferMapper::get()),
      mInitCheck(NO_ERROR), mId(getUniqueId()), mGenerationNumber(0){
    ...
    handle = NULL;
    mInitCheck = initSize(inWidth, inHeight, inFormat, inUsage,
            std::move(requestorName));
}

status_t GraphicBuffer::initSize(uint32_t inWidth, uint32_t inHeight,
        PixelFormat inFormat, uint32_t inUsage, std::string requestorName)
{
    GraphicBufferAllocator& allocator = GraphicBufferAllocator::get();
    uint32_t outStride = 0;
    <!--请求分配内存-->
    status_t err = allocator.allocate(inWidth, inHeight, inFormat, inUsage,
            &handle, &outStride, mId, std::move(requestorName));
    if (err == NO_ERROR) {
        width = static_cast<int>(inWidth);
        height = static_cast<int>(inHeight);
        format = inFormat;
        usage = static_cast<int>(inUsage);
        stride = static_cast<int>(outStride);
    }
    return err;
}

 status_t GraphicBufferAllocator::allocate(uint32_t width, uint32_t height,
        PixelFormat format, uint32_t usage, buffer_handle_t* handle,
        uint32_t* stride, uint64_t graphicBufferId, std::string requestorName)
{
     ...
    auto descriptor = mDevice->createDescriptor();
    auto error = descriptor->setDimensions(width, height);
    error = descriptor->setFormat(static_cast<android_pixel_format_t>(format));
    error = descriptor->setProducerUsage(
            static_cast<gralloc1_producer_usage_t>(usage));
    error = descriptor->setConsumerUsage(
            static_cast<gralloc1_consumer_usage_t>(usage));
    <!--这里的device就是抽象的硬件设备-->
    error = mDevice->allocate(descriptor, graphicBufferId, handle);
    error = mDevice->getStride(*handle, stride);
    ...
    return NO_ERROR;
}

上面代码的mDevice就是利用hw_get_module及gralloc1_open获取到的硬件抽象层device，hw_get_module装载HAL模块，会加载相应的.so文件gralloc.default.so，它实现位于 hardware/libhardware/modules/gralloc.cpp中，最后将device映射的函数操作加载进来。这里我们关心的是allocate函数，先分析普通图形缓冲区的分配，它最终会调用gralloc_alloc_buffer()利用匿名共享内存进行分配，之前的文章Android匿名共享内存（Ashmem）原理分析了Android是如何通过匿名共享内存进行通信的，这里就直接用了：

static int gralloc_alloc_buffer(alloc_device_t* dev,
        size_t size, int usage, buffer_handle_t* pHandle)
{
    int err = 0;
    int fd = -1;
    size = roundUpToPageSize(size);
    // 创建共享内存，并且设定名字跟size
    fd = ashmem_create_region("gralloc-buffer", size);
    if (err == 0) {
        private_handle_t* hnd = new private_handle_t(fd, size, 0);
        gralloc_module_t* module = reinterpret_cast<gralloc_module_t*>(
                dev->common.module);
         // 执行mmap，将内存映射到自己的进程
        err = mapBuffer(module, hnd);
        if (err == 0) {
            *pHandle = hnd;
        }
    }

    return err;
}

mapBuffer会进一步调用ashmem的驱动，在tmpfs新建文件，同时开辟虚拟内存，

int mapBuffer(gralloc_module_t const* module,
            private_handle_t* hnd)
    {
        void* vaddr; 
        // vaddr有个毛用？
        return gralloc_map(module, hnd, &vaddr);
    }

static int gralloc_map(gralloc_module_t const* module,
        buffer_handle_t handle,
        void** vaddr)
{
    private_handle_t* hnd = (private_handle_t*)handle;
    if (!(hnd->flags & private_handle_t::PRIV_FLAGS_FRAMEBUFFER)) {
        size_t size = hnd->size;
        void* mappedAddress = mmap(0, size,
                PROT_READ|PROT_WRITE, MAP_SHARED, hnd->fd, 0);
        if (mappedAddress == MAP_FAILED) {
            return -errno;
        }
        hnd->base = intptr_t(mappedAddress) + hnd->offset;
    }
    *vaddr = (void*)hnd->base;
    return 0;
}

View绘制内存的传递

分配之后，会继续利用BpGraphicBufferProducer的requestBuffer，申请将共享内存给映射到当前进程：

virtual status_t requestBuffer(int bufferIdx, sp<GraphicBuffer>* buf) {
    Parcel data, reply;
    data.writeInterfaceToken(IGraphicBufferProducer::getInterfaceDescriptor());
    data.writeInt32(bufferIdx);
    status_t result =remote()->transact(REQUEST_BUFFER, data, &reply);
    if (result != NO_ERROR) {
        return result;
    }
    bool nonNull = reply.readInt32();
    if (nonNull) {
        *buf = new GraphicBuffer();
        reply.read(**buf);
    }
    result = reply.readInt32();
    return result;
}

private_handle_t对象用来抽象图形缓冲区，其中存储着与共享内存对应tmpfs文件的fd，GraphicBuffer对象会通过序列化，将这个fd会利用Binder通信传递给App进程，APP端获取到fd之后，便可以同mmap将共享内存映射到自己的进程空间，进而进行图形绘制。等到APP端对GraphicBuffer的反序列化的时候，会将共享内存mmap到当前进程空间：

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;  
}

进而调用GraphicBuffer::unflatten：

status_t GraphicBuffer::unflatten(void const* buffer, size_t size,
        int fds[], size_t count)
{
   ...
    mOwner = ownHandle;
    <!--将共享内存映射当前内存空间-->
    if (handle != 0) {
        status_t err = mBufferMapper.registerBuffer(handle);
    }
    return NO_ERROR;
}

mBufferMapper.registerBuffer函数对应gralloc_register_buffer

struct private_module_t HAL_MODULE_INFO_SYM = {
    .base = {
        .common = {
            .tag = HARDWARE_MODULE_TAG,
            .version_major = 1,
            .version_minor = 0,
            .id = GRALLOC_HARDWARE_MODULE_ID,
            .name = "Graphics Memory Allocator Module",
            .author = "The Android Open Source Project",
            .methods = &gralloc_module_methods
        },
        .registerBuffer = gralloc_register_buffer,
        .unregisterBuffer = gralloc_unregister_buffer,
        .lock = gralloc_lock,
        .unlock = gralloc_unlock,
    },
    .framebuffer = 0,
    .flags = 0,
    .numBuffers = 0,
    .bufferMask = 0,
    .lock = PTHREAD_MUTEX_INITIALIZER,
    .currentBuffer = 0,
};

最后会调用gralloc_register_buffer，通过mmap真正将tmpfs文件映射到进程空间：

static int gralloc_register_buffer(gralloc_module_t const* module,
                                   buffer_handle_t handle)
{
    ...
    if (cb->ashmemSize > 0 && cb->mappedPid != getpid()) {
        void *vaddr;
        <!--mmap-->
        int err = map_buffer(cb, &vaddr);
        cb->mappedPid = getpid();
    }

    return 0;
}

终于我们用到tmpfs中文件对应的描述符fd0->cb->fd

static int map_buffer(cb_handle_t *cb, void **vaddr)
{
    if (cb->fd < 0 || cb->ashmemSize <= 0) {
        return -EINVAL;
    }

    void *addr = mmap(0, cb->ashmemSize, PROT_READ | PROT_WRITE,
                      MAP_SHARED, cb->fd, 0);
    cb->ashmemBase = intptr_t(addr);
    cb->ashmemBasePid = getpid();
    *vaddr = addr;
    return 0;
}

到这里内存传递成功，App端就可以应用这块内存进行图形绘制了。

View绘制内存的使用

关于内存的使用，我们回到之前的Surface lock函数，内存经过反序列化，拿到内存地址后，会封装一个ANativeWindow_Buffer返回给上层调用：

status_t Surface::lock(
        ANativeWindow_Buffer* outBuffer, ARect* inOutDirtyBounds)
{
     ...
        void* vaddr;
        <!--lock获取地址-->
        status_t res = backBuffer->lock(
                GRALLOC_USAGE_SW_READ_OFTEN | GRALLOC_USAGE_SW_WRITE_OFTEN,
                newDirtyRegion.bounds(), &vaddr);

        if (res != 0) {
            err = INVALID_OPERATION;
        } else {
            mLockedBuffer = backBuffer;
            outBuffer->width  = backBuffer->width;
            outBuffer->height = backBuffer->height;
            outBuffer->stride = backBuffer->stride;
            outBuffer->format = backBuffer->format;

                <!--关键点 设置虚拟内存的地址-->
            outBuffer->bits   = vaddr;
        }
    }
    return err;
}

ANativeWindow_Buffer的数据结构如下，其中bits字段与虚拟内存地址对应，

typedef struct ANativeWindow_Buffer {
    // The number of pixels that are show horizontally.
    int32_t width;

    // The number of pixels that are shown vertically.
    int32_t height;

    // The number of *pixels* that a line in the buffer takes in
    // memory.  This may be >= width.
    int32_t stride;

    // The format of the buffer.  One of WINDOW_FORMAT_*
    int32_t format;

    // The actual bits.
    void* bits;

    // Do not touch.
    uint32_t reserved[6];
} ANativeWindow_Buffer;

如何使用，看下Canvas的draw

static void nativeLockCanvas(JNIEnv* env, jclass clazz,
        jint nativeObject, jobject canvasObj, jobject dirtyRectObj) {
    sp<Surface> surface(reinterpret_cast<Surface *>(nativeObject));
    ...
    status_t err = surface->lock(&outBuffer, &dirtyBounds);
    ...
    <!--SkBitmap-->
    SkBitmap bitmap;
    ssize_t bpr = outBuffer.stride * bytesPerPixel(outBuffer.format);
    <!--为SkBitmap填充配置-->
    bitmap.setConfig(convertPixelFormat(outBuffer.format), outBuffer.width, outBuffer.height, bpr);
    <!--为SkBitmap填充格式-->
    if (outBuffer.format == PIXEL_FORMAT_RGBX_8888) {
        bitmap.setIsOpaque(true);
    }
    <!--为SkBitmap填充内存-->
    if (outBuffer.width > 0 && outBuffer.height > 0) {
        bitmap.setPixels(outBuffer.bits);
    } else {
        // be safe with an empty bitmap.
        bitmap.setPixels(NULL);
    }

    <!--创建native SkCanvas-->
    SkCanvas* nativeCanvas = SkNEW_ARGS(SkCanvas, (bitmap));
    swapCanvasPtr(env, canvasObj, nativeCanvas);
   ...
}

对于2D绘图，会用skia库会填充Bitmap对应的共享内存，如此即可完成绘制，本文不深入Skia库，有兴趣自行分析。绘制完成后，通过unlock直接通知SurfaceFlinger服务进行图层合成。

总结

Android View的绘制建立匿名共享内存的基础上，APP端与SurfaceFlinger通过共享内存的方式避免了View视图数据的拷贝，提高了系统同的视图处理能力。

作者：看书的小蜗牛
原文链接:Android窗口管理分析（4）：Android View绘制内存的分配、传递、使用
仅供参考，欢迎指正