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<IBinder> 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<IGraphicBufferProducer> 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<IGraphicBufferProducer> gbp向SurfaceFlinger申请的，那么首先看一下APP端如何获得sp<IGraphicBufferProducer> 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<IGraphicBufferProducer> 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<IGraphicBufferProducer> 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() { // Creates a custom BufferQueue for SurfaceFlingerConsumer to use sp<IGraphicBufferProducer> producer; sp<IGraphicBufferConsumer> consumer; BufferQueue::createBufferQueue(&producer, &consumer); <!--创建producer与consumer--> mProducer = new MonitoredProducer(producer, mFlinger); mSurfaceFlingerConsumer = new SurfaceFlingerConsumer(consumer, mTextureName); mSurfaceFlingerConsumer->setConsumerUsageBits(getEffectiveUsage(0)); mSurfaceFlingerConsumer->setContentsChangedListener(this); mSurfaceFlingerConsumer->setName(mName); <!--三缓冲还是双缓冲--> #ifdef TARGET_DISABLE_TRIPLE_BUFFERING #warning "disabling triple buffering" mSurfaceFlingerConsumer->setDefaultMaxBufferCount(2); #else mSurfaceFlingerConsumer->setDefaultMaxBufferCount(3); #endif const sp<const DisplayDevice> hw(mFlinger->getDefaultDisplayDevice()); updateTransformHint(hw); }
通过TARGET_DISABLE_TRIPLE_BUFFERING看是用三缓冲还是双缓冲，MaxBufferCoun。
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<IGraphicBufferProducer> 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，为了方便理解，先看软件绘制：ViewRootImpl中的drawSoftware
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，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可以对应多块（6.0好像是64）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。
Android View局部重绘的原理
拿TextView来说，如果内容发生了改变，就会触发重绘，加入当前视图中还包含其他View，这个时候，可能只会触发TextView及其父层级View的重绘，其他View不重绘，为什么呢？这个时候传递给SurfaceFlinger的UI数据如何保证完整呢？其实在lockCanvas的时候，默认是又一次数据拷贝的，也就是将之前绘制的UI数据拷贝到最新的申请内存中去，而新的重绘是从拷贝之后开始的，也就是在原来视图的基础上进行脏区域重绘：
status_t Surface::lock( ANativeWindow_Buffer* outBuffer, ARect* inOutDirtyBounds) { <!--申请内存--> status_t err = dequeueBuffer(&out, &fenceFd); ALOGE_IF(err, "dequeueBuffer failed (%s)", strerror(-err)); if (err == NO_ERROR) { <!--如果需要就尽心拷贝--> sp<GraphicBuffer> backBuffer(GraphicBuffer::getSelf(out)); const Rect bounds(backBuffer->width, backBuffer->height); ... const sp<GraphicBuffer>& frontBuffer(mPostedBuffer); const bool canCopyBack = (frontBuffer != 0 && backBuffer->width == frontBuffer->width && backBuffer->height == frontBuffer->height && backBuffer->format == frontBuffer->format); // 是否能够拷贝到当前backBuffer中来？必须两个样式一样，才能拷贝，如果不一样不用 if (canCopyBack) { // copy the area that is invalid and not repainted this round const Region copyback(mDirtyRegion.subtract(newDirtyRegion)); if (!copyback.isEmpty()) { // 拷贝 copyBlt(backBuffer, frontBuffer, copyback, &fenceFd); } } else { // 如果不能拷贝，那就整块绘制，终于找到了入口 入江口 入口啊 newDirtyRegion.set(bounds); mDirtyRegion.clear(); Mutex::Autolock lock(mMutex); for (size_t i=0 ; i<NUM_BUFFER_SLOTS ; i++) { mSlots[i].dirtyRegion.clear(); } } .... }
对于通过lockCanvas获取的内存，要么被上次绘制的UI数据填充，要么整体重绘，如果被上次填充，那么这次就只需要绘制脏区域相关的视图，这就是Android局部重绘的原理
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。
