Android Surface-GraphicBuffer-BufferQueue

 

Android的UI控件最终在Surface上进行绘制；Surface要进行绘制，须要申请显存，绘制，提交显存进行显示。

申请显存

Android的显存由两个部分表示，对APP的接口体现为Surface（native/libs/gui/Surface.cpp），对graphics部分（CPU/GPU/OPENGL)体现为GraphicBuffer。

Surface说明

Surface自己有两个含义，一个是表明UI系统的Canvas，另外一个是表明本地window系统，为跨平台的OPENGL(EGL)提供接口。

UI通常基于Canvas绘制，参考UI的始祖View的draw函数：

public void draw(Canvas canvas)

全部UI控件继承自View，都会基于Canvs来绘制本身；UI组件的draw是谁触发的，canvas是怎么建立的?这些秘密在ViewRootImpl里面，每一个Activity在setContentView以后，系统会为其建立一个ViewRootImpl对象，该对象代替Activity管理其view系统，并和window系统创建关联（Activity的window就是在该类中建立的），而且ViewRootImpl会创建和SurfaceFlinger的链接，监听SurfaceFlinger的VSYNC信号，一旦VSYNC信号发生，ViewRootImpl就会进入到framecallback中进行绘制。其中ViewRootImpl拥有window对应的Surface对象：

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff, boolean scalingRequired, Rect dirty, Rect surfaceInsets) { // Draw with software renderer.
        final Canvas canvas;try { ...... canvas = mSurface.lockCanvas(dirty); ...... if (!canvas.isOpaque() || yoff != 0 || xoff != 0) { canvas.drawColor(0, PorterDuff.Mode.CLEAR); } try { canvas.translate(-xoff, -yoff); 　　　　　　　　　　....... mView.draw(canvas); } finally { ...... } } finally { try { surface.unlockCanvasAndPost(canvas); } catch (IllegalArgumentException e)  ...... } 

对于Canvas的使用流程：

Surface.lockCanvas->View.draw(Canvas)-> Surface.unlockCanvasAndPost(Canvas)

在Surface.lockCanvas中会调用native的对象android_view_Surface.cpp->Surface.dequeueBuffer->BufferQueueProducer.dequeueBuffer获得struct ANativeWindowBuffer 的对象，其实就是一个GraphicBuffer对象，与此同时还返回了FenceID。　 　　

 ANativeWindow_Buffer outBuffer; status_t err = surface->lock(&outBuffer, dirtyRectPtr); if (err < 0) { const char* const exception = (err == NO_MEMORY) ? OutOfResourcesException : "java/lang/IllegalArgumentException"; jniThrowException(env, exception, NULL); return 0; } SkImageInfo info = SkImageInfo::Make(outBuffer.width, outBuffer.height, convertPixelFormat(outBuffer.format), outBuffer.format == PIXEL_FORMAT_RGBX_8888 ? kOpaque_SkAlphaType : kPremul_SkAlphaType, GraphicsJNI::defaultColorSpace()); SkBitmap bitmap; ssize_t bpr = outBuffer.stride * bytesPerPixel(outBuffer.format); bitmap.setInfo(info, bpr); if (outBuffer.width > 0 && outBuffer.height > 0) { bitmap.setPixels(outBuffer.bits); } else { // be safe with an empty bitmap.
 bitmap.setPixels(NULL); } Canvas* nativeCanvas = GraphicsJNI::getNativeCanvas(env, canvasObj); nativeCanvas->setBitmap(bitmap);

由上面的lockCanvas代码片断来看，根据ANativeWindowBuffer构建了一个SKBitmap对象，将该对象设置给nativeCanvas（SkiaCanvas），而后就返回到Java空间了。

上面提到了Canvas，咱们看一下Canvas的处理流程。

 

 大部分基于Canvas的操做最后会落到SKCanvas上面去,这个在Skia 2D库里面。若是想搞清楚流程，能够拿TextView或者Android任意一个UI控件，看一下他的draw是怎么利用canvas API来实现的。也能够看一下skia库实现。

提交显存

在UI绘制完成后，须要将绘制的内容提交显示，这里用到了Surface::unlockAndPost:

status_t Surface::unlockAndPost() { if (mLockedBuffer == 0) { ALOGE("Surface::unlockAndPost failed, no locked buffer"); return INVALID_OPERATION; } int fd = -1; status_t err = mLockedBuffer->unlockAsync(&fd); ALOGE_IF(err, "failed unlocking buffer (%p)", mLockedBuffer->handle); err = queueBuffer(mLockedBuffer.get(), fd); ALOGE_IF(err, "queueBuffer (handle=%p) failed (%s)", mLockedBuffer->handle, strerror(-err)); mPostedBuffer = mLockedBuffer; mLockedBuffer = 0; return err; }

其中主要就是将GraphicBuffer 提交到BufferQueue上等待SurfaceFlinger（comsumer）显示出来。

 GraphicBufferProducer诞生流程

BufferQueue的基本结构以下：

 GraphicBuffer就与基于BufferQueueProducer产生的，在Surface.cpp里面有一个sp<IGraphicsBufferProducer> mGraphicBufferProducer;全部对GraphicBuffer的queue/dequeue/cancel等都是经过mBufferProducer产生的，咱们看一下这个对象是怎么产生，谁在server端为其服务，client和server的链接是怎么创建的。

从前面Surface的说明里面咱们提到一点，就是ViewRootImpl；ViewRootImpl里面的Surface为全部View的绘制提供canvas，咱们看一下这个Surface是怎么建立的就能搞清楚Surface.mGraphicBufferProducer是怎么实例化的。ViewRootImpl是Activity View管理者，也是Activity对应window的建立者，在其中有几个步骤：

create window，就是建立Activity对应的window对象，是和WMS创建通信建立窗口

try { mOrigWindowType = mWindowAttributes.type; mAttachInfo.mRecomputeGlobalAttributes = true; collectViewAttributes(); res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes, getHostVisibility(), mDisplay.getDisplayId(), mWinFrame, mAttachInfo.mContentInsets, mAttachInfo.mStableInsets, mAttachInfo.mOutsets, mAttachInfo.mDisplayCutout, mInputChannel); } catch (RemoteException e) { mAdded = false;

relayout window，测量窗口大小位置等：

int relayoutResult = mWindowSession.relayout(mWindow, mSeq, params, (int) (mView.getMeasuredWidth() * appScale + 0.5f), (int) (mView.getMeasuredHeight() * appScale + 0.5f), viewVisibility, insetsPending ? WindowManagerGlobal.RELAYOUT_INSETS_PENDING : 0, frameNumber, mWinFrame, mPendingOverscanInsets, mPendingContentInsets, mPendingVisibleInsets, mPendingStableInsets, mPendingOutsets, mPendingBackDropFrame, mPendingDisplayCutout, mPendingMergedConfiguration, mSurface);

在WMS里面上面两个接口分别调用addWindow以及relayoutWindow；其中relayoutWindow中建立了实际的surface，也就是说实在WMS中显示窗口的时候去建立了实际的surface，其建立过程以下：

最后是调用surface.copyFrom(SurfaceControl)获得真实的surface；SurfaceControl是在WMS里面建立的，SurfaceControl建立的时候就会向SurfaceComposerClient申请建立surface：

static jlong nativeCreate(JNIEnv* env, jclass clazz, jobject sessionObj, jstring nameStr, jint w, jint h, jint format, jint flags, jlong parentObject, jint windowType, jint ownerUid) { ScopedUtfChars name(env, nameStr); sp<SurfaceComposerClient> client(android_view_SurfaceSession_getClient(env, sessionObj)); SurfaceControl *parent = reinterpret_cast<SurfaceControl*>(parentObject); sp<SurfaceControl> surface; status_t err = client->createSurfaceChecked( String8(name.c_str()), w, h, format, &surface, flags, parent, windowType, ownerUid); if (err == NAME_NOT_FOUND) { jniThrowException(env, "java/lang/IllegalArgumentException", NULL); return 0; } else if (err != NO_ERROR) { jniThrowException(env, OutOfResourcesException, NULL); return 0; } surface->incStrong((void *)nativeCreate); return reinterpret_cast<jlong>(surface.get()); }

 

在createSurfaceChecked里面想surfaceFlinger申请建立Surface，并基于建立的Surface建立新的SurfaceControl。而后一步步返回，ViewRootImpl里面的Surface就具有真正的显存了。可是咱们前面是要知道GraphicBufferProducer是谁建立的，这个秘密就在ComposerSurfaceClient.createSurfaceChecked函数里面。ComposerSurfaceClient有一个成员变量mClient，这是SurfaceFlinger.Client的客户端，经过这个mClient和SurfaceFlinger创建通信。

status_t SurfaceComposerClient::createSurfaceChecked(...) { sp<SurfaceControl> sur; status_t err = mStatus; if (mStatus == NO_ERROR) { sp<IBinder> handle; sp<IBinder> parentHandle; sp<IGraphicBufferProducer> gbp; if (parent != nullptr) { parentHandle = parent->getHandle(); } err = mClient->createSurface(name, w, h, format, flags, parentHandle, windowType, ownerUid, &handle, &gbp); ALOGE_IF(err, "SurfaceComposerClient::createSurface error %s", strerror(-err)); if (err == NO_ERROR) { *outSurface = new SurfaceControl(this, handle, gbp, true /* owned */); } } return err; }

在这里能够看到调用了mClient的createSurface，而后返回了gbp（也就是IGraphicBufferProducer）；mClient是SurfaceFlinger.Client的客户端，因而可知GraphicBufferProducer实际是有SurfaceFlinger进程建立的。mClient和SurfaceFlinger的对象关系以下图所示：

 

 那么进入SurfaceFlinger看一下究竟是怎么建立GraphicBufferProducer的；Client.createSurface->SurfaceFlinger.createLayer-> new BufferLayer，实际是在BufferLayer::onFirstRef里面建立的：

void BufferLayer::onFirstRef() { // Creates a custom BufferQueue for SurfaceFlingerConsumer to use
    sp<IGraphicBufferProducer> producer; sp<IGraphicBufferConsumer> consumer; BufferQueue::createBufferQueue(&producer, &consumer, true); mProducer = new MonitoredProducer(producer, mFlinger, this); mConsumer = new BufferLayerConsumer(consumer, mFlinger->getRenderEngine(), mTextureName, this); mConsumer->setConsumerUsageBits(getEffectiveUsage(0)); mConsumer->setContentsChangedListener(this); mConsumer->setName(mName); if (mFlinger->isLayerTripleBufferingDisabled()) { mProducer->setMaxDequeuedBufferCount(2); } const sp<const DisplayDevice> hw(mFlinger->getDefaultDisplayDevice()); updateTransformHint(hw); }

 

由上面的建立过程也能够看出来，Surface提交的GraphicBuffer由BufferLayerConsumer来消耗。