Android之Surface绘制原理

一、Surface的Buffer是从哪里来的？

　　源码：frameworks/base/core/java/android/view/ViewRootImpl.java

　　View触发绘制是通过requestLayout()函数或者setLayoutParms()函数：

　　performTravsersals()函数实现：

private void performTraversals() {
    ……
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
    performLayout(lp, mWidth, mHeight);
    performDraw();
    ……
}

　　perfomrDraw()函数调用draw()函数开始绘制：

private void performDraw() {
    ……
    boolean canUseAsync = draw(fullRedrawNeeded);
    ……
}

　　ViewRootImpl.draw()函数实现：

private boolean draw(boolean fullRedrawNeeded) {
    Surface surface = mSurface;
    ……
    if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty, surfaceInsets)) {
        return false;
    }
    ……
    return useAsyncReport;
}

　　drawSoftware()软件绘制，默认是软件绘制。

　　drawSoftware()函数软件绘制流程：

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
        boolean scalingRequired, Rect dirty, Rect surfaceInsets) {

    // Draw with software renderer.
    final Canvas canvas;
    ……
    canvas = mSurface.lockCanvas(dirty);
    ……
    mView.draw(canvas);
    ……
    surface.unlockCanvasAndPost(canvas);

}

　　获取：通过lockCanvas函数获取Canvas对象，

　　绘制：再通过mView.draw(canvas)函数向在canvas上绘制，

　　提交：最后通过surface.unlockCanvasAndPost(canvas)函数提交Canvas。

　　通过lockCanvas()函数获取Canvas对象，lockCanvas()函数如何获取Canvas对象。

　　lockCanvas()函数实现：

/**
 * Gets a {@link Canvas} for drawing into this surface.
 *
 * After drawing into the provided {@link Canvas}, the caller must
 * invoke {@link #unlockCanvasAndPost} to post the new contents to the surface.
 *
 * @param inOutDirty A rectangle that represents the dirty region that the caller wants
 * to redraw.  This function may choose to expand the dirty rectangle if for example
 * the surface has been resized or if the previous contents of the surface were
 * not available.  The caller must redraw the entire dirty region as represented
 * by the contents of the inOutDirty rectangle upon return from this function.
 * The caller may also pass <code>null</code> instead, in the case where the
 * entire surface should be redrawn.
 * @return A canvas for drawing into the surface.
 *
 * @throws IllegalArgumentException If the inOutDirty rectangle is not valid.
 * @throws OutOfResourcesException If the canvas cannot be locked.
 */
public Canvas lockCanvas(Rect inOutDirty)
        throws Surface.OutOfResourcesException, IllegalArgumentException {
    synchronized (mLock) {
        checkNotReleasedLocked();
        if (mLockedObject != 0) {
            // Ideally, nativeLockCanvas() would throw in this situation and prevent the
            // double-lock, but that won't happen if mNativeObject was updated.  We can't
            // abandon the old mLockedObject because it might still be in use, so instead
            // we just refuse to re-lock the Surface.
            throw new IllegalArgumentException("Surface was already locked");
        }
        mLockedObject = nativeLockCanvas(mNativeObject, mCanvas, inOutDirty);
        return mCanvas;
    }
}

　　通过Native层android_view_Surface.cpp的nativeLockCanvas(mNativeObject, mCanvas, inOutDirty)函数获取，mNativeOjbect参数是Java层的Surface在Native层对应的Surface对象的指针。mCanvas是Surface的变量，在lockCanvas()函数调用时mCanvas是空的。

　　在调用nativeLockCanvas()函数后mCanvas就有值了，最后返回mCanvas对象。

static jlong nativeLockCanvas(JNIEnv* env, jclass clazz,
        jlong nativeObject, jobject canvasObj, jobject dirtyRectObj) {
    // (1)
    sp<Surface> surface(reinterpret_cast<Surface *>(nativeObject));
    ……
    // (2)
    ANativeWindow_Buffer buffer;
    status_t err = surface->lock(&buffer, dirtyRectPtr);
    ……
    // (3)
    graphics::Canvas canvas(env, canvasObj);
    canvas.setBuffer(&buffer, static_cast<int32_t>(surface->getBuffersDataSpace()));
    ……
    // (4)
    // Create another reference to the surface and return it.  This reference
    // should be passed to nativeUnlockCanvasAndPost in place of mNativeObject,
    // because the latter could be replaced while the surface is locked.
    sp<Surface> lockedSurface(surface);
    lockedSurface->incStrong(&sRefBaseOwner);
    return (jlong) lockedSurface.get();
}

　　(1) 获取Native层的Surface对象。

　　(2) 获取Native层的Surface对象的Buffer。

　　(3) 将Buffer设置给Canvas，这里Canvas就有一个Buffer了。在每次都申请一个新的Buffer给Canvas对象。

　　(4) 向Java层返回Native的Surface对象，这里返回的是一个Long型数据，这个Long型数据是Surface指针。

　　获取Buffer实现，surface -> lock(&buffer, )，这里传入Buffer地址：

　　源码：frameworks/native/libs/gui/Surface.cpp

status_t Surface::lock(ANativeWindow_Buffer* outBuffer, ARect* inOutDirtyBounds)
{
    ……
    // (1)
    ANativeWindowBuffer* out;
    status_t err = dequeueBuffer(&out, &fenceFd);
    ……
    // (2)
    sp<GraphicBuffer> backBuffer(GraphicBuffer::getSelf(out));
    ……
    // (3)
    void* vaddr;
    status_t res = backBuffer->lockAsync(GRALLOC_USAGE_SW_READ_OFTEN | GRALLOC_USAGE_SW_WRITE_OFTEN, newDirtyRegion.bounds(), &vaddr, fenceFd);
    ……
    // (4)
    mLockedBuffer = backBuffer;
    // (5)
    outBuffer->bits   = vaddr;
    ……
}

　　(1) 获取dequeueBuffer()函数在SurfaceFlinger的Buffer队列中获取Buffer。

　　(2) 创建GraphicBuffer对象backBuffer。在SharedBufferStack中有双缓冲机制，分别为FontBuffer和BackBuffer。

- FontBuffer：代表当前将显示在屏幕的Buffer数据。属于前台Buffer。
- BackBuffer：代表绘制的Buffer数据，是准备渲染的数据Buffer。属于后台Buffer。

　　(3) 锁定Buffer，并将Buffer地址返回，将返回的Buffer地址给Canvas的Buffer。

　　(4) 切换Buffer，将后台BackBuffer切换到前台，交给mLockedBuffer。FontBuffer的变量就是mLockedBuffer。

　　(5) 将vaddr赋值给outBuffer->bits，bits最后赋值给Canvas的Buffer，就是BkBitmap，作为Canvas的缓冲区。

　　dequeteBuffer()是如何获取Buffer的，dequeteBuffer()函数实现：

int Surface::dequeueBuffer(android_native_buffer_t** buffer, int* fenceFd) {
    ……
    int buf = -1;
    // (1)
    status_t result = mGraphicBufferProducer->dequeueBuffer(&buf, &fence, reqWidth, reqHeight,
                                                            reqFormat, reqUsage, &mBufferAge,
                                                            enableFrameTimestamps ? &frameTimestamps
                                                                                  : nullptr);
    ……
    // (2)
    sp<GraphicBuffer>& gbuf(mSlots[buf].buffer);
    ……
    // (3)
    if ((result & IGraphicBufferProducer::BUFFER_NEEDS_REALLOCATION) || gbuf == nullptr) {
        if (mReportRemovedBuffers && (gbuf != nullptr)) {
            mRemovedBuffers.push_back(gbuf);
        }
        result = mGraphicBufferProducer->requestBuffer(buf, &gbuf);
        if (result != NO_ERROR) {
            ALOGE("dequeueBuffer: IGraphicBufferProducer::requestBuffer failed: %d", result);
            mGraphicBufferProducer->cancelBuffer(buf, fence);
            return result;
        }
    }
    ……
    // (4)
    *buffer = gbuf.get();
}

　　(1) 通过mGaphicBufferProducer->dequeteBuffer()函数在远端的Buffer slots中获得一个空闲的Buffer，返回远端Buffer地址指针。

　　(2) 通过gbp从本地Buffer Slots里获取Buffer，在(1)中从远端，在(2)中从本地，这里涉及远端Buffer queue与本地Buffer queue同步问题。

　　(3) 负责本地Buffer与远端Buffer同步，远端返回的Buffer的result是BUFFER_NEEDS_REALLOCATION或者本地的gbp是null，通过gbp的requestBuffer()获取新的远端Buffer指针地址。mGaphicBufferProducer->requestBuffer()函数。

　　(4) 获取Buffer。

二、Surface的Buffer是如何提交的？

　　通过surface.unlockCanvasAndPost(canvas)向远端提交更新的Buffer，unlockCanvasAndPost()函数实现：

/**
 * Posts the new contents of the {@link Canvas} to the surface and
 * releases the {@link Canvas}.
 *
 * @param canvas The canvas previously obtained from {@link #lockCanvas}.
 */
public void unlockCanvasAndPost(Canvas canvas) {
    synchronized (mLock) {
        unlockSwCanvasAndPost(canvas);
    }
}

private void unlockSwCanvasAndPost(Canvas canvas) {
    try {
        nativeUnlockCanvasAndPost(mLockedObject, canvas);
    } finally {
        nativeRelease(mLockedObject);
        mLockedObject = 0;
    }
}

　　最后调用到Native层的nativeUnlockCanvasAndPost(mLockedObject, canvas)。

　　Native层，nativeUnlockCanvasAndPost()函数实现：

static void nativeUnlockCanvasAndPost(JNIEnv* env, jclass clazz,
        jlong nativeObject, jobject canvasObj) {
    // (1)
    sp<Surface> surface(reinterpret_cast<Surface *>(nativeObject));

    // (2)
    // detach the canvas from the surface
    graphics::Canvas canvas(env, canvasObj);
    canvas.setBuffer(nullptr, ADATASPACE_UNKNOWN);

    // (3)
    // unlock surface
    status_t err = surface->unlockAndPost();
}

　　(1) 获取对应Java层的Native层的Surface对象。

　　(2) 获取对应Java层的Native层的Canvas对象。

　　(3) 将本地Buffer更新到远端的Buffer queue中。

　　Native层更新远端Buffer queue，surface->unlockAndPost()函数实现：

　　源码：frameworks/native/libs/gui/Surface.cpp

status_t Surface::unlockAndPost()
{
    if (mLockedBuffer == nullptr) {
        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 = nullptr;
    return err;
}

　　通过函数queueBuffer(mLockedBuffer.get(), )函数实现更新：

int Surface::queueBuffer(android_native_buffer_t* buffer, int fenceFd) {
    ……
    // (1)
    int i = getSlotFromBufferLocked(buffer);
    ……
    // (2)
    status_t err = mGraphicBufferProducer->queueBuffer(i, input, &output);
    ……
    return err;
}

　　(1) 获取Buffer的index。

　　(2) 通过mGraphicBufferProducer->queueBuffer(i, )函数，将本地的Buffer同步到远端Buffer queue中。