本文内容接着上一篇文章Android的UI显示原理之Surface的创建继续来看
Surface渲染的大致过程。
对于Surface的渲染可以转化为ViewRootImpl的渲染。因此我们从ViewRootImpl.draw()来看一下它的渲染逻辑。这个方法最终会调用到ViewRootImpl.drawSoftward():
private boolean drawSoftware(Surface surface, AttachInfo attachInfo,...) {
// Draw with software renderer.
final Canvas canvas;
...
canvas = mSurface.lockCanvas(dirty); //step 1
...
mView.draw(canvas); //setp 2
...
mSurface.unlockCanvasAndPost(canvas); //step 3
}
mView是ViewRootImpl的根View。mSurface即为上一篇文章分析的所创建的Surface，它的实际对象在nativie层。
上面3步大致描绘了ViewRootImpl的绘制原理，本文就逐一分析这3步，来大致了解ViewRootImpl的渲染逻辑。
mSurface.lockCanvas()
public Canvas lockCanvas(Rect inOutDirty){
...
mLockedObject = nativeLockCanvas(mNativeObject, mCanvas, inOutDirty);
return mCanvas;
}
这个方法会直接调用到native方法nativeLockCanvas(mNativeObject, mCanvas, inOutDirty)。
mNativeObject就是Surface创建时对应的nativeSurface指针，inOutDirty指明了lock的区域。
android_view_Surface.cpp
static jlong nativeLockCanvas(JNIEnv* env, jclass clazz,jlong nativeObject, jobject canvasObj, jobject dirtyRectObj) {
sp surface(reinterpret_cast(nativeObject)); //转换指针
...
Rect dirtyRect(Rect::EMPTY_RECT);
Rect* dirtyRectPtr = NULL;
//获取 lock 区域
if (dirtyRectObj) {
dirtyRect.left = env->GetIntField(dirtyRectObj, gRectClassInfo.left);
dirtyRect.top = env->GetIntField(dirtyRectObj, gRectClassInfo.top);
dirtyRect.right = env->GetIntField(dirtyRectObj, gRectClassInfo.right);
dirtyRect.bottom = env->GetIntField(dirtyRectObj, gRectClassInfo.bottom);
dirtyRectPtr = &dirtyRect;
}
ANativeWindow_Buffer outBuffer;
status_t err = surface->lock(&outBuffer, dirtyRectPtr);
...
sp lockedSurface(surface);
return (jlong) lockedSurface.get(); //返回surface指针
}
这个方法主要新建了一个Rect对象，确定要lock的区域的参数，然后调用surface->lock(&outBuffer, dirtyRectPtr):
Surface.cpp
status_t Surface::lock(ANativeWindow_Buffer* outBuffer, ARect* inOutDirtyBounds){
ANativeWindowBuffer* out; int fenceFd = -1;
status_t err = dequeueBuffer(&out, &fenceFd); //从 GraphicBufferProduce 中 拿出来一个 buffer
sp backBuffer(GraphicBuffer::getSelf(out)); // 构建一个 backbuffer
status_t res = backBuffer->lockAsync(...);
}
可以看出lock方法，首先通过dequeueBuffer来获取一个ANativeWindowBuffer,然后利用它构造一个GraphicBuffer,它被称为backBuffer,然后调用它的backBuffer->lockAsync(...),那么怎么获取一个ANativeWindowBuffer呢？
dequeueBuffer()
Surface.cpp
int Surface::dequeueBuffer(android_native_buffer_t** buffer, int* fenceFd) {
...
status_t result = mGraphicBufferProducer->dequeueBuffer(&buf, &fence, reqWidth, reqHeight,
reqFormat, reqUsage, &mBufferAge,
enableFrameTimestamps ? &frameTimestamps
: nullptr);
...经过一系列的操作，buffer最终会指向&buf
}
我们上一篇文章中已经介绍过了GraphicBufferProducer，它是一个Layer的graphic buffer producer，Layer在绘制时，会从GraphicBufferProducer取出一个GraphicBuffer来绘制。所以可以理解为GraphicBufferProducer存放着一个Layer待绘制的一帧。dequeueBuffer()所做的事情就是:从GraphicBufferProducer取出一个GraphicBuffer。
至于backBuffer->lockAsync(...)所做的操作就不细看了，可以猜想就是把这个GraphicBuffer锁上，保证一个GraphicBuffer绘制操作的不可重入。
综上，surface.lockCanvas()的主要逻辑可以使用下面这张图来表示:
即mSurface.lockCanvas最终是lock了一个GraphicBuffer。继续看mView.draw(canvas):
mView.draw(canvas)
View.draw(canvas)所做的事情其实就是:根据View的状态来把带绘制的数据保存到Canvas。对Canvas我们到最后再来看一下它和Surface的关系。
mSurface.unlockCanvasAndPost(canvas)
在canvas的绘制数据准备ok后，Surface就可以开始绘制了，而绘制操作的发起点就是Surface.unlockCanvasAndPost()方法,这个方法会调用到:
private void unlockSwCanvasAndPost(Canvas canvas) {
...
try {
nativeUnlockCanvasAndPost(mLockedObject, canvas); //mLockedObject其实就是native的那个surface
} finally {
nativeRelease(mLockedObject);
mLockedObject = 0;
}
}
继续看nativeUnlockCanvasAndPost()
android_view_Surface.cpp
static void nativeUnlockCanvasAndPost(JNIEnv* env, jclass clazz,jlong nativeObject, jobject canvasObj) {
sp surface(reinterpret_cast(nativeObject));
...
// detach the canvas from the surface
Canvas* nativeCanvas = GraphicsJNI::getNativeCanvas(env, canvasObj); // 把java canvas指针转化为native 指针
nativeCanvas->setBitmap(SkBitmap());
// unlock surface
status_t err = surface->unlockAndPost();
}
即调用了surface->unlockAndPost():
Surface.cpp
status_t Surface::unlockAndPost()
{
...
int fd = -1;
status_t err = mLockedBuffer->unlockAsync(&fd);
err = queueBuffer(mLockedBuffer.get(), fd);
mPostedBuffer = mLockedBuffer;
mLockedBuffer = 0;
return err;
}
这里mLockedBuffer其实就是前面的backBuffer。mLockedBuffer->unlockAsync(&fd)的操作其实很简单，就是解除GraphicBuffer的lock状态，主要看一下queueBuffer(mLockedBuffer.get(), fd)
Surface.cpp
int Surface::queueBuffer(android_native_buffer_t* buffer, int fenceFd) {
...
int i = getSlotFromBufferLocked(buffer);
...
IGraphicBufferProducer::QueueBufferOutput output;
IGraphicBufferProducer::QueueBufferInput input(timestamp, isAutoTimestamp,
static_cast(mDataSpace), crop, mScalingMode,
mTransform ^ mStickyTransform, fence, mStickyTransform,
mEnableFrameTimestamps);
...
status_t err = mGraphicBufferProducer->queueBuffer(i, input, &output);
...
mQueueBufferCondition.broadcast();
return err;
}
对于input和output这里先不做细追，我猜测input中应该包含绘制的数据。但getSlotFromBufferLocked(buffer)是干什么的呢？它其实就是获取真正的GraphicBuffer的在mSlots集合中真正的index:
Surface.cpp
int Surface::getSlotFromBufferLocked(android_native_buffer_t* buffer) const {
for (int i = 0; i < NUM_BUFFER_SLOTS; i++) {
if (mSlots[i].buffer != NULL && mSlots[i].buffer->handle == buffer->handle) {
return i;
}
}
return BAD_VALUE;
}
对于mSlots集合，可以认为他就是按顺序保存GraphicBuffer的数组即可:
Surface.h
// mSlots stores the buffers that have been allocated for each buffer slot.
// It is initialized to null pointers, and gets filled in with the result of
// IGraphicBufferProducer::requestBuffer when the client dequeues a buffer from a
// slot that has not yet been used. The buffer allocated to a slot will also
// be replaced if the requested buffer usage or geometry differs from that
// of the buffer allocated to a slot.
BufferSlot mSlots[NUM_BUFFER_SLOTS];
继续看mGraphicBufferProducer->queueBuffer(i, input, &output), 通过上一篇文章已经知道mGraphicBufferProducer是BufferQueueProducer的实例:
BufferQueueProducer.cpp
status_t BufferQueueProducer::queueBuffer(int slot, const QueueBufferInput &input, QueueBufferOutput *output) {
//从input中获取一些列参数
input.deflate(&requestedPresentTimestamp, &isAutoTimestamp, &dataSpace,
&crop, &scalingMode, &transform, &acquireFence, &stickyTransform,
&getFrameTimestamps);
sp frameAvailableListener;
sp frameReplacedListener;
BufferItem item; //可以理解为一个待渲染的帧
...下面就是对item的一系列赋值操作
item.mAcquireCalled = mSlots[slot].mAcquireCalled;
item.mGraphicBuffer = mSlots[slot].mGraphicBuffer; //根据slot获取GraphicBuffer。
item.mCrop = crop;
item.mTransform = transform &
~static_cast(NATIVE_WINDOW_TRANSFORM_INVERSE_DISPLAY);
item.mTransformToDisplayInverse =
(transform & NATIVE_WINDOW_TRANSFORM_INVERSE_DISPLAY) != 0;
item.mScalingMode = static_cast(scalingMode);
item.mTimestamp = requestedPresentTimestamp;
item.mIsAutoTimestamp = isAutoTimestamp;
...
if (frameAvailableListener != NULL) {
frameAvailableListener->onFrameAvailable(item); //item是一个frame，准备完毕，要通知外界
} else if (frameReplacedListener != NULL) {
frameReplacedListener->onFrameReplaced(item);
}
addAndGetFrameTimestamps(&newFrameEventsEntry,etFrameTimestamps ? &output->frameTimestamps : nullptr);
return NO_ERROR;
}
其实从
queueBuffer()可以看出,mGraphicBufferProducer中存放的都是可以被渲染的GraphicBuffer,这个buffer可能被渲染完毕，也可能处于待渲染状态。
queueBuffer的主要操作是根据输入参数完善一个BufferItem,然后通知外界去绘制这个BufferItem。这里这个frameAvailableListener是什么呢？有兴趣的同学可以去跟一下, 不过最终回到BufferLayer.onFrameAvailable():
BufferLayer.cpp
// ---------------------------------------------------------------------------
// Interface implementation for SurfaceFlingerConsumer::ContentsChangedListener
// ---------------------------------------------------------------------------
void BufferLayer::onFrameAvailable(const BufferItem& item) {
...
mFlinger->signalLayerUpdate();
}
这个方法直接调用了mFlinger->signalLayerUpdate(),看样是要让SurfaceFlinger来渲染了:
SurfaceFlinger.cpp
void SurfaceFlinger::signalLayerUpdate() {
mEventQueue->invalidate();
}
至于SurfaceFlinger是如何渲染的，本文就不继续追踪了。用下面这张图总结一下mSurface.unlockCanvasAndPost(canvas):
Canvas与Surface
Canvas是一个绘图的工具类，其API提供了一系列绘图指令供开发者使用。根据绘制加速模式的不同，Canvas有软件Canvas与硬件Canvas只分。Canvas的绘图指令可以分为两个部分:
- 绘制指令:这些最常用的指令由一系列名为
drawXXX的方法提供。他们用来实现实际的绘制行为，例如绘制点、线、圆以及方块等。 - 辅助指令:这些用于提供辅助功能的指令将会影响后续绘制指令的效果，如设置变换、裁剪区域等。
Canvas还提供了save与resotore用于撤销一部分辅助指令的效果。
对于软件Canvas来说，其绘制目标是一个位图Bitmap。在Surface.unlockAndPost时，这个Bitmap所描述的内容会反映到Surface的Buffer中。可以用下面这张图来表示:
最后:
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