源码：8.x系统
我们知道ViewRootImpl是处理绘制流程的地方，具体分析如下：
ViewRootImpl.java
private void performTraversals() {
······
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
······
performLayout(lp, mWidth, mHeight);
······
performDraw();
······
}
测量、布局、绘制，我们重点看绘制performDraw
private void performDraw() {
······
draw(fullRedrawNeeded);
······
}
private void draw(boolean fullRedrawNeeded) {
······
if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
if (mAttachInfo.mThreadedRenderer != null && mAttachInfo.mThreadedRenderer.isEnabled()) {
······//硬件加速
} else {
·····
//drawSoftware为软件加速
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)){
······
}
}
}
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
······
canvas = mSurface.lockCanvas(dirty);
······
mView.draw(canvas);
······
surface.unlockCanvasAndPost(canvas);
······
}
终于来到了，我们需要重点分析的地方，接下来我们重点分析：
1.mSurface.lockCanvas(dirty)
2.mView.draw(canvas)是否可以用其他的替代?
3.surface.unlockCanvasAndPost(canvas)
重点分析一：mSurface.lockCanvas(dirty)
Surface.java
public Canvas lockCanvas(Rect inOutDirty)
throws Surface.OutOfResourcesException, IllegalArgumentException {
·····
mLockedObject = nativeLockCanvas(mNativeObject, mCanvas, inOutDirty);
return mCanvas;
}
首先，我们重点关注点在返回值mCanvas，而mCanvas初始化的地方为private final Canvas mCanvas = new CompatibleCanvas();
，说明这个mCanvas初始化的时候是空壳。提问：canvas和surface有什么必然的关联？
其次，我们重点看nativeLockCanvas具体做了什么？
android_view_Surface.cpp
static jlong nativeLockCanvas(JNIEnv* env, jclass clazz,
jlong nativeObject, jobject canvasObj, jobject dirtyRectObj) {
sp surface(reinterpret_cast(nativeObject));
······
//关注点1
ANativeWindow_Buffer outBuffer;
status_t err = surface->lock(&outBuffer, dirtyRectPtr);
······
//关注点2
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);
·····
}
初步分析:
关注点1：
一个buffer跟surface绑定在一起
关注点2：
bitmap和canvas绑定在一起，而bimap和buffer绑定在一起
小结论：
surface和canvas绑定在一起，用了一个bitmap，进一步说，是一个buffer。
详细分析：
关注点1：
status_t err = surface->lock(&outBuffer, dirtyRectPtr);
Surface.cpp
status_t Surface::lock(
ANativeWindow_Buffer* outBuffer, ARect* inOutDirtyBounds)
{
//2.backBuffer来自out，而out由dequeueBuffer函数赋值
ANativeWindowBuffer* out;
int fenceFd = -1;
status_t err = dequeueBuffer(&out, &fenceFd);
·····
sp backBuffer(GraphicBuffer::getSelf(out));
//1.逆向思维分析，outBuffer是被赋值的？
//是被backBuffer，backBuffer是怎么来的？
mLockedBuffer = backBuffer;
outBuffer->width = backBuffer->width;
outBuffer->height = backBuffer->height;
outBuffer->stride = backBuffer->stride;
outBuffer->format = backBuffer->format;
outBuffer->bits = vaddr;
}
int Surface::dequeueBuffer(android_native_buffer_t** buffer, int* fenceFd) {
······
//gbuf由mGraphicBufferProducer(GBP)产生
result = mGraphicBufferProducer->requestBuffer(buf, &gbuf);
······
//逆向思维分析：buffer来自gbuf
*buffer = gbuf.get();
}
关注点2：
java层的canvas是一个空壳，通过native层赋予了值，如下：
nativeCanvas->setBitmap(bitmap);
所以，canvas的关键在于对这个bitmap赋予生机。
进一步小结：
surface和canvas绑定在一起，用了一个bitmap，进一步说，是一个buffer。
这个buffer是由Surface的mGraphicBufferProducer生产。
重点分析二： mView.draw(canvas)
顾明思意，canvas会把view中元素draw到自身身上。那是否可以通过其他途径来实现此意图呢？
mCanvas.drawBitmap
这种方法，就是直接赋值一张图片，简单粗暴。
例如，现在车载产品上流行的UsbCamera，在绘制usbcamera成像时，就会采用这种方法。
具体操作可以这样做：
1.SurfaceView负责展示成像
2.我们把SurfaceView中的Surface拿出来
3.之后通过Surface中的canvas.drawBitmap进行设置成像图案
因为surface可以进行跨进程传递，所以，负责展示和图像处理，可以分为两个进程执行。
这样就减小了耦合。
重点分析三：surface.unlockCanvasAndPost(canvas)
Surface.java
public void unlockCanvasAndPost(Canvas canvas) {
if (mHwuiContext != null) {//硬件绘制
mHwuiContext.unlockAndPost(canvas);
} else {//软件绘制
unlockSwCanvasAndPost(canvas);
}
}
我们关注软件绘制
private void unlockSwCanvasAndPost(Canvas canvas) {
······
try {
nativeUnlockCanvasAndPost(mLockedObject, canvas);
} finally {
······
}
······
}
android_view_Surface.cpp
static void nativeUnlockCanvasAndPost(JNIEnv* env, jclass clazz,
jlong nativeObject, jobject canvasObj) {
······
// detach the canvas from the surface
Canvas* nativeCanvas = GraphicsJNI::getNativeCanvas(env, canvasObj);
nativeCanvas->setBitmap(SkBitmap());
// unlock surface
status_t err = surface->unlockAndPost();
if (err < 0) {
doThrowIAE(env);
}
}
这里重点处理了两块：
a.把canvas的bitmap设置为了空壳，也就是此时surface与canvas已经失去深入的关联
b.我们具体分析surface->unlockAndPost()
Surface.cpp
status_t Surface::unlockAndPost()
{
······
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;
}
这里，我们可以看到native层的Surface有两个buffer，一个是mPostedBuffer(frontBuffer)，一个是mLockedBuffer(backBuffer),这两个buffer在相互切换。
也就是说，当unlock的时候，mLockedBuffer就切换到了前台，而后台的buffer就变成了0.
int Surface::queueBuffer(android_native_buffer_t* buffer, int fenceFd) {
····
status_t err = mGraphicBufferProducer->queueBuffer(i, input, &output);
····
}
小结：
Surface的buffer绘制完后，通过mGraphicBufferProducer提交。
参考学习
