Android绘制流程6 - Android硬件加速与软件绘制区别
基于android-8.1.0_r60
软件绘制
ViewRootImpl初始化会创建一个Surface.java文件，用于提供整个绘制过程中的画布。
其内部Canvas对象的初始化过程如下：
// Surface.java
mCanvas = new CompatibleCanvas();
---------------------
// Canvas.java
public Canvas() {
if (!isHardwareAccelerated()) {
mNativeCanvasWrapper = nInitRaster(null);
}
}
---------------------
// frameworks/base/core/android_graphics_Canvas.cpp
initRaster(jobject jbit) {
SkBitmap b;
GraphicsJNI::getSkBitmap(env, jbit, &b);
return reinterpret_cast(Canvas::create_canvas(b));
}
---------------------
// frameworks/base/libs/hwui/SkiaCanvas.cpp
Canvas* Canvas::create_canvas(const SkBitmap& b) {
return new SkiaCanvas(b);
}
而在绘制获取Canvas对象前会调用Surface的lockCanvas函数，这是一步与开启硬件加速区别的关键一步：
// frameworks/base/core/jni/android_view_Surface.cpp
static jlong nativeLockCanvas(JNIEnv* env, jclass clazz,
jlong nativeObject, jobject canvasObj, jobject dirtyRectObj) {
// ...
SkBitmap bitmap;
ssize_t bpr = outBuffer.stride * bytesPerPixel(outBuffer.format);
bitmap.setInfo(info, bpr);
// ...
Canvas* nativeCanvas = GraphicsJNI::getNativeCanvas(env, canvasObj);
nativeCanvas->setBitmap(bitmap);
// ...
sp<Surface> lockedSurface(surface);
lockedSurface->incStrong(&sRefBaseOwner);
return (jlong) lockedSurface.get();
}
可以看到每一帧的绘制都对应着新SkBitmap的创建。而其中nativeCanvas就是上面的SkiaCanvas，而Canvas->setBitmap用于更新SkiaCanvas内部的SkCanvas实例(mCanvas)。如下：
// frameworks/base/libs/hwui/SkiaCanvas.cpp
void SkiaCanvas::setBitmap(const SkBitmap& bitmap) {
sk_sp<SkColorSpace> cs = bitmap.refColorSpace();
std::unique_ptr<SkCanvas> newCanvas =
std::unique_ptr<SkCanvas>(new SkCanvas(bitmap, SkCanvas::ColorBehavior::kLegacy));
std::unique_ptr<SkCanvas> newCanvasWrapper = SkCreateColorSpaceXformCanvas(newCanvas.get(),
cs == nullptr ? SkColorSpace::MakeSRGB() : std::move(cs));
// deletes the previously owned canvas (if any)
mCanvasOwned = std::move(newCanvas);
mCanvasWrapper = std::move(newCanvasWrapper);
mCanvas = mCanvasWrapper.get();
// clean up the old save stack
mSaveStack.reset(nullptr);
}
这里会根据SkBitmap生成行的mCanvas，用于实现SkiaCanvas对外暴露的Canvas接口。
硬件绘制(开启硬件加速)
其中Canvas是在更新RootDisplayList时创建的，而其最终实例为DisplayListCanvas。下面看看DisplayListCanvas初始化时与软件绘制时的区别。
// DisplayListCanvas.java
private DisplayListCanvas(RenderNode node, int w, int h) {
super(nCreateDisplayListCanvas(node.mNativeRenderNode, w, h));
}
---------------------
// android_view_DisplayListCanvas.cpp
android_view_DisplayListCanvas_createDisplayListCanvas(jlong n,
jint w, jint h) {
RenderNode* rn = reinterpret_cast<RenderNode*>(n);
return reinterpret_cast(Canvas::create_recording_canvas(w, h, rn));
}
---------------------
// base/libs/hwui/hwui/Canvas.cpp
Canvas::create_recording_canvas(int w, int h, RenderNode* n) {
if (Properties::isSkiaEnabled()) {
return new skiapipeline::SkiaRecordingCanvas(n, w, h);
}
return new RecordingCanvas(w, h);
}
---------------------
// base/libs/hwui/pipeline/skia/SkiaRecordingCanvas.h
SkiaRecordingCanvas(RenderNode* n, int w, int h) {
initDisplayList(n, w, h);
}
---------------------
// base/libs/hwui/pipeline/skia/SkiaRecordingCanvas.cpp
void initDisplayList(RenderNode* n, int w, int h) {
mDisplayList->attachRecorder(&mRecorder);
SkiaCanvas::reset(&mRecorder);
}
---------------------
// frameworks/base/libs/hwui/SkiaCanvas.cpp
void SkiaCanvas::reset(SkCanvas* skiaCanvas) {
mCanvas = skiaCanvas;
}
其中mRecorder是一个SkLiteRecorder实例，主要用于记录View对于Canvas操作的OP。如下：
// http://androidxref.com/8.1.0_r33/xref/external/skia/src/core/SkLiteRecorder.h
/**
* A SkiaCanvas implementation that records drawing operations for deferred rendering backed by a
* SkLiteRecorder and a SkiaDisplayList.
*/
class SkLiteRecorder final : public SkNoDrawCanvas {
public:
SkLiteRecorder();
}
---------
// http://androidxref.com/8.1.0_r33/xref/external/skia/src/core/SkLiteRecorder.cpp
SkLiteRecorder::SkLiteRecorder()
: INHERITED(1, 1)
, fDL(nullptr) {}
void SkLiteRecorder::reset(SkLiteDL* dl, const SkIRect& bounds) {
this->resetCanvas(bounds.right(), bounds.bottom());
fDL = dl;
}
void SkLiteRecorder::onDrawPaint(const SkPaint& paint) {
fDL->drawPaint(paint);
}
void SkLiteRecorder::onDrawPath(const SkPath& path, const SkPaint& paint) {
fDL->drawPath(path, paint);
}
---------
// http://androidxref.com/8.1.0_r33/xref/external/skia/src/core/SkLiteDL.cpp
void SkLiteDL::drawPaint(const SkPaint& paint) {
this->push<DrawPaint>(0, paint);
}
void SkLiteDL::drawPath(const SkPath& path, const SkPaint& paint) {
this->push<DrawPath>(0, path, paint);
}
通过SkLiteRecorder头文件的注释既可知：准备render的操作都会被记录在SkLiteRecorder或者SkiaDisplayList中。最终到OpenGLPipeLine进行绘制的时候才交给GPU进行绘制。
区别
- 软件绘制：view绘制时，每一步都直接操作底层的SkCanvas。
- 硬件绘制：view绘制时，每一步都被记录在底层的recorder或者displaylist。直到最后一起交给GPU进行绘制。