add Surface 绘制原理

blogs/Android/Framework/Interview/UI 体系相关/Surface 的绘制原理.md

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+---
+Surface 的绘制原理
+---
+
+1. Surface 绘制的 buffer 是怎么来的？
+2. buffer 绘制完了又是怎么提交的？
+
+```java
+private void performTraversals(){
+    performMeasure();
+    performLayout();
+    performDraw();
+}
+private void draw(boolean fullRedrawNeeded){
+    Surface surface = mSurface;
+    drawSoftware(surface, mAttachInfo, ..., dirty);
+}
+private boolean drawSoftware(Surface surface, ...){
+    final Canvas canvas;
+    canvas = mSurface.lockCanvas(dirty);
+    mView.draw(canvas);
+    surface.unlockCanvasAndPost(canvas);
+}
+static jlong nativeLockCanvas(JNIEnv* env, jclass clazz, jlong nativeObject, jobject canvasObj, jobject dirtyRectObj){
+    sp<Surface> surface(reinterpert_cast<Surface *>(nativeObject));
+    ANativeWindow_Buffer outBuffer;
+    surface->lock(&outBuffer, dirtyRectPtr);
+    SkBitmap bitmap;
+    bitmap.setPixels(outBuffer.bits);
+    Canvas* nativeCanvas = GraphicsJNI::getNativeCanvas(env, canvasObj);
+    nativeCanvas->setBitmap(bitmap);
+    sp<Surface> lockedSurface(surface);
+    return (jlong)lockedSurface.get();
+}
+```
+
+每次绘制都要重新申请一块 buffer，它是怎么申请的？
+
+```c++
+status_t Surface::lock(ANativeWindow_Buffer* outBuffer, ...){
+	ANativeWindowBuffer* out;
+    dequeueBuffer(&out, &fenceFd);
+    sp<GraphicBuffer> backBuffer(GraphicBuffer::getSelf(out));
+    void* vaddr;
+    status_t res = backBuffer->lockAsync(..., &vaddr, fenceFd);
+    mLockedBuffer = backBuffer;
+    outBuffer->bits = vaddr;
+}
+int Surface::dequeueBuffer(android_native_buffer_t** buffer, ...){
+    int buf = -1;
+    status_t result = mGraphicBufferProduct->dequeueBuffer(&buf, ...);
+    sp<GraphicBuffer>& gbuf(mSlot[buf].buffer);
+    if((result&IGraphicBufferProduct::BUFFER_NEEDS_REALLOCATION)||gbuf == 0){
+        result = mGraphicBufferProduct->requestBuffer(buf, &gbuf);
+    }
+    *buffer = gbuf.get();
+    return OK;
+}
+```
+
+Buffer 是怎么提交的？
+
+```c++
+public void unlockCanvasAndPost(Canvas canvas){
+	synchronized(mLock){
+        unlockSwCanvasAndPost(canvas);
+    }
+}
+private void unlockSwCanvasAndPost(Canvas canvas){
+    nativeUnlockCanvasAndPost(mLockedObject, canvas);
+    nativeRelease(mLockedObject);
+    mLockedObject = 0;
+}
+static void nativeUnlockCanvasAndPost(JNIEnv* env, jclass clazz, jlong nativeObject, jobject canvasObj) {
+    sp<Surface> surface(reinterpret_cast<Surface *>(nativeObject));
+    Canvas* nativeCanvas = GraphicsJNI::getNativeCanvas(env, canvasObj);
+    nativeCanvas->setBitmap(SkBitmap());
+    surface->unlockAndPost();
+}
+status_t Surface::unlockAndPost(){
+    mLockedBuffer->unlockAsync(...);
+    queueBuffer(mLockedBuffer.get(), ...);
+    mPostedBuffer = mLockedBuffer;
+    mLockedBuffer = 0;
+    return err;
+}
+int Surface::queueBuffer(android_native_buffer_t* buffer, ...){
+    int i = getSlotFromBufferLocked(buffer);
+    mGraphicBufferProducer->queueBuffer(i, ...);
+}
+```
+
+![](https://i.loli.net/2020/03/26/l1QyIsgBtPSvLpF.png)