首先先做一些预定义工作：
namespace {#define WGL_CONTEXT_MAJOR_VERSION_ARB 0x2091#define WGL_CONTEXT_MINOR_VERSION_ARB 0x2092#define WGL_CONTEXT_PROFILE_MASK_ARB 0x9126#define WGL_CONTEXT_CORE_PROFILE_BIT_ARB 0x00000001 typedef HGLRC(WINAPI* PFNWGLCREATECONTEXTATTRIBSARBPROC)(HDC hDC, HGLRC hShareContext, const int* attribList);PFNWGLCREATECONTEXTATTRIBSARBPROC wglCreateContextAttribsARB = nullptr;extern "C" {__declspec(dllexport) int NvOptimusEnablement = 1;__declspec(dllexport) int AmdPowerXpressRequestHighPerformance = 1;}
}上述代码最后两句 dllexport 目的是在Windows系统上强制你的OpenGL程序使用高性能独立显卡（NVIDIA或AMD），而不是默认的集成显卡。这很重要，关系到多屏幕用户环境的一个重要BUG。
再定义几个类变量：
HGLRC hglrc;
sk_sp<GrDirectContext> grContext;
sk_sp<const GrGLInterface> backendContext;
SkSurfaceProps surfaceProps(SkSurfaceProps::kUseDeviceIndependentFonts_Flag, kRGB_H_SkPixelGeometry);
GrGLFramebufferInfo fbInfo;现在初始化hglrc、grContext、backendContext和fbInfo
void OpenGL::init()
{HDC hdc = GetDC(win->hwnd);PIXELFORMATDESCRIPTOR pfd = {};pfd.nSize = sizeof(pfd);pfd.nVersion = 1;pfd.dwFlags = PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;pfd.iPixelType = PFD_TYPE_RGBA;pfd.cColorBits = 32;pfd.cDepthBits = 24;pfd.cAlphaBits = 8;pfd.iLayerType = PFD_MAIN_PLANE;int pixelFormat = ChoosePixelFormat(hdc, &pfd);auto flag = SetPixelFormat(hdc, pixelFormat, &pfd);HGLRC oldContext = wglCreateContext(hdc);wglMakeCurrent(hdc, oldContext);wglCreateContextAttribsARB = (PFNWGLCREATECONTEXTATTRIBSARBPROC)wglGetProcAddress("wglCreateContextAttribsARB");int attribs[] = {WGL_CONTEXT_MAJOR_VERSION_ARB, 3,WGL_CONTEXT_MINOR_VERSION_ARB, 3,WGL_CONTEXT_PROFILE_MASK_ARB,WGL_CONTEXT_CORE_PROFILE_BIT_ARB,0};HGLRC hglrc;HGLRC newContext = wglCreateContextAttribsARB(hdc, 0, attribs);if (newContext) {wglMakeCurrent(nullptr, nullptr);wglDeleteContext(oldContext);hglrc = newContext;}else {hglrc = oldContext;}wglMakeCurrent(hdc, hglrc);ReleaseDC(win->hwnd, hdc);backendContext = GrGLMakeNativeInterface();GrGLint buffer;backendContext.get()->fFunctions.fGetIntegerv(GR_GL_FRAMEBUFFER_BINDING, &buffer);fbInfo.fFBOID = buffer;fbInfo.fFormat = GR_GL_RGBA8;fbInfo.fProtected = skgpu::Protected(false);grContext = GrDirectContexts::MakeGL(backendContext);
}获取SkSurface的代码如下所示：
sk_sp<SkSurface> OpenGL::getSurface()
{if (nullptr == surface) {auto backendRT = GrBackendRenderTargets::MakeGL(win->w, win->h,16, //sampleCount8, //stencilBitsfbInfo);surface = SkSurfaces::WrapBackendRenderTarget(grContext.get(),backendRT,kBottomLeft_GrSurfaceOrigin,kRGBA_8888_SkColorType,nullptr,&surfaceProps);}return surface;
}这段代码中：sampleCount 表示多重采样抗锯齿的采样数。stencilBits 表示模板缓冲区的位深度。
窗口对应的SkSurface对象是缓存，窗口大小不改变SkSurface对象也不会改变。
当窗口大小改变时：
void OpenGL::resize()
{surface.reset(nullptr);if (grContext) {grContext->flushAndSubmit();}
}窗口大小改变时，我们释放了SkSurface对象，迫使程序去重新获取SkSurface。
值得注意的是，这段代码与Skia的示例Demo不一样的，Skia的示例Demo有内存泄漏的问题。
在处理WM_PAINT消息时，你可以通过如下代码把SkSurface中的内容同步到窗口中：
case WM_PAINT: {PAINTSTRUCT ps;HDC hdc = BeginPaint(hwnd, &ps);onPaint(backend->getCanvas()); //你自己的绘图逻辑grContext->flushAndSubmit(surface.get());SwapBuffers(dc);ReleaseDC(hwnd, hdc);EndPaint(hwnd, &ps);return 0;
}当窗口销毁时，我们也要释放相关的资源和对象，如下代码所示：
OpenGL::~OpenGL()
{surface.reset(nullptr);wglMakeCurrent(nullptr, nullptr);wglDeleteContext(hglrc);grContext.reset(nullptr);backendContext.reset(nullptr);hglrc = NULL;
}如下是我使用上述代码创建的一个半透明窗口，如你所见，资源消耗并不高
