void drawBG(SkCanvas* canvas) {

    SkColor radColors[] = {0xFFFFFFFF, 0xFFFFFFFF, 0x00FFFFFF};

    auto rad = SkGradientShader::MakeRadial(SkPoint::Make(128, 128), 128, radColors, nullptr, 3,

                                            SkTileMode::kClamp);

​

    SkMatrix rotMtx;

    rotMtx.setRotate(-90, 128, 128);

    SkColor sweepColors[] = {0xFFFF00FF, 0xFFFF0000, 0xFFFFFF00, 0xFF00FF00,

                             0xFF00FFFF, 0xFF0000FF, 0xFFFF00FF};

    auto sweep = SkGradientShader::MakeSweep(128, 128, sweepColors, nullptr, 7, 0, &rotMtx);

​

    auto comp = SkShaders::Blend(SkBlendMode::kModulate, std::move(rad), std::move(sweep));

    SkPaint p;

    p.setShader(std::move(comp));

​

    canvas->drawPaint(p);

}

​

void draw(SkCanvas* canvas) {

    SkBlendMode blendModes[] = {

        SkBlendMode::kDst,

        SkBlendMode::kSrc,

        SkBlendMode::kSrcOver,

        SkBlendMode::kDstOver,

        SkBlendMode::kSrcIn,

        SkBlendMode::kDstIn,

        SkBlendMode::kSrcOut,

        SkBlendMode::kDstOut,

        SkBlendMode::kSrcATop,

        SkBlendMode::kDstATop,

        SkBlendMode::kXor,

        SkBlendMode::kPlus,

        SkBlendMode::kModulate,

        SkBlendMode::kScreen,

        SkBlendMode::kOverlay,

        SkBlendMode::kDarken,

        SkBlendMode::kLighten,

    };

​

    SkPaint labelPaint;

    labelPaint.setAntiAlias(true);

    SkFont font(fontMgr->matchFamilyStyle(nullptr, {}), 12);

​

    for (auto mode : blendModes) {

        SkPaint layerPaint;

        layerPaint.setBlendMode(mode);

​

        canvas->save();

        canvas->clipRect(SkRect::MakeWH(256, 256));

​

        drawBG(canvas);

​

        canvas->saveLayer(nullptr, &layerPaint);

        const SkScalar r = 80;

        SkPaint discP;

        discP.setAntiAlias(true);

        discP.setBlendMode(SkBlendMode::kPlus);

        discP.setColor(SK_ColorGREEN); canvas->drawCircle(128, r, r, discP);

        discP.setColor(SK_ColorRED);   canvas->drawCircle(r, 256 - r, r, discP);

        discP.setColor(SK_ColorBLUE);  canvas->drawCircle(256 - r, 256 - r, r, discP);

        canvas->restore();

​

        canvas->drawSimpleText(SkBlendMode_Name(mode), strlen(SkBlendMode_Name(mode)),

                               SkTextEncoding::kUTF8, 10, 10, font, labelPaint);

        canvas->restore();

        canvas->translate(0, 256);

    }

}