Skia深入分析5——skia文字绘制的实现

文字绘制主要包括编码转换（主要是中文）、字形解析（点线或image）和实际渲染三个步骤。在这个过程中，字形解析和实际渲染均是耗时步骤。Skia对文字解析的结果做了一套缓存机制。在中文字较多，使用多种字体，绘制的样式（粗/斜体）有变化时，这个缓存会变得很大，因此Skia文字缓存做了内存上的限制。

1、SkPaint

文字绘制与SkPaint的属性相关很大，先回头看下SkPaint相关的属性

class SkPaint
{
private
    SkTypeface*     fTypeface;//字体
    SkPathEffect*   fPathEffect;//路径绘制效果
    SkShader*       fShader;//取色器
    SkXfermode*     fXfermode;//混合模式，类似OpenGL里面的Blend设置
    SkColorFilter*  fColorFilter;//图像绘制时，自定义图像采样函数时使用
    SkMaskFilter*   fMaskFilter;//路径绘制时，按有无像素做进一步自定义改进处理时使用
    SkRasterizer*   fRasterizer;//路径绘制时自定义生成像素点的算法时使用
    SkDrawLooper*   fLooper;//循环绘制，SkCanvas里面的第二重循环，一般不用关注
    SkImageFilter*  fImageFilter;//SkCanvas的第一重循环，绘制后做后处理用，一般不用关注
    SkAnnotation*   fAnnotation;//暂时没用到的属性

    SkScalar        fTextSize;//文字大小

    SkScalar        fTextScaleX;//文字水平方向上的拉伸，仅用于PDF绘制
    SkScalar        fTextSkewX;//文字横向扭曲度，仅用于PDF绘制

    SkColor         fColor;//纯色，在fShader为空时使用
    SkScalar        fWidth;//带边界时（kStroke_Style/kStrokeAndFill_Style）生效，边界的宽度 
    SkScalar        fMiterLimit;//drawPath时，连接各个path片断时，要求的圆滑连接阈值，Join 类型为默认的kMiter_Join时无效
    /*一组不超过32位的属性*/
    union {
        struct {
            // all of these bitfields should add up to 32
            unsigned        fFlags : 16;//包含所有的0/1二值属性:
            /*
               kAntiAlias_Flag       = 0x01,//是否抗锯齿
               kDither_Flag          = 0x04,//是否做抖动处理
               kUnderlineText_Flag   = 0x08,//是否绘制文字下划线
               kStrikeThruText_Flag  = 0x10,//目前未看到其作用
               kFakeBoldText_Flag    = 0x20,
               kLinearText_Flag      = 0x40,
               kSubpixelText_Flag    = 0x80,//文字像素精确采样
               kDevKernText_Flag     = 0x100
               kLCDRenderText_Flag   = 0x200
               kEmbeddedBitmapText_Flag = 0x400,
               kAutoHinting_Flag     = 0x800,
               kVerticalText_Flag    = 0x1000,//是否竖向绘制文字
               kGenA8FromLCD_Flag    = 0x2000,
               kDistanceFieldTextTEMP_Flag = 0x4000,
               kAllFlags = 0xFFFF
             */

            unsigned        fTextAlign : 2;//文字对齐方式，取值如下：
            /*
            enum Align {
                kLeft_Align,//左对齐
                kCenter_Align,//居中
                kRight_Align,//右对齐
            };
            */

            unsigned        fCapType : 2;//边界连接类型，分无连接，圆角连接，半方形连接
            unsigned        fJoinType : 2;//Path片断连接类型
            
            unsigned        fStyle : 2;//绘制模式，填充边界/区域
            /*
               enum Style {
               kFill_Style, //填充区域
               kStroke_Style,//绘制边界
               kStrokeAndFill_Style,//填充区域并绘制边界
               };
             */

            unsigned        fTextEncoding : 2;//文字编码格式，支持如下几种
            enum TextEncoding {
                kUTF8_TextEncoding,//utf-8，默认格式
                kUTF16_TextEncoding,
                kUTF32_TextEncoding,
                kGlyphID_TextEncoding
            };

            unsigned        fHinting : 2;
            unsigned        fFilterLevel : 2;//在图像绘制时提到的采样质量要求
            //unsigned      fFreeBits : 2;
        };
        uint32_t fBitfields;
    };
    uint32_t fDirtyBits;//记录哪些属性被改变了，以便更新相关的缓存
};

2、字体绘制基本流程

SkDraw

两种绘制方式：

（1）将文字解析为路径，然后绘制路径，缓存路径（drawText_asPaths）。

void SkDraw::drawText_asPaths(const char text[], size_t byteLength,
                              SkScalar x, SkScalar y,
                              const SkPaint& paint) const {
    SkDEBUGCODE(this->validate();)

    SkTextToPathIter iter(text, byteLength, paint, true);

    SkMatrix    matrix;
    matrix.setScale(iter.getPathScale(), iter.getPathScale());
    matrix.postTranslate(x, y);

    const SkPath* iterPath;
    SkScalar xpos, prevXPos = 0;

    while (iter.next(&iterPath, &xpos)) {
        matrix.postTranslate(xpos - prevXPos, 0);
        if (iterPath) {
            const SkPaint& pnt = iter.getPaint();
            if (fDevice) {
                fDevice->drawPath(*this, *iterPath, pnt, &matrix, false);
            } else {
                this->drawPath(*iterPath, pnt, &matrix, false);
            }
        }
        prevXPos = xpos;
    }
}

（2）将文字解析为Mask（32*32的A8图片），然后绘制模板，缓存模板。

SkDrawCacheProc glyphCacheProc = paint.getDrawCacheProc();

    SkAutoGlyphCache    autoCache(paint, &fDevice->fLeakyProperties, fMatrix);
    SkGlyphCache*       cache = autoCache.getCache();

    // transform our starting point
    {
        SkPoint loc;
        fMatrix->mapXY(x, y, &loc);
        x = loc.fX;
        y = loc.fY;
    }

    // need to measure first
    if (paint.getTextAlign() != SkPaint::kLeft_Align) {
        SkVector    stop;

        measure_text(cache, glyphCacheProc, text, byteLength, &stop);

        SkScalar    stopX = stop.fX;
        SkScalar    stopY = stop.fY;

        if (paint.getTextAlign() == SkPaint::kCenter_Align) {
            stopX = SkScalarHalf(stopX);
            stopY = SkScalarHalf(stopY);
        }
        x -= stopX;
        y -= stopY;
    }

    const char* stop = text + byteLength;

    SkAAClipBlitter     aaBlitter;
    SkAutoBlitterChoose blitterChooser;
    SkBlitter*          blitter = NULL;
    if (needsRasterTextBlit(*this)) {
        blitterChooser.choose(*fBitmap, *fMatrix, paint);
        blitter = blitterChooser.get();
        if (fRC->isAA()) {
            aaBlitter.init(blitter, &fRC->aaRgn());
            blitter = &aaBlitter;
        }
    }

    SkAutoKern          autokern;
    SkDraw1Glyph        d1g;
    SkDraw1Glyph::Proc  proc = d1g.init(this, blitter, cache, paint);

    SkFixed fxMask = ~0;
    SkFixed fyMask = ~0;
    if (cache->isSubpixel()) {
        SkAxisAlignment baseline = SkComputeAxisAlignmentForHText(*fMatrix);
        if (kX_SkAxisAlignment == baseline) {
            fyMask = 0;
            d1g.fHalfSampleY = SK_FixedHalf;
        } else if (kY_SkAxisAlignment == baseline) {
            fxMask = 0;
            d1g.fHalfSampleX = SK_FixedHalf;
        }
    }

    SkFixed fx = SkScalarToFixed(x) + d1g.fHalfSampleX;
    SkFixed fy = SkScalarToFixed(y) + d1g.fHalfSampleY;

    while (text < stop) {
        const SkGlyph& glyph = glyphCacheProc(cache, &text, fx & fxMask, fy & fyMask);

        fx += autokern.adjust(glyph);

        if (glyph.fWidth) {
            proc(d1g, fx, fy, glyph);
        }

        fx += glyph.fAdvanceX;
        fy += glyph.fAdvanceY;
    }

cacheProc是翻译字符编码的函数，由SkPaint::getDrawCacheProc产生：

SkDrawCacheProc SkPaint::getDrawCacheProc() const {
    static const SkDrawCacheProc gDrawCacheProcs[] = {
        sk_getMetrics_utf8_00,
        sk_getMetrics_utf16_00,
        sk_getMetrics_utf32_00,
        sk_getMetrics_glyph_00,

        sk_getMetrics_utf8_xy,
        sk_getMetrics_utf16_xy,
        sk_getMetrics_utf32_xy,
        sk_getMetrics_glyph_xy
    };

    unsigned index = this->getTextEncoding();
    if (fFlags & kSubpixelText_Flag) {
        index += 4;
    }

    SkASSERT(index < SK_ARRAY_COUNT(gDrawCacheProcs));
    return gDrawCacheProcs[index];
}

SkGlyphCache：

字形解析的结果缓存。

SkScalerContext：

负责字形的解析，有多种实现。Android中是用FreeType：SkScalerContext_FreeType。主要是generateImage和generatePath两个方法：

generateImage：

void SkScalerContext_FreeType::generateImage(const SkGlyph& glyph) {
    SkAutoMutexAcquire  ac(gFTMutex);

    FT_Error    err;

    if (this->setupSize()) {
        goto ERROR;
    }

    err = FT_Load_Glyph( fFace, glyph.getGlyphID(fBaseGlyphCount), fLoadGlyphFlags);
    if (err != 0) {
        SkDEBUGF(("SkScalerContext_FreeType::generateImage: FT_Load_Glyph(glyph:%d width:%d height:%d rb:%d flags:%d) returned 0x%x\n",
                    glyph.getGlyphID(fBaseGlyphCount), glyph.fWidth, glyph.fHeight, glyph.rowBytes(), fLoadGlyphFlags, err));
    ERROR:
        memset(glyph.fImage, 0, glyph.rowBytes() * glyph.fHeight);
        return;
    }

    emboldenIfNeeded(fFace, fFace->glyph);
    generateGlyphImage(fFace, glyph);
}
void SkScalerContext_FreeType_Base::generateGlyphImage(FT_Face face, const SkGlyph& glyph) {
    const bool doBGR = SkToBool(fRec.fFlags & SkScalerContext::kLCD_BGROrder_Flag);
    const bool doVert = SkToBool(fRec.fFlags & SkScalerContext::kLCD_Vertical_Flag);

    switch ( face->glyph->format ) {
        case FT_GLYPH_FORMAT_OUTLINE: {
            FT_Outline* outline = &face->glyph->outline;
            FT_BBox     bbox;
            FT_Bitmap   target;

            int dx = 0, dy = 0;
            if (fRec.fFlags & SkScalerContext::kSubpixelPositioning_Flag) {
                dx = SkFixedToFDot6(glyph.getSubXFixed());
                dy = SkFixedToFDot6(glyph.getSubYFixed());
                // negate dy since freetype-y-goes-up and skia-y-goes-down
                dy = -dy;
            }
            FT_Outline_Get_CBox(outline, &bbox);
            /*
                what we really want to do for subpixel is
                    offset(dx, dy)
                    compute_bounds
                    offset(bbox & !63)
                but that is two calls to offset, so we do the following, which
                achieves the same thing with only one offset call.
            */
            FT_Outline_Translate(outline, dx - ((bbox.xMin + dx) & ~63),
                                          dy - ((bbox.yMin + dy) & ~63));

            if (SkMask::kLCD16_Format == glyph.fMaskFormat) {
                FT_Render_Glyph(face->glyph, doVert ? FT_RENDER_MODE_LCD_V : FT_RENDER_MODE_LCD);
                SkMask mask;
                glyph.toMask(&mask);
                if (fPreBlend.isApplicable()) {
                    copyFT2LCD16<true>(face->glyph->bitmap, mask, doBGR,
                                       fPreBlend.fR, fPreBlend.fG, fPreBlend.fB);
                } else {
                    copyFT2LCD16<false>(face->glyph->bitmap, mask, doBGR,
                                        fPreBlend.fR, fPreBlend.fG, fPreBlend.fB);
                }
            } else {
                target.width = glyph.fWidth;
                target.rows = glyph.fHeight;
                target.pitch = glyph.rowBytes();
                target.buffer = reinterpret_cast<uint8_t*>(glyph.fImage);
                target.pixel_mode = compute_pixel_mode( (SkMask::Format)fRec.fMaskFormat);
                target.num_grays = 256;

                memset(glyph.fImage, 0, glyph.rowBytes() * glyph.fHeight);
                FT_Outline_Get_Bitmap(face->glyph->library, outline, &target);
            }
        } break;

        case FT_GLYPH_FORMAT_BITMAP: {
            FT_Pixel_Mode pixel_mode = static_cast<FT_Pixel_Mode>(face->glyph->bitmap.pixel_mode);
            SkMask::Format maskFormat = static_cast<SkMask::Format>(glyph.fMaskFormat);

            // Assume that the other formats do not exist.
            SkASSERT(FT_PIXEL_MODE_MONO == pixel_mode ||
                     FT_PIXEL_MODE_GRAY == pixel_mode ||
                     FT_PIXEL_MODE_BGRA == pixel_mode);

            // These are the only formats this ScalerContext should request.
            SkASSERT(SkMask::kBW_Format == maskFormat ||
                     SkMask::kA8_Format == maskFormat ||
                     SkMask::kARGB32_Format == maskFormat ||
                     SkMask::kLCD16_Format == maskFormat);

            if (fRec.fFlags & SkScalerContext::kEmbolden_Flag &&
                !(face->style_flags & FT_STYLE_FLAG_BOLD))
            {
                FT_GlyphSlot_Own_Bitmap(face->glyph);
                FT_Bitmap_Embolden(face->glyph->library, &face->glyph->bitmap,
                                   kBitmapEmboldenStrength, 0);
            }

            // If no scaling needed, directly copy glyph bitmap.
            if (glyph.fWidth == face->glyph->bitmap.width &&
                glyph.fHeight == face->glyph->bitmap.rows &&
                glyph.fTop == -face->glyph->bitmap_top &&
                glyph.fLeft == face->glyph->bitmap_left)
            {
                SkMask dstMask;
                glyph.toMask(&dstMask);
                copyFTBitmap(face->glyph->bitmap, dstMask);
                break;
            }

            // Otherwise, scale the bitmap.

            // Copy the FT_Bitmap into an SkBitmap (either A8 or ARGB)
            SkBitmap unscaledBitmap;
            unscaledBitmap.allocPixels(SkImageInfo::Make(face->glyph->bitmap.width,
                                                         face->glyph->bitmap.rows,
                                                         SkColorType_for_FTPixelMode(pixel_mode),
                                                         kPremul_SkAlphaType));

            SkMask unscaledBitmapAlias;
            unscaledBitmapAlias.fImage = reinterpret_cast<uint8_t*>(unscaledBitmap.getPixels());
            unscaledBitmapAlias.fBounds.set(0, 0, unscaledBitmap.width(), unscaledBitmap.height());
            unscaledBitmapAlias.fRowBytes = unscaledBitmap.rowBytes();
            unscaledBitmapAlias.fFormat = SkMaskFormat_for_SkColorType(unscaledBitmap.colorType());
            copyFTBitmap(face->glyph->bitmap, unscaledBitmapAlias);

            // Wrap the glyph's mask in a bitmap, unless the glyph's mask is BW or LCD.
            // BW requires an A8 target for resizing, which can then be down sampled.
            // LCD should use a 4x A8 target, which will then be down sampled.
            // For simplicity, LCD uses A8 and is replicated.
            int bitmapRowBytes = 0;
            if (SkMask::kBW_Format != maskFormat && SkMask::kLCD16_Format != maskFormat) {
                bitmapRowBytes = glyph.rowBytes();
            }
            SkBitmap dstBitmap;
            dstBitmap.setInfo(SkImageInfo::Make(glyph.fWidth, glyph.fHeight,
                                                SkColorType_for_SkMaskFormat(maskFormat),
                                                kPremul_SkAlphaType),
                              bitmapRowBytes);
            if (SkMask::kBW_Format == maskFormat || SkMask::kLCD16_Format == maskFormat) {
                dstBitmap.allocPixels();
            } else {
                dstBitmap.setPixels(glyph.fImage);
            }

            // Scale unscaledBitmap into dstBitmap.
            SkCanvas canvas(dstBitmap);
            canvas.clear(SK_ColorTRANSPARENT);
            canvas.scale(SkIntToScalar(glyph.fWidth) / SkIntToScalar(face->glyph->bitmap.width),
                         SkIntToScalar(glyph.fHeight) / SkIntToScalar(face->glyph->bitmap.rows));
            SkPaint paint;
            paint.setFilterLevel(SkPaint::kMedium_FilterLevel);
            canvas.drawBitmap(unscaledBitmap, 0, 0, &paint);

            // If the destination is BW or LCD, convert from A8.
            if (SkMask::kBW_Format == maskFormat) {
                // Copy the A8 dstBitmap into the A1 glyph.fImage.
                SkMask dstMask;
                glyph.toMask(&dstMask);
                packA8ToA1(dstMask, dstBitmap.getAddr8(0, 0), dstBitmap.rowBytes());
            } else if (SkMask::kLCD16_Format == maskFormat) {
                // Copy the A8 dstBitmap into the LCD16 glyph.fImage.
                uint8_t* src = dstBitmap.getAddr8(0, 0);
                uint16_t* dst = reinterpret_cast<uint16_t*>(glyph.fImage);
                for (int y = dstBitmap.height(); y --> 0;) {
                    for (int x = 0; x < dstBitmap.width(); ++x) {
                        dst[x] = grayToRGB16(src[x]);
                    }
                    dst = (uint16_t*)((char*)dst + glyph.rowBytes());
                    src += dstBitmap.rowBytes();
                }
            }

        } break;

        default:
            SkDEBUGFAIL("unknown glyph format");
            memset(glyph.fImage, 0, glyph.rowBytes() * glyph.fHeight);
            return;
    }

// We used to always do this pre-USE_COLOR_LUMINANCE, but with colorlum,
// it is optional
#if defined(SK_GAMMA_APPLY_TO_A8)
    if (SkMask::kA8_Format == glyph.fMaskFormat && fPreBlend.isApplicable()) {
        uint8_t* SK_RESTRICT dst = (uint8_t*)glyph.fImage;
        unsigned rowBytes = glyph.rowBytes();

        for (int y = glyph.fHeight - 1; y >= 0; --y) {
            for (int x = glyph.fWidth - 1; x >= 0; --x) {
                dst[x] = fPreBlend.fG[dst[x]];
            }
            dst += rowBytes;
        }
    }
#endif
}

void SkScalerContext_FreeType::generatePath(const SkGlyph& glyph,
                                            SkPath* path) {
    SkAutoMutexAcquire  ac(gFTMutex);

    SkASSERT(&glyph && path);

    if (this->setupSize()) {
        path->reset();
        return;
    }

    uint32_t flags = fLoadGlyphFlags;
    flags |= FT_LOAD_NO_BITMAP; // ignore embedded bitmaps so we're sure to get the outline
    flags &= ~FT_LOAD_RENDER;   // don't scan convert (we just want the outline)

    FT_Error err = FT_Load_Glyph( fFace, glyph.getGlyphID(fBaseGlyphCount), flags);

    if (err != 0) {
        SkDEBUGF(("SkScalerContext_FreeType::generatePath: FT_Load_Glyph(glyph:%d flags:%d) returned 0x%x\n",
                    glyph.getGlyphID(fBaseGlyphCount), flags, err));
        path->reset();
        return;
    }
    emboldenIfNeeded(fFace, fFace->glyph);

    generateGlyphPath(fFace, path);

    // The path's origin from FreeType is always the horizontal layout origin.
    // Offset the path so that it is relative to the vertical origin if needed.
    if (fRec.fFlags & SkScalerContext::kVertical_Flag) {
        FT_Vector vector;
        vector.x = fFace->glyph->metrics.vertBearingX - fFace->glyph->metrics.horiBearingX;
        vector.y = -fFace->glyph->metrics.vertBearingY - fFace->glyph->metrics.horiBearingY;
        FT_Vector_Transform(&vector, &fMatrix22);
        path->offset(SkFDot6ToScalar(vector.x), -SkFDot6ToScalar(vector.y));
    }
}

SkTypeface* SkTypeface::CreateFromStream(SkStream* stream) {
    return SkFontHost::CreateTypefaceFromStream(stream);
}

bool find_name_and_attributes(SkStream* stream, SkString* name,
                              SkTypeface::Style* style, bool* isFixedPitch) {
    FT_Library  library;
    if (FT_Init_FreeType(&library)) {
        return false;
    }

    FT_Open_Args    args;
    memset(&args, 0, sizeof(args));

    const void* memoryBase = stream->getMemoryBase();
    FT_StreamRec    streamRec;

    if (NULL != memoryBase) {
        args.flags = FT_OPEN_MEMORY;
        args.memory_base = (const FT_Byte*)memoryBase;
        args.memory_size = stream->getLength();
    } else {
        memset(&streamRec, 0, sizeof(streamRec));
        streamRec.size = stream->getLength();
        streamRec.descriptor.pointer = stream;
        streamRec.read  = sk_stream_read;
        streamRec.close = sk_stream_close;

        args.flags = FT_OPEN_STREAM;
        args.stream = &streamRec;
    }

    FT_Face face;
    if (FT_Open_Face(library, &args, 0, &face)) {
        FT_Done_FreeType(library);
        return false;
    }

    int tempStyle = SkTypeface::kNormal;
    if (face->style_flags & FT_STYLE_FLAG_BOLD) {
        tempStyle |= SkTypeface::kBold;
    }
    if (face->style_flags & FT_STYLE_FLAG_ITALIC) {
        tempStyle |= SkTypeface::kItalic;
    }

    if (name) {
        name->set(face->family_name);
    }
    if (style) {
        *style = (SkTypeface::Style) tempStyle;
    }
    if (isFixedPitch) {
        *isFixedPitch = FT_IS_FIXED_WIDTH(face);
    }

    FT_Done_Face(face);
    FT_Done_FreeType(library);
    return true;
}

对于Android，在系统初始化时，所有字体文件在预加载时即被解析，包装为SkFaceRec，存为一个全局链表。(frameworks/base/graphic 和 frameworks/base/core/jni目录下面的代码)

public class Typeface {
    /*
       .......
    */
    private static void init() {
        // Load font config and initialize Minikin state
        File systemFontConfigLocation = getSystemFontConfigLocation();
        File configFilename = new File(systemFontConfigLocation, FONTS_CONFIG);
        try {
            FileInputStream fontsIn = new FileInputStream(configFilename);
            FontListParser.Config fontConfig = FontListParser.parse(fontsIn);

            List<FontFamily> familyList = new ArrayList<FontFamily>();
            // Note that the default typeface is always present in the fallback list;
            // this is an enhancement from pre-Minikin behavior.
            for (int i = 0; i < fontConfig.families.size(); i++) {
                Family f = fontConfig.families.get(i);
                if (i == 0 || f.name == null) {
                    familyList.add(makeFamilyFromParsed(f));
                }
            }
            sFallbackFonts = familyList.toArray(new FontFamily[familyList.size()]);
            setDefault(Typeface.createFromFamilies(sFallbackFonts));

            Map<String, Typeface> systemFonts = new HashMap<String, Typeface>();
            for (int i = 0; i < fontConfig.families.size(); i++) {
                Typeface typeface;
                Family f = fontConfig.families.get(i);
                if (f.name != null) {
                    if (i == 0) {
                        // The first entry is the default typeface; no sense in
                        // duplicating the corresponding FontFamily.
                        typeface = sDefaultTypeface;
                    } else {
                        FontFamily fontFamily = makeFamilyFromParsed(f);
                        FontFamily[] families = { fontFamily };
                        typeface = Typeface.createFromFamiliesWithDefault(families);
                    }
                    systemFonts.put(f.name, typeface);
                }
            }
            for (FontListParser.Alias alias : fontConfig.aliases) {
                Typeface base = systemFonts.get(alias.toName);
                Typeface newFace = base;
                int weight = alias.weight;
                if (weight != 400) {
                    newFace = new Typeface(nativeCreateWeightAlias(base.native_instance, weight));
                }
                systemFonts.put(alias.name, newFace);
            }
            sSystemFontMap = systemFonts;

        } catch (RuntimeException e) {
            Log.w(TAG, "Didn't create default family (most likely, non-Minikin build)", e);
            // TODO: normal in non-Minikin case, remove or make error when Minikin-only
        } catch (FileNotFoundException e) {
            Log.e(TAG, "Error opening " + configFilename);
        } catch (IOException e) {
            Log.e(TAG, "Error reading " + configFilename);
        } catch (XmlPullParserException e) {
            Log.e(TAG, "XML parse exception for " + configFilename);
        }
    }

    static {
        init();
        // Set up defaults and typefaces exposed in public API
        DEFAULT         = create((String) null, 0);
        DEFAULT_BOLD    = create((String) null, Typeface.BOLD);
        SANS_SERIF      = create("sans-serif", 0);
        SERIF           = create("serif", 0);
        MONOSPACE       = create("monospace", 0);

        sDefaults = new Typeface[] {
            DEFAULT,
            DEFAULT_BOLD,
            create((String) null, Typeface.ITALIC),
            create((String) null, Typeface.BOLD_ITALIC),
        };

    }
    /*
    ......
    */
}

SkTypeface 记录一个字体的id，在使用时，到链表中查出相关的字体。

对一个字体和样式，建一个 SkGlyphCache缓存，内含一个 SkScalerContext 和一个 SkGlyph 的哈希表，SkGlyph 缓存一个字体中一个字解析出来的位图。此有内存容量限制，当超过容量时，会清除之前缓存的位图。Hash冲突时，直接生成新字形替换原来的字形。

缓存限制的内存宏详见：src/core/SkGlyphCache_Globals.h。和include/core/SkUserConfig.h中的SK_DEFAULT_FONT_CACHE_LIMIT宏

struct SkGlyph {
    void*       fImage;
    SkPath*     fPath;
    SkFixed     fAdvanceX, fAdvanceY;

    uint32_t    fID;
    uint16_t    fWidth, fHeight;
    int16_t     fTop, fLeft;

    void*       fDistanceField;
    uint8_t     fMaskFormat;
    int8_t      fRsbDelta, fLsbDelta;  // used by auto-kerning
};

当绘制字体只绘边界或者位图缓存机制不好处理时，将字体解析成点线，构成SkPath，也做缓存。