Skia基础运用（Ubuntu环境下使用BUILD.gn）

1.拉取代码

git clone https://skia.googlesource.com/skia
python tools/git-sync-deps
// 这一步可能会出现部分错误，再次执行直到成功
// 这里面拉取完三方库之后会拉取node，linux等压缩包，从google下载

上面执行完，代码就完全拿到了

2.生成ninja

gn gen out/default
// 如果要导出compile_commands.json, 加入--export-compile-commands
// 可以用导出的文件加上clangd和clang配一波智能提示

3.增加自己的demo

skia
├── demo
	├── BUILD.gn
	└── main.cpp
└── out

增加demo代码如下

#include "include/codec/SkEncodedImageFormat.h"
#include "include/core/SkBitmap.h"
#include "include/core/SkCanvas.h"
#include "include/core/SkPaint.h"
#include "include/core/SkRect.h"
#include "include/core/SkStream.h"
#include "include/core/SkTextBlob.h"
#include "include/core/SkTypeface.h"
#include "include/encode/SkJpegEncoder.h"
#include "src/core/SkDevice.h"
#include "src/encode/SkImageEncoderFns.h"
const char* pText = "Hello world!";
int main() {
    SkBitmap bitmap;
    SkImageInfo ii = SkImageInfo::Make(480, 320, kBGRA_8888_SkColorType, kPremul_SkAlphaType);
    bitmap.allocPixels(ii, ii.minRowBytes());
    SkPaint paint;
    paint.setStyle(SkPaint::kStroke_Style);
    paint.setColor(0xff1f78b4);
    paint.setStrokeWidth(8);
    bitmap.allocPixels();
    SkCanvas canvas(bitmap);
    canvas.clear(0xFF000000);  // 背景为透明色
    {
        SkRect rc;
        rc.fLeft = 123;
        rc.fTop = 0;
        rc.fRight = 222;
        rc.fBottom = 50;
        canvas.drawOval(rc, paint);
    }
    {
        paint.setColor(SK_ColorYELLOW);
        canvas.drawCircle(100, 100, 50, paint);
    }
    SkFILEWStream stream("./1bc.jpg");
    SkJpegEncoder::Encode(&stream, bitmap.pixmap(), {});
    return 0;
}

接下来增加同级目录下的BUILD.gn

import("//gn/skia.gni")
skia_executable("demo") {
  sources = [
    "main.cpp"
  ]
  deps = [
    "../.:skia"
  ]
}

目前还识别不到新增的demo编译目标，需要在总工程目录中增加如下内容

group("gdemo") {
  deps = [
    "//demo:demo"
  ]
}

再生成一遍ninja，或者先弄第三步再执行第二步

5.执行编译

ninja -C out/demo demo

待执行编译之后，out/demo中会得到名字为demo的可执行文件，执行，得到生成的jpeg图片，这样我们就能愉快的学习源码了

我绘制文字貌似有问题，之后再写

6.文字绘制（追加）

skia Fiddle

void draw(SkCanvas* ) {
    SkBitmap bitmap;
    // create a bitmap 5 wide and 11 high
    bitmap.allocPixels(SkImageInfo::MakeN32Premul(5, 11));
    SkCanvas canvas(bitmap);
    canvas.clear(SK_ColorWHITE);  // white is Unpremultiplied, in ARGB order
    SkPixmap pixmap;  // provides guaranteed access to the drawn pixels
    if (!canvas.peekPixels(&pixmap)) {
        SkDebugf("peekPixels should never fail.\n");
    }
    const SkPMColor* pixels = pixmap.addr32();  // points to top-left of bitmap
    SkPMColor pmWhite = pixels[0];  // the Premultiplied format may vary
    SkPaint paint;  // by default, draws black, 12 point text
    SkFont font = SkFont(fontMgr->matchFamilyStyle(nullptr, {}));
    canvas.drawString("!", 1, 10, font, paint);  // 1 char at baseline (1, 10)
    for (int y = 0; y < bitmap.height(); ++y) {
        for (int x = 0; x < bitmap.width(); ++x) {
            SkDebugf("%c", *pixels++ == pmWhite ? '-' : 'x');
        }
        SkDebugf("\n");
    }
}

这段代码是源码中的示例，在官网提供的链接中运行当前代码，发现可以打印感叹号

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--xx-
--xx-
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代码拿来进demo中发现缺少fontMgr变量定义，搜索源码fontMgr发现如下源码


易得网页上面的代码应该是编译的这里的源码，通过头文件中的宏定义注册测试代码到列表中运行，那么我们只要能自己获取fontMgr显示出来，那么源代码的demo就应该能显示出来。第一个就好使，然后打印个hello world吧，典

代码如下

#include "include/core/SkBitmap.h"
#include "include/core/SkCanvas.h"
#include "include/core/SkPaint.h"
#include "include/core/SkTextBlob.h"
#include "include/core/SkTypeface.h"
#include "include/ports/SkFontMgr_fontconfig.h"
#include "include/ports/SkFontScanner_FreeType.h"
#include "include/core/SkFontMgr.h"

int main() {
    SkBitmap bitmap;
    // create a bitmap 100 wide and 20 high
    bitmap.allocPixels(SkImageInfo::MakeN32Premul(100, 20));
    SkCanvas canvas(bitmap);
    canvas.clear(SK_ColorWHITE);
    SkPixmap pixmap; 
    if (!canvas.peekPixels(&pixmap)) {
        SkDebugf("peekPixels should never fail.\n");
    }
    const SkPMColor* pixels = pixmap.addr32();
    SkPMColor pmWhite = pixels[0]; 
    SkPaint paint;  // by default, draws black, 12 point text
    sk_sp<SkFontMgr> fontMgr1 = SkFontMgr_New_FontConfig(nullptr, SkFontScanner_Make_FreeType());
    SkFont font = SkFont(fontMgr1->matchFamilyStyle(nullptr, {}));
    canvas.drawString("hello world", 1, 10, font, paint);  // 1 char at baseline (1, 10)
    for (int y = 0; y < bitmap.height(); ++y) {
        for (int x = 0; x < bitmap.width(); ++x) {
            SkDebugf("%c", *pixels++ == pmWhite ? '-' : 'x');
        }
        SkDebugf("\n");
    }
    return 0;
}

7.编译库（追加）

通过上面的编译可以发现生成的可执行文件特别大，竟然有一百多兆。
查找toolchain发现，编译skia库的默认工具链如下

template("component") {
  _component_mode = "static_library"
  if (is_component_build) {
    _component_mode = "shared_library"
  }
  target(_component_mode, target_name) {
    forward_variables_from(invoker, "*")
  }
}

搜索is_component_build发现，原来是false，直接编译静态库了，怪不得这么大
那么新增参数编译动态库

gn gen out/demo --args="is_component_build=true"

或者直接在out/demo/args.gn中增加

8.尚待解决（追加）

编译动态库加上字体不会生成字体的动态库，不加字体可以。当然，静态库没毛病。