_getArcs({data, selectedFeature}) {
    if (!data || !selectedFeature) {
      return null;
    }

    const {flows, centroid} = selectedFeature.properties;

    const arcs = Object.keys(flows).map(toId => {
      const f = data[toId];
      return {
        source: centroid,
        target: f.properties.centroid,
        value: flows[toId]
      };
    });

    const scale = scaleQuantile()
      .domain(arcs.map(a => Math.abs(a.value)))
      .range(inFlowColors.map((c, i) => i));

    arcs.forEach(a => {
      a.gain = Math.sign(a.value);
      a.quantile = scale(Math.abs(a.value));
    });

    return arcs;
  }

initializeState() {
    const attributeManager = this.getAttributeManager();

    /* eslint-disable max-len */
    attributeManager.addInstanced({
      instancePositions: {
        size: 4,
        transition: true,
        accessor: [\'getSourcePosition\', \'getTargetPosition\'],
        update: this.calculateInstancePositions
      },
      instanceSourceColors: {
        size: 4,
        type: GL.UNSIGNED_BYTE,
        transition: true,
        accessor: \'getSourceColor\',
        update: this.calculateInstanceSourceColors
      },
      instanceTargetColors: {
        size: 4,
        type: GL.UNSIGNED_BYTE,
        transition: true,
        accessor: \'getTargetColor\',
        update: this.calculateInstanceTargetColors
      }
    });
    /* eslint-enable max-len */
  }

_getModel(gl) {
    let positions = [];
    const NUM_SEGMENTS = 50; // 利用50个点来模拟曲线
    /*
     *  (0, -1)-------------_(1, -1)
     *       |          _,-"  |
     *       o      _,-"      o
     *       |  _,-"          |
     *   (0, 1)"-------------(1, 1)
     */
    for (let i = 0; i < NUM_SEGMENTS; i++) { // 使用三角带的方式来绘制三角形，同时这里的-1和1也是为了在绘制宽度的时候确定法向量的偏移
      positions = positions.concat([i, -1, 0, i, 1, 0]);
    }

    const model = new Model(
      gl,
      Object.assign({}, this.getShaders(), {
        id: this.props.id,
        geometry: new Geometry({
          drawMode: GL.TRIANGLE_STRIP,
          attributes: {
            positions: new Float32Array(positions)
          }
        }),
        isInstanced: true,
        shaderCache: this.context.shaderCache // 缓存着色器，我怀疑自己写的hexagon偏慢也跟这个有关系
      })// 绘制物体，这里是5.x的版本在新的版本中还要设定instanceCount参数，来控制绘制实例的数量
    );

    model.setUniforms({numSegments: NUM_SEGMENTS});

    return model;
  }

calculateInstancePositions(attribute) {
    const {data, getSourcePosition, getTargetPosition} = this.props;
    const {value, size} = attribute;
    let i = 0;
    for (const object of data) {
      const sourcePosition = getSourcePosition(object);
      const targetPosition = getTargetPosition(object);
      value[i + 0] = sourcePosition[0];
      value[i + 1] = sourcePosition[1];
      value[i + 2] = targetPosition[0];
      value[i + 3] = targetPosition[1];
      i += size;
    }
  }

  calculateInstancePositions64Low(attribute) {
    const {data, getSourcePosition, getTargetPosition} = this.props;
    const {value, size} = attribute;
    let i = 0;
    for (const object of data) {
      const sourcePosition = getSourcePosition(object);
      const targetPosition = getTargetPosition(object);
      value[i + 0] = fp64LowPart(sourcePosition[0]);
      value[i + 1] = fp64LowPart(sourcePosition[1]);
      value[i + 2] = fp64LowPart(targetPosition[0]);
      value[i + 3] = fp64LowPart(targetPosition[1]);
      i += size;
    }
  }

  calculateInstanceSourceColors(attribute) {
    const {data, getSourceColor} = this.props;
    const {value, size} = attribute;
    let i = 0;
    for (const object of data) {
      const color = getSourceColor(object);
      value[i + 0] = color[0];
      value[i + 1] = color[1];
      value[i + 2] = color[2];
      value[i + 3] = isNaN(color[3]) ? 255 : color[3];
      i += size;
    }
  }

  calculateInstanceTargetColors(attribute) {
    const {data, getTargetColor} = this.props;
    const {value, size} = attribute;
    let i = 0;
    for (const object of data) {
      const color = getTargetColor(object);
      value[i + 0] = color[0];
      value[i + 1] = color[1];
      value[i + 2] = color[2];
      value[i + 3] = isNaN(color[3]) ? 255 : color[3];
      i += size;
    }
  }

#define SHADER_NAME arc-layer-vertex-shader

attribute vec3 positions; // 几何图形的坐标，同时这里面也编码了一些信息，x代表线段索引，y可以代表偏移方向
// 本次可用的一些实例变量
attribute vec4 instanceSourceColors;// 起点的颜色
attribute vec4 instanceTargetColors; // 终点的颜色
attribute vec4 instancePositions; // 前两个值记录了起点经纬度，后两个值记录了终点经纬度
attribute vec3 instancePickingColors;

uniform float numSegments; // 抛物线的线段数量
uniform float strokeWidth; // 线宽度
uniform float opacity;

varying vec4 vColor;

// source和target是在3d空间中的单位，ratio代表本此线段在总线段数目的比值范围在0~1，返回值时抛物线高度的平方
// 这里的方式决定高度单位与source/target的单位保持一致
float paraboloid(vec2 source, vec2 target, float ratio) {

  vec2 x = mix(source, target, ratio); // 获取该线段节点对应的直线位置
  vec2 center = mix(source, target, 0.5);// 取中心点，充分利用glsl内建函数，提升性能

  // 抛物线的公式应该是y * y = (source - center)^2 - (x - center)^2;
  float dSourceCenter = distance(source, center);
  float dXCenter = distance(x, center);
  return (dSourceCenter + dXCenter) * (dSourceCenter - dXCenter);
}

// 在屏幕空间中计算偏移值，最后在反算到裁切空间，也就是ndc空间
// offset_direction在position的y坐标中记录
// offset vector by strokeWidth pixels
// offset_direction is -1 (left) or 1 (right)
vec2 getExtrusionOffset(vec2 line_clipspace, float offset_direction) {
  // normalized direction of the line
  // ndc空间中的坐标乘以屏幕宽高像素，转换成2维屏幕像素；然后归一化成单位向量
  vec2 dir_screenspace = normalize(line_clipspace * project_uViewportSize);
  // rotate by 90 degrees
  dir_screenspace = vec2(-dir_screenspace.y, dir_screenspace.x); // 求法线向量

  // 法向量乘以偏移方向乘以宽度一半获取在屏幕空间中的偏移值
  vec2 offset_screenspace = dir_screenspace * offset_direction * strokeWidth / 2.0;
  // 将屏幕坐标反算到ndc空间
  vec2 offset_clipspace = project_pixel_to_clipspace(offset_screenspace).xy;

  return offset_clipspace; // 返回ndc空间的偏移量
}

float getSegmentRatio(float index) { // 返回线段索引在总线段数目中的比值，转换成0~1之间
  return smoothstep(0.0, 1.0, index / (numSegments - 1.0));
}

vec3 getPos(vec2 source, vec2 target, float segmentRatio) { // 获取线段节点在三维空间中的位置
  float vertex_height = paraboloid(source, target, segmentRatio); // 获取高度信息

  return vec3(
    mix(source, target, segmentRatio), // 获取节点的x/y坐标
    sqrt(max(0.0, vertex_height))// 获取节点的高度坐标
  );
}

void main(void) {
  // 将insance中编码的起终点的经纬度分别转换成瓦片像素单位
  vec2 source = project_position(instancePositions.xy);
  vec2 target = project_position(instancePositions.zw);

  float segmentIndex = positions.x;// 节点的线段索引
  float segmentRatio = getSegmentRatio(segmentIndex);
  // if it\'s the first point, use next - current as direction
  // otherwise use current - prev
  // 这里处理方式比较巧妙，充分利用内建函数优势；
  // step(edge, x) 作用如： x>=edge ? 1.0 : 0.0
  // 所以上面英文注释所说，如果是起点就使用next-curr，其他的都是用curr - prev
  //float indexDir = mix(-1.0, 1.0, step(segmentIndex, 0.0));
  float indexDir = mix(-1.0, 1.0, (segmentIndex <= 0.0 ? 1.0 : 0.0));
  // 根据indexDir获取下一段或者上一个线段节点的比值
  float nextSegmentRatio = getSegmentRatio(segmentIndex + indexDir);

  // 获取两个节点的3维世界坐标并转化成ndc坐标
  vec3 currPos = getPos(source, target, segmentRatio);
  vec3 nextPos = getPos(source, target, nextSegmentRatio);
  vec4 curr = project_to_clipspace(vec4(currPos, 1.0));
  vec4 next = project_to_clipspace(vec4(nextPos, 1.0));

  // extrude
  // 进行线宽拉伸，获取法线方向的偏移
  vec2 offset = getExtrusionOffset((next.xy - curr.xy) * indexDir, positions.y);
  gl_Position = curr + vec4(offset, 0.0, 0.0); // 获取最终节点的ndc位置

  // 根据线段节点位置计算颜色插值
  vec4 color = mix(instanceSourceColors, instanceTargetColors, segmentRatio) / 255.;
  vColor = vec4(color.rgb, color.a * opacity);// 获取最终颜色

  // Set color to be rendered to picking fbo (also used to check for selection highlight).
  picking_setPickingColor(instancePickingColors);
}