HelloLayer with drawOnInteracting
index.html
<!DOCTYPE html>
<html>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Layer development - HelloLayer with drawOnInteracting</title>
  <style type="text/css">
    html,body{margin:0px;height:100%;width:100%}
    .container{width:100%;height:100%}
  </style>
  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/maptalks.css">
  <script type="text/javascript" src="https://cdn.jsdelivr.net/npm/[email protected]/dist/maptalks.min.js"></script>
  <body>

    <div id="map" class="container"></div>

    <script>
      var map = new maptalks.Map('map', {
        center: [-0.113049,51.498568],
        zoom: 14,
        baseLayer: new maptalks.TileLayer('base', {
          urlTemplate: 'http://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}.png',
          subdomains: ['a','b','c','d'],
          attribution: '&copy; <a href="http://osm.org">OpenStreetMap</a> contributors, &copy; <a href="https://carto.com/">CARTO</a>'
        })
      });


      const options = {
        // 默认颜色
        'color' : 'Red',
        // 默认字体
        'font' : '30px san-serif'
      };

      class HelloLayer extends maptalks.Layer {
        // 构造函数
        constructor(id, data, options) {
          super(id, options);
          this.data = data;
        }

        setData(data) {
          this.data = data;
          return this;
        }

        getData() {
          return this.data;
        }
      }

      //定义默认的图层配置属性
      HelloLayer.mergeOptions(options);


      class HelloLayerRenderer extends maptalks.renderer.CanvasRenderer {
        checkResources() {
          //HelloLayer只是绘制文字, 没有外部图片, 所以返回空数组
          return [];
        }

        draw() {
          const drawn = this._drawData(this.layer.getData(), this.layer.options.color);
          //记录下绘制过的数据
          this._drawnData = drawn;
          //结束绘制:
          // 1. 触发必要的事件
          // 2. 将渲染器的canvas设为更新状态, map会加载canvas并呈现在地图上
          this.completeRender();
        }

        drawOnInteracting(evtParam) {
          if (!this._drawnData || this._drawnData.length === 0) {
            return;
          }
          this._drawData(this._drawnData, this.layer.options.color);
        }

        /**
        * 绘制数据
        */
        _drawData(data, color) {
          if (!Array.isArray(data)) {
            return;
          }
          const map = this.getMap();
          //prepareCanvas是父类CanvasRenderer中的方法
          //用于准备canvas画布
          //如果canvas不存在时, 则创建它
          //如果canvas已存在, 则清空画布
          this.prepareCanvas();
          //this.context是渲染器canvas的CanvasRenderingContext2D
          const ctx = this.context;
          //设置样式
          ctx.fillStyle = color;
          ctx.font = this.layer.options['font'];

          const containerExtent = map.getContainerExtent();
          const drawn = [];
          data.forEach(d => {
            //将中心点经纬度坐标转化为containerPoint
            //containerPoint是指相对地图容器左上角的像素坐标.
            const point = map.coordinateToContainerPoint(new maptalks.Coordinate(d.coord));
            //如果绘制的点不在屏幕范围内, 则不做绘制以提高性能
            if (!containerExtent.contains(point)) {
              return;
            }
            const text = d.text;
            const len = ctx.measureText(text);
            ctx.fillText(text, point.x - len.width / 2, point.y);
            drawn.push(d);
          });

          return drawn;
        }
      }

      HelloLayer.registerRenderer('canvas', HelloLayerRenderer);

      const layer = new HelloLayer('hello');
      layer.setData([
        {
          'coord' : map.getCenter().toArray(),
          'text' : 'Hello World'
        },
        {
          'coord' : map.getCenter().add(0.01, 0.01).toArray(),
          'text' : 'Hello World 2'
        }
      ]);
      layer.addTo(map);

    </script>
  </body>
</html>
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