概述

在Openlayers中，MultiPolygon类顾名思义就是表示由多个多边形组成的几何对象，关于Polygon类可以参考这篇文章;同Polygon类一样，MultiPolygon类继承于SimpleGeometry类。

本文主要介绍MultiPolygon类的源码实现和原理。

源码分析

MultiPolygon类的源码实现

MultiPolygon类的源码实现如下：

class MultiPolygon extends SimpleGeometry {
  constructor(coordinates, layout, endss) {
    super();
    this.endss_ = [];
    this.flatInteriorPointRevision_ = -1;
    this.flatInteriorPoints = null;
    this.maxDelta_ = -1;
    this.maxDeltaRevision_ = -1;
    this.orientedRevision_ = -1;
    this.orientedFlatCoordinates_ = null;

    if (!endss && !Array.isArray(coordinates[0])) {
      const polygons = coordinates;
      const flatCoordinates = [];
      const thisEndss = [];
      for (let i = 0, ii = polygons.length; i < ii; ++i) {
        const polygon = polygons[i];
        const offset = flatCoordinates.length;
        const ends = polygon.getEnds();
        for (let j = 0, jj = ends.length; j < jj; ++j) {
          ends[j] += offset;
        }
        extend(flatCoordinates, polygon.getFlatCoordinates());
        thisEndss.push(ends);
      }
      layout =
        polygons.length === 0 ? this.getLayout() : polygons[0].getLayout();
      coordinates = flatCoordinates;
      endss = thisEndss;
    }

    if (layout !== undefined && endss) {
      this.setFlatCoordinates(layout, coordinates);
      this.endss_ = endss;
    } else {
      this.setCoordinates(coordinates, layout);
    }
  }
  appendPolygon(polygon) {
    let ends;
    if (!this.flatCoordinates) {
      this.flatCoordinates = polygon.getFlatCoordinates().slice();
      ends = polygon.getEnds().slice();
      this.endss_.push();
    } else {
      const offset = this.flatCoordinates.length;
      extend(this.flatCoordinates, polygon.getFlatCoordinates());
      ends = polygon.getEnds().slice();
      for (let i = 0, ii = ends.length; i < ii; ++i) {
        ends[i] += offset;
      }
    }
    this.endss_.push(ends);
    this.changed();
  }
  clone() {
    const len = this.endss_.length;
    const newEndss = new Array(len);
    for (let i = 0; i < len; ++i) {
      newEndss[i] = this.endss_[i].slice();
    }

    const multiPolygon = new MultiPolygon(
      this.flatCoordinates.slice(),
      this.layout,
      newEndss
    );
    multiPolygon.applyProperties(this);

    return multiPolygon;
  }

  closestPointXY(x, y, closestPoint, minSquaredDistance) {
    if (minSquaredDistance < closestSquaredDistanceXY(this.getExtent(), x, y)) {
      return minSquaredDistance;
    }
    if (this.maxDeltaRevision_ != this.getRevision()) {
      this.maxDelta_ = Math.sqrt(
        multiArrayMaxSquaredDelta(
          this.flatCoordinates,
          0,
          this.endss_,
          this.stride,
          0
        )
      );
      this.maxDeltaRevision_ = this.getRevision();
    }

    return assignClosestMultiArrayPoint(
      this.getOrientedFlatCoordinates(),
      0,
      this.endss_,
      this.stride,
      this.maxDelta_,
      true,
      x,
      y,
      closestPoint,
      minSquaredDistance
    );
  }
  containsXY(x, y) {
    return linearRingssContainsXY(
      this.getOrientedFlatCoordinates(),
      0,
      this.endss_,
      this.stride,
      x,
      y
    );
  }
  getArea() {
    return linearRingssArea(
      this.getOrientedFlatCoordinates(),
      0,
      this.endss_,
      this.stride
    );
  }
  getCoordinates(right) {
    let flatCoordinates;
    if (right !== undefined) {
      flatCoordinates = this.getOrientedFlatCoordinates().slice();
      orientLinearRingsArray(
        flatCoordinates,
        0,
        this.endss_,
        this.stride,
        right
      );
    } else {
      flatCoordinates = this.flatCoordinates;
    }

    return inflateMultiCoordinatesArray(
      flatCoordinates,
      0,
      this.endss_,
      this.stride
    );
  }
  getEnds() {
    return this.endss_;
  }
  getFlatInteriorPoint() {
    if (this.flatInteriorPointsRevision_ != this.getRevision()) {
      const flatCenters = linearRingssCenter(
        this.flatCoordinates,
        0,
        this.endss_,
        this.stride
      );
      this.flatInteriorPoints_ = getInteriorPointsOfMultiArray(
        this.getOrientedFlatCoordinates(),
        0,
        this.endss_,
        this.stride,
        flatCenters
      );
      this.flatInteriorPointsRevision_ = this.getRevision();
    }
    return this.flatInteriorPoints_;
  }
  getInteriorPoints() {
    return new MultiPoint(this.getFlatInteriorPoints().slice(), "XYM");
  }
  getOrientedFlatCoordiantes() {
    if (this.orientedRevision_ != this.getRevision()) {
      const flatCoordinates = this.flatCoordinates;
      if (
        linearRingssAreOriented(flatCoordinates, 0, this.endss_, this.stride)
      ) {
        this.orientedFlatCoordinates_ = flatCoordinates;
      } else {
        this.orientedFlatCoordinates_ = flatCoordinates.slice();
        this.orientedFlatCoordinates_.length = orientLinearRingsArray(
          this.orientedFlatCoordinates_,
          0,
          this.endss_,
          this.stride
        );
      }
      this.orientedRevision_ = this.getRevision();
    }
    return this.orientedFlatCoordinates_;
  }
  getSimplifiedGeometryInternal(squaredTolerance) {
    const simplifiedFlatCoordinates = [];
    const simplifiedEndss = [];
    simplifiedFlatCoordinates.length = quantizeMultiArray(
      this.flatCoordinates,
      0,
      this.endss_,
      this.stride,
      Math.sqrt(squaredTolerance),
      simplifiedFlatCoordinates,
      0,
      simplifiedEndss
    );
    return new MultiPolygon(simplifiedFlatCoordinates, "XY", simplifiedEndss);
  }
  getPolygon(index) {
    if (index < 0 || this.endss_.length <= index) {
      return null;
    }
    let offset;
    if (index === 0) {
      offset = 0;
    } else {
      const prevEnds = this.endss_[index - 1];
      offset = prevEnds[prevEnds.length - 1];
    }
    const ends = this.endss_[index].slice();
    const end = ends[ends.length - 1];
    if (offset !== 0) {
      for (let i = 0, ii = ends.length; i < ii; ++i) {
        ends[i] -= offset;
      }
    }
    return new Polygon(
      this.flatCoordinates.slice(offset, end),
      this.layout,
      ends
    );
  }
  getPolygons() {
    const layout = this.layout;
    const flatCoordinates = this.flatCoordinates;
    const endss = this.endss_;
    const polygons = [];
    let offset = 0;
    for (let i = 0, ii = endss.length; i < ii; ++i) {
      const ends = endss[i].slice();
      const end = ends[ends.length - 1];
      if (offset !== 0) {
        for (let j = 0, jj = ends.length; j < jj; ++j) {
          ends[j] -= offset;
        }
      }
      const polygon = new Polygon(
        flatCoordinates.slice(offset, end),
        layout,
        ends
      );
      polygons.push(polygon);
      offset = end;
    }
    return polygons;
  }
  getType() {
    return "MultiPolygon";
  }
  intersectsExtent(extent) {
    return intersectsLinearRingMultiArray(
      this.getOrientedFlatCoordinates(),
      0,
      this.endss_,
      this.stride,
      extent
    );
  }
  setCoordinates(coordinates, layout) {
    this.setLayout(layout, coordinates, 3);
    if (!this.flatCoordinates) {
      this.flatCoordinates = [];
    }
    const endss = deflateMultiCoordinatesArray(
      this.flatCoordinates,
      0,
      coordinates,
      this.stride,
      this.endss_
    );
    if (endss.length === 0) {
      this.flatCoordinates.length = 0;
    } else {
      const lastEnds = endss[endss.length - 1];
      this.flatCoordinates.length =
        lastEnds.length === 0 ? 0 : lastEnds[lastEnds.length - 1];
    }
    this.changed();
  }
}

MultiPolygon类的构造函数

MultiPolygon类构造函数接受三个参数：坐标数据coordinates、坐标布局layout和endss每个多边形结束点数组；在Polygon类的构造函数中用this.ends_存储每个线性环的结束坐标的索引，而在MultiPolygon类中用this.endss_存储每个多边形的结束点新鲜，每个多边形的结束点是一个坐标数组；其余变量如this.flatInteriorPointRevision_等等同Polygon类中一样，都是用于优化几何对象的处理和渲染、比如计算多边形的内部点、顶点排序变化等；MultiPolygon类的构造函数还会判断，若参数endss不存在并且coordinates的第一个值不是数组，即coordinates是一个包含多个多边形对象的数组，则遍历这些多边形，获取其结束点ends并将它们根据当前的偏移调整，然后将多个多边形的坐标扁平化最后赋值给coordinates，将每个多边形的结束点数组存储到this.Endss最后赋值给endss;然后根据坐标布局风格layout和endss来决定是调用this.setFlatCoordiantes还是this.setCoordiantes设置this.endss_、this.layout、this.stride和this.flatCoordinates。

MultiPolygon类的主要方法

MultiPolygon类的主要方法如下

• appendPolygon方法：该方法是向当前几何对象添加一个多边形，接受一个参数polygon多边形；首先会判断，若this.flatCoordinates不存在，则调用polygon.getFlatCoordiantes方法获取参数多边形的坐标赋值给this.flatCoordiantes;并且获取多边形的结束点；若存在，则获取多边形的坐标添加到this.faltCoordiantes中，并且获取多边形坐标的长度，以此来设置该多边形的结束点的偏移值，然后将ends添加到this.endss_的末端，最后调用this.changed方法

• clone方法：复制当前几何对象，通过this.endss_获取每个多边形的结束点信息，然后实例化MultiPolygon类，调用实例对象的applyProperties方法应用属性，最后返回实例对象。

• closestPointXY方法：计算给定点(x,y)到当前几何对象的最近距离的平方，以及可能会修改最近点坐标closestPoint和最近距离的平方minSquaredDistance;方法内部同Polygon类中同名函数类似，会基于几何对象发生变化时重新计算this.maxDelta_

• containsXY方法：判断给定点(x,y)是否在当前几何对象内部或者边界上，内部会逐一判断每个多边形是否包含该点，若包含则返回true；否则判断下一个多边形，若都不包含，则返回false.

• getArea方法：获取当前几何对象的面积，内部调用的方法是linearRingsArea方法

• getCoordinates方法：获取几何对象的坐标，内部就是调用inflateMultiCoordinatesArray方法

• getEnds方法：获取this.endss_的值

• getFlatInteriorPoints方法：实现原理和Polygon类中的同名函数类似，不过是需要通过this.endss_变量获取每个多边形的坐标，再计算对应多边形的内部点，也就说this.flatInteriorPoints_中保存的是每个多边形的内部点

• getInteriorPoints方法：获取当前几何对象每个多边形的内部点

• getOrientedFlatCoordiantes方法：实现原理和Polygon类中的同名函数一样

• getSimplifiedGeometryInternal方法：获取简化后的几何对象，接受一个参数squaredTolerance容差平方，该值越大，表示要去除的点更多；内部是调用quantizeMultiArray方法进行简化当前几何对象，简化后对象的坐标保存在simplifiedFlatCoordiantes中，最后调用MultiPolygon实例化并返回实例对象

• getPolygon方法：返回几何对象中索引值对应的多边形，首先会计算参数index是否合法，然后通过index和this.endss_计算该索引值对应的坐标，然后调用Polygon类实例化一个多边形，最后返回该多边形的实例。

• getPolygons方法：获取几何对象的多边形，以数组形式返回；通过this.endss_变量计算其中某个多边形的坐标（起止位置），然后调用Polygon进行实例化，将其实例对象保存到数组polygons中最后返回。

• getType方法：返回当前几何对象的类型,MultiPolygon

• intersectExtent方法:判断extent是否与当前几何对象相交，内部是调用intersectsLinearRingMultiArray方法

• setCoordinates方法：内部是调用delatMultiCoordinatesArray方法，设置this.flatCoordinates、this.layout和this.stride，最后调用this.changed方法

总结

本文主要介绍了MultiPolygon类的实现原理，MultiPolygon类和Polygon类的实现原理几乎大同小异。