点击容许偏差

尝试点击地图上的线段.

本示例演示使用 hitTolerance 参数.

默认情况下，map.forEachFeatureAtPixel()函数只考虑直接位于所提供像素之下的特征。这可能会让用户难以与触屏设备上的功能进行交互。要考虑与所提供像素有一定距离的特征，请使用hitTolerance选项。例如,地图。forEachFeatureAtPixel(pixel, callback， {hitTolerance: 3})将用所有在所提供像素三个像素内的特征调用回调。默认情况下，map.forEachFeatureAtPixel()方法只考虑当前点击像素下的图形要素。这样会导致在触屏设备上很难进行交互（比如手指点击困难）。如果想要获取在当前点击像素一定范围内的图形要素，使用 hitTolerance 选项即可。比如，map.forEachFeatureAtPixel(pixel, callback, {hitTolerance: 3}) 将会返回点击像素周围三个像素点的所有图形要素。

main.js

import Feature from 'ol/Feature.js';
import LineString from 'ol/geom/LineString.js';
import Map from 'ol/Map.js';
import View from 'ol/View.js';
import {OSM, Vector as VectorSource} from 'ol/source.js';
import {Stroke, Style} from 'ol/style.js';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer.js';

const raster = new TileLayer({
  source: new OSM(),
});

const style = new Style({
  stroke: new Stroke({
    color: 'black',
    width: 1,
  }),
});

const feature = new Feature(
  new LineString([
    [-4000000, 0],
    [4000000, 0],
  ])
);

const vector = new VectorLayer({
  source: new VectorSource({
    features: [feature],
  }),
  style: style,
});

const map = new Map({
  layers: [raster, vector],
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
  }),
});

let hitTolerance;

const statusElement = document.getElementById('status');

map.on('singleclick', function (e) {
  let hit = false;
  map.forEachFeatureAtPixel(
    e.pixel,
    function () {
      hit = true;
    },
    {
      hitTolerance: hitTolerance,
    }
  );
  if (hit) {
    style.getStroke().setColor('green');
    statusElement.innerHTML = 'A feature got hit!';
  } else {
    style.getStroke().setColor('black');
    statusElement.innerHTML = 'No feature got hit.';
  }
  feature.changed();
});

const selectHitToleranceElement = document.getElementById('hitTolerance');
const circleCanvas = document.getElementById('circle');

const changeHitTolerance = function () {
  hitTolerance = parseInt(selectHitToleranceElement.value, 10);

  const size = 2 * hitTolerance + 2;
  circleCanvas.width = size;
  circleCanvas.height = size;
  const ctx = circleCanvas.getContext('2d');
  ctx.clearRect(0, 0, size, size);
  ctx.beginPath();
  ctx.arc(
    hitTolerance + 1,
    hitTolerance + 1,
    hitTolerance + 0.5,
    0,
    2 * Math.PI
  );
  ctx.fill();
  ctx.stroke();
};

selectHitToleranceElement.onchange = changeHitTolerance;
changeHitTolerance();

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Hit Tolerance</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <span id="status">Try to click the line in the map.</span>
    <form>
      <label for="hitTolerance">Hit tolerance for selecting features: &nbsp;</label>
      <select id="hitTolerance">
        <option value="0" selected>0 Pixels</option>
        <option value="5">5 Pixels</option>
        <option value="10">10 Pixels</option>
      </select>
      &nbsp; Area: &nbsp;
      <canvas id="circle" style="vertical-align: middle"></canvas>
    </form>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="./resources/elm-pep.js"></script>
    <script type="module" src="main.js"></script>
  </body>
</html>

package.json

{
  "name": "hit-tolerance",
  "dependencies": {
    "ol": "7.3.0"
  },
  "devDependencies": {
    "vite": "^3.2.3"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}