背景

專案需要地圖上實現不斷報警的閃爍樣式，之前是通過Overlay來處理的，但Overlay遮擋要素，無法很好的觸發事件，所以就轉而使用Canvas原生實現。過程主要參考：openlayers4中閃爍點的實現、canvas-flashMarker-RunJS。

改造

原flashMarker的使用

原本的做法是通過構造新的物件new FlashMarker(map, citys)，每次構造新增一個Canvas，沒有辦法清除，那麼我針對這些問題進行適應性的修改。

改造版本的使用

• 通過構造CanvasLayer進行使用，只需使用一個Canvas的可以只new一個物件。
• 得到圖層物件後呼叫addMarkers方法，一批批地將閃爍樣式加入到Canvas裡。
• 清除Canvas呼叫clear方法，直接對圖層進行清空。

不足

經測試，過多的樣式繪製會有卡頓，目前沒有太好的方法解決，不過Canvas繪製仍然效果比Openlayers好。

程式碼

這裡效果就不展示了，效果可以在背景貼的原文那裡看，我只是改動呼叫和優化實現。

(function (global, factory) {
    typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
        typeof define === 'function' && define.amd ? define(factory) :
            (global.CanvasLayer = factory());
}(this, (function () { 'use strict';
 
 
	//封裝一個相容性良好的方法
	window.requestAnimFrame= (function(){
		return window.requestAnimationFrame ||
		 window.webkitRequestAnimationFrame ||
		  window.mozRequestAnimationFrame ||
		  function(callback){
			  window.setTimeout(callback, 1000 / 60);
		  }
	})();
	window.cancelAnimFrame = (function(){
		return window.cancelAnimationFrame ||
		window.webkitCancelAnimationFrame ||
		window.mozCancelAnimationFrame ||
		function(id){
			window.clearTimeout(id);
		}
	})();
	
    function CanvasLayer(options) {
        this.options = options || {};
        this.paneName = this.options.paneName || 'labelPane';
        this.zIndex = this.options.zIndex || 0;
        this._map = options.map;
        this._lastDrawTime = null;
        this.show();
    }
 
    CanvasLayer.prototype.initialize = function () {
        var map = this._map;
        var canvas = this.canvas = document.createElement('canvas');
        var ctx = this.ctx = this.canvas.getContext('2d');
        canvas.style.cssText = 'position:absolute;' + 'left:0;' + 'top:0;' + 'z-index:' + this.zIndex + ';';
		this.animationIds = [];
        this.adjustSize();
        this.adjustRatio(ctx);
        map.getViewport().appendChild(canvas);
        var that = this;
        map.getView().on('propertychange',function(){
            $(canvas).hide();
        });
        map.on("moveend",function(){
            $(canvas).show();
            that.adjustSize();
            //that._draw();
        });
    };
	
	// Wilson
	CanvasLayer.prototype.clear = function () {
		var animationCtx = this.ctx;
		var width = this._map.getSize()[0];
        var height = this._map.getSize()[1];
		var animationIds = this.animationIds;
		for (var i = 0; i < animationIds.length; i++) {  
			window.cancelAnimationFrame(animationIds[i]);
		}
		this.animationIds = [];
		animationCtx.clearRect(0, 0, width, height);
    };
	
	// Wilson
	CanvasLayer.prototype.addMarkers = function (dataSet) {
		var animationCtx = this.ctx;
		var width = this._map.getSize()[0];
        var height = this._map.getSize()[1];
		var markers = [];
		var length = this.animationIds.length;
		var that = this;
		for (var i = 0; i < dataSet.length; i++) {
			markers.push(new Marker(dataSet[i]));
		}
		
		//開始動畫
		var startAnim = function() {
			var animationId = window.requestAnimFrame(render);
			that.animationIds[length] = animationId;
			
		}

		function render() {
			startAnim();
			
			animationCtx.fillStyle = 'rgba(0,0,0,.95)';
			var prev = animationCtx.globalCompositeOperation;
			animationCtx.globalCompositeOperation = 'destination-in';
			animationCtx.fillRect(0, 0, width, height);
			animationCtx.globalCompositeOperation = prev;
			
			for (var i = 0; i < markers.length; i++) {
				var marker = markers[i];
				marker.draw(animationCtx);
			}
		};
		
		startAnim();
    };
 
    CanvasLayer.prototype.adjustSize = function () {
        var size = this._map.getSize();
        var canvas = this.canvas;
        canvas.width = size[0];
        canvas.height = size[1];
        canvas.style.width = canvas.width + 'px';
        canvas.style.height = canvas.height + 'px';
    };
 
    CanvasLayer.prototype.adjustRatio = function (ctx) {
        var backingStore = ctx.backingStorePixelRatio || ctx.webkitBackingStorePixelRatio || ctx.mozBackingStorePixelRatio || ctx.msBackingStorePixelRatio || ctx.oBackingStorePixelRatio || ctx.backingStorePixelRatio || 1;
        var pixelRatio = (window.devicePixelRatio || 1) / backingStore;
        var canvasWidth = ctx.canvas.width;
        var canvasHeight = ctx.canvas.height;
        ctx.canvas.width = canvasWidth * pixelRatio;
        ctx.canvas.height = canvasHeight * pixelRatio;
        ctx.canvas.style.width = canvasWidth + 'px';
        ctx.canvas.style.height = canvasHeight + 'px';
        ctx.scale(pixelRatio, pixelRatio);
    };

    /*CanvasLayer.prototype._draw = function () {
        var map = this._map;
        var size = map.getSize();
        var center = map.getView().getCenter();
        if (center) {
            var pixel = map.getPixelFromCoordinate(center);
            this.canvas.style.left = pixel[0] - size[0] / 2 + 'px';
            this.canvas.style.top = pixel[1] - size[1] / 2 + 'px';
            //this.options.update && this.options.update.call(this);
        }
    };*/
 
    CanvasLayer.prototype.getContainer = function () {
        return this.canvas;
    };
 
    CanvasLayer.prototype.show = function () {
        this.initialize();
        this.canvas.style.display = 'block';
    };
 
    CanvasLayer.prototype.hide = function () {
        this.canvas.style.display = 'none';
    };
 
    CanvasLayer.prototype.setZIndex = function (zIndex) {
        this.canvas.style.zIndex = zIndex;
    };
 
    CanvasLayer.prototype.getZIndex = function () {
        return this.zIndex;
    };
	
	CanvasLayer.prototype.getZIndex = function () {
        return this.zIndex;
    };
 
    function Marker(opts) {
        this.city = opts.name;
        this.location = [opts.lnglat[0], opts.lnglat[1]];
        this.color = opts.color;
        this.type = opts.type || 'circle';
        this.speed = opts.speed || 0.15;
        this.size = 0;
        this.max = opts.max || 20;
    }
 
    Marker.prototype.draw = function (context) {
        context.save();
        context.beginPath();
        switch (this.type) {
            case 'circle':
                this._drawCircle(context);
                break;
            case 'ellipse':
                this._drawEllipse(context);
                break;
            default:
                break;
        }
        context.closePath();
        context.restore();
 
        this.size += this.speed;
        if (this.size > this.max) {
            this.size = 0;
        }
    };
 
    Marker.prototype._drawCircle = function (context) {
        var pixel = this.pixel||map.getPixelFromCoordinate(this.location);
		if(pixel) {
			context.strokeStyle = this.color;
			context.moveTo(pixel[0] + pixel.size, pixel[1]);
			context.arc(pixel[0], pixel[1], this.size, 0, Math.PI * 2);
			context.stroke();
		}
    };
 
    Marker.prototype._drawEllipse = function (context) {
        var pixel = this.pixel || map.getPixelFromCoordinate(this.location);
        if(pixel) {
			var x = pixel[0],
				y = pixel[1],
				w = this.size,
				h = this.size / 2,
				kappa = 0.5522848,
	 
				// control point offset horizontal
				ox = w / 2 * kappa,
	 
				// control point offset vertical
				oy = h / 2 * kappa,
	 
				// x-start
				xs = x - w / 2,
	 
				// y-start
				ys = y - h / 2,
	 
				// x-end
				xe = x + w / 2,
	 
				// y-end
				ye = y + h / 2;
	 
			context.strokeStyle = this.color;
			context.moveTo(xs, y);
			context.bezierCurveTo(xs, y - oy, x - ox, ys, x, ys);
			context.bezierCurveTo(x + ox, ys, xe, y - oy, xe, y);
			context.bezierCurveTo(xe, y + oy, x + ox, ye, x, ye);
			context.bezierCurveTo(x - ox, ye, xs, y + oy, xs, y);
			context.stroke();
		}
    };

	return CanvasLayer;
})));