什么是享元模式

享元模式（Flyweight Pattern）主要用于减少创建对象的数量，以减少内存占用和提高性能。这种类型的设计模式属于结构型模式，它提供了减少对象数量从而改善应用所需的对象结构的方式。

享元模式尝试重用现有的同类对象，如果未找到匹配的对象，则创建新对象。

面向对象例子：

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class Flyweight{
	constructor() {
		this.divPool = [];
	}

	createDiv(text, parent) {
		if (this.divPool.length > 0) {
			console.log(`get from pool, pool count:${this.divPool.length}`);
			let div = this.divPool.shift();
			div.innerText = text;
			parent.appendChild(div);
			return div;
		} else {
			console.log(`create a new div, because pool count:${this.divPool.length}`);
			let div = document.createElement('div');
			div.innerText = text;
			parent.appendChild(div);
			return div;
		}
	}
  
	removeDiv(node, parent) {
		parent.removeChild(node);
		this.recover(node);
		console.log(`when ui remove div, restore this div, now pool has: ${this.divPool.length}`);
	}
  
	recover(node) {
		this.divPool.push(node);
	}
}

函数式例子：

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(function () {

    var circleFactory = (function () {
        var circlePool = [];
        var parent = document.querySelector('.flyweight-body');
        return {
            create: function () {
                if (circlePool.length !== 0) {
                    return circlePool.shift();
                } else {
                    var div = document.createElement('div');
                    div.setAttribute('class', 'flyweight-child');
                    parent.appendChild(div);
                    return div;
                }
            },
            recover: function (dom) {
                return circlePool.push(dom);
            },
            remove: function (oldPool) {
                for (var i = 0; i < oldPool.length; i++) {
                    parent.removeChild(oldPool[i]);
                }
            }
        }
    })();

    var renderCircle = (function () {
        var circlePool = [];
        return function (number) {
            console.log('length: ' + circlePool.length);
            for (var j = 0; j < number.length; j++) {
                circleFactory.recover(circlePool.pop());
                circlePool.length = circlePool.length - 1;
            }
            console.log('length: ' + circlePool.length);
            circleFactory.remove(circlePool);
            circlePool = [];
            for (var i = 0; i < number; i++) {
                var circle = circleFactory.create();
                circle.style.left = Math.random() * 700 + 'px';
                circle.style.top = Math.random() * 400 + 'px';
                circlePool.push(circle);
            }
        }
    })();

    var init = function () {
        Event.listen('draw-circle', function (args) {
            var number = Number(args.number);
            console.log(number);
            renderCircle(number);
        });
    };

    init();

})();