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How to implement waterfall flow plug-in using JS

亚连Original: 2018-06-07 14:28:281227browse

This article gives you a detailed analysis of a native JS waterfall plug-in and code-related explanations. Readers who are interested in this can refer to it.

The pictures in the waterfall flow layout have a core feature—equal width and variable height. The waterfall flow layout is used to a certain extent on domestic websites, such as pinterest, petals.com, etc. Then based on this feature, we will start the waterfall flow exploration journey.

Basic function implementation

First we define a container with 20 pictures.

<body>
 <style>
  #waterfall {
   position: relative;
  }
  .waterfall-box {
   float: left;
   width: 200px;
  }
 </style>
</body>
<p id="waterfall">
  <img src="images/1.png" class="waterfall-box">
  <img src="images/2.png" class="waterfall-box">
  <img src="images/3.png" class="waterfall-box">
  <img src="images/4.png" class="waterfall-box">
  <img src="images/5.png" class="waterfall-box">
  <img src="images/6.png" class="waterfall-box">
  ...
 </p>
由于未知的 css 知识点，丝袜最长的妹子把下面的空间都占用掉了。。。
接着正文，假如如上图，每排有 5 列，那第 6 张图片应该出现前 5 张图片哪张的下面呢？当然是绝对定位到前 5 张图片高度最小的图片下方。
那第 7 张图片呢？这时候把第 6 张图片和在它上面的图片当作是一个整体后，思路和上述是一致的。代码实现如下:
Waterfall.prototype.init = function () {
 ...
 const perNum = this.getPerNum() // 获取每排图片数
 const perList = []       // 存储第一列的各图片的高度
 for (let i = 0; i < perNum; i++) {
  perList.push(imgList[i].offsetHeight)
 }
 let pointer = this.getMinPointer(perList) // 求出当前最小高度的数组下标
 for (let i = perNum; i < imgList.length; i++) {
  imgList[i].style.position = &#39;absolute&#39; // 核心语句
  imgList[i].style.left = `${imgList[pointer].offsetLeft}px`
  imgList[i].style.top = `${perList[pointer]}px`

  perList[pointer] = perList[pointer] + imgList[i].offsetHeight // 数组最小的值加上相应图片的高度
  pointer = this.getMinPointer(perList)
 }
}

Careful friends may have discovered the code to obtain pictures. The height of the offsetHeight attribute is used. The sum of the heights of this attribute is equal to the image height inner margin border. Because of this, we use padding instead of margin to set the image and image. the distance between. In addition to the offsetHeight attribute, you must also understand the differences between offsetHeight, clientHeight, offsetTop, scrollTop and other attributes. , in order to better understand this project. The css code is simple as follows:

.waterfall-box {
 float: left;
 width: 200px;
 padding-left: 10px;
 padding-bottom: 10px;
}

Implementation of scroll and resize event monitoring

After implementing the initialization function init, the next step is to implement monitoring of the scroll event. This achieves the effect of a steady stream of pictures being loaded when scrolling to the bottom of the parent node. One point to consider at this time is, at what position is the loading function triggered when scrolling? This varies from person to person. My approach is to trigger the loading function when the condition of parent container height scrolling distance> offsetTop of the last picture is met, that is, orange line purple line> blue line, code As follows:

window.onscroll = function() {
 // ...
 if (scrollPX + bsHeight > imgList[imgList.length - 1].offsetTop) {// 浏览器高度 + 滚动距离 > 最后一张图片的 offsetTop
  const fragment = document.createDocumentFragment()
  for(let i = 0; i < 20; i++) {
   const img = document.createElement(&#39;img&#39;)
   img.setAttribute(&#39;src&#39;, `images/${i+1}.png`)
   img.setAttribute(&#39;class&#39;, &#39;waterfall-box&#39;)
   fragment.appendChild(img)
  }
  $waterfall.appendChild(fragment)
 }
}

Because the parent node may customize the node, an encapsulation of the scroll monitoring function is provided. The code is as follows:

proto.bind = function () {
  const bindScrollElem = document.getElementById(this.opts.scrollElem)
  util.addEventListener(bindScrollElem || window, &#39;scroll&#39;, scroll.bind(this))
 }
 const util = {
  addEventListener: function (elem, evName, func) {
   elem.addEventListener(evName, func, false)
  },
 }

Resize event monitoring is similar to scroll event monitoring. When triggered resize function, just call the init function to reset.

Use the publish-subscribe model and inheritance to implement listening binding

Since the goal is to develop plug-ins, we cannot just be satisfied with the realization of functions, but also leave corresponding operating space for developers to do their own work deal with. Reminiscing about the drop-down loading of pictures in waterfall flows in business scenarios, they are usually obtained asynchronously through Ajax, so the loaded data must not be hard-coded in the library. It is expected that the following calls can be implemented (the usage of waterfall is used here for reference),

const waterfall = new Waterfall({options})
waterfall.on("load", function () {
 // 此处进行 ajax 同步/异步添加图片
})

Observing the calling method, it is not difficult to think of using the publish/subscribe model to implement it. Regarding the publish/subscribe model, it was introduced before in Node.js Asynchronous Async Record. The core idea is to add the function to the cache through the subscription function, and then implement the asynchronous call through the publishing function. The code implementation is given below:

function eventEmitter() {
 this.sub = {}
}
eventEmitter.prototype.on = function (eventName, func) { // 订阅函数
 if (!this.sub[eventName]) {
  this.sub[eventName] = []
 }
 this.sub[eventName].push(func) // 添加事件监听器
}
eventEmitter.prototype.emit = function (eventName) { // 发布函数
 const argsList = Array.prototype.slice.call(arguments, 1)
 for (let i = 0, length = this.sub[eventName].length; i < length; i++) {
  this.sub[eventName][i].apply(this, argsList) // 调用事件监听器
 }
}

Next, to enable Waterfall to use the publish/subscribe mode, just Let Waterfall inherit the eventEmitter function, the code is implemented as follows:

function Waterfall(options = {}) {
 eventEmitter.call(this)
 this.init(options) // 这个 this 是 new 的时候，绑上去的
}
Waterfall.prototype = Object.create(eventEmitter.prototype)
Waterfall.prototype.constructor = Waterfall

The inheritance method absorbs the advantages of two methods of writing based on constructor inheritance and prototype chain inheritance, and uses Object.create isolation Now that we have subclasses and parent classes, we can write another article for more details about inheritance, so we’ll stop here.

Small optimization

In order to prevent the scroll event from triggering multiple loading of images, you can consider using function anti-shake and throttling. Based on the publish-subscribe model, an isLoading parameter is defined to indicate whether it is loading, and whether to load is determined based on its Boolean value. The code is as follows:

let isLoading = false
const scroll = function () {
 if (isLoading) return false // 避免一次触发事件多次
 if (scrollPX + bsHeight > imgList[imgList.length - 1].offsetTop) { // 浏览器高度 + 滚动距离 > 最后一张图片的 offsetTop
  isLoading = true
  this.emit(&#39;load&#39;)
 }
}
proto.done = function () {
 this.on(&#39;done&#39;, function () {
  isLoading = false
  ...
 })
 this.emit(&#39;done&#39;)
}

At this time, you need to add # at the place of the call. ##waterfall.done, thereby informing that the current image has been loaded. The code is as follows:

const waterfall = new Waterfall({})
waterfall.on("load", function () {
 // 异步/同步加载图片
 waterfall.done()
})

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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