JavaScript 从入门到放弃

JavaScript 从入门到放弃

360前端星计划-第五课
主讲：月影老师

什么才是好的 JS 代码

各司其职

• HTML=》结构
• CSS=》样式
• JavaScript=》行为

复杂 UI 组件的设计

了解其原理，能够更好的对其应用
例子–京东轮播图

• 这样的 UI 组件如何去写
  步骤1：结构设计HTML
  • 图片结构是一个列表型结构，所以主体用
  • 使用 css 绝对定位将图片重叠在同一个位置
  • 轮播图切换的状态使用修饰符（modifier）
  • 轮播图的切换动画使用 css transition

"my-slider" class="slider-list">
  

    
"slider-list__item--selected">
      "https://p5.ssl.qhimg.com/t0119c74624763dd070.png"/>
    
    
"slider-list__item">
      "https://p4.ssl.qhimg.com/t01adbe3351db853eb3.jpg"/>
    
    
"slider-list__item">
      "https://p2.ssl.qhimg.com/t01645cd5ba0c3b60cb.jpg"/>
    
    
"slider-list__item">
      "https://p4.ssl.qhimg.com/t01331ac159b58f5478.jpg"/>
    
  

• API 设计
  

class Slider{
  constructor(id){
    this.container = document.getElementById(id);
    this.items = this.container.querySelectorAll('.slider-list__item, .slider-list__item--selected');
  }
  getSelectedItem(){
    const selected = this.container.querySelector('.slider-list__item--selected');
    return selected
  }
  getSelectedItemIndex(){
    return Array.from(this.items).indexOf(this.getSelectedItem());
  }
  slideTo(idx){
    const selected = this.getSelectedItem();
    if(selected){ 
      selected.className = 'slider-list__item';
    }
    const item = this.items[idx];
    if(item){
      item.className = 'slider-list__item--selected';
    }
  }
  slideNext(){
    const currentIdx = this.getSelectedItemIndex();
    const nextIdx = (currentIdx + 1) % this.items.length;
    this.slideTo(nextIdx);
  }
  slidePrevious(){
    const currentIdx = this.getSelectedItemIndex();
    const previousIdx = (this.items.length + currentIdx - 1) % this.items.length;
    this.slideTo(previousIdx);  
  }
}

const slider = new Slider('my-slider');
setInterval(() => {
  slider.slideNext()
}, 3000)

• 控制流设计
  控制结构=》自定义事件

class Slider{
  constructor(id, cycle = 3000){
    this.container = document.getElementById(id);
    this.items = this.container.querySelectorAll('.slider-list__item, .slider-list__item--selected');
    this.cycle = cycle;

    const controller = this.container.querySelector('.slide-list__control');
    if(controller){
      const buttons = controller.querySelectorAll('.slide-list__control-buttons, .slide-list__control-buttons--selected');
      controller.addEventListener('mouseover', evt=>{
        const idx = Array.from(buttons).indexOf(evt.target);
        if(idx >= 0){
          this.slideTo(idx);
          this.stop();
        }
      });
      
      controller.addEventListener('mouseout', evt=>{
        this.start();
      });
      
      this.container.addEventListener('slide', evt => {
        const idx = evt.detail.index
        const selected = controller.querySelector('.slide-list__control-buttons--selected');
        if(selected) selected.className = 'slide-list__control-buttons';
        buttons[idx].className = 'slide-list__control-buttons--selected';
      })
    }
    
    const previous = this.container.querySelector('.slide-list__previous');
    if(previous){
      previous.addEventListener('click', evt => {
        this.stop();
        this.slidePrevious();
        this.start();
        evt.preventDefault();
      });
    }
    
    const next = this.container.querySelector('.slide-list__next');
    if(next){
      next.addEventListener('click', evt => {
        this.stop();
        this.slideNext();
        this.start();
        evt.preventDefault();
      });
    }
  }
  getSelectedItem(){
    let selected = this.container.querySelector('.slider-list__item--selected');
    return selected
  }
  getSelectedItemIndex(){
    return Array.from(this.items).indexOf(this.getSelectedItem());
  }
  slideTo(idx){
    let selected = this.getSelectedItem();
    if(selected){ 
      selected.className = 'slider-list__item';
    }
    let item = this.items[idx];
    if(item){
      item.className = 'slider-list__item--selected';
    }
  
    const detail = {index: idx}
    const event = new CustomEvent('slide', {bubbles:true, detail})
    this.container.dispatchEvent(event)
  }
  slideNext(){
    let currentIdx = this.getSelectedItemIndex();
    let nextIdx = (currentIdx + 1) % this.items.length;
    this.slideTo(nextIdx);
  }
  slidePrevious(){
    let currentIdx = this.getSelectedItemIndex();
    let previousIdx = (this.items.length + currentIdx - 1) % this.items.length;
    this.slideTo(previousIdx);  
  }
  start(){
    this.stop();
    this._timer = setInterval(()=>this.slideNext(), this.cycle);
  }
  stop(){
    clearInterval(this._timer);
  }
}

const slider = new Slider('my-slider');
slider.start();

• 优化1：插件/依赖注入
• 优化2：改进插件/模板化
• 优化3：组件模型抽象

局部细节控制

例子：消失的方块–处理“只能执行一次”

• 有很多“只允许执行一次”的函数操作，如何进行统一的抽象？
  过程抽象
  once函数

function once(fn){
  return function(...args){
    if(fn){
      let ret = fn.apply(this, args);
      fn = null;
      return ret;
    }
  }
}

应用–点击一次

block.onclick = once(function(evt){
  console.log('hide');
  evt.target.className = 'hide';
  setTimeout(function(){
    document.body.removeChild(block);
  }, 2000);
});

节流throttle

function throttle(fn, time = 500){
  let timer;
  return function(...args){
    if(timer == null){
      fn.apply(this,  args);
      timer = setTimeout(() => {
        timer = null;
      }, time)
    }
  }
}

防抖debounce

function debounce(fn){
  let timer = null
  return function(...args){
    if(timer != null) {
      clearTimeout(timer)
    }
    timer = setTimeout(() => {
      fn.apply(this, args)
      timer = null
    }, 300)
  }
}

声明式(Declarative)编程和命令式（imperative)编程

imperative：How to do？

let list = [1, 2, 3, 4];
let map1 = [];
for(let i = 0; i < list.length; i++){
  map1.push(list[i] * 2);
}

Declarative：What to do?

let list = [1, 2, 3, 4];
const double = x => x * 2;
list.map(double);

高阶函数
它们自身输入函数或返回函数，被称为高阶函数
once、throttle、debounced、consumer、iterative等

总结

• 如何写好 JavaScript？
  各司其职：JavaScript 尽量只做状态管理
  结构、API、控制流分离设计 UI 组件
  插件和模板化，并抽象出组件模型
  运用过程抽象的技巧来抽象并优化局部 API