1. 使用场景

• 代码复用和抽象的主要形式是 组件
• 当需要对普通 DOM 元素进行底层操作，此时就会用到 自定义指令
• 但是，对于大幅度的 DOM 变动，还是应该使用 组件

2. 钩子函数

详情查阅文档 https://cn.vuejs.org/v2/guide/custom-directive.html#%E9%92%A9%E5%AD%90%E5%87%BD%E6%95%B0

3. 示例

3.1 输入框自动聚焦

// 注册一个全局自定义指令 `v-focus`
Vue.directive('focus', {
  // 当被绑定的元素插入到 DOM 中时
  inserted: function (el) {
    // 聚焦元素
    el.focus()}
})

<input v-focus>

3.2 下拉菜单

• 点击下拉菜单本身不会隐藏菜单
• 点击下拉菜单以外的区域隐藏菜单

Vue.directive('clickoutside', {bind(el, binding) {function documentHandler(e) {if (el.contains(e.target)) {return false}
      
      if (binding.expression) {binding.value(e)
      }
    }
    
    el.__vueMenuHandler__ = documentHandler
    document.addEventListener('click', el.__vueMenuHandler__)
  },
  unbind(el) {document.removeEventListener('click', el.__vueMenuHandler__)
    delete el.__vueMenuHandler__
  }
})

new Vue({
  el: '#app',
  data: {show: false},
  methods: {handleHide() {this.show = false}
  }
})

<div class="main" v-menu="handleHide">
  <button @click="show = !show"> 点击显示下拉菜单 </button>
  <div class="dropdown" v-show="show">
    <div class="item"><a href="#"> 选项 1</a></div>
    <div class="item"><a href="#"> 选项 2</a></div>
    <div class="item"><a href="#"> 选项 3</a></div>
  </div>
</div>

3.3 相对时间转换

类似微博、朋友圈发布动态后的相对时间，比如刚刚、两分钟前等等

<span v-relativeTime="time"></span>

new Vue({
  el: '#app',
  data: {time: 1565753400000}
})

Vue.directive('relativeTime', {bind(el, binding) {// Time.getFormatTime() 方法，自行补充
    el.innerHTML = Time.getFormatTime(binding.value)
    el.__timeout__ = setInterval(() => {el.innerHTML = Time.getFormatTime(binding.value)
    }, 6000)
  },
  unbind(el) {clearInterval(el.innerHTML)
    delete el.__timeout__
  }
})

3.4 滚动动画

<div id="app">
  <h1 class="centered">Scroll me</h1>
  <div class="box" v-scroll="handleScroll">
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. A atque amet harum aut ab veritatis earum porro praesentium ut corporis. Quasi provident dolorem officia iure fugiat, eius mollitia sequi quisquam.</p>
  </div>
</div>

Vue.directive('scroll', {inserted: function(el, binding) {let f = function(evt) {if (binding.value(evt, el)) {window.removeEventListener('scroll', f)
      }
    }
    window.addEventListener('scroll', f)
  }
})

// main app
new Vue({
  el: '#app',
  methods: {handleScroll: function(evt, el) {if (window.scrollY > 50) {
      TweenMax.to(el, 1.5, {
        y: -10,
        opacity: 1,
        ease: Sine.easeOut
      })
    }
    return window.scrollY > 100
    }
  }
})

body {
  font-family: 'Abhaya Libre', Times, serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  background: #000;
  color: #fff;
  overflow-x: hidden;
}

h1,
h2,
h3,
h4 {
  font-family: 'Fira Sans', Helvetica, Arial, sans-serif;
  font-weight: 800;
}

.centered {
  margin: 0 auto;
  display: table;
  font-size: 60px;
  margin-top: 100px;
}

.box {border: 1px solid rgba(255, 255, 255, 0.5);
  padding: 8px 20px;
  line-height: 1.3em;
  opacity: 0;
  color: white;
  width: 200px;
  margin: 0 auto;
  margin-top: 30px;
  transform: translateZ(0);
  perspective: 1000px;
  backface-visibility: hidden;
  background: rgba(255, 255, 255, 0.1);
}

#app {height: 2000px;}

4. 参考资料

1. 《Vue.js 实战》作者：梁灏
2. 【译】vue 自定义指令的魅力