Vue.js: カスタムディレクティブ

// directives/permission.js
export const permission = {
  mounted(el, binding) {
    const { value } = binding  // 'user.create'
    const userPermissions = getCurrentUser().permissions || []
    
    if (!userPermissions.includes(value)) {
      el.parentNode?.removeChild(el)  // If you don't have permission, remove it.
    }
  }
}

// main.js
import { permission } from './directives/permission'
app.directive('permission', permission)

// 2. Abbreviation（mounted + updated）
app.directive('color', (el, binding) => {
  el.style.color = binding.value
})

app.directive('demo', {
  // 1. created（Command Creation）
  created(el, binding) {
    console.log('1. Command Creation')
  },
  
  // 2. beforeMount（Before mounting the element）
  beforeMount(el) {
    console.log('2. Before Mounting')
  },
  
  // 3. mounted（The element has been mounted）⭐ Most Commonly Used
  mounted(el, binding) {
    console.log('3. Mounted')
  },
  
  // 4. beforeUpdate（Before the dependency update）
  beforeUpdate(el, binding) {
    console.log('4. Before the update')
  },
  
  // 5. updated（After the dependency update）
  updated(el, binding) {
    console.log('5. Updated')
  },
  
  // 6. beforeUnmount（Before Uninstalling）
  beforeUnmount(el) {
    console.log('6. Before Uninstalling')
  },
  
  // 7. unmounted（After uninstallation）⭐ For cleaning
  unmounted(el) {
    console.log('7. Uninstalled')
  }
})

// el, binding, vnode, prevVnode 4 parameter
mounted(el, binding, vnode, prevVnode) {
  // el: Elements Bound to Commands
  el.style.color = 'red'
  
  // binding: Instruction Information Object
  binding.value    // Instruction Value, e.g. v-foo="bar" → bar
  binding.arg      // Parameters, e.g. v-foo:arg → 'arg'
  binding.modifiers  // Modifiers, e.g. v-foo.bar → { bar: true }
  binding.instance  // Component Instances That Use Commands
  binding.dir       // Instruction-Defined Objects
  
  // vnode: Vue Virtual Node（Generally not used）
  // prevVnode: Previous Virtual Node
}

export const focus = {
  mounted(el, binding) {
    if (binding.value === false) return
    if (binding.arg) {
      setTimeout(() => el.focus(), parseInt(binding.arg))
    } else {
      el.focus()
    }
  }
}

初期表示: コマンドを付けた input がそのまま描画される
マウント直後に el.focus() が走り、入力欄へフォーカス
v-focus:500 なら 500ms 後、値が false なら何もしない

// directives/focus.js
export const focus = {
  mounted(el, binding) {
    if (binding.value !== false) {
      el.focus()
    }
  }
}

// directives/permission.js
import { getCurrentUser } from '@/utils/auth'

export const permission = {
  mounted(el, binding) {
    const { value, modifiers } = binding
    const user = getCurrentUser()
    
    // value: String 'user.create' or Array ['user.create', 'user.delete']
    // modifiers.disable: Disable, not remove
    const required = Array.isArray(value) ? value : [value]
    const hasPermission = required.every(p => 
      user.permissions?.includes(p)
    )
    
    if (!hasPermission) {
      if (modifiers.disable) {
        el.disabled = true
        el.title = 'No permission'
      } else {
        el.parentNode?.removeChild(el)
      }
    }
  }
}

// directives/debounce.js
export const debounce = {
  mounted(el, binding) {
    const { value, arg = 300 } = binding
    
    if (typeof value !== 'function') {
      console.warn('v-debounce: value must be a function')
      return
    }
    
    let timer = null
    el.__debounceTimer__ = timer
    
    el.addEventListener('click', () => {
      clearTimeout(timer)
      timer = setTimeout(() => value(), arg)
      el.__debounceTimer__ = timer
    })
  },
  
  unmounted(el) {
    if (el.__debounceTimer__) {
      clearTimeout(el.__debounceTimer__)
    }
  }
}

// directives/copy.js
export const copy = {
  mounted(el, binding) {
    el.addEventListener('click', async () => {
      try {
        await navigator.clipboard.writeText(binding.value)
        const original = el.textContent
        el.textContent = 'Copied!'
        setTimeout(() => { el.textContent = original }, 1500)
      } catch (err) {
        console.error('Copy failed:', err)
      }
    })
  }
}

// directives/lazyLoad.js
export const lazyLoad = {
  mounted(el, binding) {
    const observer = new IntersectionObserver(([entry]) => {
      if (entry.isIntersecting) {
        el.src = binding.value
        observer.unobserve(el)
      }
    })
    observer.observe(el)
    el.__observer__ = observer
  },
  
  unmounted(el) {
    el.__observer__?.disconnect()
  }
}

export const debounce = {
  mounted(el, binding) {
    const fn = binding.value
    const delay = parseInt(binding.arg) || 300
    
    if (typeof fn !== 'function') {
      console.warn('[v-debounce] value must be a function')
      return
    }
    
    let timer = null
    el.addEventListener('click', () => {
      clearTimeout(timer)
      timer = setTimeout(() => fn(), delay)
    })
    el._debounceTimer = timer
  },
  
  unmounted(el) {
    if (el._debounceTimer) clearTimeout(el._debounceTimer)
  }
}

ボタン表示: クリックで handler が呼ばれる
連続クリックでは最後の 1 回だけ 300ms 後に実行（v-debounce:500 なら 500ms）
アンマウント時にタイマーを破棄してリークを防ぐ

<!-- 1. v-directive="value" (value) -->
<input v-foo="username">

<!-- 2. v-directive:arg（Parameters，Fixed String） -->
<input v-foo:delay="500">

<!-- 3. v-directive.modifier（Modifiers，Boolean objects） -->
<input v-foo.bar>

<!-- 4. Combination -->
<input v-foo:delay.bar="500">

app.directive('demo', (el, binding) => {
  // v-demo="123"
  binding.value  // 123
  
  // v-demo:abc
  binding.arg  // 'abc'
  
  // v-demo.foo
  binding.modifiers  // { foo: true }
  
  // v-demo:abc.foo="123"
  binding.value    // 123
  binding.arg       // 'abc'
  binding.modifiers // { foo: true }
})

<!-- Scene 1：v-permission:disable -->
<button v-permission:disable="'user.create'">
  <!-- arg='disable', value='user.create' -->
</button>

<!-- Scene 2：v-debounce:500 -->
<button v-debounce:500="handler">
  <!-- arg='500'（500ms Image Stabilization） -->
</button>

<!-- Scene 3：v-once.lazy -->
<img v-once.lazy="imageUrl">
<!-- modifiers.lazy=true, value=imageUrl -->
</template>

import { getCurrentUser } from '@/utils/auth'

export const permission = {
  mounted(el, binding) {
    const { value, modifiers } = binding
    const user = getCurrentUser()
    const required = Array.isArray(value) ? value : [value]
    const ok = required.every(p => user.permissions?.includes(p))
    
    if (!ok) {
      if (modifiers.disable) {
        el.disabled = true
        el.style.opacity = '0.5'
        el.title = 'No permission'
      } else {
        el.parentNode?.removeChild(el)
      }
    }
  },
  
  updated(el, binding) {
    // Permissions may change（User Role Switching），Re-examine
    this.mounted(el, binding)
  }
}

権限あり: ボタンはそのまま表示されクリックできる
権限なし: ボタンが DOM から削除される
v-permission.disable なら disabled + 透明度 0.5 + title No permission

// Draw on Vue Implementation of Built-in Instructions
import { createApp } from 'vue'

const app = createApp({})

// 1. v-show（Control display）
app.directive('show', {
  mounted(el, binding) { el.style.display = binding.value ? '' : 'none' },
  updated(el, binding) { el.style.display = binding.value ? '' : 'none' }
})

// 2. v-text（Settings textContent）
app.directive('text', {
  mounted(el, binding) { el.textContent = binding.value },
  updated(el, binding) { el.textContent = binding.value }
})

// 3. v-html（Settings innerHTML，XSS Risks）
app.directive('html', {
  mounted(el, binding) { el.innerHTML = binding.value },
  updated(el, binding) { el.innerHTML = binding.value }
})

// 4. v-once（Render only once）
app.directive('once', {
  mounted(el, binding, vnode) {
    if (binding.value !== undefined) {
      vnode.context[binding.arg] = binding.value
    }
  }
})

v-show: 値に応じて display を none と空文字で切り替え
v-text / v-html: textContent / innerHTML を binding.value で更新
v-once: マウント時の 1 回だけ値を反映