if.js

var _ = require('../util')
var compile = require('../compiler/compile')
var templateParser = require('../parsers/template')
var transition = require('../transition')

module.exports = {

  bind: function () {
    var el = this.el
    if (!el.__vue__) {
      this.start = document.createComment('v-if-start')
      this.end = document.createComment('v-if-end')
      _.replace(el, this.end)
      _.before(this.start, this.end)
      if (el.tagName === 'TEMPLATE') {
        this.template = templateParser.parse(el, true)
      } else {
        this.template = document.createDocumentFragment()
        this.template.appendChild(el)
      }
      // compile the nested partial
      this.linker = compile(
        this.template,
        this.vm.$options,
        true
      )
    } else {
      this.invalid = true
      _.warn(
        'v-if="' + this.expression + '" cannot be ' +
        'used on an already mounted instance.'
      )
    }
  },

  update: function (value) {
    if (this.invalid) return
    if (value) {
      this.insert()
    } else {
      this.teardown()
    }
  },

  insert: function () {
    // avoid duplicate inserts, since update() can be
    // called with different truthy values
    if (!this.unlink) {
      this.compile(this.template) 
    }
  },

  compile: function (template) {
    var vm = this.vm
    var frag = templateParser.clone(template)
    var originalChildLength = vm._children
      ? vm._children.length
      : 0
    this.unlink = this.linker
      ? this.linker(vm, frag)
      : vm.$compile(frag)
    transition.blockAppend(frag, this.end, vm)
    this.children = vm._children
      ? vm._children.slice(originalChildLength)
      : null
    if (this.children && _.inDoc(vm.$el)) {
      this.children.forEach(function (child) {
        child._callHook('attached')
      })
    }
  },

  teardown: function () {
    if (!this.unlink) return
    transition.blockRemove(this.start, this.end, this.vm)
    if (this.children && _.inDoc(this.vm.$el)) {
      this.children.forEach(function (child) {
        if (!child._isDestroyed) {
          child._callHook('detached')
        }
      })
    }
    this.unlink()
    this.unlink = null
  }

}