【面试题】 vue为什么v-for的优先级比v-if的高？

newVue({
    el:'#app',
    template:`
        <ul>
            <li v-for="(item,index） in items" v-if="index!==0">
                {{item}}
            </li>
        </ul>
    `
})

// 其实ast有很多属性，我这里只展示涉及到分析的属性
ast = {
  'type': 1,
  'tag': 'ul',
  'attrsList': [],
  attrsMap: {},
  'children': [{
    'type': 1,
    'tag': 'li',
    'attrsList': [],
    'attrsMap': {
      'v-for': '(item,index) in data',
      'v-if': 'index!==0'
     },
     // v-if解析出来的属性'if': 'index!==0',
    'ifConditions': [{
      'exp': 'index!==0',
      'block': // 指向el自身
    }],
    // v-for解析出来的属性'for': 'items',
    'alias': 'item',
    'iterator1': 'index',
    'parent': // 指向其父节点'children': [
      'type': 2,
      'expression': '_s(item)''text': '{{item}}',
      'tokens': [
        {'@binding':'item'},
      ]
    ]
  }]
}

exportfunctiongenElement (el: ASTElement, state: CodegenState): string {
  if (el.parent) {
    el.pre = el.pre || el.parent.pre
  }
  if (el.staticRoot && !el.staticProcessed) {
    returngenStatic(el, state)
  } elseif (el.once && !el.onceProcessed) {
    returngenOnce(el, state)
  // 其实从此处可以初步知道为什么v-for优先级比v-if高，// 因为解析ast树生成渲染函数代码时，会先解析ast树中涉及到v-for的属性// 然后再解析ast树中涉及到v-if的属性// 而且genFor在会把el.forProcessed置为true，防止重复解析v-for相关属性
  } elseif (el.for && !el.forProcessed) {
    returngenFor(el, state)
  } elseif (el.if && !el.ifProcessed) {
    returngenIf(el, state)
  } elseif (el.tag === 'template' && !el.slotTarget && !state.pre) {
    returngenChildren(el, state) || 'void 0'
  } elseif (el.tag === 'slot') {
    returngenSlot(el, state)
  } else {
    // component or elementlet code
    if (el.component) {
      code = genComponent(el.component, el, state)
    } else {
      let data
      if (!el.plain || (el.pre && state.maybeComponent(el))) {
        data = genData(el, state)
      }
      const children = el.inlineTemplate ? null : genChildren(el, state, true)
      code = `_c('${el.tag}'${        data ? `,${data}` : '' // data      }${        children ? `,${children}` : '' // children      })`
    }
    // module transformsfor (let i = 0; i < state.transforms.length; i++) {
      code = state.transforms[i](el, code)
    }
    return code
  }
}

exportfunctiongenFor (el, state , altGen, altHelper) {
  const exp = el.forconst alias = el.aliasconst iterator1 = el.iterator1 ? `,${el.iterator1}` : ''const iterator2 = el.iterator2 ? `,${el.iterator2}` : ''
  el.forProcessed = true// avoid recursionreturn`${altHelper || '_l'}((${exp}),` + 
    `function(${alias}${iterator1}${iterator2}){` +
      `return ${(altGen || genElement)(el, state)}` + //递归调用genElement'})'
}

exportfunctiongenIf (el,state,altGen,altEmpty) {
  el.ifProcessed = true// avoid recursion// 调用genIfConditions主要处理el.ifConditions属性returngenIfConditions(el.ifConditions.slice(), state, altGen, altEmpty)
}
functiongenIfConditions (conditions, state, altGen, altEmpty) {
  if (!conditions.length) {
    return altEmpty || '_e()'// _e用于生成空VNode
  }
  const condition = conditions.shift()
  if (condition.exp) { //condition.exp即v-if绑定值，例子中则为'index!==0'// 生成一段带三目运算符的js代码字符串return`(${condition.exp})?${       genTernaryExp(condition.block)    }:${      genIfConditions(conditions, state, altGen, altEmpty)    }`
  } else {
    return`${genTernaryExp(condition.block)}`
  }
  // v-if with v-once should generate code like (a)?_m(0):_m(1)functiongenTernaryExp (el) {
    return altGen
      ? altGen(el, state)
      : el.once
        ? genOnce(el, state)
        : genElement(el, state)
  }
}

functionrender() {
  with(this) {
    return_c('ul', _l((items), function (item, index) {
      return (index !== 0) ? _c('li') : _e()
    }), 0)
  }
}