Priority comparison: Which is higher, v-for or v-if?

Which one has higher priority, v-for or v-if? The following article will explain the priorities of v-for and v-if from actual examples and source code. I believe you will be enlightened after reading this.

Conclusion

1. It is not recommended to use v-for and v-if at the same time. (Learning video sharing: vue video tutorial)

2. v-for in vue2 has a higher priority than v-if. Because vue2 will process v-for first and then v-if when compiling the template, the generated rendering function will first execute the loop, and then execute the conditional judgment inside the loop.

3. The problem caused by doing this is

For scenario 1:<li v-for="user in users" v-if="user.show">

Every time we re-render, we have to re-traverse the entire list. In fact, we only need part of the list, which wastes performance. The recommended approach is to filter out the parts we need through calculated attributes and then render them, which is more efficient.

For scenario 2: <li v-for="user in users" v-if="globalShow">

globalShow This judgment actually if it is false , the loop does not need to be executed, but now it is used with v-if, and the loop must be executed regardless of whether globalShow is true or not, which is a complete waste. The recommended approach is to move v-if up to the ul container.

4. It should be noted that for the breaking change of vue3, v-if has a higher priority than v-for in vue3, so if they are used at the same time, for scenario 1, the user has not yet v at this time. -if="user.show" will report an error

5. Generally, if we use eslint, it will also report an error to us. The corresponding rule is: vue/no-use-v-if-with-v- for

Actual example

For example: the following template will generate the following rendering function

    
•         {{ user.name }}     

The generated rendering function is as follows

with(this) {
    return _c('ul', _l((users), function (user) {
        return (user.isActive) ? _c('li', user.name) : _e()
    }), 0)
}

Generated from above It can be seen from the rendering function that _l will be executed first to traverse user, and conditional judgment will be made inside

source code

processing v-if and v -For's source code

src/compiler/index.js

// 模板解析，生成ast树
const ast = parse(template.trim(), options)
if (options.optimize !== false) {
    optimize(ast, options)
}
const code = generate(ast, options)

Generate code based on ast. If it is the above template, the generated ast is simplified as follows

// 可以看出v-for和v-if都解析出来了
 ast = {
     'type': 1,
     'tag': 'ul',
     'attrsList': [],
     'attrsMap': {},
     'children': [{
     'type': 1,
     'tag': 'li',
     'attrsList': [],
     'attrsMap': {
         'v-for': 'user in users',
         'v-if': 'user.show'
     },
     // v-if解析出来的属性
     'if': 'user.show',
     'ifConditions': [{
         'exp': 'user.show',
         'block': // 指向el自身
     }],
     // v-for解析出来的属性
     'for': 'users',
     'alias': 'user',
     'iterator1': 'index',
     'parent': // 指向其父节点
     'children': [
         'type': 2,
         'expression': '_s(user)'
         'text': '{{user}}',
         'tokens': [
             {'@binding':'user'},
         ]
      ]
     }]
 }

compiler/codegen/index.js

// generate 调用 genElement
const code = ast ? genElement(ast, state) : '_c("div")'
// genElement里面的处理
if (el.staticRoot && !el.staticProcessed) {
return genStatic(el, state)
} else if (el.once && !el.onceProcessed) {
return genOnce(el, state)
// 从这可以看出来，先执行genFor，处理v-for指令，在genFor里面会递归调用genElement，继续处理v-if，genFor会将forProcessed设为true，这样下次进来的时候就不会处理for了
} else if (el.for && !el.forProcessed) {
return genFor(el, state)
} else if (el.if && !el.ifProcessed) {
return genIf(el, state)
} else if (el.tag === 'template' && !el.slotTarget && !state.pre) {
return genChildren(el, state) || 'void 0'
} else if (el.tag === 'slot') {
return genSlot(el, state)
} else {
// 最后这里处理标签等
const children = el.inlineTemplate ? null : genChildren(el, state, true)
code = `_c('${el.tag}'${
data ? `,${data}` : '' // data
}${
children ? `,${children}` : '' // children
})`
}

// genFor的代码
const exp = el.for // 对应上面ast的 for: users
const alias = el.alias // alias: user
// iterator1 对应v-for的(item,key,index) in items的key
// iterator2 对应的是index
// 通常我们遍历数组 key就是index
// 假如我们遍历的是对象 key就是对象的key，index就是遍历的索引
const iterator1 = el.iterator1 ? `,${el.iterator1}` : ''
const iterator2 = el.iterator2 ? `,${el.iterator2}` : ''
el.forProcessed = true // 下次递归调用genElement的时候就不会重复处理v-for了
return `${altHelper || '_l'}((${exp}),` +
`function(${alias}${iterator1}${iterator2}){`

// 这里处理完了v-for，递归调用genElement继续处理v-if
`return ${(altGen || genElement)(el, state)}` +
'})'

will eventually generate code similar to the following and return it

_l((users), function(user, index) {
    // 如果有v-if 前面就会有个条件判断，如user.isActive
    return (user.isActive) ? _c('li', user.name) : _e()
});

(Learning video sharing : Web front-end development, Basic programming video)

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