今天你清楚vue2的nextTick的原理了吗

心已赠人 2023-10-01 08:51 21阅读 0赞

#### 文章目录 ####

*  1. 入口
 *  2. \`nextTick\`执行过程

## 1. 入口 ##

1.  `src\core\instance\index.js`里面的`renderMixin(Vue)`
2.  `src\core\instance\render.js` 里面的

export function renderMixin (Vue: Class<Component>) {
        
      installRenderHelpers(Vue.prototype)
      Vue.prototype.$nextTick = function (fn: Function) {
        
        return nextTick(fn, this)
      }
        ...
    }

1.  `src\core\util\next-tick.js`
    
        export function nextTick (cb?: Function, ctx?: Object) {
              
          let _resolve
          callbacks.push(() => {
              
            if (cb) {
              
              try {
              
                cb.call(ctx)
              } catch (e) {
              
                handleError(e, ctx, 'nextTick')
              }
            } else if (_resolve) {
              
              _resolve(ctx)
            }
          })
          if (!pending) {
              
            pending = true
        
            /**
             * 异步函数
             */
            timerFunc()
          }
          // $flow-disable-line
          if (!cb && typeof Promise !== 'undefined') {
              
            return new Promise(resolve => {
              
              _resolve = resolve
            })
          }
        }

## 2. `nextTick`执行过程 ##

1.  确定使用微任务还是宏任务，并确定`timerfunc`

if (typeof Promise !== 'undefined' && isNative(Promise)) {
        
      const p = Promise.resolve()
      timerFunc = () => {
        
        /**
         * 启动微任务
         */
        p.then(flushCallbacks)
        if (isIOS) setTimeout(noop)
      }
      isUsingMicroTask = true
    } else if (!isIE && typeof MutationObserver !== 'undefined' && (
      isNative(MutationObserver) ||
      MutationObserver.toString() === '[object MutationObserverConstructor]'
    )) {
        
      let counter = 1
      const observer = new MutationObserver(flushCallbacks)
      const textNode = document.createTextNode(String(counter))
      observer.observe(textNode, {
        
        characterData: true
      })
      timerFunc = () => {
        
        counter = (counter + 1) % 2
        textNode.data = String(counter)
      }
      isUsingMicroTask = true
    } else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
        
      timerFunc = () => {
        
        setImmediate(flushCallbacks)
      }
    } else {
        
      timerFunc = () => {
        
        setTimeout(flushCallbacks, 0)
      }
    }

1.  当`nextTick`被调用且回调函数处于非`pending`时，调用`TimerFuc`

export function nextTick (cb?: Function, ctx?: Object) {
        
      let _resolve
      callbacks.push(() => {
        
        if (cb) {
        
          try {
        
            cb.call(ctx)
          } catch (e) {
        
            handleError(e, ctx, 'nextTick')
          }
        } else if (_resolve) {
        
          _resolve(ctx)
        }
      })
      if (!pending) {
        
        pending = true
    
        /**
         * 异步函数
         */
        timerFunc()
      }
      // $flow-disable-line
      if (!cb && typeof Promise !== 'undefined') {
        
        return new Promise(resolve => {
        
          _resolve = resolve
        })
      }
    }

1.  func是一个微任务或者宏任务的函数，里面放着`flushCallbacks`清空当前回调队列的任务

function flushCallbacks () {
        
      pending = false
    
      // arrayObject.slice(start,end) 返回选定元素
      const copies = callbacks.slice(0)
      callbacks.length = 0
      for (let i = 0; i < copies.length; i++) {
        
        copies[i]()
      }
    }

注意：`flushCallbacks`里面的`copies`是调用2的`nextTick`push进去的，也就是我们定义的回调函数

if (cb) {
        
          try {
        
            cb.call(ctx)
          } catch (e) {
        
            handleError(e, ctx, 'nextTick')
          }
        } else if (_resolve) {
        
          _resolve(ctx)
        }

回顾一下我们使用Vue的时候怎么使用nextTick就清楚了

this.$nextTick(()=>{
        
    	console.log('hahha')
    })

()=>{
        
    	console.log('hahha')
    }
    
    // 这个函数就是我们的copies啦

贴个官方的图吧

![在这里插入图片描述][watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5LiA6IWU6K-X5oSP6YaJ5LqG6YWS_size_16_color_FFFFFF_t_70_g_se_x_16]

*  到这里`nextTick`的原理就很清楚啦。

[watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5LiA6IWU6K-X5oSP6YaJ5LqG6YWS_size_16_color_FFFFFF_t_70_g_se_x_16]: https://img-blog.csdnimg.cn/ba40e07eb4924e9c87e5316a45f450c3.png?x-oss-process=image/watermark,type_d3F5LXplbmhlaQ,shadow_50,text_Q1NETiBA5LiA6IWU6K-X5oSP6YaJ5LqG6YWS,size_16,color_FFFFFF,t_70,g_se,x_16