Vue source – nextTick

nextTick

// Flags whether microtasks are enabled
export let isUsingMicroTask = false
// A method to store nextTick
const callback = []
let pending = false
// Execute the nextTick method while clearing the nextTick task queue
function flushCallbacks(){
  pending = false
  const copies = callback.slice(0)
  callback.length = 0
  for(let i = 0; i < copies.length; i ++){ copies[i]() } }let timeFunc

if(typeof Promise! = ='undefined' && isNative(Promise)) {// Check whether the window environment /Node environment and Promise exist
  const p = Promise.resolve()
  // Perform the microtask calling nextTick based on the Promise
  timeFunc = () = > {
    p.then(flushCallbacks)
    if(isIOS) setTimeout(noop)
  }
  // flag to enable microtasks
  isUseingMicroTask = true
}else if(! isIE &&typeofMutationObserver ! = ='undefined' && (isNative(MutationObserver) || MutationObserver.toString() === '[object MutationObserverConstructor]')) {// Flag whether MutationObserver exists in the current non-IE environment
  let counter = 1
  const observer = new MutationObserver(flushCallbacks)
  const textNode = document.createTextNode(String(counter))
  // Monitor textNode to call nextTick
  observer.observe(textNode,{
    characterData: true
  })
  / / textNode changes
  timeFunc = () = > {
    counter = (counter + 1) %2
    textNode.data = String(counter)
  }
  // flag to enable microtasks
  isUseringMicroTask = true
}else if(typeofsetImmediate ! = ='undefined' && isNative(setImmediate)){
  // To determine the current Node environment,setImmediate exists
  // Macro task calls nextTick
  timeFunc = () = > {
    setImmediate(flushCallbacks)
  }
}else {
  // Macro task calls nextTick
  timeFunc = () = > {
    setTimeout(flushCallbacks,0)}}/ / nextTick run
export function nextTick(cb,ctx){
  let _resolve
  // Stores tasks to nextTick's task queue
  callbacks.push(() = > {
    if(cb){
      try{
        cb.call(ctx)
      }catch(e){
        handleError(e,ctx,'nextTick')}}else if(_resolve){
      _resolve(ctx)
    }
  })
  // Check whether nextTick is currently executing, otherwise execute
  if(! pending){ pending =true
    timeFunc()
  }
  // Determine that no task currently exists and return the calling object
  if(! cb &&typeof Promise! = ='undefined') {return new Promise(resolve= > {
      _resolve = resolve
    })
  }
}
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