淺談JavaScript節流和防抖函數
概念
節流函數
間隔固定的時間執行傳入的方法
目的是防止函數執行的頻率過快,影響性能.常見于跟滾動,鼠標移動事件綁定的功能.
防抖函數
對于接觸過硬件的人也許更好理解,硬件按鈕按下時,由于用戶按住時間的長短不一,會多次觸發電流的波動,加一個防抖函數就會只觸發一次,防止了無意義的電流波動引起的問題.
按鍵防反跳（Debounce）為什么要去抖動呢？機械按鍵在按下時，并非按下就接觸的很好，尤其是有簧片的機械開關，會在接觸的瞬間反復的開合多次，直到開關狀態完全改變。
應用在前端時,常見的場景是,輸入框打字動作結束一段時間后再去觸發查詢/搜索/校驗,而不是每打一個字都要去觸發,造成無意義的ajax查詢等,或者與調整窗口大小綁定的函數,其實只需要在最后窗口大小固定之后再去執行動作.
自己的實現
防抖函數
關鍵點在于每次觸發時都清空延時函數的手柄,只有最后一次觸發不會清空手柄,所以最后一次觸發會等默認的1s后去執行debounce傳入的參數函數f. debounce內部返回的閉包函數,是真正每次被調用觸發的函數,不再是原本的f,所以這里的arguments取閉包函數環境變量中的arguments并在執行f時傳給f,在setTimeout函數的外面取得.
let debounce = function(f, interval = 1000) { let handler = null; return function() { if (handler) { clearTimeout(handler); } let arg = arguments; handler = setTimeout(function() { f.apply(this, arg); clearTimeout(handler); }, interval) } }
應用:
let input = document.querySelector(’#input’); input.addEventListener(’input’, debounce(function(e) { console.log('您的輸入是',e.target.value) }))
更高級的實現還會考慮到,以leading和trailing作為參數,起始先執行一次函數并消除后面的抖動,還是最后執行一下函數,消除前面的抖動,如同我這里的例子.后面分析loadash的防抖函數時會詳細解析.
節流函數
let throttle = function(f,gap = 300){ let lastCall = 0; return function(){ let now = Date.now(); let ellapsed = now - lastCall; if(ellapsed < gap){ return } f.apply(this,arguments); lastCall = Date.now(); } }
閉包函數在不斷被調用的期間,去記錄離上一次調用間隔的時間,如果間隔時間小于節流設置的時間則直接返回,不去執行真正被包裹的函數f.只有間隔時間大于了節流函數設置的時間gap,才調用f,并更新調用時間.
應用:
document.addEventListener(’scroll’, throttle(function (e) { // 判斷是否滾動到底部的邏輯 console.log(e,document.documentElement.scrollTop); }));
lodash源碼分析
以上是對節流防抖函數最基礎簡單的實現,我們接下來分析一下lodash庫中節流防抖函數的分析.
節流函數的使用
$(window).on(’scroll’, _.debounce(doSomething, 200));
function debounce(func, wait, options) { var lastArgs, lastThis, result, timerId, lastCallTime = 0, lastInvokeTime = 0, leading = false, maxWait = false, trailing = true; if (typeof func != ’function’) { throw new TypeError(FUNC_ERROR_TEXT); } wait = wait || 0; if (isObject(options)) { leading = !!options.leading; maxWait = ’maxWait’ in options && Math.max((options.maxWait) || 0, wait); trailing = ’trailing’ in options ? !!options.trailing : trailing; } function invokeFunc(time) { var args = lastArgs, thisArg = lastThis; lastArgs = lastThis = undefined; lastInvokeTime = time; result = func.apply(thisArg, args); return result; } function leadingEdge(time) { console.log('leadingEdge setTimeout') // Reset any `maxWait` timer. lastInvokeTime = time; // Start the timer for the trailing edge. timerId = setTimeout(timerExpired, wait); // Invoke the leading edge. return leading ? invokeFunc(time) : result; } function remainingWait(time) { var timeSinceLastCall = time - lastCallTime, timeSinceLastInvoke = time - lastInvokeTime, result = wait - timeSinceLastCall; console.log('remainingWait',result) return maxWait === false ? result : Math.min(result, maxWait - timeSinceLastInvoke); } function shouldInvoke(time) { console.log('shouldInvoke') var timeSinceLastCall = time - lastCallTime, timeSinceLastInvoke = time - lastInvokeTime; console.log('time',time,'lastCallTime',lastCallTime,'timeSinceLastCall',timeSinceLastCall) console.log('time',time,'lastInvokeTime',lastInvokeTime,'timeSinceLastInvoke',timeSinceLastInvoke) console.log('should?',(!lastCallTime || (timeSinceLastCall >= wait) || (timeSinceLastCall < 0) || (maxWait !== false && timeSinceLastInvoke >= maxWait))) // Either this is the first call, activity has stopped and we’re at the // trailing edge, the system time has gone backwards and we’re treating // it as the trailing edge, or we’ve hit the `maxWait` limit. return (!lastCallTime || (timeSinceLastCall >= wait) || (timeSinceLastCall < 0) || (maxWait !== false && timeSinceLastInvoke >= maxWait)); } function timerExpired() { console.log('timerExpired') var time = Date.now(); if (shouldInvoke(time)) { return trailingEdge(time); } console.log('Restart the timer.',time,remainingWait(time)) // Restart the timer. console.log('timerExpired setTimeout') timerId = setTimeout(timerExpired, remainingWait(time)); } function trailingEdge(time) { clearTimeout(timerId); timerId = undefined; // Only invoke if we have `lastArgs` which means `func` has been // debounced at least once. console.log('trailing',trailing,'lastArgs',lastArgs) if (trailing && lastArgs) { return invokeFunc(time); } lastArgs = lastThis = undefined; return result; } function cancel() { if (timerId !== undefined) { clearTimeout(timerId); } lastCallTime = lastInvokeTime = 0; lastArgs = lastThis = timerId = undefined; } function flush() { return timerId === undefined ? result : trailingEdge(Date.now()); } function debounced() { var time = Date.now(), isInvoking = shouldInvoke(time); console.log('time',time); console.log('isInvoking',isInvoking); lastArgs = arguments; lastThis = this; lastCallTime = time; if (isInvoking) { if (timerId === undefined) { return leadingEdge(lastCallTime); } // Handle invocations in a tight loop. clearTimeout(timerId); console.log('setTimeout') timerId = setTimeout(timerExpired, wait); return invokeFunc(lastCallTime); } return result; } debounced.cancel = cancel; debounced.flush = flush; return debounced; }
ref
https://css-tricks.com/debouncing-throttling-explained-examples/
https://github.com/lodash/lodash/blob/4.7.0/lodash.js#L9840
https://jinlong.github.io/2016/04/24/Debouncing-and-Throttling-Explained-Through-Examples/
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