概念
节流函数
间隔固定的时间执行传入的方法
目的是防止函数执行的频率过快,影响性能.常见于跟滚动,鼠标移动事件绑定的功能.
防抖函数
对于接触过硬件的人也许更好理解,硬件按钮按下时,由于用户按住时间的长短不一,会多次触发电流的波动,加一个防抖函数就会只触发一次,防止了无意义的电流波动引起的问题.
按键防反跳（Debounce）为什么要去抖动呢？机械按键在按下时，并非按下就接触的很好，尤其是有簧片的机械开关，会在接触的瞬间反复的开合多次，直到开关状态完全改变。
应用在前端时,常见的场景是,输入框打字动作结束一段时间后再去触发查询/搜索/校验,而不是每打一个字都要去触发,造成无意义的ajax查询等,或者与调整窗口大小绑定的函数,其实只需要在最后窗口大小固定之后再去执行动作.
自己的实现
防抖函数
关键点在于每次触发时都清空延时函数的手柄,只有最后一次触发不会清空手柄,所以最后一次触发会等默认的1s后去执行debounce传入的参数函数f. debounce内部返回的闭包函数,是真正每次被调用触发的函数,不再是原本的f,所以这里的arguments取闭包函数环境变量中的arguments并在执行f时传给f,在setTimeout函数的外面取得.
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)
}
}
应用:
let input = document.querySelector('#input');
input.addEventListener('input', debounce(function(e) {
console.log("您的输入是",e.target.value)
}))
更高级的实现还会考虑到,以leading和trailing作为参数,起始先执行一次函数并消除后面的抖动,还是最后执行一下函数,消除前面的抖动,如同我这里的例子.后面分析loadash的防抖函数时会详细解析.
节流函数
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();
}
}
闭包函数在不断被调用的期间,去记录离上一次调用间隔的时间,如果间隔时间小于节流设置的时间则直接返回,不去执行真正被包裹的函数f.只有间隔时间大于了节流函数设置的时间gap,才调用f,并更新调用时间.
应用:
document.addEventListener('scroll', throttle(function (e) {
// 判断是否滚动到底部的逻辑
console.log(e,document.documentElement.scrollTop);
}));
lodash源码分析
以上是对节流防抖函数最基础简单的实现,我们接下来分析一下lodash库中节流防抖函数的分析.
节流函数的使用
$(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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