有人可以解释Java语言中的“反跳”功能吗

2020/11/06 23:22 · javascript ·  · 0评论

我对javascript中的“反跳”功能感兴趣,写在这里:http : //davidwalsh.name/javascript-debounce-function

不幸的是,代码的解释不够清楚,我无法理解。谁能帮我弄清楚它是如何工作的(我在下面留下了我的评论)。简而言之,我真的不明白这是如何工作的

   // Returns a function, that, as long as it continues to be invoked, will not
   // be triggered. The function will be called after it stops being called for
   // N milliseconds.


function debounce(func, wait, immediate) {
    var timeout;
    return function() {
        var context = this, args = arguments;
        var later = function() {
            timeout = null;
            if (!immediate) func.apply(context, args);
        };
        var callNow = immediate && !timeout;
        clearTimeout(timeout);
        timeout = setTimeout(later, wait);
        if (callNow) func.apply(context, args);
    };
};

编辑:复制的代码片段以前callNow在错误的位置。

问题中的代码与链接中的代码略有不同。在链接中,(immediate && !timeout)在创建新的timout之前要进行检查之后拥有它会导致即时模式从不触发。我已经更新了答案,以便从链接中注释工作版本。

function debounce(func, wait, immediate) {
  // 'private' variable for instance
  // The returned function will be able to reference this due to closure.
  // Each call to the returned function will share this common timer.
  var timeout;

  // Calling debounce returns a new anonymous function
  return function() {
    // reference the context and args for the setTimeout function
    var context = this,
      args = arguments;

    // Should the function be called now? If immediate is true
    //   and not already in a timeout then the answer is: Yes
    var callNow = immediate && !timeout;

    // This is the basic debounce behaviour where you can call this 
    //   function several times, but it will only execute once 
    //   [before or after imposing a delay]. 
    //   Each time the returned function is called, the timer starts over.
    clearTimeout(timeout);

    // Set the new timeout
    timeout = setTimeout(function() {

      // Inside the timeout function, clear the timeout variable
      // which will let the next execution run when in 'immediate' mode
      timeout = null;

      // Check if the function already ran with the immediate flag
      if (!immediate) {
        // Call the original function with apply
        // apply lets you define the 'this' object as well as the arguments 
        //    (both captured before setTimeout)
        func.apply(context, args);
      }
    }, wait);

    // Immediate mode and no wait timer? Execute the function..
    if (callNow) func.apply(context, args);
  }
}

/////////////////////////////////
// DEMO:

function onMouseMove(e){
  console.clear();
  console.log(e.x, e.y);
}

// Define the debounced function
var debouncedMouseMove = debounce(onMouseMove, 50);

// Call the debounced function on every mouse move
window.addEventListener('mousemove', debouncedMouseMove);

这里要注意的重要事情是debounce产生一个“封闭”变量函数timeouttimeout即使在debounce返回产生的函数之后变量在每次调用产生的函数期间仍保持可访问状态,并且可以在不同的调用之间进行切换。

的总体思路debounce如下:

  1. 从没有超时开始。
  2. 如果调用了产生的函数,请清除并重置超时。
  3. 如果超时,请调用原始函数。

第一点是正义var timeout;,的确是正义undefined幸运的是,clearTimeout它的输入相当宽松:传递undefined计时器标识符会使它什么都不做,它不会引发错误或其他任何事情。

第二点由产生的函数完成。它首先在变量中存储有关该调用的一些信息(this上下文和arguments),以便以后可以将它们用于去抖动的调用。然后清除超时(如果有一组),然后创建一个新的以替换它setTimeout请注意,这会覆盖的值,timeout并且该值会在多个函数调用中持续存在!这样可以使防抖动实际起作用:如果多次调用该函数,timeout则使用新的计时器多次覆盖该函数如果不是这种情况,则多次调用将导致启动多个计时器,这些计时器保持活动状态-调用只会延迟,而不会被去抖。

第三点是在超时回调中完成的。它取消设置timeout变量,并使用存储的调用信息进行实际的函数调用。

immediate标志应该控制是在计时器之前还是之后调用该函数如果为false,则在点击计时器后才调用原始函数如果为true,则将首先调用原始函数,直到点击计时器,该函数才会被调用。

但是,我确实认为if (immediate && !timeout)检查是错误的:timeout刚刚将其设置为返回的计时器标识符,setTimeout因此!timeout始终false在该点处进行,因此永远不能调用该函数。当前版本的underscore.js似乎有一些不同的检查,它immediate && !timeout 调用之前对其进行评估setTimeout(算法也有所不同,例如,它不使用clearTimeout。)因此,您应该始终尝试使用最新版本的库。:-)

防反跳函数在调用时不会执行,它们会在可配置的持续时间内等待调用暂停。每次新的调用都会重新启动计时器。

受限制的函数将执行,然后等待一段可配置的持续时间,然后才有资格再次触发。

防抖功能可用于按键事件。当用户开始键入然后暂停时,您将所有按键提交作为单个事件提交,从而减少了处理调用。

节流非常适合实时端点,您只希望允许用户在设定的时间段内调用一次即可。

还要查看Underscore.js的实现。

在JavaScript撰写了一篇题为Demistifying Debounce的文章,其中我确切地解释了去抖动功能的工作原理并包括一个演示。

当我第一次遇到反跳功能时,我也不太了解。尽管尺寸相对较小,但是它们实际上采用了一些相当高级的JavaScript概念!牢牢把握范围,闭包和setTimeout方法将有所帮助。

话虽如此,以下是我上面引用的我的帖子中解释和演示的基本去抖动功能。

成品

// Create JD Object
// ----------------
var JD = {};

// Debounce Method
// ---------------
JD.debounce = function(func, wait, immediate) {
    var timeout;
    return function() {
        var context = this,
            args = arguments;
        var later = function() {
            timeout = null;
            if ( !immediate ) {
                func.apply(context, args);
            }
        };
        var callNow = immediate && !timeout;
        clearTimeout(timeout);
        timeout = setTimeout(later, wait || 200);
        if ( callNow ) { 
            func.apply(context, args);
        }
    };
};

说明

// Create JD Object
// ----------------
/*
    It's a good idea to attach helper methods like `debounce` to your own 
    custom object. That way, you don't pollute the global space by 
    attaching methods to the `window` object and potentially run in to
    conflicts.
*/
var JD = {};

// Debounce Method
// ---------------
/*
    Return a function, that, as long as it continues to be invoked, will
    not be triggered. The function will be called after it stops being 
    called for `wait` milliseconds. If `immediate` is passed, trigger the 
    function on the leading edge, instead of the trailing.
*/
JD.debounce = function(func, wait, immediate) {
    /*
        Declare a variable named `timeout` variable that we will later use 
        to store the *timeout ID returned by the `setTimeout` function.

        *When setTimeout is called, it retuns a numeric ID. This unique ID
        can be used in conjunction with JavaScript's `clearTimeout` method 
        to prevent the code passed in the first argument of the `setTimout`
        function from being called. Note, this prevention will only occur
        if `clearTimeout` is called before the specified number of 
        milliseconds passed in the second argument of setTimeout have been
        met.
    */
    var timeout;

    /*
        Return an anomymous function that has access to the `func`
        argument of our `debounce` method through the process of closure.
    */
    return function() {

        /*
            1) Assign `this` to a variable named `context` so that the 
               `func` argument passed to our `debounce` method can be 
               called in the proper context.

            2) Assign all *arugments passed in the `func` argument of our
               `debounce` method to a variable named `args`.

            *JavaScript natively makes all arguments passed to a function
            accessible inside of the function in an array-like variable 
            named `arguments`. Assinging `arguments` to `args` combines 
            all arguments passed in the `func` argument of our `debounce` 
            method in a single variable.
        */
        var context = this,   /* 1 */
            args = arguments; /* 2 */

        /*
            Assign an anonymous function to a variable named `later`.
            This function will be passed in the first argument of the
            `setTimeout` function below.
        */
        var later = function() {

            /*      
                When the `later` function is called, remove the numeric ID 
                that was assigned to it by the `setTimeout` function.

                Note, by the time the `later` function is called, the
                `setTimeout` function will have returned a numeric ID to 
                the `timeout` variable. That numeric ID is removed by 
                assiging `null` to `timeout`.
            */
            timeout = null;

            /*
                If the boolean value passed in the `immediate` argument 
                of our `debouce` method is falsy, then invoke the 
                function passed in the `func` argument of our `debouce`
                method using JavaScript's *`apply` method.

                *The `apply` method allows you to call a function in an
                explicit context. The first argument defines what `this`
                should be. The second argument is passed as an array 
                containing all the arguments that should be passed to 
                `func` when it is called. Previously, we assigned `this` 
                to the `context` variable, and we assigned all arguments 
                passed in `func` to the `args` variable.
            */
            if ( !immediate ) {
                func.apply(context, args);
            }
        };

        /*
            If the value passed in the `immediate` argument of our 
            `debounce` method is truthy and the value assigned to `timeout`
            is falsy, then assign `true` to the `callNow` variable.
            Otherwise, assign `false` to the `callNow` variable.
        */
        var callNow = immediate && !timeout;

        /*
            As long as the event that our `debounce` method is bound to is 
            still firing within the `wait` period, remove the numerical ID  
            (returned to the `timeout` vaiable by `setTimeout`) from 
            JavaScript's execution queue. This prevents the function passed 
            in the `setTimeout` function from being invoked.

            Remember, the `debounce` method is intended for use on events
            that rapidly fire, ie: a window resize or scroll. The *first* 
            time the event fires, the `timeout` variable has been declared, 
            but no value has been assigned to it - it is `undefined`. 
            Therefore, nothing is removed from JavaScript's execution queue 
            because nothing has been placed in the queue - there is nothing 
            to clear.

            Below, the `timeout` variable is assigned the numerical ID 
            returned by the `setTimeout` function. So long as *subsequent* 
            events are fired before the `wait` is met, `timeout` will be 
            cleared, resulting in the function passed in the `setTimeout` 
            function being removed from the execution queue. As soon as the 
            `wait` is met, the function passed in the `setTimeout` function 
            will execute.
        */
        clearTimeout(timeout);

        /*
            Assign a `setTimout` function to the `timeout` variable we 
            previously declared. Pass the function assigned to the `later` 
            variable to the `setTimeout` function, along with the numerical 
            value assigned to the `wait` argument in our `debounce` method. 
            If no value is passed to the `wait` argument in our `debounce` 
            method, pass a value of 200 milliseconds to the `setTimeout` 
            function.  
        */
        timeout = setTimeout(later, wait || 200);

        /*
            Typically, you want the function passed in the `func` argument
            of our `debounce` method to execute once *after* the `wait` 
            period has been met for the event that our `debounce` method is 
            bound to (the trailing side). However, if you want the function 
            to execute once *before* the event has finished (on the leading 
            side), you can pass `true` in the `immediate` argument of our 
            `debounce` method.

            If `true` is passed in the `immediate` argument of our 
            `debounce` method, the value assigned to the `callNow` variable 
            declared above will be `true` only after the *first* time the 
            event that our `debounce` method is bound to has fired.

            After the first time the event is fired, the `timeout` variable
            will contain a falsey value. Therfore, the result of the 
            expression that gets assigned to the `callNow` variable is 
            `true` and the function passed in the `func` argument of our
            `debounce` method is exected in the line of code below.

            Every subsequent time the event that our `debounce` method is 
            bound to fires within the `wait` period, the `timeout` variable 
            holds the numerical ID returned from the `setTimout` function 
            assigned to it when the previous event was fired, and the 
            `debounce` method was executed.

            This means that for all subsequent events within the `wait`
            period, the `timeout` variable holds a truthy value, and the
            result of the expression that gets assigned to the `callNow`
            variable is `false`. Therefore, the function passed in the 
            `func` argument of our `debounce` method will not be executed.  

            Lastly, when the `wait` period is met and the `later` function
            that is passed in the `setTimeout` function executes, the 
            result is that it just assigns `null` to the `timeout` 
            variable. The `func` argument passed in our `debounce` method 
            will not be executed because the `if` condition inside the 
            `later` function fails. 
        */
        if ( callNow ) { 
            func.apply(context, args);
        }
    };
};

您要执行的操作如下:如果尝试立即调用另一个函数,则应取消第一个函数,而新的函数应等待给定的超时,然后执行。因此,实际上,您需要某种方式来取消第一个函数的超时?但是如何?可以调用该函数,并传递返回的超时ID,然后将该ID传递给任何新函数。但是上面的解决方案更优雅。

它的作用是有效地使timeout变量在返回函数的范围内可用。因此,当触发“调整大小”事件时,它不会debounce()再次调用,因此timeout内容不会更改(!),并且仍可用于“下一个函数调用”。

基本上,这里的关键是每次遇到resize事件时,我们都会调用内部函数。如果我们想象所有的resize-events都在一个数组中,则可能更清楚:

var events = ['resize', 'resize', 'resize'];
var timeout = null;
for (var i = 0; i < events.length; i++){
    if (immediate && !timeout) func.apply(this, arguments);
    clearTimeout(timeout); // does not do anything if timeout is null.
    timeout = setTimeout(function(){
        timeout = null;
        if (!immediate) func.apply(this, arguments);
    }
}

您看到timeout可供下次迭代使用吗?而且也没有理由,在我看来,重命名this,以contentargumentsargs

这是一个变体,总是在第一次调用去抖动功能时触发它,并使用更具描述性的变量:

function debounce(fn, wait = 1000) {
  let debounced = false;
  let resetDebouncedTimeout = null;
  return function(...args) {
    if (!debounced) {
      debounced = true;
      fn(...args);
      resetDebouncedTimeout = setTimeout(() => {
        debounced = false;
      }, wait);
    } else {
      clearTimeout(resetDebouncedTimeout);
      resetDebouncedTimeout = setTimeout(() => {
        debounced = false;
        fn(...args);
      }, wait);
    }
  }
};

JavaScript中的简单反跳方法

<!-- Basic HTML -->
<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>Debounce Method</title>
</head>
<body>
  <button type="button" id="debounce">Debounce Method</button><br />
  <span id="message"></span>
</body>
</html>

  // JS File
  var debouncebtn = document.getElementById('debounce');
    function debounce(func, delay){
      var debounceTimer;
      return function () {
        var context = this, args = arguments;
        clearTimeout(debounceTimer);
        debounceTimer = setTimeout(function() {
          func.apply(context, args)
        }, delay);
      }
    }

// Driver Code
debouncebtn.addEventListener('click', debounce(function() {
    document.getElementById('message').innerHTML += '<br/> Button only triggeres is every 3 secounds how much every you fire an event';
  console.log('Button only triggeres in every 3 secounds how much every you fire an event');
},3000))

运行时示例JSFiddle:https ://jsfiddle.net/arbaazshaikh919/d7543wqe/10/

简单的反跳功能:-

HTML:-

<button id='myid'>Click me</button>

Javascript:-

    function debounce(fn, delay) {
      let timeoutID;
      return function(...args){
          if(timeoutID) clearTimeout(timeoutID);
          timeoutID = setTimeout(()=>{
            fn(...args)
          }, delay);
      }
   }

document.getElementById('myid').addEventListener('click', debounce(() => {
  console.log('clicked');
},2000));
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