_.debounceは何をしますか？

Question

私が取り組んでいるプロジェクトでは、_。debounce（）を使用しています。

デバウンスのアンダースコアJSドキュメント は次のようになります。

debounce_.debounce(function, wait, [immediate])

最後に呼び出されてからwaitミリ秒が経過するまで実行を延期する、渡された関数の新しいデバウンスバージョンを作成して返します。

これは明らかに、debounce()が何をするかを知りたいと思っている人は誰でも、「デバウンス」が何を意味するかをすでに知っていることを前提としています。

デバウンスは実際に何をしますか？

Answer 1

基本的には、呼び出しを調整するため、短時間に複数回呼び出された場合、1つのインスタンスのみが呼び出されます。

なぜそれを使用するのですか？

Window.onresizeなどのイベントは、連続して複数回発生します。新しい位置で多くの計算を行う必要がある場合、計算を複数回実行する必要はありません。ユーザーがサイズ変更イベントを終了したときにのみ起動します。

Answer 2

Underscore.jsの ソースコード からの説明：

呼び出され続ける限り、トリガーされない関数を返します。この関数は、Nミリ秒の呼び出しが停止した後に呼び出されます。 「イミディエート」が渡された場合、トレーリングではなくリーディングエッジで関数をトリガーします。

自己コーディング：

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

Answer 3

Debounce Debounce in JavaScriptというタイトルの投稿を書きました。ここで正確に説明します デバウンス関数の仕組み とデモを含めます。

デバウンス関数は、関数の実行を「スロットル」する方法を提供します。通常、関数は、連続して起動するイベントにバインドされている状況で使用されます。ウィンドウのサイズ変更やスクロールでデバウンス機能が使用されるのはよくあることです。

アンダースコアであろうと他のJavaScriptライブラリであろうと、すべてのデバウンス関数はJavaScriptのネイティブsetTimeoutメソッドで構築されています。したがって、デバウンス関数の機能を理解する前に、徹底的な WindowTimersの理解 （MDNへのリンク）を用意することをお勧めします。

さらに、スコープとクロージャーを十分に理解する必要があります。サイズは比較的小さいですが、デバウンス関数は実際にはかなり高度なJavaScriptの概念を採用しています！

そうは言っても、上記の私の投稿で説明し、デモした基本的なデバウンス機能は次のとおりです。

完成品

// 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);
        }
    };
};

説明

// 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);
        }
    };
};

Answer 4

タイムアウトが期限切れになるまで関数の実行を保持します。これは、必要でないときに関数が連続して実行されるのを避けるためです。 underscore.debounce（）は複雑なコードに依存しているため、注意してください。ほとんどの場合、関数内の単純な「if」ステートメントは、デバウンスよりもはるかに高速です。カウンターを実装し、N回の反復ごとにメソッドを実行するか、タイムアウトを設定して、少なくとも一定のミリ秒が経過したことを確認できます。