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Filename : throttle.js
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import requestAnimationFrame from '../shims/requestAnimationFrame.js';

// Defines minimum timeout before adding a trailing call.
const trailingTimeout = 2;

/**
 * Creates a wrapper function which ensures that provided callback will be
 * invoked only once during the specified delay period.
 *
 * @param {Function} callback - Function to be invoked after the delay period.
 * @param {number} delay - Delay after which to invoke callback.
 * @returns {Function}
 */
export default function (callback, delay) {
    let leadingCall = false,
        trailingCall = false,
        lastCallTime = 0;

    /**
     * Invokes the original callback function and schedules new invocation if
     * the "proxy" was called during current request.
     *
     * @returns {void}
     */
    function resolvePending() {
        if (leadingCall) {
            leadingCall = false;

            callback();
        }

        if (trailingCall) {
            proxy();
        }
    }

    /**
     * Callback invoked after the specified delay. It will further postpone
     * invocation of the original function delegating it to the
     * requestAnimationFrame.
     *
     * @returns {void}
     */
    function timeoutCallback() {
        requestAnimationFrame(resolvePending);
    }

    /**
     * Schedules invocation of the original function.
     *
     * @returns {void}
     */
    function proxy() {
        const timeStamp = Date.now();

        if (leadingCall) {
            // Reject immediately following calls.
            if (timeStamp - lastCallTime < trailingTimeout) {
                return;
            }

            // Schedule new call to be in invoked when the pending one is resolved.
            // This is important for "transitions" which never actually start
            // immediately so there is a chance that we might miss one if change
            // happens amids the pending invocation.
            trailingCall = true;
        } else {
            leadingCall = true;
            trailingCall = false;

            setTimeout(timeoutCallback, delay);
        }

        lastCallTime = timeStamp;
    }

    return proxy;
}