(function (global, factory) {
    typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
        typeof define === 'function' && define.amd ? define(factory) :
            (global.ResizeObserver = factory());
}(this, (function () { 'use strict';

    /**
     * A collection of shims that provide minimal functionality of the ES6 collections.
     *
     * These implementations are not meant to be used outside of the ResizeObserver
     * modules as they cover only a limited range of use cases.
     */
    /* eslint-disable require-jsdoc, valid-jsdoc */
    var MapShim = (function () {
        if (typeof Map !== 'undefined') {
            return Map;
        }

        /**
         * Returns index in provided array that matches the specified key.
         *
         * @param {Array<Array>} arr
         * @param {*} key
         * @returns {number}
         */
        function getIndex(arr, key) {
            var result = -1;

            arr.some(function (entry, index) {
                if (entry[0] === key) {
                    result = index;

                    return true;
                }

                return false;
            });

            return result;
        }

        return (function () {
            function anonymous() {
                this.__entries__ = [];
            }

            var prototypeAccessors = { size: { configurable: true } };

            /**
             * @returns {boolean}
             */
            prototypeAccessors.size.get = function () {
                return this.__entries__.length;
            };

            /**
             * @param {*} key
             * @returns {*}
             */
            anonymous.prototype.get = function (key) {
                var index = getIndex(this.__entries__, key);
                var entry = this.__entries__[index];

                return entry && entry[1];
            };

            /**
             * @param {*} key
             * @param {*} value
             * @returns {void}
             */
            anonymous.prototype.set = function (key, value) {
                var index = getIndex(this.__entries__, key);

                if (~index) {
                    this.__entries__[index][1] = value;
                } else {
                    this.__entries__.push([key, value]);
                }
            };

            /**
             * @param {*} key
             * @returns {void}
             */
            anonymous.prototype.delete = function (key) {
                var entries = this.__entries__;
                var index = getIndex(entries, key);

                if (~index) {
                    entries.splice(index, 1);
                }
            };

            /**
             * @param {*} key
             * @returns {void}
             */
            anonymous.prototype.has = function (key) {
                return !!~getIndex(this.__entries__, key);
            };

            /**
             * @returns {void}
             */
            anonymous.prototype.clear = function () {
                this.__entries__.splice(0);
            };

            /**
             * @param {Function} callback
             * @param {*} [ctx=null]
             * @returns {void}
             */
            anonymous.prototype.forEach = function (callback, ctx) {
                var this$1 = this;
                if ( ctx === void 0 ) ctx = null;

                for (var i = 0, list = this$1.__entries__; i < list.length; i += 1) {
                    var entry = list[i];

                    callback.call(ctx, entry[1], entry[0]);
                }
            };

            Object.defineProperties( anonymous.prototype, prototypeAccessors );

            return anonymous;
        }());
    })();

    /**
     * Detects whether window and document objects are available in current environment.
     */
    var isBrowser = typeof window !== 'undefined' && typeof document !== 'undefined' && window.document === document;

// Returns global object of a current environment.
    var global$1 = (function () {
        if (typeof global !== 'undefined' && global.Math === Math) {
            return global;
        }

        if (typeof self !== 'undefined' && self.Math === Math) {
            return self;
        }

        if (typeof window !== 'undefined' && window.Math === Math) {
            return window;
        }

        // eslint-disable-next-line no-new-func
        return Function('return this')();
    })();

    /**
     * A shim for the requestAnimationFrame which falls back to the setTimeout if
     * first one is not supported.
     *
     * @returns {number} Requests' identifier.
     */
    var requestAnimationFrame$1 = (function () {
        if (typeof requestAnimationFrame === 'function') {
            // It's required to use a bounded function because IE sometimes throws
            // an "Invalid calling object" error if rAF is invoked without the global
            // object on the left hand side.
            return requestAnimationFrame.bind(global$1);
        }

        return function (callback) { return setTimeout(function () { return callback(Date.now()); }, 1000 / 60); };
    })();

// Defines minimum timeout before adding a trailing call.
    var 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}
     */
    var throttle = function (callback, delay) {
        var 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$1(resolvePending);
        }

        /**
         * Schedules invocation of the original function.
         *
         * @returns {void}
         */
        function proxy() {
            var 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;
    };

// Minimum delay before invoking the update of observers.
    var REFRESH_DELAY = 20;

// A list of substrings of CSS properties used to find transition events that
// might affect dimensions of observed elements.
    var transitionKeys = ['top', 'right', 'bottom', 'left', 'width', 'height', 'size', 'weight'];

// Check if MutationObserver is available.
    var mutationObserverSupported = typeof MutationObserver !== 'undefined';

    /**
     * Singleton controller class which handles updates of ResizeObserver instances.
     */
    var ResizeObserverController = function() {
        this.connected_ = false;
        this.mutationEventsAdded_ = false;
        this.mutationsObserver_ = null;
        this.observers_ = [];

        this.onTransitionEnd_ = this.onTransitionEnd_.bind(this);
        this.refresh = throttle(this.refresh.bind(this), REFRESH_DELAY);
    };

    /**
     * Adds observer to observers list.
     *
     * @param {ResizeObserverSPI} observer - Observer to be added.
     * @returns {void}
     */


    /**
     * Holds reference to the controller's instance.
     *
     * @private {ResizeObserverController}
     */


    /**
     * Keeps reference to the instance of MutationObserver.
     *
     * @private {MutationObserver}
     */

    /**
     * Indicates whether DOM listeners have been added.
     *
     * @private {boolean}
     */
    ResizeObserverController.prototype.addObserver = function (observer) {
        if (!~this.observers_.indexOf(observer)) {
            this.observers_.push(observer);
        }

        // Add listeners if they haven't been added yet.
        if (!this.connected_) {
            this.connect_();
        }
    };

    /**
     * Removes observer from observers list.
     *
     * @param {ResizeObserverSPI} observer - Observer to be removed.
     * @returns {void}
     */
    ResizeObserverController.prototype.removeObserver = function (observer) {
        var observers = this.observers_;
        var index = observers.indexOf(observer);

        // Remove observer if it's present in registry.
        if (~index) {
            observers.splice(index, 1);
        }

        // Remove listeners if controller has no connected observers.
        if (!observers.length && this.connected_) {
            this.disconnect_();
        }
    };

    /**
     * Invokes the update of observers. It will continue running updates insofar
     * it detects changes.
     *
     * @returns {void}
     */
    ResizeObserverController.prototype.refresh = function () {
        var changesDetected = this.updateObservers_();

        // Continue running updates if changes have been detected as there might
        // be future ones caused by CSS transitions.
        if (changesDetected) {
            this.refresh();
        }
    };

    /**
     * Updates every observer from observers list and notifies them of queued
     * entries.
     *
     * @private
     * @returns {boolean} Returns "true" if any observer has detected changes in
     *  dimensions of it's elements.
     */
    ResizeObserverController.prototype.updateObservers_ = function () {
        // Collect observers that have active observations.
        var activeObservers = this.observers_.filter(function (observer) {
            return observer.gatherActive(), observer.hasActive();
        });

        // Deliver notifications in a separate cycle in order to avoid any
        // collisions between observers, e.g. when multiple instances of
        // ResizeObserver are tracking the same element and the callback of one
        // of them changes content dimensions of the observed target. Sometimes
        // this may result in notifications being blocked for the rest of observers.
        activeObservers.forEach(function (observer) { return observer.broadcastActive(); });

        return activeObservers.length > 0;
    };

    /**
     * Initializes DOM listeners.
     *
     * @private
     * @returns {void}
     */
    ResizeObserverController.prototype.connect_ = function () {
        // Do nothing if running in a non-browser environment or if listeners
        // have been already added.
        if (!isBrowser || this.connected_) {
            return;
        }

        // Subscription to the "Transitionend" event is used as a workaround for
        // delayed transitions. This way it's possible to capture at least the
        // final state of an element.
        document.addEventListener('transitionend', this.onTransitionEnd_);

        window.addEventListener('resize', this.refresh);

        if (mutationObserverSupported) {
            this.mutationsObserver_ = new MutationObserver(this.refresh);

            this.mutationsObserver_.observe(document, {
                attributes: true,
                childList: true,
                characterData: true,
                subtree: true
            });
        } else {
            document.addEventListener('DOMSubtreeModified', this.refresh);

            this.mutationEventsAdded_ = true;
        }

        this.connected_ = true;
    };

    /**
     * Removes DOM listeners.
     *
     * @private
     * @returns {void}
     */
    ResizeObserverController.prototype.disconnect_ = function () {
        // Do nothing if running in a non-browser environment or if listeners
        // have been already removed.
        if (!isBrowser || !this.connected_) {
            return;
        }

        document.removeEventListener('transitionend', this.onTransitionEnd_);
        window.removeEventListener('resize', this.refresh);

        if (this.mutationsObserver_) {
            this.mutationsObserver_.disconnect();
        }

        if (this.mutationEventsAdded_) {
            document.removeEventListener('DOMSubtreeModified', this.refresh);
        }

        this.mutationsObserver_ = null;
        this.mutationEventsAdded_ = false;
        this.connected_ = false;
    };

    /**
     * "Transitionend" event handler.
     *
     * @private
     * @param {TransitionEvent} event
     * @returns {void}
     */
    ResizeObserverController.prototype.onTransitionEnd_ = function (ref) {
        var propertyName = ref.propertyName; if ( propertyName === void 0 ) propertyName = '';

        // Detect whether transition may affect dimensions of an element.
        var isReflowProperty = transitionKeys.some(function (key) {
            return !!~propertyName.indexOf(key);
        });

        if (isReflowProperty) {
            this.refresh();
        }
    };

    /**
     * Returns instance of the ResizeObserverController.
     *
     * @returns {ResizeObserverController}
     */
    ResizeObserverController.getInstance = function () {
        if (!this.instance_) {
            this.instance_ = new ResizeObserverController();
        }

        return this.instance_;
    };

    ResizeObserverController.instance_ = null;

    /**
     * Defines non-writable/enumerable properties of the provided target object.
     *
     * @param {Object} target - Object for which to define properties.
     * @param {Object} props - Properties to be defined.
     * @returns {Object} Target object.
     */
    var defineConfigurable = (function (target, props) {
        for (var i = 0, list = Object.keys(props); i < list.length; i += 1) {
            var key = list[i];

            Object.defineProperty(target, key, {
                value: props[key],
                enumerable: false,
                writable: false,
                configurable: true
            });
        }

        return target;
    });

    /**
     * Returns the global object associated with provided element.
     *
     * @param {Object} target
     * @returns {Object}
     */
    var getWindowOf = (function (target) {
        // Assume that the element is an instance of Node, which means that it
        // has the "ownerDocument" property from which we can retrieve a
        // corresponding global object.
        var ownerGlobal = target && target.ownerDocument && target.ownerDocument.defaultView;

        // Return the local global object if it's not possible extract one from
        // provided element.
        return ownerGlobal || global$1;
    });

// Placeholder of an empty content rectangle.
    var emptyRect = createRectInit(0, 0, 0, 0);

    /**
     * Converts provided string to a number.
     *
     * @param {number|string} value
     * @returns {number}
     */
    function toFloat(value) {
        return parseFloat(value) || 0;
    }

    /**
     * Extracts borders size from provided styles.
     *
     * @param {CSSStyleDeclaration} styles
     * @param {...string} positions - Borders positions (top, right, ...)
     * @returns {number}
     */
    function getBordersSize(styles) {
        var positions = [], len = arguments.length - 1;
        while ( len-- > 0 ) positions[ len ] = arguments[ len + 1 ];

        return positions.reduce(function (size, position) {
            var value = styles['border-' + position + '-width'];

            return size + toFloat(value);
        }, 0);
    }

    /**
     * Extracts paddings sizes from provided styles.
     *
     * @param {CSSStyleDeclaration} styles
     * @returns {Object} Paddings box.
     */
    function getPaddings(styles) {
        var positions = ['top', 'right', 'bottom', 'left'];
        var paddings = {};

        for (var i = 0, list = positions; i < list.length; i += 1) {
            var position = list[i];

            var value = styles['padding-' + position];

            paddings[position] = toFloat(value);
        }

        return paddings;
    }

    /**
     * Calculates content rectangle of provided SVG element.
     *
     * @param {SVGGraphicsElement} target - Element content rectangle of which needs
     *      to be calculated.
     * @returns {DOMRectInit}
     */
    function getSVGContentRect(target) {
        var bbox = target.getBBox();

        return createRectInit(0, 0, bbox.width, bbox.height);
    }

    /**
     * Calculates content rectangle of provided HTMLElement.
     *
     * @param {HTMLElement} target - Element for which to calculate the content rectangle.
     * @returns {DOMRectInit}
     */
    function getHTMLElementContentRect(target) {
        // Client width & height properties can't be
        // used exclusively as they provide rounded values.
        var clientWidth = target.clientWidth;
        var clientHeight = target.clientHeight;

        // By this condition we can catch all non-replaced inline, hidden and
        // detached elements. Though elements with width & height properties less
        // than 0.5 will be discarded as well.
        //
        // Without it we would need to implement separate methods for each of
        // those cases and it's not possible to perform a precise and performance
        // effective test for hidden elements. E.g. even jQuery's ':visible' filter
        // gives wrong results for elements with width & height less than 0.5.
        if (!clientWidth && !clientHeight) {
            return emptyRect;
        }

        var styles = getWindowOf(target).getComputedStyle(target);
        var paddings = getPaddings(styles);
        var horizPad = paddings.left + paddings.right;
        var vertPad = paddings.top + paddings.bottom;

        // Computed styles of width & height are being used because they are the
        // only dimensions available to JS that contain non-rounded values. It could
        // be possible to utilize the getBoundingClientRect if only it's data wasn't
        // affected by CSS transformations let alone paddings, borders and scroll bars.
        var width = toFloat(styles.width),
            height = toFloat(styles.height);

        // Width & height include paddings and borders when the 'border-box' box
        // model is applied (except for IE).
        if (styles.boxSizing === 'border-box') {
            // Following conditions are required to handle Internet Explorer which
            // doesn't include paddings and borders to computed CSS dimensions.
            //
            // We can say that if CSS dimensions + paddings are equal to the "client"
            // properties then it's either IE, and thus we don't need to subtract
            // anything, or an element merely doesn't have paddings/borders styles.
            if (Math.round(width + horizPad) !== clientWidth) {
                width -= getBordersSize(styles, 'left', 'right') + horizPad;
            }

            if (Math.round(height + vertPad) !== clientHeight) {
                height -= getBordersSize(styles, 'top', 'bottom') + vertPad;
            }
        }

        // Following steps can't be applied to the document's root element as its
        // client[Width/Height] properties represent viewport area of the window.
        // Besides, it's as well not necessary as the <html> itself neither has
        // rendered scroll bars nor it can be clipped.
        if (!isDocumentElement(target)) {
            // In some browsers (only in Firefox, actually) CSS width & height
            // include scroll bars size which can be removed at this step as scroll
            // bars are the only difference between rounded dimensions + paddings
            // and "client" properties, though that is not always true in Chrome.
            var vertScrollbar = Math.round(width + horizPad) - clientWidth;
            var horizScrollbar = Math.round(height + vertPad) - clientHeight;

            // Chrome has a rather weird rounding of "client" properties.
            // E.g. for an element with content width of 314.2px it sometimes gives
            // the client width of 315px and for the width of 314.7px it may give
            // 314px. And it doesn't happen all the time. So just ignore this delta
            // as a non-relevant.
            if (Math.abs(vertScrollbar) !== 1) {
                width -= vertScrollbar;
            }

            if (Math.abs(horizScrollbar) !== 1) {
                height -= horizScrollbar;
            }
        }

        return createRectInit(paddings.left, paddings.top, width, height);
    }

    /**
     * Checks whether provided element is an instance of the SVGGraphicsElement.
     *
     * @param {Element} target - Element to be checked.
     * @returns {boolean}
     */
    var isSVGGraphicsElement = (function () {
        // Some browsers, namely IE and Edge, don't have the SVGGraphicsElement
        // interface.
        if (typeof SVGGraphicsElement !== 'undefined') {
            return function (target) { return target instanceof getWindowOf(target).SVGGraphicsElement; };
        }

        // If it's so, then check that element is at least an instance of the
        // SVGElement and that it has the "getBBox" method.
        // eslint-disable-next-line no-extra-parens
        return function (target) { return target instanceof getWindowOf(target).SVGElement && typeof target.getBBox === 'function'; };
    })();

    /**
     * Checks whether provided element is a document element (<html>).
     *
     * @param {Element} target - Element to be checked.
     * @returns {boolean}
     */
    function isDocumentElement(target) {
        return target === getWindowOf(target).document.documentElement;
    }

    /**
     * Calculates an appropriate content rectangle for provided html or svg element.
     *
     * @param {Element} target - Element content rectangle of which needs to be calculated.
     * @returns {DOMRectInit}
     */
    function getContentRect(target) {
        if (!isBrowser) {
            return emptyRect;
        }

        if (isSVGGraphicsElement(target)) {
            return getSVGContentRect(target);
        }

        return getHTMLElementContentRect(target);
    }

    /**
     * Creates rectangle with an interface of the DOMRectReadOnly.
     * Spec: https://drafts.fxtf.org/geometry/#domrectreadonly
     *
     * @param {DOMRectInit} rectInit - Object with rectangle's x/y coordinates and dimensions.
     * @returns {DOMRectReadOnly}
     */
    function createReadOnlyRect(ref) {
        var x = ref.x;
        var y = ref.y;
        var width = ref.width;
        var height = ref.height;

        // If DOMRectReadOnly is available use it as a prototype for the rectangle.
        var Constr = typeof DOMRectReadOnly !== 'undefined' ? DOMRectReadOnly : Object;
        var rect = Object.create(Constr.prototype);

        // Rectangle's properties are not writable and non-enumerable.
        defineConfigurable(rect, {
            x: x, y: y, width: width, height: height,
            top: y,
            right: x + width,
            bottom: height + y,
            left: x
        });

        return rect;
    }

    /**
     * Creates DOMRectInit object based on the provided dimensions and the x/y coordinates.
     * Spec: https://drafts.fxtf.org/geometry/#dictdef-domrectinit
     *
     * @param {number} x - X coordinate.
     * @param {number} y - Y coordinate.
     * @param {number} width - Rectangle's width.
     * @param {number} height - Rectangle's height.
     * @returns {DOMRectInit}
     */
    function createRectInit(x, y, width, height) {
        return { x: x, y: y, width: width, height: height };
    }

    /**
     * Class that is responsible for computations of the content rectangle of
     * provided DOM element and for keeping track of it's changes.
     */
    var ResizeObservation = function(target) {
        this.broadcastWidth = 0;
        this.broadcastHeight = 0;
        this.contentRect_ = createRectInit(0, 0, 0, 0);

        this.target = target;
    };

    /**
     * Updates content rectangle and tells whether it's width or height properties
     * have changed since the last broadcast.
     *
     * @returns {boolean}
     */


    /**
     * Reference to the last observed content rectangle.
     *
     * @private {DOMRectInit}
     */


    /**
     * Broadcasted width of content rectangle.
     *
     * @type {number}
     */
    ResizeObservation.prototype.isActive = function () {
        var rect = getContentRect(this.target);

        this.contentRect_ = rect;

        return rect.width !== this.broadcastWidth || rect.height !== this.broadcastHeight;
    };

    /**
     * Updates 'broadcastWidth' and 'broadcastHeight' properties with a data
     * from the corresponding properties of the last observed content rectangle.
     *
     * @returns {DOMRectInit} Last observed content rectangle.
     */
    ResizeObservation.prototype.broadcastRect = function () {
        var rect = this.contentRect_;

        this.broadcastWidth = rect.width;
        this.broadcastHeight = rect.height;

        return rect;
    };

    var ResizeObserverEntry = function(target, rectInit) {
        var contentRect = createReadOnlyRect(rectInit);

        // According to the specification following properties are not writable
        // and are also not enumerable in the native implementation.
        //
        // Property accessors are not being used as they'd require to define a
        // private WeakMap storage which may cause memory leaks in browsers that
        // don't support this type of collections.
        defineConfigurable(this, { target: target, contentRect: contentRect });
    };

    var ResizeObserverSPI = function(callback, controller, callbackCtx) {
        this.activeObservations_ = [];
        this.observations_ = new MapShim();

        if (typeof callback !== 'function') {
            throw new TypeError('The callback provided as parameter 1 is not a function.');
        }

        this.callback_ = callback;
        this.controller_ = controller;
        this.callbackCtx_ = callbackCtx;
    };

    /**
     * Starts observing provided element.
     *
     * @param {Element} target - Element to be observed.
     * @returns {void}
     */


    /**
     * Registry of the ResizeObservation instances.
     *
     * @private {Map<Element, ResizeObservation>}
     */


    /**
     * Public ResizeObserver instance which will be passed to the callback
     * function and used as a value of it's "this" binding.
     *
     * @private {ResizeObserver}
     */

    /**
     * Collection of resize observations that have detected changes in dimensions
     * of elements.
     *
     * @private {Array<ResizeObservation>}
     */
    ResizeObserverSPI.prototype.observe = function (target) {
        if (!arguments.length) {
            throw new TypeError('1 argument required, but only 0 present.');
        }

        // Do nothing if current environment doesn't have the Element interface.
        if (typeof Element === 'undefined' || !(Element instanceof Object)) {
            return;
        }

        if (!(target instanceof getWindowOf(target).Element)) {
            throw new TypeError('parameter 1 is not of type "Element".');
        }

        var observations = this.observations_;

        // Do nothing if element is already being observed.
        if (observations.has(target)) {
            return;
        }

        observations.set(target, new ResizeObservation(target));

        this.controller_.addObserver(this);

        // Force the update of observations.
        this.controller_.refresh();
    };

    /**
     * Stops observing provided element.
     *
     * @param {Element} target - Element to stop observing.
     * @returns {void}
     */
    ResizeObserverSPI.prototype.unobserve = function (target) {
        if (!arguments.length) {
            throw new TypeError('1 argument required, but only 0 present.');
        }

        // Do nothing if current environment doesn't have the Element interface.
        if (typeof Element === 'undefined' || !(Element instanceof Object)) {
            return;
        }

        if (!(target instanceof getWindowOf(target).Element)) {
            throw new TypeError('parameter 1 is not of type "Element".');
        }

        var observations = this.observations_;

        // Do nothing if element is not being observed.
        if (!observations.has(target)) {
            return;
        }

        observations.delete(target);

        if (!observations.size) {
            this.controller_.removeObserver(this);
        }
    };

    /**
     * Stops observing all elements.
     *
     * @returns {void}
     */
    ResizeObserverSPI.prototype.disconnect = function () {
        this.clearActive();
        this.observations_.clear();
        this.controller_.removeObserver(this);
    };

    /**
     * Collects observation instances the associated element of which has changed
     * it's content rectangle.
     *
     * @returns {void}
     */
    ResizeObserverSPI.prototype.gatherActive = function () {
        var this$1 = this;

        this.clearActive();

        this.observations_.forEach(function (observation) {
            if (observation.isActive()) {
                this$1.activeObservations_.push(observation);
            }
        });
    };

    /**
     * Invokes initial callback function with a list of ResizeObserverEntry
     * instances collected from active resize observations.
     *
     * @returns {void}
     */
    ResizeObserverSPI.prototype.broadcastActive = function () {
        // Do nothing if observer doesn't have active observations.
        if (!this.hasActive()) {
            return;
        }

        var ctx = this.callbackCtx_;

        // Create ResizeObserverEntry instance for every active observation.
        var entries = this.activeObservations_.map(function (observation) {
            return new ResizeObserverEntry(observation.target, observation.broadcastRect());
        });

        this.callback_.call(ctx, entries, ctx);
        this.clearActive();
    };

    /**
     * Clears the collection of active observations.
     *
     * @returns {void}
     */
    ResizeObserverSPI.prototype.clearActive = function () {
        this.activeObservations_.splice(0);
    };

    /**
     * Tells whether observer has active observations.
     *
     * @returns {boolean}
     */
    ResizeObserverSPI.prototype.hasActive = function () {
        return this.activeObservations_.length > 0;
    };

// Registry of internal observers. If WeakMap is not available use current shim
// for the Map collection as it has all required methods and because WeakMap
// can't be fully polyfilled anyway.
    var observers = typeof WeakMap !== 'undefined' ? new WeakMap() : new MapShim();

    /**
     * ResizeObserver API. Encapsulates the ResizeObserver SPI implementation
     * exposing only those methods and properties that are defined in the spec.
     */
    var ResizeObserver = function(callback) {
        if (!(this instanceof ResizeObserver)) {
            throw new TypeError('Cannot call a class as a function.');
        }
        if (!arguments.length) {
            throw new TypeError('1 argument required, but only 0 present.');
        }

        var controller = ResizeObserverController.getInstance();
        var observer = new ResizeObserverSPI(callback, controller, this);

        observers.set(this, observer);
    };

// Expose public methods of ResizeObserver.
    ['observe', 'unobserve', 'disconnect'].forEach(function (method) {
        ResizeObserver.prototype[method] = function () {
            return (ref = observers.get(this))[method].apply(ref, arguments);
            var ref;
        };
    });

    var index = (function () {
        // Export existing implementation if available.
        if (typeof global$1.ResizeObserver !== 'undefined') {
            return global$1.ResizeObserver;
        }

        return ResizeObserver;
    })();

    return index;

})));