angular material service to stack multiple toasts (demo at https://codepen.io/z00bs/pen/vGrjvX)

notification.service.js

(function () {
    'use strict';

    angular
        .module('app')
        .service('Notification', Notification);

    Notification.$inject = ['$mdToast', '$timeout'];
    function Notification($mdToast, $timeout) {
        var stack = [];
        var debounce;
        var lastMsg;
        var cooldown = 500;

        var service = {};
        service.show = show;

        return service;

        function show(options) {
            hideDelay(options, 5000);
            internalShow(basicToast(options));
        }

        function hideDelay(options, delay) {
            if (options.hideDelay == undefined) {
                options.hideDelay = delay;
            }
        }

        /**
         * We defer the showing in case too many toast are created in too little time (seems to be an issue with ng-material).
         *
         * @param toast
         */
        function internalShow(toast) {
            (function () {
                if (lastMsg === toast.locals.message) {
                    $timeout.cancel(debounce);
                }
                lastMsg = toast.locals.message;
                debounce = $timeout(function () {
                    lastMsg = null;
                    // we stack toasts with different msg not showing within 500 millis (cooldown)
                    stack.push(toast);
                    if (stack.length === 1) {
                        showStacked();
                    }
                }, cooldown);
            })();
        }

        function showStacked() {
            if (!stack.length) {
                return;
            }
            var toast = stack[0];
            $mdToast.show(toast).then(function () {
                stack.shift();
                showStacked();
            });
        }

        function basicToast(options) {
            return {
                templateUrl: 'toast-template.tpl.html',
                controller: 'ToastController',
                controllerAs: 'vm',
                locals: {
                    message: options.message
                },
                position: 'top right',
                hideDelay: options.hideDelay
            };
        }
    }
})();