Track pending web requests with AngularJS

Navigation Guard

In the previous two posts (here and here), I covered the AngularJS factory, saNavigationGuard that I had created. This factory provides a simple way to guard against inconvenient page navigation in our Angular applications. However, up until now, I have not demonstrated one of the coolest uses for this. Before we look at this awesome use of saNavigationGuard, let us take a short trip into late last year.

I was getting started on my first big project with AngularJS and I had added a busy indicator to my user experience. To control when the indicator was active, I used a counter. This counter was incremented and decremented before and after using Angular resources. It was a nest of promises and error handlers, just to make sure the counter worked properly. I was not entirely happy with it but it was the best I could work out from my knowledge of Angular and JavaScript, so I submitted a pull request. My colleague reviewed the work and, looking at the busy indicator and the code to control the counter, stated, "there must be a better way."

I did not know at the time, but he was right, there is a better way and it uses an Angular feature called "interceptors".

Interceptors

Interceptors provide hooks into web requests, responses and their errors allowing us to modify or handle them in different ways. They are provided via an AngularJS provider such as a factory or service and injected into the $httpProvider using config as follows.

angular.module('somewhatabstract').config(['$httpProvider', function($httpProvider) {
	$httpProvider.interceptors.push('saHttpActivityInterceptor');
}]);

In this snippet, the name of the interceptor, saHttpActivityInterceptor, is added to the array of interceptors on $httpProvider.

The interceptor itself is a little more complex.

angular.module('somewhatabstract').factory('saHttpActivityInterceptor', ['$q', 'saNavigationGuard', function($q, saNavigationGuard) {
	var pendingRequestsCounter = 0;

	var updateCounter = function(method, delta) {
		if (method != 'POST' && method != 'PUT' && method != 'DELETE') {
		    return false;
		}
		pendingRequestsCounter += delta;
		return true;
	};

	saNavigationGuard.registerGuardian(function() {
		return pendingRequestsCounter > 0 ? 'There are changes pending.' : undefined;
	});

	return {
		request: function(request) {
			request.saTracked = updateCounter(request.method, 1);
			return request;
		},

		response: function(response) {
			if (response.config && response.config.saTracked) {
				updateCounter(response.config.method, -1);
			}
			return response;
		},

		responseError: function(rejection) {
			if (rejection.config && rejection.config.saTracked) {
				updateCounter(response.config.method, -1);
			}
			return $q.reject(rejection);
		}
	};
}]);

The interceptor factory returns an object that, in this case, has three methods: request, response and responseError. A fourth method, requestError, can also be included in interceptors if needed. Before returning our interceptor, the interceptor factory registers a guardian with saNavigationGuard that will guard against navigation if the pendingRequestsCounter is greater than zero.

The interceptor monitors requests and responses. On each request, the request method is checked, if it is POST, PUT or DELETE, the pendingRequestsCounter variable is incremented by one and the request is tagged to indicate it is being tracked. We flag it so that we know to pay attention when the corresponding response or response error occurs. In the response and response error handlers, we decrement our counter based on the tracking flag and the method.

Finally…

The outcome of using this interceptor is that if the user tries to navigate away from the page after a request has been made but before its response has been received, they will see a message asking them to consider postponing the navigation.

In the next post, we will look at testing both this interceptor and the saNavigationGuard functionality using my preferred combination of jasmine, CoffeeScript and jasmine-given.

As always, please consider leaving a comment if you have found this post useful or have any alternatives.

Navigation guard using AngularJS: $locationChangeStart

Where were we?

In the last post, I introduced saNavigationGuard, an AngularJS factory that provided a mechanism for different components to guard against the user navigating away from the page when it was not desirable. This used the $window.onbeforeunload event and therefore, only works when navigating outside of the AngularJS application. For in-app navigation, we need to monitor a different event that Angular provides called $locationChangeStart.

$locationChangeStart

In the following code I have expanded saNavigationGuard to include support for $locationChangeStart.

angular.module('somewhatabstract').factory('saNavigationGuard', ['$window', '$rootScope', function($window, $rootScope) {
	var guardians = [];

	var onBeforeUnloadHandler = function($event) {
		var message = getGuardMessage();
		if (message) {
			($event || $window.event).returnValue = message;
			return message;
		} else {
			return undefined;
		}
	};

	var locationChangeStartHandler = function($event) {
		var message = getGuardMessage();
		if (message && !$window.confirm(message))
		{
			if ($event.stopPropagation) $event.stopPropagation();
			if ($event.preventDefault) $event.preventDefault();
			$event.cancelBubble = true;
			$event.returnValue = false;
		}
	};

	var getGuardMessage = function () {
	    var message = undefined;
	    _.any(guardians, function(guardian) { return !!(message = guardian()); });
	    return message;
	};

	var registerGuardian = function(guardianCallback) {
		guardians.unshift(guardianCallback);
		return function() {
			var index = guardians.indexOf(guardianCallback);
			if (index >= 0) {
				guardians.splice(index, 1);
			}
		};
	};

	if ($window.addEventListener) {
		$window.addEventListener('beforeunload', onBeforeUnloadHandler);
	} else {
		$window.onbeforeunload = onBeforeUnloadHandler;
	}

	$rootScope.on('$locationChangeStart', locationChangeStartHandler);

	return {
		registerGuardian: registerGuardian
	};
}]);

In this new version of saNavigationGuard, the getGuardMessage method has been introduced that is responsible for querying the guardians for a reason not to allow navigation. In addition, an event handler has been added on $rootScope to handle $locationChangeStart. This handler is then used to query the guardians and display a confirmation message.

To display the confirmation message, I settled on using $window.confirm. Originally, I had thought about allowing a custom experience instead, but I felt that, due to any alternative experience being non-modal, the state management to allow a navigation to continue would be overly complicated.

Finally…

I was expecting the modifications to support $locationChangeStart to be more difficult than it turned out, though there are certainly some improvements that can be made¹.

In the next post, I am going to show how we can track pending web requests and prevent navigation accordingly. In the meantime, please comment and let me know if you have other uses for saNavigationGuard or even alternative implementations.

For example, the old and new URLs that are given as arguments to the $locationChangeStart handler could be passed to the guardians in order for them to make more informed decisions. [↩]

Navigation guard using AngularJS: onbeforeunload

onbeforeunload

I have been working on an add-on for one of our products that includes some data entry components. As part of this, I wanted to make sure that any changes to the data were saved before the user lost interest and went searching the Internet for cat pictures. I quickly turned to the window.onbeforeunload event.

The onbeforeunload event is the last chance for a website to get the user's attention before they have navigated away to something more shiny or sparkly. In AngularJS, since a real navigation is not going to occur in a single-page application, two events are provided, $locationChangeStart and $locationChangeSuccess, that can be used to similarly intercept in-app navigations.

Through onbeforeunload and the subsequent browser-based dialog, or the AngularJS events and a subsequent app-based dialog, the user is given the opportunity to cancel the navigation and address something important like unsaved data or a cute cat photo that they almost missed.

So, with events available to intercept navigation, I started hooking them up in all the places that needed them. A directive here, a factory there; before long I was duplicating almost the same code in nearly 20 places. There had to be a better way, so I created the saNavigationGuard service¹.

saNavigationGuard

The saNavigationGuard service provides consumers with the ability to register callbacks, known as guardians, that will be called if a navigation event is detected. If a guardian wishes to prevent that navigation, it returns a message explaining why. The saNavigationGuard then signs up to the relevant events and when they occur, asks each guardian if it should go ahead. Whichever guardian returns a string first, wins, and the guardian acts accordingly to prevent the navigation.

In the case of onbeforeunload , the browser continues the event handling by offering the user a choice to continue or cancel the navigation. In the case of the $locationChangeStart event, this part must be handled by the application. We will come back to how in another post; for right now, let's just look at the onbeforeunload scenario.

Here is the saNavigationGuard factory declaration:

angular.module('somewhatabstract').factory('saNavigationGuard', ['$window', function($window) {
	var guardians = [];

	var onBeforeUnloadHandler = function(event) {
		var message;
		if (_.any(guardians, function(guardian) { return !!(message = guardian()); })) {
			(event || $window.event).returnValue = message;
			return message;
		} else {
			return undefined;
		}
	}

	var registerGuardian = function(guardianCallback) {
		guardians.unshift(guardianCallback);
		return function() {
			var index = guardians.indexOf(guardianCallback);
			if (index >= 0) {
				guardians.splice(index, 1);
			}
		};
	};

	if ($window.addEventListener) {
		$window.addEventListener('beforeunload', onBeforeUnloadHandler);
	} else {
		$window.onbeforeunload = onBeforeUnloadHandler;
	}

	return {
		registerGuardian: registerGuardian
	};
}]);

First thing to note is on line 6 where I have used the any method from underscore (or lodash). This calls a predicate for each element in an array until the predicate returns true. In this case, we are passing our array of guardians and the predicate is calling that guardian to see if it currently wants to stop navigation. If it does, it will return a message.

If one of the guardians returns a message, the message it returns is captured and the predicate returns true. The message is then passed to the event so that the browser will show its dialog. If no guardian returns a message, then the browser is allowed to continue with the navigation unhindered (at least by this code).

Because of how factories are instantiated in Angular, the initialization code that actually signs-up to the onbeforeunload event only occurs once per Angular application. So, injecting this factory into your directives, factories, etc. means the event handling will be initialized just once. This gives a central point to control interception of navigation while allowing individual components to prevent navigation for their own reasons.

Usage

Here is a simple example of how this might be used:

angular.module('somewhatabstract').controller('saEditorThingyCtrl', ['$scope', 'saNavigationGuard', function($scope, saNavigationGuard) {
    var editing = false;
    var navigationGuardian = function() {
        return editing ? "Edit in progress" : undefined;
    };
    
    saNavigationGuard.registerGuardian(navigationGuardian);

    $scope.startEdit = function(){
        if (editing) return;
        editing = true;
        // Stuff happens
    };
    $scope.endEdit = function() {
        if (!editing) return;
        editing = false; 
    };
}]);

Of course, this is a contrived example but it illustrates how easy it is to put the saNavigationGuard to work. In fact, if you didn't want your guardian to be called all the time because you knew it usually would not block navigation, you could only have it registered when needed.

    ...
    var unregisterGuardian;
    
    $scope.startEdit = function(){
        ...
        unregisterGuardian = saNavigationGuard.registerGuardian(navigationGuardian);
        ...
    };
    $scope.endEdit = function() {
        ...
        unregisterGuardian();
        unregisterGuardian = undefined;
        ...
    };
    ...

This ability to register and unregister guardians brings me to my second noteworthy aspect of saNavigationGuard. When registering a new guardian, it is added to the start of the array of guardians rather than the end. This is because, in my use case, the more recently registered guardians are often the most urgent. I suspect this may be common for most uses.

Finally…

This post is the first in a series covering some useful things I have created in my experiences with AngularJS. In the posts to follow, I will be covering how we can expand saNavigationGuard to cover in-app navigation and how we can intercept web requests to provide both navigation guards and busy indication.

I hope you find the saNavigationGuard factory as useful as I have on my current project where I have guardians for active edits and when there are pending POST, PUT or DELETE requests.

If you use this or anything else that I have written about in your project or want to suggest some tweaks or alternatives, please don't forget to leave a comment.

It's really a factory in the AngularJS world, but it's providing a service…oh, I'm not getting into this discussion again, just look up service vs. factory and read about why it doesn't matter as long as you choose. [↩]