Saturday, December 20, 2014

Initial thoughts on Responsive Single Page Applications

Is it feasible to build a responsive, mobile-first Single Page Application (SPA)? Here are my initial thoughts on the feasibility and challenges of building a responsive SPA. I plan to do more research in the coming weeks to hopefully answer this question more completely.

I was recently working on a mobile version of FuelMyRoute. I started by using jQuery Mobile and Backbone, but I found trying to use jQuery Mobile as a UI framework frustrating so I decided to give the Bootstrap CSS framework a try. I was able to convert my code over to Bootstrap fairly easily, but I started reading about and thinking about the responsive features of Bootstrap. This got me to thinking maybe I could make a responsive Single Page Application, one website that would work well on mobile devices and on desktop computers.

The benefits are pretty obvious I think. I don't relish the idea of maintaining two separate web applications. Especially when a lot of the non-UI code would be very similar. I could get a mobile site for my application while at the same time updating and modernizing the existing desktop site. That's a huge win!

I should probably define what I mean by responsive SPA. First of all, a Single Page Application is a web application where there is only an initial full page load. From that point forward AJAX is used to interact with the server to fetch data (e.g. get new mail) or to perform server side actions (e.g. send an email). The DOM is manipulated as appropriate in response to user actions and requested information. A responsive SPA is one where the UI of the SPA changes based on the size of the viewport, so that it is more appropriate/usable on smaller viewports but also can adapt and take advantage of more screen space on larger viewports. Responsive to me means also taking a mobile-first approach to web application development, where the mobile phone/small tablet use case is given primary (or at least equal) consideration.

But, I don't really see or hear much about responsive SPAs. I'm talking strictly about web applications, not web sites. If you are building, for example, a blog, then there is a lot of advice and support for making it responsive. However, for applications, it's different. The received wisdom is that it is better to have two different sites, one for mobile and one for desktop. Why? This is a question I want to answer along with the following:

  • What are the problems/tradeoffs that are encountered when making a responsive SPA?
  • Is it possible to use only CSS, only media queries to switch between different layouts for mobile and desktop usage? Or would it be necessary to have the JavaScript code be aware of whether the application is in mobile or desktop mode? (Ideally the responsiveness would be implemented in CSS only)

In this blog post I cover my initial thoughts about how a responsive SPA could work and what the challenges are likely to be.

Responsive Mobile UI Patterns

How would this work in practice? In this section I'll go over a couple of common mobile UI patterns and how they would adapt to a larger viewport.

Separate "pages" (mobile) vs in-page controls (desktop)

Sometimes the ideal view on mobile is very different from the ideal view on desktop. A common pattern here is that on desktop you have enough room to display all controls on the same screen, but on mobile you might want a separate full page view for certain actions. For example, imagine an airplane flight search application. On desktop you could have the fields for departing city and arriving city and dates on one page and display the results under those controls. On mobile, there isn't enough room for this so you would rather have a separate full page view for specifying search criteria.

In some ways, this is the least ideal situation. It is always better to be able to reuse the same controls and just style them differently using media queries. However, we must anticipate that this might not always be possible and think about what we could do in that situation.

To accomplish this, we could simply include the mobile full page alternate view in the page, hidden unless the width of the page is wide enough. By default, the mobile view is hidden.

#mobile {
    background-color: #fff;
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
@media only screen and (max-width: 480px) {
    .desktop-only {
        display: none;
    #mobile {
        display: block;

I created a very simple demo of this concept using the airplane ticket search example. In practice, you would want to have your application code add and remove CSS classes from a top level element to drive hiding and showing different views, but I've left that sort of thing out.

This "separate page" view could possible have a use on the desktop layout. You might have a situation where not all controls fit on the desktop layout so the same view to show all controls can be used on mobile and desktop. On mobile it might be styled as a full page view, whereas on desktop it could be styled as a modal popup.

Data binding is a little complicated in such a situation. Ideally, if the user changes the size of the browser window then switching from "mobile" to "desktop" or vice versa should be seamless from a user input perspective. Using the airplane ticket search example, if I am in the "mobile" view and I type in a Departing from city and then decide to expand my browser and get the "desktop" view, then ideally the Departing from control in the "desktop" layout should be populated with what I had typed on the "mobile" view. This means that I need to bind to the keypress event and update my model as each key is typed in Departing from and keep the "mobile" and "desktop" Departing from fields in sync.

Another challenge with this approach is that you, obviously, have two different views that have to be maintained. Arguably, this is one of the benefits of adopting MVC in the first place: your models are independent of views and models can have more than one view. However, it is still something extra that has to be maintained. It is probably better than having two separate sites though.

List and detail views on separate pages (mobile) vs both on the same page (desktop)

A common pattern with mobile apps is that you have a view(s) that display lists of data and tapping on a list item takes you to a view that displays details for that item. Examples include inbox listing/email detail and contact listing/contact details.

On desktop however there is enough space to show both the list and the detail, typically with the list on the left hand side and the detail view on the right. For example, an email client.

Media queries can be used to achieve both of these layouts. One nice thing here is that the same views are used on mobile and on desktop. Its just on mobile they are separate "pages" and on desktop they are all on one page.

How would you achieve this with media queries? I won't flesh out an entire example here, but I will outline how it would be done. The key I think is to have your SPA router add/remove CSS classes to a root element of the DOM. When the "list" class is added to the root element, the mobile media query will make just the list view visible, hiding the detail view. For desktop, there is no additional work needed. Likewise, when transitioning to the detail route, the "list" class is removed from the root element and a "detail" class is added. The mobile media query would then hide the list view and make the detail view visible. On desktop, the presence of the "detail" class on the root element might cause the currently selected item to be highlighted.

One challenge that this approach highlights (and this was true of the "separate view" UI pattern above) is that although on mobile the user is seeing only one view at a time, those other views are still there. For example, in an email client app, only the detail view may be visible, but the list view is still there and still responding to model updates. So for example, if you have a periodic check for new email, then the list view is updating to display new email received. This is more of a problem for mobile where off screen updates may cause performance to drag. On desktop the presence of alternate views that aren't on screen is unlikely to cause much of a problem.

Challenges with doing responsive SPA

Some of these challenges were mentioned above. Here I'll summarize them and also discuss how to address some of them. Not having experience developing a responsive SPA, this is mostly speculation about what I anticipate would be challenges building one.

Sizing of controls

It's possible to use media queries to size controls differently. You could for example make buttons large, more "tap friendly" on mobile screens and smaller on desktop screens.

However, one would have to assume that small screens are tapped on and large screens are clicked on. And that assumption isn't valid in a lot of cases. For example, a mobile phone might have a stylus that is able to make very precise touches. As another example, there are now several Windows 8 devices, some that are tablet/laptop hybrids, that are basically desktop devices but allow for touch interaction.

So I think a reasonable approach is to just size all controls for tapping. The tradeoff is that on desktop the controls are going to be "fatter" than necessary, meaning you won't be able to fit as many controls on the same page, but I don't think there is any reasonable alternative unless you want to aggravate uses with large touch screens.

Also, the fact that you can't have as many controls on a desktop view is not necessarily a bad thing. It will lead to a simplification of the UI, which will probably end up being easier to use.

Off screen view updates on mobile

I mentioned this one above. What can be done? For one, I think some testing and validation that this causes an issue is necessary first. Are DOM updates that are off screen as intensive as on screen ones for the browser to handle? Maybe, hopefully, not. I think it can very much depend on the application and might only affect certain views but not others.

If it becomes a problem I think one thing that could be done is to delay re-rendering the view if the view is off screen. Whenever it is ready to be brought back on screen the view could first be fully re-rendered. The problem here is that JS code now needs to know if the app is in "mobile" mode or "desktop" mode and so far I've been trying to keep the differences purely in CSS media queries.

Duplicate UIs

This one is mentioned above too. This is most applicable when the "mobile" version of the app needs a completely different view from the "desktop" version.

To mitigate this, first, obviously, try to use the same views for "mobile" and "desktop" and simply have them laid out differently.

Also it should be said that the duplication of effort and maintenance is probably not nearly so much as having two separate sites.

Also as mentioned above, there is also the data binding consideration: you'll want to keep the "mobile" and "desktop" views in sync at all times because the user could at any time transition from one to the other. In practice I think this requires only a little extra effort. See the discussion on listening for keypress events above.


Well that's it for my initial thoughts. I hope to investigate this further and try some of these techniques out.