Programming user interfaces has many challenges. Fetching the data from remote service, populating the UI controls, tracking user input against the predefined set of validation rules, persisting the data back to the server, handling transitions between different application screens, supporting offline mode, handling error cases in the communication layer. All of these are just part of our daily programming tasks. While all of these relate directly to what the end user sees on the screen, there is a small subset that greatly affects the overall user experience.

The main two items in this subset are pixel perfection and application responsiveness. It is important to remember that the end user does not care about all of the wonderful underlying layers that take up a larger part of our day. As bad as analogies usually are, i would compare it to the vast majority of the drivers. I personally do not care how many people have worked on the specific model, how intricate are the internals of the engine and how many technical challenges had to be overcome during the development and testing. All i care about is that when i turn the key, the engine starts, when i turn the wheel / press the brake / turn on the AC, it responds correctly and immediately, and that it feels good to be sitting behind the wheel or in the passenger seat.

It is thus rather unfortunate that a lot of user interfaces are not implemented with these two main goals in mind. It would seem that programmers would rather enjoy letting the implementation complexity leaking into the UI layer – if it was hard to develop, it should at least look like that on the screen. There are multiple factors contributing to this. The farther you are from the target audience, the less you are able to judge the usability of the application, especially when you are immersed in its internals for long periods of time. When you operate at the level of data objects (with perhaps direct mapping to the optimized backend data storage), the users don’t. They see your application as a means to accomplish the specific flow that they have in mind. If you fail to crystallize the flows during the design stage, your users will see your application as unintuitive, time wasting and counter productive.

And even if you get the flows right – at least for your target audience – there is the issue of responsiveness. Imagine the following scenario. You’re in the kitchen, and want to heat that last slice of pizza from yesterday’s party. You put it in the microwave, press the button – and the entire kitchen freezes for one whole minute. You can look at the fridge, but you cannot open it. You remember that you pressed the microwave button, but the tray is not spinning and it does not make any noises. You step in your living room, and are not able to get back into the kitchen.

This is what happens when your code does I/O, database or any network operation on the UI thread. It does not really matter how small the operation is or how fast your machine is. If it locks the UI for even a fraction of a second, people will notice. You can say – well, this operation here that I’m doing is persisting the current screen state to the backend, so i cannot really let the user interact with the UI while i’m doing that. Or maybe it’s just not that important and the user can wait. Or maybe it’s just going to add to the development complexity of the UI layer.

Doing this right is hard. First, you need to understand what is right – and that varies depending on the specific scenario. Do you prevent the user from doing anything with the application? Or maybe let him browse some subparts of the screen while you’re running this long operation? Or maybe even let him change information elsewhere in the application? Do you show the progress of that operation? Do you allow canceling the operation? Do you allow creating dependencies between executing / scheduled operations?

And after you know what you want to do, the next step is actually implementing and testing it. At this point the word “multi-threading” will be your friend and nemesis. I cannot say that we are at a stage where doing multi-threading in UI is easy (although i certainly am not an expert in all the modern UI toolkits and libraries). It is certainly easier than it was a few years ago, but it’s still a mess. The words of Graham Hamilton from five years ago are still true today:

I believe you can program successfully with multi-threaded GUI toolkits if the toolkit is very carefully designed; if the toolkit exposes its locking methodology in gory detail; if you are very smart, very careful, and have a global understanding of the whole structure of the toolkit. If you get one of these things slightly wrong, things will mostly work, but you will get occasional hangs (due to deadlocks) or glitches (due to races). This multithreaded approach works best for people who have been intimately involved in the design of the toolkit.

Unfortunately I don’t think this set of characteristics scale to widespread commercial use. What you tend to end up with is normal smart programmers building apps that don’t quite work reliably for reasons that are not at all obvious. So the authors get very disgruntled and frustrated and use bad words on the poor innocent toolkit. (Like me when I first started using AWT. Sorry!)

I believe that any help we can get in writing correct multi-threaded code that deals with UI is welcome. This is why i continue enforcing the Swing threading rules in Substance. It is by far the biggest source of complaints ever since it was introduced about a year and a half ago in version 5.0. The original blog entry on the subject implied – rather unfortunately – that i wanted to make my job easier and not handle bugs that originate from UI threading violations. Allow me to clarify my position on the subject and repost my reply from one of Substance forum postings:

I do not intend to provide such API (disabling the threading checks) in the core Substance library. Once such an API exists, people will have little to no incentive to make their code more robust and compliant with the toolkit threading rules.

If the code you control violates the threading rules – and you *know* it – you should fix it. Does not matter if you’re using Substance or not.

If the code you do not control violates the threading rules – either put pressure on the respective developers to change it or stop using it.

It may be painful in the short term. I may lose potential users because of this. It may cause internal forks of the code base. I am aware of these issues. In my personal view, all of them are dwarfed by what is right in the long term interests of both Substance itself and Swing applications in general.

The followup posting by Adam Armistead provides a deeper look into why this matters – and i thank Adam for allowing me to repost it here:

I would just like to say I strongly support Kirill in this and I am very glad to see he is sticking to his guns on this. I feel that it is too easy to violate threading rules in UI code and that I run across entirely too much code that does. I feel that if someone doesn’t strictly enforce these threading rules then there is not enough pressure on developers to fix the problem. If there was enough pressure I wouldn’t see so damn much of this.

As for it causing problems due to 3rd party dependencies having bad UI code, there are tons of solutions for this. I have personally put pressure on developers to fix problems, submitted patches to open source projects to fix code myself, extended classes, forked codebases, used bytecode manipulation, proxy classes, and Spring and AspectJ method injection, method replacement, as well as adding before/after/around advice to methods. I have yet to encounter a situation where a little work or ingenuity on my part has not been able to overcome someone else’s crappy code. In the worst cases I have written my own libraries that don’t suck, but in most cases I didn’t have to go to this extreme.

I sincerely believe that having substance crash my applications has made MY applications as well as those I interact with better. I have seen people in comment sections in blog posts and on forums advise using Substance in order to assist developers in finding UI threading violations. I have fixed open source code and seen others do the same because Substance throws this exception. I can also say, I know may coders that if they had the choice to just log it, they would do so and just say, “aww, I’m busy, I’ll fix it later.” and likely never get around to it…

They’re always busy, so when one thing gets done there’s always half a dozen more that pop up. If it’s not crashing their code its not critical to fix. Besides, its too easy to violate these rules. I’m a Swing developer and I know the rules, and sometimes when I’m hammering out features I slip up and do things incorrectly. Personally, I am glad Substance catches it so I can fix it now.

What are the chances that Swing will start enforcing threading rules in the core? Zero. Between focusing all the client-side resources on JavaFX and sticking to binary compatibility (even when any reasonably sized Swing application needs code changes when it is ported between different JDKs), there is no chance. However, as SWT and Android show, a toolkit / library that aggressively enforces its own threading rules from the very beginning is not such a bad thing. And who knows, your users may even thank you for it.