Using JS for Qt logic

Sorry, but I’m going to have to take sides on this one: Electron sucks. At their core, they convert a program into an embedded web browser with somewhat looser restrictions on interfacing with the host system, only to make it possible for web developers to develop for the desktop without learning a new language. The benefits of Electron are heavily biased toward developers and not the end-user, which is really the target of the entire user experience.

The problem is that HTML5 makes for an abstract machine that is far too abstract, and the more layers of abstraction that sit between the software and the hardware, the more overhead there will exist from translation and just-in-time compilation. The result is a web browser that uses 120 MB of shared memory and 200 MB of resident memory per tab.

For a web browser, though, abstraction is good. It makes development easy without sacrificing user experience for what is essentially a meta-program. But for any other desktop application with a specific defined functionality, this abstraction is excessive.

This is generally why I like Qt: because it is one of the only libraries left that continues to support native desktop and embedded application development without sacrificing performance. The one problem with it, however, is that performance requires the use of C++, and most people do not know how to work C++. Moreover, it requires double the work if the program is also being targeted for the web.

There does exist QML, which removes most of the C++ and exposes a nice declarative syntax that combines both layout and logic into a single file. However, it has two significant problems: first, it adds even more cruft to the output program, and custom functionality still requires interfacing with C++ code, which can get a little difficult.

Qt’s main Achilles’ heel for a long time has been targeting the web. There are various experimental solutions available, but none of them are stable or fast enough to do the job yet.

I’ve been coming up with an idea. Qt exposes its V4 JavaScript engine (a JIT engine) for use in traditional C++ desktop programs. What I could do is the following:

  • Write JS code that both the browser and desktop clients share in common, and then make calls to some abstract interface.
  • Implement the interface uniquely for each respective platform.

For instance, the wiring for most UI code can be written in C++, which then exposes properties and calls events in JS-land. Heck, Qt already does most of that work for us with meta-objects.

How do I maintain the strong contract of an interface? You need a little strong typing, don’t you? Of course, of course – we can always slap in TypeScript, which, naturally, compiles to standards-compliant JavaScript.

The one problem is supporting promises in the JS code that gets run, which mostly relies on the capabilities of the V4 engine. I think they support promises, but it does not seem well documented. Based on this post about invoking async C++ functions asynchronously, I think that I need to write callback-based functions on the C++ side and then promisify the functions when connecting between the JS interface and the C++ side. That shouldn’t be too hard.

Note that important new features for QtJsEngine, such as ES6, were only added in Qt 5.12. This might complicate distribution for Linux (since Qt continues to lag behind in Debian and Ubuntu), but we’ll get there when we get there – it is like thinking about tripping on a rock at the summit of a mountain when we are still at home base.

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