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Isomorphic Desktop Apps with Rust

Electron + WASM = ☣

  • rust
  • javascript
  • webassembly

Forgive me, but this is going to be a bit of a schizophrenic post. I both despise Javascript and the modern ECMAScript ecosystem, and I’m stunned by its success doing some really cool things. It’s this duality that’s led me to a couple of (very) late nights over the past weeks trying to reconcile myself as I bootstrap a simple desktop application.

See, as much as Webassembly isn’t trying to replace Javascript, I want Javascript gone. There are plenty of people who don’t share my views, and they are probably nicer and more fun at parties. But I cringe every time “Webpack” is mentioned, and I think it’s hilarious that the language specification dramatically outpaces anyone’s actual implementation. The answer to this conundrum is of course to recompile code from newer versions of the language to older versions of the same language before running. At least Babel is a nice tongue-in-cheek reference.

Yet for as much hate as Electron receives, it does a stunningly good job at solving a really hard problem: how the hell do I put a button on the screen and react when the user clicks it? GUI programming is hard, straight up. But if browsers are already able to run everywhere, why don’t we take advantage of someone else solving the hard problems for us? I don’t like that I have to use Javascript for it, but I really don’t feel inclined to whip out good ol’ wxWidgets.

Now there are other native solutions (libui-rs, conrod, oh hey wxWdidgets again!), but those also have their own issues with distribution, styling, etc. With Electron, I can yarn create electron-app my-app and just get going, knowing that packaging/upgrades/etc. are built in.

My question is: given recent innovations with WASM, are we Electron yet?

No, not really.

Instead, what would it take to get to a point where we can skip Javascript in Electron apps?

Setting the Stage

Truth is, WASM/Webassembly is a pretty new technology and I’m a total beginner in this area. There may already be solutions to the issues I discuss, but I’m totally unaware of them, so I’m going to try and organize what I did manage to discover.

I should also mention that the content and things I’m talking about here are not intended to be prescriptive, but more “if someone else is interested, what do we already know doesn’t work?” I expect everything in this post to be obsolete within two months. Even over the course of writing this, a separate blog post had to be modified because upstream changes broke a Rust tool the post tried to use. The post ultimately got updated, but all this happened within the span of a week. Things are moving quickly.

I’ll also note that we’re going to skip asm.js and emscripten. Truth be told, I couldn’t get either of these to output anything, and so I’m just going to say here be dragons. Everything I’m discussing here uses the wasm32-unknown-unknown target.

The code that I did get running is available over here. Feel free to use it as a starting point, but I’m mostly including the link as a reference for the things that were attempted.

An Example Running Application

So, I did technically get a running application:

Electron app using WASM

…which you can also try out if you want:

git clone https://github.com/bspeice/isomorphic_rust.git
cd isomorphic_rust/percy
yarn install && yarn start

…but I wouldn’t really call it a “high quality” starting point to base future work on. It’s mostly there to prove this is possible in the first place. And that’s something to be proud of! There’s a huge amount of engineering that went into showing a window with the text “It’s alive!”.

There’s also a lot of usability issues that prevent me from recommending anyone try Electron and WASM apps at the moment, and I think that’s the more important thing to discuss.

Issue the First: Complicated Toolchains

I quickly established that wasm-bindgen was necessary to “link” my Rust code to Javascript. At that point you’ve got an Electron app that starts an HTML page which ultimately fetches your WASM blob. To keep things simple, the goal was to package everything using webpack so that I could just load a bundle.js file on the page. That decision was to be the last thing that kinda worked in this process.

The first issue I ran into while attempting to bundle everything via webpack is a detail in the WASM spec:

This function accepts a Response object, or a promise for one, and … [if it] does not match the application/wasm MIME type, the returned promise will be rejected with a TypeError;

WebAssembly - Additional Web Embedding API

Specifically, if you try and load a WASM blob without the MIME type set, you’ll get an error. On the web this isn’t a huge issue, as the server can set MIME types when delivering the blob. With Electron, you’re resolving things with a file:// URL and thus can’t control the MIME type:

TypeError: Incorrect response MIME type. Expected 'application/wasm'.

There are a couple of solutions depending on how far into the deep end you care to venture:

  • Embed a static file server in your Electron application
  • Use a custom protocol and custom protocol handler
  • Host your WASM blob on a website that you resolve at runtime

But all these are pretty bad solutions and defeat the purpose of using WASM in the first place. Instead, my workaround was to open a PR with webpack and use regex to remove calls to instantiateStreaming in the build script:

cargo +nightly build --target=wasm32-unknown-unknown && \
    wasm-bindgen "$WASM_DIR/debug/$WASM_NAME.wasm" --out-dir "$APP_DIR" --no-typescript && \
    # Have to use --mode=development so we can patch out the call to instantiateStreaming
    "$DIR/node_modules/webpack-cli/bin/cli.js" --mode=development "$APP_DIR/app_loader.js" -o "$APP_DIR/bundle.js" && \
    sed -i 's/.*instantiateStreaming.*//g' "$APP_DIR/bundle.js"

Once that lands, the build process becomes much simpler:

cargo +nightly build --target=wasm32-unknown-unknown && \
    wasm-bindgen "$WASM_DIR/debug/$WASM_NAME.wasm" --out-dir "$APP_DIR" --no-typescript && \
    "$DIR/node_modules/webpack-cli/bin/cli.js" --mode=production "$APP_DIR/app_loader.js" -o "$APP_DIR/bundle.js"

But we’re not done yet! After we compile Rust into WASM and link WASM to Javascript (via wasm-bindgen and webpack), we still have to make an Electron app. For this purpose I used a starter app from Electron Forge, and then a prestart script to actually handle starting the application.

The final toolchain looks something like this:

  • yarn start triggers the prestart script
  • prestart checks for missing tools (wasm-bindgen-cli, etc.) and then:
    • Uses cargo to compile the Rust code into WASM
    • Uses wasm-bindgen to link the WASM blob into a Javascript file with exported symbols
    • Uses webpack to bundle the page start script with the Javascript we just generated
      • Uses babel under the hood to compile the wasm-bindgen code down from ES6 into something browser-compatible
  • The start script runs an Electron Forge handler to do some sanity checks
  • Electron actually starts

…which is complicated. I think more work needs to be done to either build a high-quality starter app that can manage these steps, or another tool that “just handles” the complexity of linking a compiled WASM file into something the Electron browser can run.

Issue the Second: WASM tools in Rust

For as much as I didn’t enjoy the Javascript tooling needed to interface with Rust, the Rust-only bits aren’t any better at the moment. I get it, a lot of projects are just starting off, and that leads to a fragmented ecosystem. Here’s what I can recommend as a starting point:

Don’t check in your Cargo.lock files to version control. If there’s a disagreement between the version of wasm-bindgen-cli you have installed and the wasm-bindgen you’re compiling with in Cargo.lock, you get a nasty error:

it looks like the Rust project used to create this wasm file was linked against
a different version of wasm-bindgen than this binary:

rust wasm file: 0.2.21
    this binary: 0.2.17

Currently the bindgen format is unstable enough that these two version must
exactly match, so it's required that these two version are kept in sync by
either updating the wasm-bindgen dependency or this binary.

Not that I ever managed to run into this myself (coughs nervously).

There are two projects attempting to be “application frameworks”: percy and yew. Between those, I managed to get two examples running using percy, but was unable to get an example running with yew because of issues with “missing modules” during the webpack step:

ERROR in ./dist/electron_yew_wasm_bg.wasm
Module not found: Error: Can't resolve 'env' in '/home/bspeice/Development/isomorphic_rust/yew/dist'
 @ ./dist/electron_yew_wasm_bg.wasm
 @ ./dist/electron_yew_wasm.js
 @ ./dist/app.js
 @ ./dist/app_loader.js

If you want to work with the browser APIs directly, your choices are percy-webapis or stdweb (or eventually web-sys). See above for my percy examples, but when I tried an example with stdweb, I was unable to get it running:

ERROR in ./dist/stdweb_electron_bg.wasm
Module not found: Error: Can't resolve 'env' in '/home/bspeice/Development/isomorphic_rust/stdweb/dist'
 @ ./dist/stdweb_electron_bg.wasm
 @ ./dist/stdweb_electron.js
 @ ./dist/app_loader.js

At this point I’m pretty convinced that stdweb is causing issues for yew as well, but can’t prove it.

I did also get a minimal example running that doesn’t depend on any tools besides wasm-bindgen. However, it requires manually writing “extern C” blocks for everything you need from the browser. Es no bueno.

Finally, from a tools and platform view, there are two up-and-coming packages that should be mentioned: js-sys and web-sys. Their purpose is to be fundamental building blocks that exposes the browser’s APIs to Rust. If you’re interested in building an app framework from scratch, these should give you the most flexibility. I didn’t touch either in my research, though I expect them to be essential long-term.

So there’s a lot in play from the Rust side of things, and it’s just going to take some time to figure out what works and what doesn’t.

Issue the Third: Known Unknowns

Alright, so after I managed to get an application started, I stopped there. It was a good deal of effort to chain together even a proof of concept, and at this point I’d rather learn Typescript than keep trying to maintain an incredibly brittle pipeline. Blasphemy, I know…

The important point I want to make is that there’s a lot unknown about how any of this holds up outside proofs of concept. Things I didn’t attempt:

  • Testing
  • Packaging
  • Updates
  • Literally anything related to why I wanted to use Electron in the first place

What it Would Take

Much as I don’t like Javascript, the tools are too shaky for me to recommend mixing Electron and WASM at the moment. There’s a lot of innovation happening, so who knows? Someone might have an application in production a couple months from now. But at the moment, I’m personally going to stay away.

Let’s finish with a wishlist then - here are the things that I think need to happen before Electron/WASM/Rust can become a thing:

  • Webpack still needs some updates. The necessary work is in progress, but hasn’t landed yet (#7983)
  • Browser API libraries (web-sys and stdweb) need to make sure they can support running in Electron (see module error above)
  • Projects need to stabilize. There’s talk of stdweb being turned into a Rust API on top of web-sys, and percy moving to web-sys, both of which are big changes
  • wasm-bindgen is great, but still in the “move fast and break things” phase
  • A good “boilerplate” app would dramatically simplify the start-up costs; electron-react-boilerplate comes to mind as a good project to imitate
  • More blog posts/contributors! I think Electron + Rust could be cool, but I have no idea what I’m doing