First draft of Isomorphic apps in 2020
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layout: post
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title: "More Isomorphic Desktop Apps with Rust"
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description: "They suck less now."
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category:
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tags: [rust, javascript, webassembly]
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---
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I wasn't expecting to write this, but it's 2020 and we could all use a win. When last I addressed
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using WASM + Electron to write desktop applications in Rust, there were ultimately too many issues
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to recommend this combination as feasible. Since then, there's been a lot of progress, and after
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finding out [the biggest problem] has been addressed, I decided it was time to take another look at
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where things stand.
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# Loading local WASM blobs
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Previously, the most significant issue was trying to actually load WASM blobs in Electron. This
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problem was the result of a combination of factors:
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1. When using streaming WASM blobs
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([`WebAssembly.instantiateStreaming()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/instantiateStreaming)),
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the blob must be returned with a `Content-Type` header of `application/wasm`.
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- If the full WASM blob is loaded into memory first, the MIME type restriction does not apply.
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2. When Chrome loads `file://` URLs, the `Content-Type` is unset.
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3. Webpack prioritizes streaming WASM whenever available, and if it fails, has no graceful fallback.
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4. When building Rust WASM binaries using `wasm-bindgen`, webpack is used to generate JS bindings
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for the WASM blob.
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Putting all this together:
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- Electron loads the JS created by `wasm-bindgen`/webpack
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- This JS file calls attempts to load the WASM blob, which just so happens to be located on disk
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- Because the MIME type isn't set, loading fails
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This was a well-known issue;
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[emscripten](https://github.com/emscripten-core/emscripten/blob/8914c5cd5e4ac35a806430e8c77c88cd8c65b234/src/preamble.js#L2295)
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even included a graceful fallback for this scneario. It was possible to tweak the JS created by
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`wasm-bindgen` to load WASM, but using `sed` to edit generated code will eventually lead to madness.
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I'm unable to figure out when exactly it changed, but requesting WASM blobs from `file://` URLs in
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Chrome now sets the MIME type, and thus the blob is loaded correctly. Additionally, recent changes
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to be released in Webpack 5 (specifically the `asyncWebAssembly` and `importAsync`
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[experiments](https://webpack.js.org/configuration/experiments/)) enable loading WASM without a
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separate launcher script.
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# The evolution of Rust
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Rust as a language has also made a great deal of progress since late 2018. Previously, some
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widely-used crates (like `stdweb`) required a `nightly` Rust compiler to function. Now, nearly
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everything compiles on `stable`. In addition, now that Rust supports `async/await`, it's much easier
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to interact with Javascript. It's still necessary to use some crates like `wasm_bindgen` to assist
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the interaction, but Rust can now make use of the same asynchronous paradigms that have proven to be
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incredibly effective in Javascript.
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There's also been great progress on some crates to interact directly with the browser; `web-sys` and
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`js-sys` enable easier interoperation with the browser, where previously users didn't have these
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options available.
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The tooling and documentation has improved as well. `wasm-pack` has proven itself as a reliable "one
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stop shop" tool for managing WASM projects. While using `wasm-bindgen` and `webpack` directly are
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still necessary for building Electron apps (due to Webpack v5 not yet released), this should change
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in the near future as well.
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# New examples
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(need to put some screenshots and link to the new examples here)
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# Outstanding issues
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While I haven't been directly involved in any of the progress made to improve Rust + WASM, it's
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incredibly encouraging to see just how far everything has come. Seeing where the ecosystem stands
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now, I think using Electron + Rust to build desktop applications is _feasible_. Not necessarily a
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_good_ idea, not that it offers any specific benefit over using Javascript/Typescript, just that
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it's now _feasible_.
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Looking forward, the things I think could be beneficial to address are:
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- Template/starter project examples
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- Being able to `yarn create` a project and have it already set up with two-way JS to Rust
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bindings would go a long way towards reducing the currently painful setup using either
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`wasm-bindgen` or `wasm-pack`.
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- Comparisons to Typescript and Neon
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- Is there a development or performance benefit that comes from using Rust instead of Typescript?
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WASM might execute faster, but Rust might be more tedious to develop. It would be useful to port
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an existing (small) application to Rust and see what the differences are. That said, while
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benchmarking WASM vs. Javascript is at least theoretically possible to do in a systematic
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manner, evaluating the developer experience is like a case-by-case determination.
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- Instead of embedding Rust in Electron by way of WASM, [Neon] can be used to develop extensions
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that run natively and are "glued" to Electron via Javascript. Further investigation to clarify
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the pros/cons of each approach would be helpful; are there situations in which WASM offers
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benefits over Neon? Vice-versa? Both WASM and Neon already require more complex setups than
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typical JS/TS setups.
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