Android support

[stackotter]

This is a tracking issue for Android support. I've started some work locally but haven't published any of my progress yet. I've mostly been trying to figure out the bundling side of things to start off, but I have made some progress on how SwiftCrossUI can seamlessly integrate with JVM callback stuff;

JVM callback stuff

Since SwiftCrossUI apps get built as executable products, their only required interface is a main function. To integrate into an Android app, the plan is to link this user-written executable code into a library. We somehow need to implement an entry point that a Java shim can call into, and this entry point should ideally be defined and implemented seamlessly on the user's behalf.

My idea is to forward declare int main(int argc, char **argv) in a c file within SwiftCrossUI. This forward declaration gets resolved at link time, so it gets resolved to the main symbol that SwiftPM generates for the user's executable code. With this, SwiftCrossUI can define an androidBackendEntryPoint function (name isn't important) that takes a java VM instance and whatever else as input, stores these inputs for later use within the backend, and then calls into the user's original entry point. I've tested all parts of this plan in a test project and they all appear to work.

Remaining tasks

• Figure out bundling (ideally without Gradle, but an initial prototype could use Gradle)
• Implement enough basic backend functionality to get the main structure of the backend figured out

Once those are completed, we can probably just merge things into main and continue improving things from there.

[finagolfin]

Hey, mentioned this project on our first Android workgroup video call on Wednesday since we were made official, in a discussion of possible Android UI options. Are you using the new swift-java work for your backend here?

Looking forward to seeing what you come up with for Android, and I'm sure we will try it out. 😺

[stackotter]

That’s great to hear! The formation of the Android workgroup is quite exciting.

Yeah I’m using swift-java via colemancda’s AndroidKit. I was originally doing JNI stuff manually cause it was quick to get up and running, but the nice bindings made a huge difference hahah.

I’m currently away but I plan to merge the prototype AndroidBackend into the repo soon (currently sitting in a separate testing repo on my laptop).

In case you’re interested, here’s the recording of a Twitch stream during which I worked on the prototype, adding support for text, buttons and stacks. That was before I switched to AndroidKit, so a lot of that stream was me struggling with interop issues. I switched over to AndroidKit soon after, once I figured out some environment configuration issues.

I’ve also uploaded the current AndroidBackend source code to the general channel in the SwiftCrossUI Discord server because someone inquired while I was away. I sourced the files from my cloud backup via the mobile interface.

I’ve solved most of the foundational issues to get most views up and running now. Just need to spend the time going through backend views one by one and implementing them. One of the main remaining challenges is bundling. I plan to extend Swift Bundler to support bundling SwiftPM executable targets into Android APKs. I’ve solved the main challenges for doing so (such as compiling executable libraries to dynamic libraries, and calling into the user provided main function from a specialised Android entry point provided by SwiftCrossUI. I just need to spend the time implementing the techniques within Swift Bundler. I plan to do things in a way that doesn’t lock people to using SwiftCrossUI.

[colemancda]

@stackotter Take a look at AndroidSwiftUI and feel free to copy any code that helps you. I would much rather focus on my AndroidBluetooth library instead of SwiftUI if you plan on adding an Android backend to your library. My code is based on Tokamak and OpenCombine.

[stackotter]

Yeah I had a quick poke around at that code. It helped me see how to use AndroidKit a bit even though Tokamak and SwiftCrossUI work quite differently under the hood.

Thanks so much for making AndroidKit! It’s been a pleasure to use so far. I did have to add a couple of missing bindings, but I’ll PR those once I’m done.

[colemancda]

Glad someone else is using it! Feel free to send that PR anytime.