Support deferred components (On-demand Resources) on iOS

[GaryQian]

Deferred components was implemented for Android in #57617. We should support similar functionality on iOS.

Deferred components allows AOT dart code and assets to be downloaded and installed at runtime. See go/deferred-components-instructions

We can make use of App store's On-demand resources https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/On_Demand_Resources_Guide/index.html

This work would require supporting the engine platform view deferred components API in the iOS embedder and tooling to read and package the assets/.so files in an on-demand resource.

It would be interesting to find out of custom implementations are something that enterprise customers desire, as iOS is more closed than Android and all distribution occurs via the App store.

[chinmaygarde]

I am not sure on-demand resources can contain executables. But its worthwhile to investigate this nonetheless.

[GaryQian]

A data file can contain any sort of data except for executable Swift, Objective-C, C, or C++ code. Files generated by scripting languages can be on-demand resources.

@chinmaygarde This is on the iOS docs, I believe it only bans executables of the above specified languages? I'm reading this as compiled dart code should be allowed.

[chinmaygarde]

Compiled Dart code is machine code in a dynamic library. Same as Swift, ObjC, etc.. So the same restrictions ought to apply. In any case, I suggest trying it for yourself.

[GaryQian]

I created a test app that includes split .so files in the on-demand resources, and so far, it has been allowed up to testflight beta testing.

Is this indicative that it would be allowed in final release? How thorough is the testflight review process? There does not seem to be any particular xcode/appstore automatic detection/banning of dart .so files in on-demand resources.

[chinmaygarde]

You mean .dylib files? If you can dlopen and run code from these, then it ought to be fine.

[GaryQian]

cc @jmagman for reference