Open source aside, it would be great if any developer could do this on iOS. Unfortunately -- in contrast to macOS -- Apple keeps the XPC API on iOS private, so

Open source aside, it would be great if any developer could do this on iOS. Unfortunately -- in contrast to macOS -- Apple keeps the XPC API on iOS private, so that it is impossible for "normal" app developers to properly sandbox the more critical and exposed parts of their application. Only Apple can provide the described security for their messenger, but other messengers which face similar challenges (rendering untrusted content) cannot protect themselves.