Engine

Shipping your own code

When a patch needs more than a few instructions, write Java and ship it in the bundle. Reseam compiles it against android.jar into a DEX file and links that file into the app the first time a patch refers to one of its classes. These modules are extensions.

Build time: an extension module with src/main/java and compile-only stubs under src/stubs/java goes through d8 and lands in the bundle as one DEX file. Patch time: patch code declares an ExtClass and calls it; the engine merges the extension DEX into the app on that first reference, plus every extension it refers to.

apps/example/extensions/ads/
  src/main/java/app/example/ext/AdBlocker.java
  src/stubs/java/com/example/app/FeedItem.java

src/main/java is compiled and dexed. src/stubs/java holds compile-time stand-ins for classes the app already has and is never dexed. A module needs a build script only to compile against another extension: compileOnly(project(":shared:settings-runtime")).

A patch declares the extension classes it calls:

object AdBlocker : ExtClass("app.example.ext.AdBlocker") {
    val init = static("init", Type.Context)
    val onFeedLoad = static("onFeedLoad", Type.List)
}

The first reference to a class an extension defines merges that DEX into the app, plus every extension it refers to. Two extensions defining one class fail the bundle at load. A reference nothing defines is logged once: ... is not defined by the app or any extension in the bundle.

Warning

That warning means a typo in the ExtClass name or a module missing from the bundle (reseam bundle list prints the DEX files); the app crashes with NoClassDefFoundError when the call runs. Stubs are promises too: a stub method the real class lacks throws NoSuchMethodError in the app.

Toggle gates call app.reseam.runtime.settings.ReseamSettings from the settings-runtime shared extension; the engine links it when the first gate is emitted.

Next: Raw bytecode.

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