Fourth step of the Compose-backed architecture (#18 teardown, #19 core, #20 interpreter): the JNI bridge connecting them, wired desktop-first exactly as planned — the five hand-matched signatures were shaken out on the macOS JVM before Android saw them, and they worked on the first live run.

The bridge

• AndroidSwiftUIBridge (new platform-neutral target, no android.* imports — builds as a macOS dylib and cross-compiles for Android identically):
  • @JavaClass bindings for ViewNode and TreeStore — the safe Swift→Kotlin direction.
  • Materializer: walks the core's RenderNode IR building Kotlin nodes, one JNI call per node with flat arrays; prop values cross as JSON literals (typed model, no per-field crossings, no whole-tree strings).
  • SwiftCallbackSink @JavaImplementation: the entire Kotlin→Swift surface — five methods, documented on both sides as never growing per-view entries.
  • BridgeRuntime: evaluate → materialize → TreeStore.update on every state change. Event callbacks arrive on the platform main thread outside composition, so the synchronous path is safe; a scheduler comes with async state in R5.
• ViewNode gains the bridge constructor (flat arrays → typed model); the sink lives in a JVM source set both Kotlin targets share.

Desktop rig goes live

SwiftRuntime.load() loads libSwiftJava.dylib first — swift-java initializes in JNI_OnLoad, which the JVM only fires for explicitly loaded libraries — then the app dylib. The rig renders live Swift-evaluated trees, falling back to fixtures when unconfigured.

Android host

SwiftUIHostView (interpreter in a ComposeView) + AndroidSwiftUIApp.run(root:) installing it as the activity content. The umbrella @_exporteds the core, so app code writes plain SwiftUI with one import — AndroidKit must not be imported by app code (its AndroidView.View collides with the core's View); a log helper covers the one thing the demo needed it for.

Verification

• Desktop (headless, automated): :desktop:jvmTest runs the full round trip — native Swift evaluates the counter, JNI materializes, Compose renders "Count: 0", clicks dispatch through the sink into Swift @State, re-evaluation pushes new trees, UI shows "Count: 1" then "Count: 2". Passing.
• Emulator: the same counter demo — launches, renders Material 3 button/switch, two taps advance "Count: 0" → "Count: 2" (uiautomator-verified).

Notes

• The host view doesn't yet inset for system bars (content draws under the status bar) — R5 polish.
• The exact six exported JNI symbols were verified with nm before any JVM loaded them.