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Migrating off Skip to AndroidSwiftUI

The app previously used Skip to compile its SwiftUI codebase for Android. It now uses the system SwiftUI on Apple platforms and PureSwift/AndroidSwiftUI on Android.

What Skip was doing

Skip's footprint was smaller than it appeared — nine import lines and one Logger — but it also owned the build:

Skip piece Replacement
SkipFuseUI — SwiftUI for Android AndroidSwiftUI, linked only .when(platforms: [.android])
SkipModel — Observation for Android import Observation directly; the Swift Android SDK ships Observation.swiftmodule
SkipFuse — Foundation shims, Logger AppLogger in Sources/BluetoothExplorer/Logging.swift (os.Logger on Apple, print-based elsewhere)
skipstone build plugin + three skip.yml nothing — plain SwiftPM targets
/* SKIP @bridge */ on the root view and app delegate nothing — Darwin/Sources/Main.swift was already a real @main App
ComposeView / #if SKIP Compose interop in ContentView removed with the rest of the Skip template scaffolding
Skip.env removed

Views now import conditionally:

#if canImport(SwiftUI)
import SwiftUI
#else
import AndroidSwiftUI
#endif

The dependency-graph side effect

This removed the whole Skip fork stack — skip-fuse, skip-fuse-ui, skip-android-bridge, skip-ui, skip-foundation, skip-model, and their transitive Java/JNI packages. The package dropped from ~30 resolved dependencies to 20, and the graph now resolves from a clean checkout:

swift package resolve   # exit 0, no mirrors, no SWIFTPM_ENABLE_MACROS=0

Every blocker recorded in Documentation/DependencyState.md traced back to that stack:

  • the swift-android-native two-URL conflict needed skip-android-bridge as one of the two claimants — with Skip gone there is only one claimant left;
  • the swift-java / swift-syntax 603-vs-602 conflict came in through the same graph;
  • skip-fuse-ui was the package that could not build against current skip-ui.

Two PureSwift fixes are still required and are still open as PRs — PureSwift/Android#43 and PureSwift/AndroidBluetooth#4. Until they merge, local SwiftPM mirrors pointing at fixed clones are needed; AndroidBluetooth otherwise fails with product 'AndroidManifest' ... not found in package 'Android'.

What AndroidSwiftUI needed

The app's UI relies on SwiftUI that AndroidSwiftUI did not implement. These were added to AndroidSwiftUI first (branch feature/swiftui-containers):

  • NavigationStack — reusing the existing NavigationContext, so NavigationLink push and hardware-back pop are shared with NavigationView rather than duplicated
  • LazyVStack / LazyHStack — eager, mapping onto the same LinearLayout path as VStack/HStack
  • TabView with .tabItem and selection — tab items are read back by walking the modifier chain for trait-writing modifiers; only the selected tab is mounted
  • .sheet(isPresented:content:) — a full-screen overlay in the same view hierarchy rather than an Android Dialog, because the fiber renderer mounts children into the parent's ViewGroup and has no path to a Dialog's separate decor view
  • .refreshable — stores a RefreshAction in the environment as SwiftUI does, but no gesture triggers it: binding SwipeRefreshLayout would need an androidx dependency the package lacks
  • Swift Observation support — .environment(object) and @Environment(Type.self), with invalidation driven by withObservationTracking

.fileImporter is not needed on Android: the plugin import button is already gated behind #if !os(Android).

Known gaps in those additions

All four compile and were verified to build together for Android, but none has been exercised on a device or emulator. Notable limitations, each documented in the source:

  • Observation: @Bindable is not implemented, and @Environment(Store.self) var store: Store? (the optional form) is unsupported — a missing injection traps rather than yielding nil.
  • TabView: unselected tabs are unmounted, so their @State resets; no TabViewStyle, paging or badges.
  • Sheet: no animation, detents or drag-to-dismiss, and no @Environment(\.dismiss) — content must dismiss itself through the binding.
  • NavigationStack: NavigationStack(path:) and value-based navigationDestination(for:) are not implemented; NavigationContext.path holds type-erased views, not a Hashable data path.

Platform status

  • Apple platforms — the package resolves and every target builds with no Skip involvement.
  • Android — the dependency wiring is in place, but the Android app build has not been verified end to end. It additionally needs the two PureSwift PRs above, and the Kotlin JNI peers under Sources/BluetoothExplorer/Skip/ (ScanCallback.kt, BluetoothGattCallback.kt) rehomed into the Android app project — they are peers for AndroidBluetooth, unrelated to Skip, and were only living in that directory because skipstone collected them.

Building an Android app archive

CI (.github/workflows/ci.yml) archives the iOS app but does not produce an Android .aab. Three things block it, each verified by trying:

  1. The app does not cross-compile for Android. swift build --swift-sdk … --target BluetoothExplorer fails inside AndroidBluetooth: its @JavaImplementation macro generates invalid Swift for LowEnergyScanCallback.swiftOnScanFailed(error:) — the parameter named error is emitted into a context where it parses as a keyword, producing declaration name '…' is not covered by macro 'JavaImplementation'. This is an upstream swift-java/AndroidBluetooth bug.
  2. A whole-package Android build additionally fails in WasmKit's SystemExtras: it does st_mode & S_IFMT, but st_mode is UInt32 in the Android sysroot while swift-system's CInterop.Mode is UInt16. This target is not reachable from the app, so building specific targets avoids it; a plain swift build does not.
  3. The Android project is still Skip's. Android/settings.gradle.kts shells out to skip plugin --prebuild, and Android/app/build.gradle.kts applies skip-build-plugin with a skip { } block that reads the now-deleted Skip.env. With Skip removed none of that resolves.

Replacing (3) means writing a conventional Android app project, using AndroidSwiftUI's Demo/ as the template: its app/src/main/java/com/pureswift/swiftandroid/ Kotlin classes (MainActivity, Application, NativeLibrary, ListViewAdapter, Fragment, …) are the peers AndroidSwiftUI's @JavaClass bindings bind to, and build-swift.sh shows the packaging step — build the Swift package for the target architecture, then copy the resulting .so into app/src/main/jniLibs/<abi>/. The Kotlin JNI peers currently under Sources/BluetoothExplorer/Skip/ (ScanCallback.kt, BluetoothGattCallback.kt) belong in that project too.

None of this is worth doing until (1) is fixed upstream, since the .so cannot be produced.