Skip Fuse

Skip Fuse

Fuse is Skip's "native Swift on Android" mode. Instead of transpiling Swift to Kotlin (Lite), Fuse compiles the same Swift source with the official Swift SDK for Android and generates JNI bridges so the Swift code can call into Kotlin/Java and vice versa. The SwiftUI surface still goes through SkipUI → Compose; only the Swift-language code is compiled rather than transpiled.

This skill covers the Fuse-specific workflow. For the rest of the Skip workflow (UI components, icons, localization, tests, deployment) the Lite skills apply unchanged.

When to choose Fuse over Lite

| Choose Fuse when… | Choose Lite when… | |---|---| | You depend on Swift language features that transpile poorly: variadic generics, complex macros, reflection, key-paths-as-values, Mirror. | You're targeting the broadest base of Swift libraries and want the smallest APK. | | You need C interop without SkipFFI boilerplate (Fuse can call C directly via the Swift Android SDK). | You're shipping a small, focused app where 60 MB of Swift runtime is the wrong trade-off. | | You're consuming Swift packages that weren't written for Skip and want them to "just work". | You want maximum agent / tooling support — most Skip skills, examples, and sample apps target Lite. | | You need bidirectional Swift↔Kotlin interop in the same project (a Swift Package that also calls Android SDK APIs). | You don't need to call Kotlin or Java APIs directly; SkipFoundation / SkipUI cover what you use. |

Default to Lite unless one of the left-column reasons applies. Fuse is more capable but more nascent — fewer skills, sample apps, and CI templates document it.

Mode detection (always do this first)

Before writing any code, determine the project mode. The Swift you can safely write depends on it.

# Check each module's skip.yml
grep -l "mode: 'native'" Sources/*/Skip/skip.yml

• mode: 'native' → Fuse (this skill applies).
• mode: 'transpiled' or no mode set → Lite ([skip-lite-transpilation](../skip-lite-transpilation/SKILL.md) applies).
• mode: 'automatic' → depends on the resolved dependency graph. If SkipFuse/SkipFuseUI is pulled in (typically via SKIP_MODE=fuse at build time), it's Fuse; otherwise Lite.

Prerequisites

Before starting a Fuse project, install the Swift SDK for Android and run the Fuse-specific environment checkup:

skip android sdk install        # one-time: download the Swift SDK for Android
skip checkup --native           # verify the native toolchain (above and beyond plain `skip checkup`)

skip checkup --native runs everything in skip checkup plus the additional checks needed for Fuse compilation. If you skip these, the first skip app launch will fail with a confusing toolchain error.

Creating a Fuse project

skip init --native-app --appid=com.example.someapp some-app SomeApp

The flags that matter:

• --native-app — selects Fuse mode (vs. --transpiled-app for Lite).
• A Fuse app typically uses a single module — no separate SomeAppModel. The bridge generator works better with one module per app.
• All the other skip init flags from [skip-project-creation](../skip-project-creation/SKILL.md) (--appid, --version, --no-build, --git-repo, --icon) apply unchanged.

The generated Sources/SomeApp/Skip/skip.yml will contain:

skip:
  mode: 'native'
  bridging: true

bridging: true is shorthand for the auto-bridging form (bridging.auto: true) — see [Bridging options](#bridging-options) below for the full schema and when you'd want to customise it.

@Observable requires import SkipFuse (warning, not error)

Any Swift file that declares an @Observable type in a Fuse module needs import SkipFuse (or import SkipFuseUI, which re-exports it). Without it, the state-tracking glue that synchronises Swift observables with Jetpack Compose's recomposition is not installed, and the Android UI silently stops updating when the model changes.

import Foundation
import SkipFuse        // installs the @Observable bridge for Compose
import Observation

@Observable final class ViewModel {
    var items: [Item] = []
    var isLoading = false
}

This is enforced as a warning, not a compile error. The build emits:

> This file contains @Observables, but they will not be able to power your Android UI unless you 'import SkipFuse' or 'import SkipFuseUI'

The code compiles, the iOS build runs correctly, and the Android build runs without crashing — the bug only shows up at runtime as a UI that never updates when the model mutates. Treat the warning as a blocker during code review.

Visibility rules for views

Views and their SwiftUI properties must use default (internal) or public visibility. private views and private @State / @Binding properties are not visible to the bridge generator and will not render on Android.

// BAD on Android — bridge generator can't see private members
private struct DetailRow: View {
    @State private var expanded = false
    var body: some View { ... }
}

// GOOD — default internal visibility
struct DetailRow: View {
    @State var expanded = false
    var body: some View { ... }
}

This applies in both Fuse and Lite, but it bites harder in Fuse because the bridge generator runs against the public API surface; in Lite the transpiler reaches private members through its own AST traversal.

#if os(Android) vs. #if SKIP in Fuse

In Lite, #if os(Android) and #if SKIP are equivalent (everything is transpiled to Kotlin). In Fuse they're different:

| Conditional | Fuse behaviour | |---|---| | #if os(Android) | Compiled as native Swift for Android. Cannot call Kotlin/Java APIs directly. | | #if SKIP | Transpiled to Kotlin. Can call Kotlin/Java API directly. | | #if !SKIP | Native Swift on both iOS and Android. The right place for shared logic that should not be transpiled. | | #if !os(Android) | iOS only. |

Rules of thumb:

• Need to call android.content.Context, java.util.UUID, a Kotlin coroutine? Use #if SKIP.
• Need platform-specific Swift that uses Foundation/Swift APIs only (e.g. an Android-specific file path)? #if os(Android) is fine and stays native.
• Shared business logic that should run as native Swift on iOS and Android? #if !SKIP is what you want.

Bridging options

bridging: true in skip.yml is shorthand for the auto-bridging form. The full schema:

skip:
  mode: 'native'
  bridging:
    auto: true
    options: 'kotlincompat'    # optional

| Form | Effect | |---|---| | bridging: true | Shorthand for bridging.auto: true. Auto-generates JNI bridges for every public API in the module. The default for skip init --native-app. | | bridging.auto: true | Same as above, expanded. | | bridging.options: 'kotlincompat' | Generated Kotlin wrappers use kotlin.collections.List / kotlin.collections.Map / java.net.URI instead of skip.lib.Array / skip.foundation.URL. Use this when the bridged API is consumed from native Kotlin / Compose code rather than from another transpiled Swift module. |

Without kotlincompat, the bridged surface is shaped for Skip Lite consumers (uses the Skip wrapper types). With kotlincompat, it's shaped for raw Kotlin consumers (uses standard Kotlin collections). For an app that mixes Lite and Fuse modules, leave it off. For a Fuse library you want to ship to Kotlin-side consumers, turn it on.

Per-declaration control via SKIP comment directives:

| Directive | Effect | |---|---| | // SKIP @bridge | Bridge this specific declaration even if auto-bridging is off. | | // SKIP @bridgeMembers | Bridge all members of this type matching its visibility (all public if the type is public). | | // SKIP @nobridge | Exclude this member from auto-bridging. |

Use @nobridge to selectively trim what crosses the JNI boundary — for example, a public method that returns a non-bridgeable type, where you only want it visible to native Swift callers.

AnyDynamicObject — calling Kotlin/Java from Fuse without a wrapper

When a Fuse module needs to use a Kotlin or Java type from the Android SDK or a Kotlin library and there's no transpiled Swift wrapper for it, the documented escape hatch is AnyDynamicObject. It's a SkipFuse type that wraps any JVM object and lets native Swift call its methods and properties via Swift's dynamic dispatch features.

Raw form (no dynamicroot):

import SkipFuse

#if os(Android)
let date = try AnyDynamicObject(className: "java.util.Date", 999)

// Call instance methods. Return types and `throws` are mandatory.
let time: Int64 = try date.getTime()!

// Property syntax works for getters and setters.
let t = date.time!
date.time = 1001

// Chain calls freely.
let s = try date.instant?.toString()
#endif

Important characteristics:

• All calls are unchecked at compile time. You name the method, supply the args, and assert the return type. Wrong method name or wrong return type produces a runtime error, not a build error.
• All calls are throws and ?-prefixed. Kotlin methods can throw, and Swift can't know whether they return null.
• Android-only. Always wrap in #if os(Android).

dynamicroot for cleaner syntax

Repeatedly typing className: "java.util.Date" is tedious. The dynamicroot option in skip.yml creates a magic namespace prefix for fully-qualified types:

skip:
  mode: 'native'
  dynamicroot: 'D'        # any single uppercase letter or short identifier

With dynamicroot: 'D':

import SkipFuse

#if os(Android)
let date = try D.java.util.Date(999)        // no string class name
typealias JDate = D.java.util.Date          // typealias for repeated use
let time: Int64 = try date.getTime()!

// Static methods use Companion()
let parsed = try D.java.util.Date.Companion().parse(dateString)!

// Convert a base AnyDynamicObject to a typed one
let dateAsTyped = baseAnyDynamicObject.as(D.java.util.Date.self)
#endif

dynamicroot is purely syntactic sugar over AnyDynamicObject. There's no runtime difference; the build plugin rewrites D.foo.bar.Baz(args) into the equivalent AnyDynamicObject(className: "foo.bar.Baz", args).

When to use AnyDynamicObject vs. a transpiled wrapper

| Use AnyDynamicObject when… | Write a transpiled wrapper when… | |---|---| | You need one or two Kotlin types in a Fuse app. | You're consuming a whole Kotlin library across many call sites. | | The API surface is small enough to live with unchecked calls. | You want Swift-side type checking. | | You don't want to maintain a parallel Swift declaration file. | You want IDE autocomplete and refactoring. |

For the deeper story (including how dynamicroot reaches into reflection, performance characteristics, and what it does for class hierarchies), see the modes documentation.

SKIP_BRIDGE — the bridge-mode preprocessor symbol

When a transpiled module is built as a bridge target for native Swift consumption, the build plugin defines a SKIP_BRIDGE preprocessor symbol. Use #if !SKIP_BRIDGE to guard Swift code that would produce duplicate-symbol errors when the module is built in bridge mode:

#if !SKIP_BRIDGE
#if os(Android)
import android.content.Context
let context = ProcessInfo.processInfo.androidContext        // Lite-style call
#endif
#endif

The skipstone plugin emits a warning when it detects Android-specific code in a transpiled module that lacks the #if !SKIP_BRIDGE guard. Treat the warning as a blocker — without the guard, the module won't link in mixed Lite/Fuse projects.

Setting SKIP_BRIDGE=1 as an environment variable when running swift build forces every library product in the resolved graph to be dynamic (.dynamic rather than .automatic / .static). This is occasionally needed when a Fuse module consumes a peer Fuse package across the JNI boundary:

SKIP_BRIDGE=1 swift build
SKIP_BRIDGE=1 swift test
SKIP_BRIDGE=1 skip app launch

Use it for the build of a Fuse-library-plus-Fuse-app project; you don't need it for a single-module Fuse app or for pure Lite projects.

Binary size — the 60 MB Swift runtime

Fuse apps add approximately 60 MB to the Android APK for the Swift runtime, Foundation, and internationalisation libraries. This is a fixed cost that does not grow with app complexity. It will decrease as the Swift Android SDK matures, but plan for it today:

• Verify your Play Store listing's APK size limit before shipping a Fuse app.
• Profile the APK with apkanalyzer (Android Studio) — confirm the 60 MB lives in lib/ (the Swift stdlib .so files) and not in app code.
• Lite apps land at ~10–15 MB for the same UI surface. If your app is content-heavy on the iOS side, the relative overhead is smaller; if it's small, the overhead dominates.

Debugging Fuse on Android

Two limitations to plan around:

Breakpoint debugging of native Swift on Android is not yet supported

LLDB-on-Android for the Swift Android SDK is in development but not generally available. Today:

• Use OSLog.Logger (not print()) for runtime tracing. OSLog is bridged to Logcat in Fuse, so logger.info(...) lands in adb logcat -s . print() goes to stdout, which Android does not route to Logcat.
• For crash investigation, run swift demangle on the mangled Swift symbols in the crash dump — they look like $s9SomeApp11ViewModelC4loadyyYaKF and demangle to SomeApp.ViewModel.load() async throws -> ().

import OSLog
let logger = Logger(subsystem: "com.example.someapp", category: "ViewModel")

func loadItems() async {
    logger.info("starting load")    // visible in adb logcat
    // print("starting load")       // NOT visible in Logcat, even though it works on iOS
}

Kotlin inline classes in Android SDK APIs

Some Android SDK and Kotlin-library APIs (common in Compose, LiveKit, AndroidX) expose properties whose types are Kotlin inline classes (@JvmInline value class Dp(...), Color, TextUnit, etc.). The JVM-level getter for an inline-class property has a mangled name (getColor-impl rather than getColor), and Fuse's bridge generator can't resolve .stringValue or direct casts against these.

The reliable workaround is string interpolation:

#if SKIP
import androidx.compose.ui.unit.Dp

let height: Dp = ...
// BAD — fails to resolve the mangled getter
let s = height.stringValue

// GOOD — string interpolation forces a toString() call
let s = "\(height)"
#endif

This trades type safety for compile-time guarantee and is documented in the Skip codebase as the recommended pattern.

Running tests on Fuse

swift test runs the Swift-side tests as usual. For the Android side, Fuse has its own runner:

skip android test               # Fuse CLI mode — fast, no Android APIs available
skip android test --apk         # Fuse APK mode — full Android APIs, slower

| Mode | What runs | When to use | |---|---|---| | swift test | Swift tests on the host (macOS/Linux). | All pure-Swift logic. | | skip android test | Fuse tests in a CLI environment on the emulator/device. No Android Context, no UI. | Fast iteration on cross-platform logic. | | skip android test --apk | Tests run inside the packaged APK on the emulator/device with full Android APIs. | Anything that touches ProcessInfo.processInfo.dynamicAndroidContext(), system services, or asset bundles. |

The --android-serial flag scopes to a specific device when multiple are connected:

skip android test --android-serial emulator-5554
skip android test --android-serial auto   # auto-detect, prefer emulators

For the broader test-suite story (parity testing across iOS and Android, XCSkipTests harness, Robolectric — all of which apply to Lite, not Fuse), see [skip-testing](../../../skip-testing-deployment/skills/skip-testing/SKILL.md).

Accessing the A

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Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: skiptools
• Source: skiptools/skills
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.