Expo Migrate Module

Migrate an Expo Module

Migrate the Swift side of an existing Expo module without changing its observable JS API. Treat the current JS/TypeScript surface and tests as the compatibility contract. Leave Kotlin on the 1.0 DSL unless the user explicitly expands the task.

Prerequisite

Use expo 57.0.21 or newer. Earlier 57.x versions can compile the macros but lack many of the 2.0 features and performance optimizations, so do not target them. Before editing, check the target's installed version (expo in package.json/lockfile, or npm ls expo). If it is older, stop and tell the user to upgrade first.

Check the example app's own package.json too, not only the module root. The example app is the integration surface you build and launch in step 4, so it needs the same floor.

This is a floor, not a guarantee: the exact macro and core surface still varies within 57.x, so step 2 must still verify the checked-out source.

SDK 57 ships the macros as experimental and undocumented, with the official beta in SDK 58. The API can still change under you. Say this to the user before a large migration, and prefer incremental mixed mode over converting a module wholesale.

References

• Read references/migration-map.md before changing source. It contains the 1.0-to-2.0 mappings, semantic traps, and mixed-mode rules.
• Read references/example.md for a full before/after walkthrough of one module through mixed mode to a complete migration. Consult it when you need to see how the per-member rules compose.
• Read references/compatibility.md when the checked-out expo-modules-core version or branch is not known to support every requested macro. It explains how to verify the actual compile-time and runtime surface instead of guessing from a version number, and lists what the macros plugin gained through 0.10.0.

Workflow

1. Establish the contract

Inspect repository instructions and the worktree before editing. Locate the Swift module classes, records, shared objects, native views, JS/TS bindings, tests, example app, podspec, expo-module.config.json, and installed or checked-out expo-modules-core.

Inventory every exported item before rewriting it:

• module and shared-object JS names
• function names, arity, labels, defaults, nullability, sync/async behavior, errors, and queue/thread semantics (note which AsyncFunction bodies do blocking or long-running work, and any .runOnQueue(...))
• property names, mutability, and constant caching behavior
• event wire names and payload shapes
• record field names, defaults, requiredness, and nullability
• shared-object constructors and instance/static placement (Function vs StaticFunction/StaticAsyncFunction)
• lifecycle hooks and views

Use the TypeScript declarations and JS call sites to resolve ambiguity. Do not silently "improve" requiredness, rename an event, or change sync behavior during a syntax migration.

2. Verify the available 2.0 surface

Inspect the macro declarations and matching core hooks in the dependency actually used by the target. Do not assume that all items in the 2.0 design are present because one macro compiles.

Classify each 1.0 item as:

• Migrate: both its macro and required core runtime support exist.
• Keep in DSL: mixed mode preserves it safely, or 2.0 lacks an equivalent.
• Blocked: migration would alter the JS contract or requires unavailable runtime support.

Prefer an incremental mixed-mode result over speculative generated code. Keep definition() for any remaining DSL elements; delete it only when it is empty and the resolved module name is preserved by @ExpoModule.

3. Apply the migration

Migrate one semantic group at a time: module naming, functions, properties/constants, events, shared objects, then records. Keep the diff narrow.

Follow these invariants:

• Preserve every existing JS-visible name explicitly when Swift naming rules or macro defaults differ.
• Keep original optional/default behavior. An optional 1.0 record field must not become required merely because 2.0 can express required fields.
• Do not migrate same-JS-name overloads unless the checked-out macro groups and dispatches them.
• Preserve async threading behavior. A 1.0 AsyncFunction body ran off the JS thread; a 2.0 async @JS member starts on it and leaves only at the first await. A body that never awaits therefore blocks the JS thread, with no change to the JS signature. Audit what runs before the first await, and never leave blocking I/O on the JS actor. Fix with @JS(.concurrent), or restructure onto Swift Concurrency or a continuation. Queue-pinned functions get the same treatment. See the threading section in references/migration-map.md.
• Verify core support before migrating views, unions, synchronous events, shared-object static members, or free-form Any arguments. The macros plugin has shipped ahead of core on every one of these, so check the checked-out core rather than a plugin version, and leave unsupported members on the 1.0 DSL. references/compatibility.md has the per-capability checks.
• Keep expo-module.config.json listing every module class under apple.modules. The entries are bare Swift class names, so renaming a class during migration detaches the module silently: it builds, then is absent at runtime. A migrated class must also stay public or open. SDK 58's auto-discovery is what retires these entries.
• Do not change Kotlin, JS wrappers, or public .d.ts files unless the user requested an API change.
• Never write macro-generated symbols into the module's source. The macro emits them; hand-writing or overriding them is not part of a migration.

After each group, search for old DSL entries and call sites that should have moved. Avoid broad formatting or unrelated cleanup.

When a 2.0 equivalent is missing or a group fails

When step 2 classified an item as Blocked, or a migrated group fails to build or breaks the contract, do not force it. Stop on that group and:

1. Ask the user how to proceed for that item, with two options:

• Co-exist: keep the item in the 1.0 definition() DSL alongside the migrated @ExpoModule (mixed mode) and continue with the other groups.
• Revert: back out the group's edits, leaving it untouched on 1.0, and move on.

Default to co-existence when mixed mode is verified safe, since it preserves the most progress. Revert when the half-applied change left the module in a non-building state and cannot be salvaged incrementally.

1. Open a tracking issue on `expo/expo` noting the functionality that 2.0 does not yet cover, so the gap is recorded rather than silently worked around. Use gh issue create --repo expo/expo and confirm with the user before posting (per repo conventions, do not post outward-facing comments without approval). Include:

• the 1.0 member and its JS contract
• the specific macro or core hook that is missing (cite the evidence gap from references/compatibility.md)
• the expo-modules-core version/branch checked out

Reference the issue in the handoff so the remaining DSL entry is traceable to a known limitation.

Keep going with the groups that do migrate cleanly; one blocked member does not block the rest.

4. Verify behavior

Run the narrowest available checks first, then the real integration surface:

1. Build or type-check the Apple module against the target expo-modules-core.
2. Run native unit tests and JS/TS tests.
3. Build and launch the example app when the repository provides one.
4. Confirm expo-module.config.json still names every module class under apple.modules, matching the Swift class names as they now stand. A stale entry builds clean and fails only at runtime, so the example app must actually resolve the module, not merely compile.
5. Compare the final exported surface with the inventory from step 1.