feat(bindings): add the Flutter plugin

.github/workflows/ci.yml

@@ -1327,3 +1327,130 @@ jobs:
       - name: Build a minified application against the published library
         working-directory: bindings/kotlin/consumer-check
         run: gradle --no-daemon --stacktrace -PVERSION_NAME=0.0.0 :app:assembleRelease
+
+  flutter-binding:
+    name: Flutter Binding
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          # paths-filter compares the PR diff to the base branch.
+          fetch-depth: 0
+
+      # Native AAR gate: structural/UniFFI paths or lockfile (engine-only PRs skip
+      # NDK/Gradle; pushes to main always run the full job).
+      - uses: dorny/paths-filter@de90cc6fb38fc0963ad72b210f1f284cd68cea36 # v3.0.2
+        id: flutter_paths
+        with:
+          filters: |
+            flutter_dart:
+              - 'bindings/flutter/**'
+              - 'docs/bridges/flutter.md'
+              - '.github/workflows/ci.yml'
+              - 'crates/offline-protocol-uniffi/src/tests/flutter_bridge_guards.rs'
+            flutter_native:
+              # The plugin's own Kotlin and Gradle build: `flutter analyze`
+              # excludes android/**, so without these a pull request that
+              # touches only them never compiles them, and main is where it
+              # breaks.
+              - 'bindings/flutter/offline_protocol/android/**'
+              - 'bindings/flutter/offline_protocol/example/android/**'
+              - 'bindings/flutter/baselines/**'
+              - 'bindings/flutter/scripts/**'
+              - 'bindings/react-native/android/**'
+              - 'scripts/generate-bindings.sh'
+              - 'crates/offline-protocol-uniffi/Cargo.toml'
+              - 'crates/offline-protocol-uniffi/build.rs'
+              - 'crates/offline-protocol-uniffi/src/lib.rs'
+              - 'crates/offline-protocol-uniffi/src/offline_protocol.udl'
+              - 'crates/offline-protocol-uniffi/src/capi/**'
+              - 'crates/offline-protocol-uniffi/src/host_log.rs'
+            flutter_lock:
+              - 'Cargo.lock'
+              - 'Cargo.toml'
+
+      - name: Decide Flutter native (NDK + AAR) gate
+        id: flutter_native_gate
+        run: |
+          run=false
+          if [[ "${{ github.event_name }}" == "push" ]]; then
+            run=true
+          elif [[ "${{ steps.flutter_paths.outputs.flutter_native }}" == "true" ]]; then
+            run=true
+          elif [[ "${{ steps.flutter_paths.outputs.flutter_lock }}" == "true" ]]; then
+            run=true
+          fi
+          echo "run=$run" >> "$GITHUB_OUTPUT"
+
+      - name: Skip (no Flutter-related changes on this PR)
+        if: >-
+          github.event_name == 'pull_request'
+          && steps.flutter_paths.outputs.flutter_dart != 'true'
+          && steps.flutter_paths.outputs.flutter_native != 'true'
+          && steps.flutter_paths.outputs.flutter_lock != 'true'
+        run: echo "No matching paths; Flutter Binding checks skipped for this PR."
+
+      - uses: subosito/flutter-action@1a449444c387b1966244ae4d4f8c696479add0b2 # v2.23.0
+        if: >-
+          github.event_name == 'push'
+          || steps.flutter_paths.outputs.flutter_dart == 'true'
+          || steps.flutter_paths.outputs.flutter_native == 'true'
+          || steps.flutter_paths.outputs.flutter_lock == 'true'
+        with:
+          channel: stable
+          flutter-version: "3.27.4"
+          cache: true
+
+      - name: Dart unit tests and analyzer
+        if: >-
+          github.event_name == 'push'
+          || steps.flutter_paths.outputs.flutter_dart == 'true'
+          || steps.flutter_paths.outputs.flutter_native == 'true'
+          || steps.flutter_paths.outputs.flutter_lock == 'true'
+        working-directory: bindings/flutter/offline_protocol
+        run: |
+          flutter pub get
+          flutter test
+          flutter analyze
+
+      - uses: actions/setup-java@v6
+        if: steps.flutter_native_gate.outputs.run == 'true'
+        with:
+          distribution: temurin
+          java-version: "17"
+
+      - name: Setup Android NDK
+        if: steps.flutter_native_gate.outputs.run == 'true'
+        uses: nttld/setup-ndk@ed92fe6cadad69be94a966a7ee3271275e62f779 # v1.6.0
+        id: setup-ndk
+        with:
+          ndk-version: r26b
+          add-to-path: true
+
+      - uses: dtolnay/rust-toolchain@stable
+        if: steps.flutter_native_gate.outputs.run == 'true'
+        with:
+          targets: aarch64-linux-android
+
+      - uses: Swatinem/rust-cache@v2
+        if: steps.flutter_native_gate.outputs.run == 'true'
+        with:
+          shared-key: ci-flutter-binding
+
+      - name: Build arm64 UniFFI for Flutter Android
+        if: steps.flutter_native_gate.outputs.run == 'true'
+        env:
+          ANDROID_NDK_HOME: ${{ steps.setup-ndk.outputs.ndk-path }}
+        run: bash bindings/flutter/scripts/ci-build-android-jni-arm64.sh
+
+      # The example's Gradle wrapper is committed and the next step runs it,
+      # so its jar is checked against Gradle's published checksums first.
+      - uses: gradle/actions/wrapper-validation@v6
+        if: steps.flutter_native_gate.outputs.run == 'true'
+
+      - name: Assemble release AAR and check size baseline
+        if: steps.flutter_native_gate.outputs.run == 'true'
+        env:
+          ANDROID_NDK_HOME: ${{ steps.setup-ndk.outputs.ndk-path }}
+        run: bindings/flutter/scripts/measure-plugin-aar-size.sh --check

.github/workflows/release.yml

@@ -1103,7 +1103,19 @@ jobs:
             exit 1
           fi
 
-          echo "Workspace and Python versions ($CARGO_VERSION) match tag v$TAG_VERSION."
+          # The Flutter plugin's podspec and Gradle build read the React Native
+          # package.json, but pub has no way to read another file, so the
+          # pubspec carries the number itself and is checked here. Without
+          # this a release ships a plugin that says the previous version.
+          FLUTTER_VERSION="$(grep -m1 '^version: ' bindings/flutter/offline_protocol/pubspec.yaml \
+            | sed 's/^version: *//' || true)"
+
+          if [ "$TAG_CORE" != "$FLUTTER_VERSION" ]; then
+            echo "::error title=Flutter plugin version does not match the tag::Tag v$TAG_VERSION wants $TAG_CORE, but bindings/flutter/offline_protocol/pubspec.yaml reads '${FLUTTER_VERSION}'. It moves with the workspace version: bump it, then delete and re-push the tag (nothing has published yet)."
+            exit 1
+          fi
+
+          echo "Workspace, Python and Flutter versions ($CARGO_VERSION) match tag v$TAG_VERSION."
 
   # ---------------------------------------------------------------------------
   # Release

CHANGELOG.md

@@ -15,6 +15,17 @@ archived by series under [docs/changelog/](docs/changelog/); see the
 
 ### Added
 
+- **A Flutter plugin (preview).** `bindings/flutter/offline_protocol` hosts
+  the engine on iOS and Android from Dart: create, MLS with the platform
+  secure stores, start, stop, destroy, the local address, `sendMessage`, the
+  Internet, BLE and Wi-Fi Direct transports, and core events with the same
+  sequence numbers and gap replay as React Native. Its Android and iOS hosts
+  compile the React Native bridge sources where they are, less a named list
+  of the ones that need React, so a transport fix reaches both. Nostr and
+  Reticulum are not hosted yet, and a config that enables either is refused
+  at create. Until it is on pub.dev it is a path dependency. CI runs the Dart
+  tests and assembles the Android plugin against a size baseline; nothing in
+  CI compiles the iOS plugin yet. The contract is `docs/bridges/flutter.md`.
 - **A Java binding.** `bindings/java` is a typed Java API generated from the
   interface definition by `uniffi-bindgen-java`, the fourth member of the
   artifact set that `scripts/generate-bindings.sh` writes with Swift, Kotlin

CONTRIBUTING.md

@@ -269,7 +269,8 @@ Version files to bump together:
 | `Cargo.lock` | any `cargo` command refreshes it once the manifests change |
 | `tools/embedded-footprint/Cargo.lock`, `tools/mls-interop/Cargo.lock` | Separate workspaces that depend on the SDK crates by path, so their lockfiles pin the versions too. CI builds both with `--locked`, so a stale one fails the run. `cargo metadata --manifest-path <tool>/Cargo.toml` refreshes each |
 | `bindings/python/pyproject.toml` | `version` — release.yml gates this one against the tag too |
-| `bindings/react-native/package.json` + `package-lock.json` | `npm version X.Y.Z --no-git-tag-version --allow-same-version` from `bindings/react-native/` |
+| `bindings/react-native/package.json` + `package-lock.json` | `npm version X.Y.Z --no-git-tag-version --allow-same-version` from `bindings/react-native/`. The Flutter plugin's podspec and Gradle build read their version from this file |
+| `bindings/flutter/offline_protocol/pubspec.yaml` | `version`. Pub cannot read another file, so this one carries the number itself, and release.yml gates it against the tag |
 | `THIRD-PARTY-NOTICES.md` (×3) | `scripts/generate-third-party-notices.sh` — the notices list our own crates by version, and the CI drift gate fails on a stale copy |
 
 Then the prose: `CHANGELOG.md` (replace `## [Unreleased]` with

README.md

@@ -35,6 +35,7 @@ Every channel carries the version number of the release it came from.
 | Swift, iOS (preview) | [`offline-protocol-swift`](https://github.com/Offline-Protocol/offline-protocol-swift), product `OfflineProtocolSDK` | Swift Package Manager, see below |
 | Android, Kotlin (preview) | A download on each [GitHub release](https://github.com/Offline-Protocol/offline-protocol-sdk/releases); not on Maven Central yet | See below |
 | C, and any language that calls C | `offline_protocol.h` and the native library in each [GitHub release](https://github.com/Offline-Protocol/offline-protocol-sdk/releases)'s desktop archives | See [bindings/c](bindings/c/README.md) |
+| Flutter (iOS, Android, preview) | [`offline_protocol`](bindings/flutter/README.md) plugin in this repository | Path dependency; see [bindings/flutter](bindings/flutter/README.md) |
 | Java (JVM 22+, desktop and server) | Generated sources in [bindings/java](bindings/java/README.md), over the native library in each release's desktop archives; not on Maven Central yet | See [bindings/java](bindings/java/README.md) |
 
 **Python.** Wheels are published for Python 3.10 or later on macOS arm64

THIRD-PARTY-NOTICES.md

@@ -826,7 +826,7 @@ DEALINGS IN THE SOFTWARE.
 
 Used by:
 
-- [webpki-roots 1.0.9](https://github.com/rustls/webpki-roots)
+- [webpki-roots 1.0.10](https://github.com/rustls/webpki-roots)
 
 ```
 # Community Data License Agreement - Permissive - Version 2.0
@@ -949,7 +949,7 @@ CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
 Used by:
 
-- [rustls-webpki 0.103.15](https://github.com/rustls/webpki)
+- [rustls-webpki 0.103.16](https://github.com/rustls/webpki)
 
 ```
 Except as otherwise noted, this project is licensed under the following

bindings/flutter/.gitignore

@@ -0,0 +1,14 @@
+.dart_tool/
+.packages
+build/
+**/example/build/
+**/example/.dart_tool/
+**/example/.flutter-plugins-dependencies
+**/example/ios/Flutter/
+**/example/ios/Pods/
+**/example/ios/.symlinks/
+**/example/ios/Runner/GeneratedPluginRegistrant.h
+**/example/ios/Runner/GeneratedPluginRegistrant.m
+**/example/android/.gradle/
+**/example/android/local.properties
+**/example/android/app/src/main/java/io/flutter/plugins/