feat(android): add native mobile ink support

README.md

@@ -14,7 +14,7 @@ Production-grade React Native ink primitives extracted from the MathNotes canvas
   <img src="https://raw.githubusercontent.com/mathnotes-app/mobile-ink/main/docs/assets/mobile-ink-cover.jpg" alt="Mobile Ink cover drawn inside the example canvas" width="680" />
 </p>
 
-`@mathnotes/mobile-ink` is an iOS-first native drawing engine for React Native apps. It gives you Apple Pencil input, Skia/Metal rendering, stroke serialization, selection, zoom, momentum scrolling, and a continuous notebook surface backed by a fixed native engine pool.
+`@mathnotes/mobile-ink` is a native drawing engine for React Native apps. It gives you Apple Pencil and stylus input, Skia-backed native rendering, stroke serialization, selection, zoom, momentum scrolling, and a continuous notebook surface backed by a fixed native engine pool.
 
 ## A Note To Contributors
 
@@ -37,12 +37,12 @@ mobile-ink is currently used in production in MathNotes: https://apps.apple.com/
 | Area | Current support |
 | --- | --- |
 | iOS Apple Pencil drawing | Used in production |
-| Native rendering | Custom `MTKView` backed by C++ Skia/Metal |
+| Android stylus/finger drawing | Native Android view backed by the same C++ engine |
+| Native rendering | iOS `MTKView` with Skia/Metal; Android `GLSurfaceView` with Skia/OpenGL upload |
 | Continuous notebooks | Fixed native engine pool with momentum scroll and pinch zoom |
 | Tools | Pen, highlighter, crayon, calligraphy, eraser, selection, and shape recognition |
 | Serialization | JSON notebook payloads plus native page load/save/export helpers |
 | Example app | Expo dev-client app with blank continuous notebook, tools, selection, save/reload, and zoom |
-| Android | Not supported yet |
 | Expo Go | Not supported because this package includes native code |
 
 ## Demos
@@ -68,7 +68,7 @@ npm install @mathnotes/mobile-ink \
 cd ios && pod install
 ```
 
-For Expo apps, use a dev client or prebuild. Expo Go cannot load this native module.
+For Expo apps, use a dev client or prebuild. Expo Go cannot load this native module. Android consumers need the normal React Native Android toolchain because the package builds a Kotlin/C++ Android library.
 
 Your app Babel config must include the Reanimated/Worklets plugin expected by your React Native/Reanimated version. For Expo SDK 54/Reanimated 4:
 
@@ -121,7 +121,7 @@ export function Notebook() {
 
 ## Public Surface
 
-- `NativeInkCanvas`: low-level native Skia/Metal drawing view.
+- `NativeInkCanvas`: low-level native Skia drawing view.
 - `ZoomableInkViewport`: production pinch, pan, momentum, focal-point zoom, and Apple Pencil/finger gesture routing.
 - `ContinuousEnginePool`: fixed-size native canvas pool for continuous notebooks.
 - `InfiniteInkCanvas`: full vertical continuous notebook shell with pooled native engines, trailing page creation, dirty-page serialization, zoom, momentum scroll, and generic page backgrounds.
@@ -144,6 +144,12 @@ For a simulator:
 npx expo run:ios
 ```
 
+For Android:
+
+```sh
+npx expo run:android
+```
+
 ## Documentation
 
 - [Architecture](docs/architecture.md)
@@ -159,7 +165,7 @@ Near-term work is focused on making the public package easier to adopt and easie
 - Tighten selection transform performance for large stroke groups.
 - Improve edge-case zoom behavior near page and canvas boundaries.
 - Continue hardening the example app as a small regression harness.
-- Explore Android after the iOS API surface has settled.
+- Keep Android behavior aligned with the iOS API surface as new native capabilities land.
 
 ## Development
 

android/.gitignore

@@ -0,0 +1,2 @@
+/.cxx/
+/build/

android/build.gradle

@@ -0,0 +1,92 @@
+buildscript {
+    def kotlinVersion = rootProject.ext.has("kotlinVersion")
+        ? rootProject.ext.get("kotlinVersion")
+        : "2.1.20"
+
+    repositories {
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        classpath("com.android.tools.build:gradle:8.10.1")
+        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion")
+    }
+}
+
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
+static def findNodeModules(baseDir) {
+    def basePath = baseDir.toPath().normalize()
+    while (basePath != null) {
+        def nodeModulesPath = basePath.resolve("node_modules").toFile()
+        def reactNativePath = new File(nodeModulesPath, "react-native")
+        if (nodeModulesPath.exists() && reactNativePath.exists()) {
+            return nodeModulesPath.absolutePath
+        }
+        basePath = basePath.getParent()
+    }
+    throw new GradleException("@mathnotes/mobile-ink: failed to find node_modules")
+}
+
+def reactNativeArchitectures() {
+    def value = project.getProperties().get("reactNativeArchitectures")
+    return value ? value.split(",") : ["armeabi-v7a", "x86", "x86_64", "arm64-v8a"]
+}
+
+apply plugin: "com.android.library"
+apply plugin: "kotlin-android"
+
+def nodeModules = findNodeModules(projectDir)
+
+android {
+    namespace "com.mathnotes.mobileink"
+    compileSdkVersion safeExtGet("compileSdkVersion", 35)
+
+    if (rootProject.ext.has("ndkPath")) {
+        ndkPath rootProject.ext.ndkPath
+    }
+    if (rootProject.ext.has("ndkVersion")) {
+        ndkVersion rootProject.ext.ndkVersion
+    }
+
+    defaultConfig {
+        minSdkVersion safeExtGet("minSdkVersion", 24)
+        targetSdkVersion safeExtGet("targetSdkVersion", 35)
+
+        externalNativeBuild {
+            cmake {
+                cppFlags "-std=c++17", "-frtti", "-fexceptions", "-DSK_BUILD_FOR_ANDROID", "-DON_ANDROID", "-DONANDROID"
+                arguments "-DNODE_MODULES_DIR=${nodeModules}",
+                    "-DANDROID_STL=c++_shared",
+                    "-DANDROID_SUPPORT_FLEXIBLE_PAGE_SIZES=ON"
+                abiFilters(*reactNativeArchitectures())
+            }
+        }
+    }
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
+    externalNativeBuild {
+        cmake {
+            path "src/main/cpp/CMakeLists.txt"
+            version "3.22.1"
+        }
+    }
+
+    packagingOptions {
+        excludes = [
+            "**/libc++_shared.so",
+            "META-INF/**"
+        ]
+    }
+}
+
+dependencies {
+    implementation "com.facebook.react:react-android"
+}

android/src/main/AndroidManifest.xml

@@ -0,0 +1 @@
+<manifest xmlns:android="http://schemas.android.com/apk/res/android" />

android/src/main/cpp/CMakeLists.txt

@@ -0,0 +1,64 @@
+cmake_minimum_required(VERSION 3.13)
+project(mobileink)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+# Node modules path passed from Gradle
+set(NODE_MODULES_DIR ${NODE_MODULES_DIR})
+
+# Skia paths from react-native-skia
+set(SKIA_DIR "${NODE_MODULES_DIR}/@shopify/react-native-skia")
+set(SKIA_INCLUDE_DIR "${SKIA_DIR}/cpp/skia")
+set(SKIA_LIB_DIR "${SKIA_DIR}/libs/android/${ANDROID_ABI}")
+
+# Shared C++ drawing engine from this package root.
+set(NATIVE_DRAWING_CPP_DIR "${CMAKE_CURRENT_LIST_DIR}/../../../../cpp")
+
+message(STATUS "mobileink: NODE_MODULES_DIR=${NODE_MODULES_DIR}")
+message(STATUS "mobileink: SKIA_DIR=${SKIA_DIR}")
+message(STATUS "mobileink: NATIVE_DRAWING_CPP_DIR=${NATIVE_DRAWING_CPP_DIR}")
+message(STATUS "mobileink: ANDROID_ABI=${ANDROID_ABI}")
+
+# Include directories
+include_directories(
+    ${SKIA_INCLUDE_DIR}
+    ${SKIA_INCLUDE_DIR}/include
+    ${SKIA_INCLUDE_DIR}/modules/pathops/include
+    ${NATIVE_DRAWING_CPP_DIR}
+)
+
+# Add source files
+add_library(
+    mobileink
+    SHARED
+    ${NATIVE_DRAWING_CPP_DIR}/SkiaDrawingEngine.cpp
+    ${NATIVE_DRAWING_CPP_DIR}/DrawingSelection.cpp
+    ${NATIVE_DRAWING_CPP_DIR}/BackgroundRenderer.cpp
+    ${NATIVE_DRAWING_CPP_DIR}/DrawingSerialization.cpp
+    ${NATIVE_DRAWING_CPP_DIR}/PathRenderer.cpp
+    ${NATIVE_DRAWING_CPP_DIR}/EraserRenderer.cpp
+    ${NATIVE_DRAWING_CPP_DIR}/StrokeSplitter.cpp
+    ${NATIVE_DRAWING_CPP_DIR}/BatchExporter.cpp
+    ${NATIVE_DRAWING_CPP_DIR}/ActiveStrokeRenderer.cpp
+    jni_bridge.cpp
+)
+
+# Link Skia library
+add_library(skia STATIC IMPORTED)
+set_target_properties(skia PROPERTIES IMPORTED_LOCATION "${SKIA_LIB_DIR}/libskia.a")
+
+add_library(pathops STATIC IMPORTED)
+set_target_properties(pathops PROPERTIES IMPORTED_LOCATION "${SKIA_LIB_DIR}/libpathops.a")
+
+# Link libraries
+target_link_libraries(
+    mobileink
+    skia
+    pathops
+    android
+    log
+    GLESv2
+    EGL
+    jnigraphics
+)