cpp: add Android backend support

.github/workflows/ci.yaml

@@ -678,6 +678,27 @@ jobs:
             - name: Build usecases demo
               run: cargo apk build -p usecases --target aarch64-linux-android --lib
 
+    android_cpp:
+        needs: files-changed
+        if: ${{ needs.files-changed.outputs.internal == 'true' || needs.files-changed.outputs.api_cpp == 'true' || needs.files-changed.outputs.examples == 'true' }}
+        runs-on: ubuntu-latest
+        env:
+            CARGO_INCREMENTAL: false
+            CARGO_PROFILE_DEV_DEBUG: 0
+        steps:
+            - uses: actions/checkout@v6
+            - uses: ./.github/actions/install-linux-dependencies
+            - uses: ./.github/actions/setup-rust
+              with:
+                  target: aarch64-linux-android
+            - name: Build Slint and usecases demo for Android (CMake cross-compile)
+              run: |
+                  cmake -B build-android -S . \
+                    -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK_LATEST_HOME/build/cmake/android.toolchain.cmake \
+                    -DANDROID_ABI=arm64-v8a -DANDROID_PLATFORM=android-26 \
+                    -DSLINT_BUILD_EXAMPLES=ON
+                  cmake --build build-android --target usecases
+
     test-figma-inspector:
         needs: files-changed
         if: ${{ needs.files-changed.outputs.figma_inspector == 'true' }}

REUSE.toml

@@ -325,3 +325,15 @@ path = ["examples/bevy/bevy-hosts-slint/**.gltf"]
 precedence = "aggregate"
 SPDX-FileCopyrightText = "Copyright (c) 2024 e-verse"
 SPDX-License-Identifier = "MIT"
+
+[[annotations]]
+path = ["api/cpp/android/**/gradle-wrapper.properties", "examples/**/android/**/gradle-wrapper.properties"]
+precedence = "aggregate"
+SPDX-FileCopyrightText = "Copyright © SixtyFPS GmbH <info@slint.dev>"
+SPDX-License-Identifier = "GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0"
+
+[[annotations]]
+path = ["demos/**/android/**/gradle-wrapper.properties"]
+precedence = "aggregate"
+SPDX-FileCopyrightText = "Copyright © SixtyFPS GmbH <info@slint.dev>"
+SPDX-License-Identifier = "MIT"

api/cpp/CMakeLists.txt

@@ -24,6 +24,39 @@ endif ()
 list(PREPEND CMAKE_MODULE_PATH ${Corrosion_SOURCE_DIR}/cmake)
 find_package(Rust 1.92 REQUIRED MODULE)
 
+# When building for Android (NDK toolchain sets CMAKE_SYSTEM_NAME to "Android"),
+# auto-configure the Rust target, enable the android backend, and set sensible defaults.
+# This block must run before the feature options are defined so that the forced cache values
+# are picked up by define_cargo_feature / define_cargo_dependent_feature.
+if(ANDROID)
+    # Map Android ABI to Rust target triple
+    if(ANDROID_ABI STREQUAL "arm64-v8a")
+        set(Rust_CARGO_TARGET "aarch64-linux-android" CACHE STRING "" FORCE)
+    elseif(ANDROID_ABI STREQUAL "armeabi-v7a")
+        set(Rust_CARGO_TARGET "armv7-linux-androideabi" CACHE STRING "" FORCE)
+    elseif(ANDROID_ABI STREQUAL "x86_64")
+        set(Rust_CARGO_TARGET "x86_64-linux-android" CACHE STRING "" FORCE)
+    elseif(ANDROID_ABI STREQUAL "x86")
+        set(Rust_CARGO_TARGET "i686-linux-android" CACHE STRING "" FORCE)
+    else()
+        message(FATAL_ERROR "Unsupported ANDROID_ABI: ${ANDROID_ABI}")
+    endif()
+
+    # Android requires a shared library
+    set(BUILD_SHARED_LIBS ON CACHE BOOL "" FORCE)
+
+    # Enable skia renderer and disable desktop backends for Android
+    set(SLINT_FEATURE_RENDERER_SKIA ON CACHE BOOL "" FORCE)
+    set(SLINT_FEATURE_BACKEND_WINIT OFF CACHE BOOL "" FORCE)
+    set(SLINT_FEATURE_BACKEND_QT OFF CACHE BOOL "" FORCE)
+    set(SLINT_FEATURE_RENDERER_FEMTOVG OFF CACHE BOOL "" FORCE)
+
+    # Default to Material style on Android
+    if(NOT SLINT_STYLE)
+        set(SLINT_STYLE "material" CACHE STRING "" FORCE)
+    endif()
+endif()
+
 option(BUILD_SHARED_LIBS "Build Slint as shared library" ON)
 option(SLINT_FEATURE_COMPILER "Enable support for compiling .slint files to C++ ahead of time" ON)
 add_feature_info(SLINT_FEATURE_COMPILER SLINT_FEATURE_COMPILER "Enable support for compiling .slint files to C++ ahead of time")
@@ -306,6 +339,41 @@ if (SLINT_BUILD_RUNTIME)
         )
     endif()
 
+    # Propagate Android NDK/SDK paths to the Rust build so that the android-activity
+    # backend can compile Java sources and produce DEX bytecode.
+    if(ANDROID)
+        if(DEFINED ENV{ANDROID_NDK_ROOT})
+            set_property(
+                TARGET slint_cpp
+                APPEND
+                PROPERTY CORROSION_ENVIRONMENT_VARIABLES
+                "ANDROID_NDK_ROOT=$ENV{ANDROID_NDK_ROOT}"
+            )
+        elseif(CMAKE_ANDROID_NDK)
+            set_property(
+                TARGET slint_cpp
+                APPEND
+                PROPERTY CORROSION_ENVIRONMENT_VARIABLES
+                "ANDROID_NDK_ROOT=${CMAKE_ANDROID_NDK}"
+            )
+        endif()
+        if(DEFINED ENV{ANDROID_HOME})
+            set_property(
+                TARGET slint_cpp
+                APPEND
+                PROPERTY CORROSION_ENVIRONMENT_VARIABLES
+                "ANDROID_HOME=$ENV{ANDROID_HOME}"
+            )
+        elseif(CMAKE_ANDROID_SDK)
+            set_property(
+                TARGET slint_cpp
+                APPEND
+                PROPERTY CORROSION_ENVIRONMENT_VARIABLES
+                "ANDROID_HOME=${CMAKE_ANDROID_SDK}"
+            )
+        endif()
+    endif()
+
     if(SLINT_FEATURE_RENDERER_SKIA OR SLINT_FEATURE_RENDERER_SKIA_OPENGL OR SLINT_FEATURE_RENDERER_SKIA_VULKAN)
         find_program(CLANGCC clang)
         find_program(CLANGCXX clang++)

api/cpp/Cargo.toml

@@ -51,6 +51,7 @@ std = [
   "i-slint-core/default",
   "i-slint-core/image-default-formats",
   "i-slint-backend-selector",
+  "i-slint-backend-selector/backend-android-activity",
   "i-slint-renderer-software?/std",
   "i-slint-core/raw-window-handle-06",
   "i-slint-backend-selector/raw-window-handle-06",
@@ -74,6 +75,9 @@ esp-backtrace = { version = "0.17.0", features = ["panic-handler", "println"], o
 esp-println = { version = "0.15.0", default-features = false, features = ["auto", "log-04"], optional = true }
 unicode-segmentation = { workspace = true }
 
+[target.'cfg(target_os = "android")'.dependencies]
+i-slint-backend-android-activity = { workspace = true, features = ["native-activity", "aa-06"] }
+
 [build-dependencies]
 anyhow = "1.0"
 cbindgen = { workspace = true }

api/cpp/cbindgen.rs

@@ -797,6 +797,7 @@ namespace slint {
         using types::IntRect;
         using types::Size;
         using types::MouseEvent;
+        using ::slint::Timer;
     }
     template<typename ModelData> class Model;
 }",

api/cpp/include/private/slint_callbacks.h

@@ -3,10 +3,28 @@
 
 #pragma once
 #include <tuple>
+#include <utility>
 #include "private/slint_properties_internal.h"
 
 namespace slint::private_api {
 
+namespace detail {
+// Custom apply implementation to replace std::apply.
+// NDK r27's libc++ rejects std::apply with const tuple references in some configurations.
+template<typename F, typename Tuple, std::size_t... I>
+decltype(auto) apply_impl(F &&f, Tuple &&t, std::index_sequence<I...>)
+{
+    return std::forward<F>(f)(std::get<I>(std::forward<Tuple>(t))...);
+}
+template<typename F, typename Tuple>
+decltype(auto) apply(F &&f, Tuple &&t)
+{
+    return apply_impl(
+            std::forward<F>(f), std::forward<Tuple>(t),
+            std::make_index_sequence<std::tuple_size_v<std::remove_reference_t<Tuple>>> {});
+}
+} // namespace detail
+
 /// A Callback stores a function pointer with no parameters and no return value.
 /// It's possible to set that pointer via set_handler() and it can be invoked via call(). This is
 /// used to implement callbacks in the `.slint` language.
@@ -33,9 +51,9 @@ struct Callback<Ret(Arg...)>
         cbindgen_private::slint_callback_set_handler(
                 &inner,
                 [](void *user_data, const void *arg, void *ret) {
+                    Tuple args = *reinterpret_cast<const Tuple *>(arg);
                     *reinterpret_cast<Ret *>(ret) =
-                            std::apply(*reinterpret_cast<F *>(user_data),
-                                       *reinterpret_cast<const Tuple *>(arg));
+                            detail::apply(*reinterpret_cast<F *>(user_data), std::move(args));
                 },
                 new F(std::move(binding)),
                 [](void *user_data) { delete reinterpret_cast<F *>(user_data); });
@@ -77,8 +95,8 @@ struct Callback<void(Arg...)>
         cbindgen_private::slint_callback_set_handler(
                 &inner,
                 [](void *user_data, const void *arg, void *) {
-                    std::apply(*reinterpret_cast<F *>(user_data),
-                               *reinterpret_cast<const Tuple *>(arg));
+                    Tuple args = *reinterpret_cast<const Tuple *>(arg);
+                    detail::apply(*reinterpret_cast<F *>(user_data), std::move(args));
                 },
                 new F(std::move(binding)),
                 [](void *user_data) { delete reinterpret_cast<F *>(user_data); });

api/cpp/include/private/slint_timer.h

@@ -125,7 +125,7 @@ struct Timer
     }
 
 private:
-    uint64_t id = 0;
+    uintptr_t id = 0;
 };
 
 } // namespace slint

api/cpp/lib.rs

@@ -38,6 +38,22 @@ pub use i_slint_backend_testing;
 #[cfg(feature = "slint-interpreter")]
 pub use slint_interpreter;
 
+#[cfg(target_os = "android")]
+mod android {
+    unsafe extern "C" {
+        fn slint_main();
+    }
+
+    #[unsafe(no_mangle)]
+    fn android_main(app: i_slint_backend_android_activity::AndroidApp) {
+        i_slint_core::platform::set_platform(alloc::boxed::Box::new(
+            i_slint_backend_android_activity::AndroidPlatform::new(app),
+        ))
+        .unwrap();
+        unsafe { slint_main() };
+    }
+}
+
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn slint_windowrc_init(out: *mut WindowAdapterRcOpaque) {
     assert_eq!(

demos/CMakeLists.txt

@@ -12,6 +12,7 @@ endif()
 
 if (TARGET Slint::slint-compiler)
     add_subdirectory(printerdemo/cpp/)
+    add_subdirectory(usecases/cpp/)
 endif()
 if (SLINT_FEATURE_INTERPRETER)
     add_subdirectory(printerdemo/cpp_interpreted/)

demos/usecases/cpp/CMakeLists.txt

@@ -8,8 +8,14 @@ if (NOT TARGET Slint::Slint)
     find_package(Slint REQUIRED)
 endif()
 
-set(SLINT_STYLE "cosmic-light" CACHE STRING "Style for demo" FORCE)
+if(NOT SLINT_STYLE)
+    set(SLINT_STYLE "cosmic-light" CACHE STRING "Style for demo" FORCE)
+endif()
 
-add_executable(usecases main.cpp)
+if(ANDROID)
+    add_library(usecases SHARED main.cpp)
+else()
+    add_executable(usecases main.cpp)
+endif()
 target_link_libraries(usecases PRIVATE Slint::Slint)
 slint_target_sources(usecases ../ui/app.slint)

demos/usecases/cpp/android/app/build.gradle.kts

@@ -0,0 +1,38 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: MIT
+
+plugins {
+    id("com.android.application")
+}
+
+android {
+    namespace = "dev.slint.usecases"
+    compileSdk = 35
+
+    defaultConfig {
+        applicationId = "dev.slint.usecases"
+        // API 26 required for InMemoryDexClassLoader used by the Slint Android backend
+        minSdk = 26
+        targetSdk = 35
+        versionCode = 1
+        versionName = "1.0"
+
+        ndk {
+            // Add other ABIs as needed: "armeabi-v7a", "x86_64", "x86"
+            abiFilters += "arm64-v8a"
+        }
+    }
+
+    buildTypes {
+        release {
+            isMinifyEnabled = false
+        }
+    }
+
+    externalNativeBuild {
+        cmake {
+            path = file("src/main/cpp/CMakeLists.txt")
+            version = "3.22.1"
+        }
+    }
+}

demos/usecases/cpp/android/app/src/main/AndroidManifest.xml

@@ -0,0 +1,22 @@
+<!-- Copyright © SixtyFPS GmbH <info@slint.dev> -->
+<!-- SPDX-License-Identifier: MIT -->
+
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
+    <application
+        android:label="Slint Usecases"
+        android:hasCode="false">
+        <activity
+            android:name="android.app.NativeActivity"
+            android:exported="true"
+            android:configChanges="orientation|screenSize|keyboardHidden">
+            <meta-data
+                android:name="android.app.lib_name"
+                android:value="usecases" />
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
+        </activity>
+    </application>
+</manifest>

demos/usecases/cpp/android/app/src/main/cpp/CMakeLists.txt

@@ -0,0 +1,15 @@
+# Copyright © SixtyFPS GmbH <info@slint.dev>
+# SPDX-License-Identifier: MIT
+
+cmake_minimum_required(VERSION 3.21)
+project(SlintUsecases LANGUAGES CXX)
+
+# Use the local Slint source tree
+add_subdirectory(${SLINT_SOURCE_DIR} slint_build)
+
+# Shared source directory: app/src/main/cpp -> android -> cpp (the desktop example)
+cmake_path(SET _usecases_cpp_dir NORMALIZE "${CMAKE_CURRENT_SOURCE_DIR}/../../../..")
+
+add_library(usecases SHARED "${_usecases_cpp_dir}/main.cpp")
+target_link_libraries(usecases PRIVATE Slint::Slint)
+slint_target_sources(usecases "${_usecases_cpp_dir}/../ui/app.slint")

demos/usecases/cpp/android/build.gradle.kts

@@ -0,0 +1,6 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: MIT
+
+plugins {
+    id("com.android.application") version "8.7.0" apply false
+}

demos/usecases/cpp/android/gradle.properties

@@ -0,0 +1,4 @@
+# Copyright © SixtyFPS GmbH <info@slint.dev>
+# SPDX-License-Identifier: MIT
+
+android.useAndroidX=true

demos/usecases/cpp/android/gradle/wrapper/gradle-wrapper.properties

@@ -0,0 +1,8 @@
+# Copyright © SixtyFPS GmbH <info@slint.dev>
+# SPDX-License-Identifier: MIT
+
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.11-bin.zip
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists

demos/usecases/cpp/android/settings.gradle.kts

@@ -0,0 +1,20 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: MIT
+
+pluginManagement {
+    repositories {
+        google()
+        mavenCentral()
+        gradlePluginPortal()
+    }
+}
+
+dependencyResolution {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+rootProject.name = "SlintUsecases"
+include(":app")