FirebaseAndroid: introduce the new JNI module for Android support

In order to interact with the C++ Firebase SDK, we need to have access
to the JVM and the JavaEnvironment. Introduce a JNI layer which allows
us to capture this information when loaded. Additionally, register a
`company.thebrowser.Native.RegisterActivity` method to allow us access
to the `Activity`. This is required to be called before the Firebase
layer is accessed by the application.

CMakeLists.txt

@@ -3,6 +3,11 @@
 cmake_minimum_required(VERSION 3.25)
 project(swift-firebase
   LANGUAGES Swift)
+if(ANDROID)
+  enable_language(C)
+  include(FindJava)
+  include(UseJava)
+endif()
 
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
@@ -39,6 +44,10 @@ else()
   message(FATAL_ERROR "unsupported firebase-cpp-sdk platform")
 endif()
 
+if(ANDROID)
+  add_subdirectory(Sources/FirebaseAndroid)
+endif()
+
 add_library(FirebaseCore SHARED
   Sources/FirebaseCore/FirebaseApp+Swift.swift
   Sources/FirebaseCore/FirebaseConfiguration.swift
@@ -55,6 +64,10 @@ target_link_libraries(FirebaseCore PRIVATE
   firebase_app
   flatbuffers
   zlibstatic)
+if(ANDROID)
+  target_link_libraries(FirebaseCore PRIVATE
+    FirebaseAndroidJNI)
+endif()
 
 add_library(FirebaseAuth SHARED
   Sources/FirebaseAuth/AuthStateDidChangeListenerHandle.swift

Sources/FirebaseAndroid/CMakeLists.txt

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: BSD-3-Clause
+
+add_library(FirebaseAndroidJNI SHARED
+  jni.c)
+target_include_directories(FirebaseAndroidJNI PUBLIC
+  include)
+target_link_libraries(FirebaseAndroidJNI PRIVATE
+  log)
+

Sources/FirebaseAndroid/abi.h

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+
+#ifndef SwiftFirebase_FirebaseAndroidShim_abi_h
+#define SwiftFirebase_FirebaseAndroidShim_abi_h
+
+#if defined(FirebaseAndroidJNI_EXPORTS)
+#define FIREBASE_ANDROID_ABI __attribute__((__visibility__("default")))
+#else
+#define FIREBASE_ANDROID_ABI __attribute__((__visibility__("default")))
+#endif
+
+#endif

Sources/FirebaseAndroid/include/FirebaseAndroid.h

@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+
+#ifndef SwiftFirebase_FirebaseAndroidShim_FirebaseAndroid_h
+#define SwiftFirebase_FirebaseAndroidShim_FirebaseAndroid_h
+
+#include <jni.h>
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+jobject SwiftFirebase_GetActivity(void);
+JNIEnv *SwiftFirebase_GetJavaEnvironment(void);
+JavaVM *SwiftFirebase_GetJVM(void);
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif

Sources/FirebaseAndroid/include/module.modulemap

@@ -0,0 +1,6 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+
+module FirebaseAndroid {
+  header "FirebaseAndroid.h"
+  export *
+}

Sources/FirebaseAndroid/jni.c

@@ -0,0 +1,79 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+
+#include "FirebaseAndroid.h"
+#include "abi.h"
+#include "log.h"
+
+#include <assert.h>
+
+static JavaVM *g_VM;
+static JNIEnv *g_Env;
+static jobject *g_Activity;
+
+#define N_ELEMENTS(array) (sizeof((array)) / sizeof(*(array)))
+
+static const char kClassPath[] = "company/thebrowser/Native";
+
+static jboolean
+SwiftFirebase_RegisterActivity(JNIEnv *env, jobject *this, jobject *activity)
+{
+  assert(g_Activity == NULL && "re-registeration of activity");
+  if (g_Activity) return JNI_FALSE;
+
+  g_Activity = activity;
+  return JNI_TRUE;
+}
+
+static JNINativeMethod kMethods[] = {
+  { "RegisterActivity", "()Z", SwiftFirebase_RegisterActivity },
+};
+
+static void
+RegisterNativeMethods(JNIEnv *env)
+{
+  jclass class;
+  jint result;
+
+  class = (*env)->FindClass(env, kClassPath);
+  if (class == NULL) {
+    LOG_ERROR("unable to find class '%s'", kClassPath);
+    return;
+  }
+  LOG_DEBUG("located class path '%s': %p", kClassPath, class);
+
+  result = (*env)->RegisterNatives(env, class, kMethods, N_ELEMENTS(kMethods));
+  if (result < 0) {
+    LOG_ERROR("JVM.RegisterNatives(%s): %u", kClassPath, result);
+    return;
+  }
+  LOG_DEBUG("registered %u methods", N_ELEMENTS(kMethods));
+}
+
+FIREBASE_ANDROID_ABI
+jint JNI_OnLoad(JavaVM *vm, void *reserved)
+{
+  g_VM = vm;
+  if ((*g_VM)->GetEnv(g_VM, (void **)&g_Env, JNI_VERSION_1_6) == JNI_OK)
+    return JNI_VERSION_1_6;
+  RegisterNativeMethods(g_Env);
+  return -1;
+}
+
+FIREBASE_ANDROID_ABI
+jobject SwiftFirebase_GetActivity(void)
+{
+  assert(g_Activity && "`GetActivity` invoked before `RegisterActivity`");
+  return *g_Activity;
+}
+
+FIREBASE_ANDROID_ABI
+JNIEnv *SwiftFirebase_GetJavaEnvironment(void)
+{
+  return g_Env;
+}
+
+FIREBASE_ANDROID_ABI
+JavaVM *SwiftFirebase_GetJVM(void)
+{
+  return g_VM;
+}

Sources/FirebaseAndroid/log.h

@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+
+#ifndef SwiftFirebase_FirebaseAndroidShim_logging_h
+#define SwiftFirebase_FirebaseAndroidShim_logging_h
+
+#include <android/log.h>
+
+#define FIREBASE_ANDROID_LOG(level, tag, ...) __android_log_print(level, tag, __VA_ARGS__)
+#define FIREBASE_ANDROID_TAG                  "company.thebrowser.firebase"
+
+#define LOG_DEBUG(...)   FIREBASE_ANDROID_LOG(ANDROID_LOG_DEBUG, FIREBASE_ANDROID_TAG, __VA_ARGS__)
+#define LOG_VERBOSE(...) FIREBASE_ANDROID_LOG(ANDROID_LOG_VERBOSE, FIREBASE_ANDROID_TAG, __VA_ARGS__)
+#define LOG_INFO(...)    FIREBASE_ANDROID_LOG(ANDROID_LOG_INFO, FIREBASE_ANDROID_TAG, __VA_ARGS__)
+#define LOG_WARN(...)    FIREBASE_ANDROID_LOG(ANDROID_LOG_WARN, FIREBASE_ANDROID_TAG, __VA_ARGS__)
+#define LOG_ERROR(...)   FIREBASE_ANDROID_LOG(ANDROID_LOG_ERROR, FIREBASE_ANDROID_TAG, __VA_ARGS__)
+
+#endif