Update and improve android_gradle (#965)

* Update CMakeLists.txt files so we can build libamber_ndk.so for
Android via CMake.

* `android_gradle/`:

  * Update Gradle version, settings, and build files to match the latest
  Android sample. Importantly, this removes the use of jcenter and
  bintray repositories, which were shut down.

  * Specify NDK version so that Gradle can install required native build
  tools automatically.

  * Define externalNativeBuild so Gradle can perform the Android CMake
  build automatically.

* Replace `main` with `android_main` when building the Amber tool as a
library for Android to avoid compiler warnings, and rename other
Android-related symbols to avoid confusion.

Fixes #964

4 files changed, 31 insertions, 7 deletions

diff --git a/samples/Android.mk b/samples/Android.mk
index 6ebdbc8..1428ead 100644
--- a/samples/Android.mk
+++ b/samples/Android.mk
@@ -19,7 +19,6 @@ LOCAL_MODULE:=amber_ndk
 LOCAL_CPP_EXTENSION := .cc .cpp .cxx
 LOCAL_SRC_FILES:= \
     amber.cc \
-    android_main.cc \
     config_helper.cc \
     config_helper_vulkan.cc \
     log.cc \
@@ -31,8 +30,11 @@ LOCAL_LDLIBS:=-landroid -lvulkan -llog
 LOCAL_CXXFLAGS:=-std=c++11 -fno-exceptions -fno-rtti -Werror -Wno-unknown-pragmas -DAMBER_ENGINE_VULKAN=1 -DAMBER_ENABLE_LODEPNG=1
 LOCAL_STATIC_LIBRARIES:=amber lodepng
 include $(BUILD_EXECUTABLE)
+
 LOCAL_MODULE:=amber_ndk_sharedlib
 LOCAL_MODULE_FILENAME:=libamber_ndk
+LOCAL_SRC_FILES+=android_helper.cc
+LOCAL_CXXFLAGS+=-DAMBER_ANDROID_MAIN=1
 include $(BUILD_SHARED_LIBRARY)
 
 include $(LOCAL_PATH)/../Android.mk
diff --git a/samples/CMakeLists.txt b/samples/CMakeLists.txt
index 3fa1882..1e91c4d 100644
--- a/samples/CMakeLists.txt
+++ b/samples/CMakeLists.txt
@@ -74,3 +74,11 @@ target_include_directories(image_diff PRIVATE "${CMAKE_BINARY_DIR}")
 target_link_libraries(image_diff libamber "lodepng")
 amber_default_compile_options(image_diff)
 set_target_properties(image_diff PROPERTIES OUTPUT_NAME "image_diff")
+
+if (ANDROID)
+  add_library(amber_ndk SHARED android_helper.cc ${AMBER_SOURCES})
+  target_include_directories(amber_ndk PRIVATE "${CMAKE_BINARY_DIR}")
+  target_link_libraries(amber_ndk libamber ${AMBER_EXTRA_LIBS})
+  amber_default_compile_options(amber_ndk)
+  target_compile_definitions(amber_ndk PRIVATE AMBER_ANDROID_MAIN=1)
+endif()
diff --git a/samples/amber.cc b/samples/amber.cc
index d037ba4..5973dba 100644
--- a/samples/amber.cc
+++ b/samples/amber.cc
@@ -447,7 +447,15 @@ std::string disassemble(const std::string& env,
 
 }  // namespace
 
+#ifdef AMBER_ANDROID_MAIN
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wmissing-prototypes"
+#pragma ide diagnostic ignored "OCUnusedGlobalDeclarationInspection"
+int android_main(int argc, const char** argv) {
+#pragma clang diagnostic pop
+#else
 int main(int argc, const char** argv) {
+#endif
   std::vector<std::string> args(argv, argv + argc);
   Options options;
   SampleDelegate delegate;
diff --git a/samples/android_main.cc b/samples/android_helper.cc
index b24a9bb..7da69ad 100644
--- a/samples/android_main.cc
+++ b/samples/android_helper.cc
@@ -18,16 +18,20 @@
 #include <string>
 #include <vector>
 
-extern int main(int argc, const char** argv);
+extern int android_main(int argc, const char** argv);
 
-extern "C" JNIEXPORT JNICALL int Java_com_google_amber_Amber_androidMain(
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wmissing-prototypes"
+#pragma ide diagnostic ignored "OCUnusedGlobalDeclarationInspection"
+
+extern "C" JNIEXPORT JNICALL int Java_com_google_amber_Amber_androidHelper(
     JNIEnv* env,
     jobject,
     jobjectArray args,
     jstring stdoutFile,
     jstring stderrFile) {
-  const char* stdout_file_cstr = env->GetStringUTFChars(stdoutFile, NULL);
-  const char* stderr_file_cstr = env->GetStringUTFChars(stderrFile, NULL);
+  const char* stdout_file_cstr = env->GetStringUTFChars(stdoutFile, nullptr);
+  const char* stderr_file_cstr = env->GetStringUTFChars(stderrFile, nullptr);
 
   // Redirect std output to a file
   freopen(stdout_file_cstr, "w", stdout);
@@ -43,7 +47,7 @@ extern "C" JNIEXPORT JNICALL int Java_com_google_amber_Amber_androidMain(
 
   for (jsize i = 0; i < arg_count; i++) {
     jstring js = static_cast<jstring>(env->GetObjectArrayElement(args, i));
-    const char* arg_cstr = env->GetStringUTFChars(js, NULL);
+    const char* arg_cstr = env->GetStringUTFChars(js, nullptr);
     argv_string.push_back(arg_cstr);
     env->ReleaseStringUTFChars(js, arg_cstr);
   }
@@ -53,5 +57,7 @@ extern "C" JNIEXPORT JNICALL int Java_com_google_amber_Amber_androidMain(
   for (const std::string& arg : argv_string)
     argv.push_back(arg.c_str());
 
-  return main(argv.size(), argv.data());
+  return android_main(static_cast<int>(argv.size()), argv.data());
 }
+
+#pragma clang diagnostic pop