Add option to use Amber as a shared library on Android (google#708)

samples/Android.mk

@@ -19,6 +19,7 @@ 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 \
@@ -30,5 +31,8 @@ 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
+include $(BUILD_SHARED_LIBRARY)
 
 include $(LOCAL_PATH)/../Android.mk

samples/android_main.cc

@@ -0,0 +1,45 @@
+// Copyright 2019 The Amber Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <jni.h>
+
+#include <sstream>
+#include <string>
+#include <vector>
+
+extern int main(int argc, const char** argv);
+
+extern "C" JNIEXPORT JNICALL int Java_com_google_amber_Amber_androidMain(
+    JNIEnv* env,
+    jobject,
+    jstring args_jstring) {
+  // Redirect std output to a file
+  freopen("/sdcard/amberlog.txt", "w", stdout);
+  freopen("/sdcard/amberlog.txt", "a", stderr);
+
+  std::string args(env->GetStringUTFChars(args_jstring, NULL));
+
+  // Parse argument string and add -d by default
+  std::stringstream ss(args);
+  std::vector<std::string> argv_string{std::istream_iterator<std::string>{ss},
+                                       std::istream_iterator<std::string>{}};
+  std::vector<const char*> argv;
+  argv.push_back("amber");
+  argv.push_back("-d");
+
+  for (auto s : argv_string)
+    argv.push_back(s.c_str());
+
+  return main(argv.size(), argv.data());
+}