Gradle (#649)

* Gradle

Gradle support to make ease of import in Android Java projects.

* Gradle

Gradle support for Android projects.
For command line build, `gradle assembe` will trigger build phase

 - Config for Android .so file generation
 - Commented build.gradle
 - Automatic file copy after build
 - Dummy AndroidManifest file

* Build with Gradle

 - Build job for Travis CI
 - Moved gradle file to support/

* Gradle Path

 - Absolute path for Gradle binary

* File check after Gradle build

 - Additional script for build success
 - Check the Gradle's `assemble.doLast` task

.gitignore

+.vscode/
+
+*.iml
+.idea/
+.externalNativeBuild/
+.gradle/
+gradle/
+gradlew*
+local.properties
+build/
+
 bin/
 /_CPack_Packages
 /CMakeScripts

.travis.yml

@@ -21,6 +21,36 @@ matrix:
   exclude:
     - os: osx
       env: BUILD=Doc
+  include:
+    - language: android
+      android:
+      components:
+        - tools
+        - platform-tools
+        - android-21
+        - sys-img-armeabi-v7a-android-21
+      env:
+        - ANDROID=true
+      before_install:
+        - git submodule update --init --recursive
+        - sudo apt-get install wget unzip tree
+      install:
+        # Accept SDK Licenses + Install NDK
+        - yes | sdkmanager --update > /dev/null 2>&1
+        - sdkmanager ndk-bundle > /dev/null 2>&1
+        # Download Gradle 4.3.1 
+        - wget https://services.gradle.org/distributions/gradle-4.3.1-bin.zip
+        - mkdir -p gradle
+        - unzip -q -d ./gradle gradle-4.3.1-bin.zip
+        - export GRADLE=${TRAVIS_BUILD_DIR}/gradle/gradle-4.3.1/bin/gradle
+      before_script:
+        - bash $GRADLE --version
+        - cd ./support
+      script:
+        - bash $GRADLE clean assemble
+      after_success:
+        - cd ${TRAVIS_BUILD_DIR}
+        - tree ./libs
 
 # Install gcc-6 for extended constexpr support.
 addons:

support/AndroidManifest.xml

+<manifest package="fmt" />
\ No newline at end of file

support/build.gradle

+// General gradle arguments for root project
+buildscript {    
+    repositories {
+        google()
+        jcenter()
+    }
+
+    dependencies {
+        classpath 'com.android.tools.build:gradle:3.0.1'
+    }
+}
+
+// Output: Shared library (.so) for Android 
+apply plugin: 'com.android.library'
+
+android {
+    compileSdkVersion 25    // Android 7.0
+
+    // Target ABI
+    //  - This option controls target platform of module
+    //  - The platform might be limited by compiler's support
+    //    some can work with Clang(default), but some can work only with GCC...
+    //    if bad, both toolchains might not support it
+    splits {
+        abi {
+            enable true
+            // Be general, as much as possible ...
+            // universalApk true 
+
+            // Specify platforms for Application
+            reset()
+            include "x86", "x86_64", "armeabi-v7a", "arm64-v8a"
+        }
+    }
+
+    defaultConfig {
+        minSdkVersion 21    // Android 5.0+
+        targetSdkVersion 25 // Follow Compile SDK
+        versionCode 16      // Follow release count
+        versionName "4.1.0" // Follow Official version
+        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
+        
+        externalNativeBuild {
+            cmake {
+                arguments "-DANDROID_STL=c++_shared"    // Specify Android STL
+                arguments "-DBUILD_SHARED_LIBS=true"    // Build shared object
+                arguments "-DFMT_TEST=false"            // Skip test
+                arguments "-DFMT_DOC=false"             // Skip document
+                cppFlags  "-std=c++14"
+            }
+        }
+        println("Gradle CMake Plugin: ")
+        println(externalNativeBuild.cmake.cppFlags)
+        println(externalNativeBuild.cmake.arguments)
+    }
+
+    // External Native build
+    //  - Use existing CMakeList.txt
+    //  - Give path to CMake. This gradle file should be 
+    //    neighbor of the top level cmake
+    externalNativeBuild {
+        cmake {
+            path "../CMakeLists.txt"
+            // buildStagingDirectory "./build"  // Custom path for cmake output
+        }
+        //println(cmake.path)
+    }
+    
+    sourceSets{
+        // Android Manifest for Gradle
+        main {
+            manifest.srcFile 'AndroidManifest.xml'
+        }
+    }
+}
+
+assemble.doLast
+{
+    // Instead of `ninja install`, Gradle will deploy the files.
+    // We are doing this since FMT is dependent to the ANDROID_STL after build
+    copy {
+        from 'build/intermediates/cmake'
+        into '../libs'
+    }
+    // Copy debug binaries
+    copy {
+        from '../libs/debug/obj'
+        into '../libs/debug'
+    }
+    // Copy Release binaries
+    copy {
+        from '../libs/release/obj'
+        into '../libs/release'
+    }
+    // Remove empty directory
+    delete '../libs/debug/obj'
+    delete '../libs/release/obj'
+}