Add auto downloader for native lib

Update document & changelog

CHANGELOG.md

@@ -1,4 +1,2 @@
 ## 0.0.1
-
-No release yet.
-Still under development.
+Provide simplest IkeV2-eap VPN service.

README.md

@@ -24,8 +24,22 @@ Add the service in the `application` part your `AndroidManifest.xml`.
         android:permission="android.permission.BIND_VPN_SERVICE"/>
 </application>
 ```
-
-Add native libs inside your `android/app/src/main/libs` .  
-The native libs can be build from strongswan. 
-You can also download the prebuild native libs [here](https://github.com/X-dea/Flutter_VPN/releases).
+Modify your `app/build.gradle` to use Java 8 and avoid [#22397](https://github.com/flutter/flutter/issues/22397).
+```gradle
+android {
+    ...
+    lintOptions {
+        ...
+        // To avoid error.
+        checkReleaseBuilds false
+    }
+
+    // Plugin requires Java 8.
+    compileOptions {
+        targetCompatibility 1.8
+        sourceCompatibility 1.8
+    }
+}
+```
+The plugin will automatically download pre-build native libraries from [here](https://github.com/X-dea/Flutter_VPN/releases) if they haven't been downloaded.
 

android/build.gradle

@@ -11,9 +11,31 @@ buildscript {
     dependencies {
         classpath 'com.android.tools.build:gradle:3.2.1'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+        classpath 'de.undercouch:gradle-download-task:3.4.3'
     }
 }
 
+apply plugin: 'de.undercouch.download'
+
+// Download pre-build native libs.
+task downloadNaticeLib(type: Download) {
+    src 'https://github.com/X-dea/Flutter_VPN/releases/download/v0.0.1/strongswan_libs_v5.7.2.zip'
+    dest "$buildDir/native.zip"
+}
+
+task getNativeFile(dependsOn: downloadNaticeLib, type: Copy) {
+    from zipTree("$buildDir/native.zip")
+    into "src/main/libs"
+}
+
+// Automatically download native libs before build when doesn't exist.
+tasks.whenTaskAdded {
+    task ->
+        if (task.name == "preBuild" && !(file('./src/main/libs').exists())) {
+            preBuild.dependsOn getNativeFile
+        }
+}
+
 rootProject.allprojects {
     repositories {
         google()
@@ -34,7 +56,7 @@ android {
     }
 
     defaultConfig {
-        minSdkVersion 19
+        minSdkVersion 16
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
     }
     lintOptions {

example/lib/main.dart

@@ -57,7 +57,7 @@ class _MyAppState extends State<MyApp> {
             ),
             RaisedButton(
               child: Text('Connect'),
-              onPressed: () => FlutterVpn.connect(
+              onPressed: () => FlutterVpn.simpleConnect(
                     _addressController.text,
                     _usernameController.text,
                     _passwordController.text,

lib/flutter_vpn.dart

@@ -19,16 +19,26 @@ import 'package:flutter/services.dart';
 class FlutterVpn {
   static const MethodChannel _channel = const MethodChannel('flutter_vpn');
 
+  /// Prepare for vpn connection.
+  ///
+  /// For first connection it will show a dialog to ask for permission.
+  /// When your connection was interrupted by another VPN connection,
+  /// you should prepare again before reconnection.
   static Future<bool> prepare() async {
     return await _channel.invokeMethod('prepare');
   }
 
-  static Future<Null> connect(
+  /// Connect to VPN.
+  ///
+  /// Use given credentials to connect VPN (ikev2-eap).
+  /// This will create a background VPN service.
+  static Future<Null> simpleConnect(
       String address, String username, String password) async {
     await _channel.invokeMethod('connect',
         {'address': address, 'username': username, 'password': password});
   }
 
+  /// Disconnect will stop current VPN service.
   static Future<Null> disconnect() async {
     await _channel.invokeMethod('disconnect');
   }