flutter_js 0.1.0+1

A Javascript engine to use with flutter. It uses quickjs on Android and JavascriptCore on IOS

Flutter JS plugin

A Javascript engine to use with flutter. It uses JavascriptCore both on Android (we intend to use QuickJS for a small footprint - but it is crashing on some devices) and IOS. The Javascript runtimes runs synchronously through the dart ffi in both platforms. So now you can run javascript code as a native citzen inside yours Flutter Mobile Apps.

In the previous versions we only get the result of evaluated expressions as String.

BUT NOW we can do more with flutter_js, like run xhr and fetch http calls through Dart http library. We are supporting Promises as well.

With flutter_js Flutter applications can take advantage of great javascript libraries such as ajv (json schema validation), moment (DateTime parser and operations) running natively (no PlatformChannels needed, so the javascript evaluation runs synchronously) on mobile devices, both Android and iOS.

On IOS this library relies on the native JavascriptCore provided by iOS SDK. In Android it uses the amazing and small Javascript Engine QuickJS https://bellard.org/quickjs/ (A spetacular work of the Fabrice Bellard and Charlie Gordon). It was ported to be used in Android through jni in this project i recently found on Github: https://github.com/seven332/quickjs-android. We used seven332/quickjs-android in the very first versions of flutter_js. Thanks to seven332

Recently we found the oasis-jsbridge-android repository which brings quickjs integration to Android to a new level (Close to what JavascriptCore offers in iOS). So, since version 0.0.2+1 we are using oasis-jsbridge-android quickjs library as our javascript engine under the hood. So thanks to the guys of p7s1digital team to theirs amazing work.

On Android it uses JavascriptCore also (soon it will be replaced by the amazing and small Javascript Engine QuickJS https://bellard.org/quickjs/). On IOS, it uses the Javascript Core. In both platforms we rely on dart ffi to make calls to the js runtime engines, which make javascript code evaluation an first class citzen inside Flutter Mobile Apps. We could use it to execute validations logic of TextFormField, also we can execute rule engines or redux logic shared from our web applications. The opportunities are huge.

The project is open source under MIT license.

The bindings for use to communicate with JavascriptCore through dart:ffi we took it from the package flutter_jscore.

Flutter JS provided the implementation to the QuickJS dart ffi bindings ourselves and also constructed a wrapper API to Dart which provides a unified API to evaluate javascript and communicate between Dart and Javascript.

This library also allows to call xhr and fetch on Javascript through Dart Http calls. We also provide the implementation which allows to evaluate promises returns.

Features:

Instalation

dependencies:
  flutter_js: 0.1.0+0

iOS

Since flutter_js uses the native JavascriptCore, no action is needed.

Android

Change the minimum Android sdk version to 21 (or higher) in your android/app/build.gradle file.

minSdkVersion 21

Examples

Here is a small flutter app showing how to evaluate javascript code inside a flutter app

import 'package:flutter/material.dart';
import 'dart:async';

import 'package:flutter/services.dart';
import 'package:flutter_js/flutter_js.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  String _jsResult = '';
  JavascriptRuntime flutterJs;
  @override
  void initState() {
    super.initState();
    
    flutterJs = getJavascriptRuntime();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('FlutterJS Example'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Text('JS Evaluate Result: $_jsResult\n'),
              SizedBox(height: 20,),
              Padding(padding: EdgeInsets.all(10), child: Text('Click on the big JS Yellow Button to evaluate the expression bellow using the flutter_js plugin'),),
              Padding(
                padding: const EdgeInsets.all(8.0),
                child: Text("Math.trunc(Math.random() * 100).toString();", style: TextStyle(fontSize: 12, fontStyle: FontStyle.italic, fontWeight: FontWeight.bold),),
              )
            ],
          ),
        ),
        floatingActionButton: FloatingActionButton(
          backgroundColor: Colors.transparent, 
          child: Image.asset('assets/js.ico'),
          onPressed: () async {
            try {
              JsEvalResult jsResult = flutterJs.evaluate(
                  "Math.trunc(Math.random() * 100).toString();");
              setState(() {
                _jsResult = jsResult.stringResult;
              });
            } on PlatformException catch (e) {
              print('ERRO: ${e.details}');
            }
          },
        ),
      ),
    );
  }
}

How to call dart from Javascript

You can add a channel on JavascriptRuntime objects to receive calls from the Javascript engine:

In the dart side:

javascriptRuntime.onMessage('someChannelName', (dynamic args) {
     print(args);
});

Now, if ou javascript code call sendMessage('someChannelName', JSON.stringify([1,2,3]); the dart function above will be called with a List with the elements 1, 2 and 3.

Alternatives (and also why we think our library is better)

There were another packages which provides alternatives to evaluate javascript in flutter projects:

https://pub.dev/packages/flutter_liquidcore

Good, is based on https://github.com/LiquidPlayer/LiquidCore

It is based on V8 engine so the exectuable library is huge (20Mb). So the final app will be huge too.

https://pub.dev/packages/interactive_webview

Allows to evaluate javascript in a hidden webview. Does not add weight to size of the app, but a webview means a entire browser is in memory just to evaluate javascript code. So we think an embeddable engine is a way better solution.

https://pub.dev/packages/jsengine

Based on jerryscript which is slower than quickjs. The jsengine package does not have implementation to iOS.

https://pub.dev/packages/flutter_jscore

Uses Javascript Core in Android and IOS. We got the JavascriptCore bindings from this amazing package. But, by default we provides QuickJS as the javascript runtime on Android because it provides a smaller footprint. Also our library adds support to ConsoleLog, SetTimeout, Xhr, Fetch and Promises to be used in the scripts evaluation and allows your Flutter app to provide dartFunctions as channels through onMessage function to be called inside your javascript code.

Small Apk size

A hello world flutter app, according flutter docs has 4.2 Mb or 4.6 Mb in size.

https://flutter.dev/docs/perf/app-size#android

Bellow you can see the apk sizes of the example app generated with flutter_js:

|master ✓| → flutter build apk --split-per-abi

✓ Built build/app/outputs/apk/release/app-armeabi-v7a-release.apk (5.4MB).
✓ Built build/app/outputs/apk/release/app-arm64-v8a-release.apk (5.9MB).
✓ Built build/app/outputs/apk/release/app-x86_64-release.apk (6.1MB).

Ajv

We just added an example of use of the amazing js library Ajv which allow to bring state of the art json schema validation features to the Flutter world. We can see the Ajv examples here: https://github.com/abner/flutter_js/blob/master/example/lib/ajv_example.dart

See bellow the screens we added to the example app:

IOS

Android