flutter_qjs

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This plugin is a simple js engine for flutter using the quickjs project with dart:ffi. Plugin currently supports all the platforms except web!

Getting Started

Basic usage

Firstly, create a FlutterQjs object, then call dispatch to establish event loop:

final engine = FlutterQjs(
  stackSize: 1024 * 1024, // change stack size here.
);
engine.dispatch();

Use evaluate method to run js script, it runs synchronously, you can use await to resolve Promise:

try {
  print(engine.evaluate(code ?? ''));
} catch (e) {
  print(e.toString());
}

Method close can destroy quickjs runtime that can be recreated again if you call evaluate. Parameter port should be close to stop dispatch loop when you do not need it. Reference leak exception will be thrown since v0.3.3

try {
  engine.port.close(); // stop dispatch loop
  engine.close();      // close engine
} on JSError catch(e) { 
  print(e);            // catch reference leak exception
}
engine = null;

Data conversion between dart and js are implemented as follow:

dart	js
Bool	boolean
Int	number
Double	number
String	string
Uint8List	ArrayBuffer
List	Array
Map	Object
Function(arg1, arg2, ..., {thisVal}) JSInvokable.invoke([arg1, arg2, ...], thisVal)	function.call(thisVal, arg1, arg2, ...)
Future	Promise
JSError	Error
Object	DartObject

Use Modules

ES6 module with import function is supported and can be managed in dart with moduleHandler:

final engine = FlutterQjs(
  moduleHandler: (String module) {
    if(module == "hello")
      return "export default (name) => `hello \${name}!`;";
    throw Exception("Module Not found");
  },
);

then in JavaScript, import function is used to get modules:

import("hello").then(({default: greet}) => greet("world"));

notice: Module handler should be called only once for each module name. To reset the module cache, call FlutterQjs.close then evaluate again.

To use async function in module handler, try run on isolate thread

Run on Isolate Thread

Create a IsolateQjs object, pass handlers to resolving modules. Async function such as rootBundle.loadString can be used now to get modules:

final engine = IsolateQjs(
  moduleHandler: (String module) async {
    return await rootBundle.loadString(
        "js/" + module.replaceFirst(new RegExp(r".js$"), "") + ".js");
  },
);
// not need engine.dispatch();

Same as run on main thread, use evaluate to run js script. In isolate, everything returns asynchronously, use await to get the result:

try {
  print(await engine.evaluate(code ?? ''));
} catch (e) {
  print(e.toString());
}

Method close can destroy isolate thread that will be recreated again if you call evaluate.

Use Dart Function (Breaking change in v0.3.0)

Js script returning function will be converted to JSInvokable. It does not extend Function, use invoke method to invoke it:

(func as JSInvokable).invoke([arg1, arg2], thisVal);

notice: evaluation returning JSInvokable may cause reference leak. You should manually call free to release JS reference.

(obj as JSRef).free();
// or JSRef.freeRecursive(obj);

Arguments passed into JSInvokable will be freed automatically. Use dup to keep the reference.

(obj as JSRef).dup();
// or JSRef.dupRecursive(obj);

Since v0.3.0, you can pass a function to JSInvokable arguments, and channel function is no longer included by default. You can use js function to set dart object globally. For example, use Dio to implement http in qjs:

final setToGlobalObject = await engine.evaluate("(key, val) => { this[key] = val; }");
await setToGlobalObject.invoke(["http", (String url) {
  return Dio().get(url).then((response) => response.data);
}]);
setToGlobalObject.free();

In isolate, top level function passed in JSInvokable will be invoked in isolate thread. Use IsolateFunction to pass a instant function:

await setToGlobalObject.invoke([
  "http",
  IsolateFunction((String url) {
    return Dio().get(url).then((response) => response.data);
  }),
]);

Libraries

flutter_qjs