I eventually solved this by using Platform channels as suggested by rmtmckenzie in this answer.
I downloaded the JS file and saved it to android/app/src/main/res/raw/ether.js
and ios/runner/ether.js
for Android and iOS respectively.
Installing dependencies
Android
Add LiquidCore as a dependency in app level build.gradle
implementation 'com.github.LiquidPlayer:LiquidCore:0.5.0'
iOS
For iOS I used the JavaScriptCore which is part of the SDK.
Platform Channel
In my case, I needed to create a Wallet based on a Mnemonic (look up BIP39) I pass in into the JS function. For this, I created a Platform channel which passes in the Mnemonic (which is basically of type String) as an argument and will return a JSON object when done.
Future<dynamic> getWalletFromMnemonic({@required String mnemonic}) {
return platform.invokeMethod('getWalletFromMnemonic', [mnemonic]);
}
Android Implementation (Java)
Inside MainActivity.java
add this after this line
GeneratedPluginRegistrant.registerWith(this);
String CHANNEL = "UNIQUE_CHANNEL_NAME";
new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler(
new MethodChannel.MethodCallHandler() {
@Override
public void onMethodCall(MethodCall methodCall, MethodChannel.Result result) {
if (methodCall.method.equals("getWalletFromMnemonic")) {
ArrayList<Object> args = (ArrayList<Object>) methodCall.arguments;
String mnemonic = (String) args.get(0);
JSObject walletFromMnemonic = getWalletFromMnemonic(mnemonic);
if (walletFromMnemonic == null) {
result.error("Could not create", "Wallet generation failed", null);
return;
}
String privateKey = walletFromMnemonic.property("privateKey").toString();
String address = walletFromMnemonic.property("address").toString();
HashMap<String, String> map = new HashMap<>();
map.put("privateKey", privateKey);
map.put("address", address);
JSONObject obj = new JSONObject(map);
result.success(obj.toString());
} else {
result.notImplemented();
}
}
}
);
Declare the following methods which perform the actual action of interacting with the JS library and returning the result to the platform channel.
@Nullable
@VisibleForTesting
private JSObject getWalletFromMnemonic(String mnemonic) {
JSContext jsContext = getJsContext(getEther());
JSObject wallet = getWalletObject(jsContext);
if (wallet == null) {
return null;
}
if (!wallet.hasProperty("fromMnemonic")) {
return null;
}
JSFunction walletFunction = wallet.property("fromMnemonic").toObject().toFunction();
return walletFunction.call(null, mnemonic).toObject();
}
@Nullable
@VisibleForTesting
private JSObject getWalletObject(JSContext context) {
JSObject jsEthers = context.property("ethers").toObject();
if (jsEthers.hasProperty("Wallet")) {
return jsEthers.property("Wallet").toObject();
}
return null;
}
@VisibleForTesting
String getEther() {
String s = "";
InputStream is = getResources().openRawResource(R.raw.ether);
try {
s = IOUtils.toString(is);
} catch (IOException e) {
s = null;
e.printStackTrace();
} finally {
IOUtils.closeQuietly(is);
}
return s;
}
@VisibleForTesting
JSContext getJsContext(String code) {
JSContext context = new JSContext();
context.evaluateScript(code);
return context;
}
iOS Implementation (Swift)
Add the following lines in AppDelegate.swift
inside the override application
method.
final let methodChannelName: String = "UNIQUE_CHANNEL_NAME"
let controller: FlutterViewController = window?.rootViewController as! FlutterViewController
let methodChannel = FlutterMethodChannel.init(name: methodChannelName, binaryMessenger: controller)
methodChannel.setMethodCallHandler({
(call: FlutterMethodCall, result: FlutterResult)-> Void in
if call.method == "getWalletFromMnemonic" {
guard let mnemonic = call.arguments as? [String] else {
return
}
if let wallet = self.getWalletFromMnemonic(mnemonic: mnemonic[0]) {
result(wallet)
} else {
result("Invalid")
}
}
})
Add the logic to interact with the JavaScriptCore.
private func getWalletFromMnemonic(mnemonic: String) -> Dictionary<String, String>? {
let PRIVATE_KEY = "privateKey"
let ADDRESS = "address"
guard let jsContext = self.initialiseJS(jsFileName: "ether") else { return nil }
guard let etherObject = jsContext.objectForKeyedSubscript("ethers") else { return nil }
guard let walletObject = etherObject.objectForKeyedSubscript("Wallet") else { return nil }
guard let walletFromMnemonicObject = walletObject.objectForKeyedSubscript("fromMnemonic") else {
return nil
}
guard let wallet = walletFromMnemonicObject.call(withArguments: [mnemonic]) else { return nil }
guard let privateKey = wallet.forProperty(PRIVATE_KEY)?.toString() else { return nil }
guard let address = wallet.forProperty(ADDRESS)?.toString() else { return nil }
var walletDictionary = Dictionary<String, String>()
walletDictionary[ADDRESS] = address
walletDictionary[PRIVATE_KEY] = privateKey
return walletDictionary
}
private func initialiseJS(jsFileName: String) -> JSContext? {
let jsContext = JSContext()
guard let jsSourcePath = Bundle.main.path(forResource: jsFileName, ofType: "js") else {
return nil
}
do {
let jsSourceContents = try String(contentsOfFile: jsSourcePath)
jsContext!.evaluateScript(jsSourceContents)
return jsContext!
} catch {
print(error.localizedDescription)
}
return nil
}