如何使用MethodChannel从Java调用参数到Dart？

我试图将Java本机代码中的一个参数调用到Dart， 我的Java代码中有一个方法-

@Override
public void onReward(final int quantity) {
    HashMap<String, Object> args = new HashMap<>();
    args.put("reward",quantity);
    Log.i("TheoremReach", "onRewardCenterOpened");
    TrsurveysPlugin.getInstance().OnMethodCallHandler("onReward", args);
}

在我的Dart代码中如何称呼它？这是我目前的尝试-

这就是我从代码的Java端获得回调的方式，但是这不会从方法返回我的

int-quantity

-

typedef TheoremListener(TheoremReachListener listener);

static final Map<String, TheoremReachListener> theoremReachListener = {
    'onReward': TheoremReachListener.onReward,
};

static Future<void> handleMethod(
    MethodCall call, TheoremListener listener) async {
    listener(theoremReachListener[call.method]);
}

typedef TheoremListener(TheoremReachListener listener);

static final Map<String, TheoremReachListener> theoremReachListener = {
    'onReward': TheoremReachListener.onReward,
};

static Future<void> handleMethod(
    MethodCall call, TheoremListener listener) async {
    listener(theoremReachListener[call.method]);
}

listener(TheoremReachListener event) {
    if (event == TheoremReachListener.onReward) {
        print('on reward');
    } 
}