DotNetBrowser and Promises

Question

I'm using DotNetBrowser as a sort of headless Javascript engine within a .Net app. It seems DotNetBrowser has no support for Promises, or I am not using invoking them properly.

In JS:

window.fooAsyncMethod = () => {
    return new Promise((resolve, reject) => {
        resolve('async value to resolve');
    });
}

In C#:

var result = await browser.MainFrame.ExecuteJavascript("await window.fooAsyncMethod()")

In this case, the result is always null. I've tried async/await as well, and always receive null:

window.fooAsyncMethod = async () => {
    return await someOtherAsyncMethod()
}

Does DotNetBrowser support running asynchronous javascript or not? And if so, how can I achieve awaiting the result of an asynchronous operation and resolving that value within C#?

Answer 1

Looking over the JS-.NET bridge examples it appears that it does support promises however they are a bit more involved to use.

If you implement a type of promise object on your C# side as such:

public static class JsObjectExtensions
{
    #region Methods

    public static JsPromise AsPromise(this IJsObject jsObject) => JsPromise.AsPromise(jsObject);

    #endregion
}

public sealed class JsPromise
{
    private readonly IJsObject jsObject;

    #region Constructors

    private JsPromise(IJsObject jsObject)
    {
        this.jsObject = jsObject;
    }

    #endregion

    public static JsPromise AsPromise(IJsObject jsObject) => !IsPromise(jsObject) ? null : new JsPromise(jsObject);

    public JsPromise Catch(Func<object, object> onRejected)
    {
        IJsObject newPromise = jsObject.Invoke("catch", onRejected) as IJsObject;
        return new JsPromise(newPromise);
    }

    public Task<Result> ResolveAsync()
    {
        TaskCompletionSource<Result> promiseTcs = new TaskCompletionSource<Result>();
        Then(obj => { promiseTcs.TrySetResult(Fulfilled(obj)); },
             obj => { promiseTcs.TrySetResult(Rejected(obj)); });

        promiseTcs.Task.ConfigureAwait(false);
        return promiseTcs.Task;
    }

    public void Then(Action<object> onFulfilled, Action<object> onRejected = null)
    {
        jsObject.Invoke("then", onFulfilled, onRejected);
    }

    public JsPromise Then(Func<object, object> onFulfilled, Func<object, object> onRejected = null)
    {
        IJsObject newPromise = jsObject.Invoke("then", onFulfilled, onRejected) as IJsObject;
        return new JsPromise(newPromise);
    }

    private Result Fulfilled(object o) => new Result(ResultState.Fulfilled, o);

    private static bool IsPromise(IJsObject jsObject)
    {
        if (jsObject == null || jsObject.IsDisposed)
        {
            return false;
        }

        IJsObject promisePrototype = jsObject.Frame.ExecuteJavaScript<IJsObject>("Promise.prototype").Result;
        return promisePrototype.Invoke<bool>("isPrototypeOf", jsObject);
    }

    private Result Rejected(object o) => new Result(ResultState.Rejected, o);

    public enum ResultState
    {
        Fulfilled,
        Rejected
    }

    public class Result
    {
        public object Data { get; }
        public ResultState State { get; }

        internal Result(ResultState state, object data)
        {
            State = state;
            Data = data;
        }
    }
}

You will then be able to call the promise like this:

                    IJsObject window = browser.MainFrame.ExecuteJavaScript<IJsObject>("window").Result;
                    //Prepare promise handlers
                    Action<object> promiseResolvedHandler = o => Console.WriteLine("Success: " + o);
                    Action<object> promiseRejectedHandler = o => Console.Error.WriteLine("Error: " + o);

                    //Create a promise that is fulfilled
                    Console.WriteLine("Create a promise that is fulfilled...");
                    IJsObject promise1 = window.Invoke<IJsObject>("CreatePromise", true);
                    //Append fulfillment and rejection handlers to the promise
                    promise1.Invoke("then", promiseResolvedHandler, promiseRejectedHandler);

                    //Create a promise that is rejected
                    Console.WriteLine("Create a promise that is rejected...");
                    IJsObject promise2 = window.Invoke<IJsObject>("CreatePromise", false);
                    //Append fulfillment and rejection handlers to the promise
                    promise2.Invoke("then", promiseResolvedHandler, promiseRejectedHandler);

                    CreatePromiseAsync(window).Wait();

Please note that all the above code was taken directly from their Promises Example.