Create stub for a class invokes Action delegate.

IUsersAPI.cs

using System;

public interface IUsersAPI
{
    // GET api/users/:id
    void GetUser(int id, Action<string> onSuccess, Action<string> onFailure);
}

User.cs

using UnityEngine;

public class User
{
    public int ID { get; private set; }

    public string Name { get; private set; }

    public static User ParseJSON(string userJSON)
    {
        var user = JsonUtility.FromJson<UserJSON>(userJSON);

        return new User {
            ID = user.id,
            Name = user.name
        };
    }

    private class UserJSON
    {
        public int id;
        public string name;
    }
}

UserProvider.cs

using System;

public class UserProvider
{
    private IUsersAPI api;

    public UserProvider(IUsersAPI api)
    {
        this.api = api;
    }

    public void Get(int id, Action<User> onSuccess, Action<string> onFailure)
    {
        api.GetUser(id, (string json) =>
        {
            var user = User.ParseJSON(json);
            onSuccess(user);
        }, onFailure);
    }
}

UserProviderTest-CreateStub.cs

public class UserProviderTest
{
    UserProvider sut;

    [SetUp]
    public void CreateStub()
    {
        var api = Substitute.For<IUsersAPI>();

        api.GetUser(Arg.Is(1),
            Arg.Invoke<string>("{ \"id\":1, \"name\":\"Aimee\" }"),
            Arg.Any<Action<string>>());
        
        api.GetUser(Arg.Is(2),
            Arg.Any<Action<string>>(),
            Arg.Invoke<string>("User not found"));

        sut = new UserProvider(api);
    }
    
    // ...
}

UserProviderTest-Get_ReturnsUser.cs

public class UserProviderTest
{
    // ...

    [Test]
    public void Get_ReturnsUser()
    {
        User result = null;

        // The stub invokes the callbacks synchronously.
        sut.Get(1, (User user) =>
        {
            result = user;
        }, (string error) =>
        {
            Assert.Fail("Failure callback should not be called");
        });

        Assert.That(result, Is.Not.Null);
        Assert.That(result.ID, Is.EqualTo(1));
    }
    
    // ...
}

UserProviderTest-Get_WhenUserDoesNotExist_ReturnsError.cs

public class UserProviderTest
{
    // ...

    [Test]
    public void Get_WhenUserDoesNotExist_ReturnsError()
    {
        string result = null;

        // The stub invokes the callbacks synchronously.
        sut.Get(2, (User user) =>
        {
            Assert.Fail("Success callback should not be called");
        }, (string error) =>
        {
            result = error;
        });

        Assert.That(result, Is.Not.Null);
        Assert.That(result, Is.EqualTo("User not found"));
    }
}

UserProviderTest.cs

using UnityEngine;
using UnityEditor;
using NUnit.Framework;
using NSubstitute;
using System;

public class UserProviderTest
{
    UserProvider sut;

    [SetUp]
    public void CreateStub()
    {
        var api = Substitute.For<IUsersAPI>();

        api.GetUser(Arg.Is(1),
            Arg.Invoke<string>("{ \"id\":1, \"name\":\"Aimee\" }"),
            Arg.Any<Action<string>>());
        
        api.GetUser(Arg.Is(2),
            Arg.Any<Action<string>>(),
            Arg.Invoke<string>("User not found"));

        sut = new UserProvider(api);
    }

    [Test]
    public void Get_ReturnsUser()
    {
        User result = null;

        // The stub invokes the callbacks synchronously.
        sut.Get(1, (User user) =>
        {
            result = user;
        }, (string error) =>
        {
            Assert.Fail("Failure callback should not be called");
        });

        Assert.That(result, Is.Not.Null);
        Assert.That(result.ID, Is.EqualTo(1));
    }

    [Test]
    public void Get_WhenUserDoesNotExist_ReturnsError()
    {
        string result = null;

        // The stub invokes the callbacks synchronously.
        sut.Get(2, (User user) =>
        {
            Assert.Fail("Success callback should not be called");
        }, (string error) =>
        {
            result = error;
        });

        Assert.That(result, Is.Not.Null);
        Assert.That(result, Is.EqualTo("User not found"));
    }
}