c# - Using Rx Framework for async calls using the void AsyncMethod(Action<T> callback) pattern

Question

I've seen tons of examples on how to use the Observable.FromAsyncPattern() in the Rx Framework to simplify Async calls, but I'm using an interface that doesn't use the standard Async pattern of IAsyncResult BeginXXX/EndXXX(IAsyncResult), so this doesn't work for me.

The library I'm working with exposes async functions with a callback patter:

void GetAllObjects(Action<List<Object>> callback)

In an ideal world I'd like to turn this:

var isLoadingUsers = true;
var isLoadingSystems = true;
var isLoadingCustomers = true;
var isLoadingRules = true;

mClient.GetAllUsers(UsersCallback);
mClient.GetAllCustomers(CustomersCallback);
mClient.GetAllRules(RulesCallback);

// set the IsLoadingXXX variables to false in callbacks
// once all are false

mClient.GetAllSystems(SystemsCallback);

into something like this:

var o = Observable.ForkJoin(
                     Observable.Start(GetAllUsers()),
                     Observable.Start(GetAllCustomers()),
                     Observable.Start(GetAllRules())
                    ).Finally(() => GetAllSystems);

How would one go about turning that pattern into something that returns an IObservable?

Answer 1

Func<IObservable<TRet>> FromListCallbackPattern(Action<Action<List<TRet>>> function)
{
    return () => {
        // We use a ReplaySubject so that if people subscribe *after* the
        // real method finishes, they'll still get all the items
        ret = new ReplaySubject<TRet>();

        function((list) => {
            // We're going to "rebroadcast" the list onto the Subject
            // This isn't the most Rx'iest way to do this, but it is the most
            // comprehensible :)
            foreach(var v in list) {
                ret.OnNext(v);
            }
            ret.OnCompleted();
        });

        return ret;
    };
}

Now, you can do something like:

var getAllUsers = FromListCallbackPattern(mClient.GetAllUsers);
getAllUsers().Subscribe(x => /* ... */);