Creating a ReactiveUI derived collection with more elements than the original

Question

Is it possible to create a ReactiveUI derived collection that has more elements in it than the original?

I\'ve seen that there is a way of filtering a collection, and se

Answer 1

Hmm...I'm not all that familiar with ReactiveUI, but just reading thru, it looks like you need to alter where your SelectMany goes:

var jam = cars.SelectMany(x => x.Passengers).CreateDerivedCollection(p => p);

Answer 2

At the moment, CreateDerivedCollection doesn't support SelectMany as a transformation, it gets too Tricky to handle removes. If you don't have many items, you can just regenerate the collection every time:

cars.Changed
    .Select(_ => cars.SelectMany(x => x.Passengers).ToList())
    .ToProperty(this, x => x.Passengers);

Edit: Alright, here we go:

var whenCarsOrPassengersInThoseCarsChange = Observable.Merge(
    cars.Changed
        .SelectMany(_ =>
            cars.Select(x => x.Passengers.Changed).Merge())
        .Select(_ => Unit.Default),
    cars.Changed.Select(_ => Unit.Default));

whenCarsOrPassengersInThoseCarsChange.StartWith(Unit.Default)
    .Select(_ => cars.SelectMany(x => x.Passengers).ToList())
    .ToProperty(this, x => x.Passengers);

So, the idea is that we've got two main situations when we want to reevaluate the passengers list:

1. When one of the passengers change in the cars
2. When one of the cars change

However, the tricky part is, we only want to watch passengers for cars in the collection (i.e. if a car is removed, we no longer care about its passengers).

Properly tracking suicidal passengers

So, the idea in that weird SelectMany is, "Every time the car list changes, build a new list of Observables that represent when the passenger collection changes, and merge them all together".

However, if we only had that statement, we would have to wait for a car to be added and its passengers change before we got a new passenger list, so we also have to update when the list of cars change too.

What's this "Unit" business?

In this case, I actually don't care about the values that these Observables put out, just when they happen. "Unit" is the Rx version of void, it only has a single value, "Unit.Default". You use it when you only care when something happens, not what the value of it is.