ObservableType+FlatMapNilWrapping.swift

extension ObservableType {
    
    func flatMap<A: AnyObject, O: ObservableType>(weak obj: A, selector: @escaping (A, Self.E) throws -> O) -> Observable<O.E> {
        return flatMap { [weak obj] value -> Observable<O.E> in
            try obj.map { try selector($0, value).asObservable() } ?? .empty()
        }
    }
    
    func flatMapFirst<A: AnyObject, O: ObservableType>(weak obj: A, selector: @escaping (A, Self.E) throws -> O) -> Observable<O.E> {
        return flatMapFirst { [weak obj] value -> Observable<O.E> in
            try obj.map { try selector($0, value).asObservable() } ?? .empty()
        }
    }
    
    func flatMapWithIndex<A: AnyObject, O: ObservableType>(weak obj: A, selector: @escaping (A, Self.E, Int) throws -> O) -> Observable<O.E> {
        return flatMapWithIndex { [weak obj] value, index -> Observable<O.E> in
            try obj.map { try selector($0, value, index).asObservable() } ?? .empty()
        }
    }
    
    func flatMapLatest<A: AnyObject, O: ObservableType>(weak obj: A, selector: @escaping (A, Self.E) throws -> O) -> Observable<O.E> {
        return flatMapLatest { [weak obj] value -> Observable<O.E> in
            try obj.map { try selector($0, value).asObservable() } ?? .empty()
        }
    }
}