Improvements.swift

// 1. Don't compare to `nil`, only to force unwrap later. Just use conditional binding:

func allOfferCards() -> [OfferCard]{
    guard let dataSource = dataSource else { return [] }

    let numberOfCards = self.dataSource.kolodaNumberOfCards(self)

    var offerCards = [OfferCard]()

    for i in 0..<numberOfCards {
        let offerCard = viewForCard(at: i)

        if let offerCard = offerCard {
            offerCards.append(offerCard as! OfferCard)
        }

    }

    return offerCards
}

// 2. use map, instead of repeated appends:

func allOfferCards() -> [OfferCard]{
    guard let dataSource = dataSource else { return [] }

    let numberOfCards = self.dataSource.kolodaNumberOfCards(self)

    var offerCards = (0..<numberOfCards).map { viewForCard(at: $0) as! OfferCard }

    return offerCards
}

// 3. Inline offerCards

func allOfferCards() -> [OfferCard]{
    guard let dataSource = dataSource else { return [] }

    let numberOfCards = self.dataSource.kolodaNumberOfCards(self)

    return (0..<numberOfCards).map { viewForCard(at: $0) as! OfferCard }
}

// 4. simplify logic (if dataSource is nil, take `numberOfCards` to be zero, which will
// make map result in an empty array)


func allOfferCards() -> [OfferCard]{
    let numberOfCards = self.dataSource?.kolodaNumberOfCards(self) ?? 0
    return (0..<numberOfCards).map { viewForCard(at: $0) as! OfferCard }
}