View don't update in real time when running a cycle - swift

Question

I'm making a card game in SwiftUI and having the following problem: when running a cycle, the view updates only on cycle stop, but don't show any changes when running. UI part of code is:
//on the table
ScrollView(.horizontal) {
HStack(alignment: .center, spacing: 0) {
if game.gameStarted {
ForEach ((0..<game.onTheTable.count), id: \.self) {number in
VStack {
Image(game.onTheTable[number].pic)
.resizable()
.modifier(CardStyle())
Text("\(ai.getPower(card: game.onTheTable[number]))")
}
}
}
}
}
It actually shows card images "on the table" when I move an item to the game.onTheTable array. But when I run a while loop like "while true" it behaves as I mentioned above. So I've created a simple code with a delay to be able to se how card images one by one appears on the table but it just doesn't work as expected. There's the code for the cycle:
func test() {
gameStarted = true
while deck.cardsInDeck.count > 0 {
onTheTable.append(deck.cardsInDeck[0])
deck.cardsInDeck.remove(at: 0)
usleep(100000)
}
}
Yes, it appends cards, but visually I see the result just when the whole cycle has finished. Any ideas how to fix that to see the cards being added in real time one by one?

Answer 1

SwiftUI is declarative, so it doesn't mesh well with imperative control flow like while loops or system timers. You don't have control over when layout happens. Instead, you need to modify the underlying state which is driving the view, and those updates must happen on the main thread.
Here's one approach, which starts the timer when the view appears. You could also trigger the timer based on user interaction.
Note that you can attach transitions to views, and those transitions can take advantage of .matchedGeometryEffect... So you could have cards animate from their position on the deck to their place on the table, and that could happen automatically as you move items from one array to another—as long as the deck and table views use the same namespace and a consistent ID for each unique card.
struct GameView: View {
#State var deckCards: [Card] = Card.standardDeck
#State var tableCards: [Card] = []
#State var timer: Timer? = nil
var body: some View {
VStack {
DeckView(cards: deckCards)
TableView(cards: tableCards)
}.onAppear {
timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { _ in
moveCard()
}
}
}
func moveCard() {
DispatchQueue.main.async {
guard deckCards.count > 0 else {
self.timer?.invalidate()
return
}
tableCards.append(deckCards.removeFirst())
}
}
}