0001-single-source-of-truth-004.swift

final class ViewModel {
    // 1.
    @Published var operation: MathOperation?
    @Published var operationText: String?
    @Published var firstNumberText: String?
    @Published var secondNumberText: String?
    @Published var result: String?
    
    @Published private var firstNumber: Int?
    @Published private var secondNumber: Int?
    
    // 2.
    private var cancellables: [AnyCancellable] = []
    
    init() {
        // 3.
        $firstNumberText
            .compactMap({ Int($0 ?? "") })
            .assign(to: \.firstNumber, on: self)
            .add(to: &cancellables)
        
        $secondNumberText
            .compactMap({ Int($0 ?? "") })
            .assign(to: \.secondNumber, on: self)
            .add(to: &cancellables)
        
        $operation
            .compactMap({ $0?.rawValue })
            .assign(to: \.operationText, on: self)
            .add(to: &cancellables)
        
        // 4.
        Publishers.CombineLatest3($firstNumber, $secondNumber, $operation)
            .tryMap { (firstNumber: Int?, secondNumber: Int?, operation: MathOperation?) throws -> String? in
                guard
                    let firstNumber = firstNumber,
                    let secondNumber = secondNumber,
                    let operation = operation
                else {
                    return nil
                }
                let result = try operation.operate(on: firstNumber, and: secondNumber)
                return result.description
            }
            .catch { (error: Error) -> Just<String?> in
                switch error {
                case MathOperation.Error.divideByZero:
                    return Just("Cannot divide by zero")
                default:
                    return Just("Something went wrong...")
                }
            }
            .assign(to: \.result, on: self)
            .add(to: &cancellables)
    }
}