问获取苹果订阅中`SKProduct`的时长

EN

private func subscriptionDuration(product: SKProduct) -> TimeInterval {

    let currentDate = Date()
        
    guard let subscriptionPeriod = product.subscriptionPeriod else {
        return 0
    }
    
    let numberOfUnits = subscriptionPeriod.numberOfUnits
    let unit = subscriptionPeriod.unit
    
    var calendarComponent: Calendar.Component? = nil
    
    switch unit {
        
    case .day:
        calendarComponent = .day
        
    case .month:
        calendarComponent = .month
        
    case .week:
        calendarComponent = .weekOfYear
        
    case .year:
        calendarComponent = .year
        
    default:
        break
        
    }
    
    guard let component = calendarComponent else {
        return 0
    }
    
    let calendar = Calendar(identifier: .iso8601)
    
    guard let newDate = calendar.date(byAdding: component, value: numberOfUnits, to: currentDate) else {
        return 0
    }
    
    return newDate.timeIntervalSince1970 - currentDate.timeIntervalSince1970
}