未包装可选类型EKReminder的值

Question

I am trying to create reminder according to this tutorial, trying to change the Swift 1 to Swift 2 code:

http://www.techotopia.com/index.php/Using_iOS_8_Event_Kit_and_Swift_to_Create_Date_and_Location_Based_Reminders

I get the error: Value for optional type not unwrapped.

func createReminder() {

    if (appDelegate!.eventStore != nil) {
        let reminder = EKReminder(eventStore: appDelegate!.eventStore) //value not unwrapped

        reminder.title = reminderText.text
        reminder.calendar =
            appDelegate!.eventStore!.defaultCalendarForNewReminders()
        let date = myDatePicker.date
        let alarm = EKAlarm(absoluteDate: date)

        reminder.addAlarm(alarm)

        var error: NSError?
        appDelegate!.eventStore!.saveReminder(reminder,
            commit: true, error: &error)

        if error != nil {
            print("Reminder failed with error \(error?.localizedDescription)")
        }
    }
}

I understand the concept of optional value (variable can be not assigned a value at this point during run time), what I do not understand is that I am inside the if block, that checks the value for not being nil. Therefore appDelegate!.eventStore is not nil in this block, therefore does not need to be unwrapped (?). I also tried setting the "!" after this variable, like this:

appDelegate!.eventStore!

but it breaks other code, it gives error around this code:

appDelegate!.eventStore!.saveReminder(reminder,
            commit: true, error: &error)  //extra argumenr 'error' in call 

Surrounding it with do{}catch{} statement does not help..

Answer 1

By the looks of it, this is more to do with the extra argument than with the implicitly unwrapped optional — Apple have updated a lot of their SDK for Swift 2, to avoid passing in the reference to the NSError object and use try instead.

eventStore.saveReminder(reminder, commit: true, error: &error)

becomes

do {
    try eventStore.saveReminder(reminder, commit: true)
}
catch error: NSError {
    print(error.localizedDescription)
}

As a general rule, I avoid implicitly unwrapped optionals like the plague — there is very little need for them and when they do exist you can always make a proper non-optional. Try:

func createReminder() throws {

    guard let eventStore = appDelegate?.eventStore else {
        let error: NSError = NSError(domain: "com.mycompany.myapp", code: 1, userInfo: [
        NSLocalizedDescriptionKey: "Unable to get event store from app delegate"
        ])
        throw error
    }

    let reminder = EKReminder(eventStore: eventStore)

    reminder.title = reminderText.text
    reminder.calendar = eventStore.defaultCalendarForNewReminders()
    let date = myDatePicker.date
    let alarm = EKAlarm(absoluteDate: date)

    reminder.addAlarm(alarm)

    try eventStore.saveReminder(reminder, commit: true)
}

That way you are not playing around with implicitly unwrapped optionals, eventStore is guaranteed to be set when you use it and whatever calls createReminder() can handle the error is there is one (you would probably also throw an error instead of printing out if the eventStore is not set).

Answer 2

Ok, I am not sure about the code, but I fixed the errors, I needed to still use the do-try-catch, but like so:

     do {
            try appDelegate!.eventStore!.saveReminder(reminder, commit: true) 
        } catch {
            print(error)
        }

Answer 3

if you check an optional value for nil

if (appDelegate!.eventStore != nil) {

you have to unwrap it in case it's not nil

let reminder = EKReminder(eventStore: appDelegate!.eventStore!)

A more suitable way is to use optional bindings

if let store = appDelegate!.eventStore {
 let reminder = EKReminder(eventStore: store)
  …
}

What is the purpose to declare appDelegate as optional?
The AppDelegate class exists in any application by default and could be never nil .