为什么我不能在Swift中使用从一个UIViewController传递到另一个UIViewController的数据来设置标签（在控制台中打印数据）？

Question

In my app I have an embedded UIViewController inside a container. In my story board I added a touch event to this viewcontroller and I called the segue: fullRequestSegue

Then in the code of that UIViewController I wrote:

class RequestDetails: UIViewController, ShowRequestDetailsFromMap {

var fullRequestDetails: FullRequestFromMap?

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if (segue.identifier == "fullRequestSegue"){
    fullRequestDetails?.changeEventDescr(self.descr, username: "adam", number: "12", created_at: self.created_at, photo: self.photo)

        fullRequestDetails = segue.destinationViewController as? FullRequestFromMap
        fullRequestDetails!.showRequestDetails = self      
    }
}
}

Then in my class FullRequestFromMap I have:

protocol ShowRequestDetailsFromMap {
    func changeEventDescr(text:String)
}

class FullRequestFromMap: UIViewController{

@IBOutlet weak var userNumber: UILabel!
var showRequestDetails:ShowRequestDetailsFromMap?
override func viewDidLoad() {
    super.viewDidLoad()
}

func changeEventDescr(description: String, username: String, number: String, created_at: NSDate, photo: String) {

    print(username)
    print(description)
    print(number)
    print(created_at)
    print(photo) //that works fine, I see all valid data in the console

    userNumber.text = number //doesn't work, I see empty label instead of filled with passed data, the same problem is with other labels

}

What is the problem here?

Answer 1

The problem is when the method changeEventDescr is called the userNumber label is not initialized. You are trying to assign to a nil object.

Create a string variable in your FullRequestFromMap class and store text in it and in your viewDidLoad method you have to assign the text to userNumber label.

class FullRequestFromMap: UIViewController {
    @IBOutlet weak var userNumber: UILabel!
    var showRequestDetails:ShowRequestDetailsFromMap?

    var userNumberLabelText:String = "Default Value"

    override func viewDidLoad() {
        super.viewDidLoad()

        userNumber.text = userNumberLabelText
    }

    func changeEventDescr(description: String, username: String, number: String, created_at: NSDate, photo: String) {

        print(username)
        print(description)
        print(number)
        print(created_at)
        print(photo)

        userNumberLabelText = number // Here you set the class variable, not the label it self
    }
}

class RequestDetails: UIViewController {
    ......

    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
        if (segue.identifier == "fullRequestSegue") {
            fullRequestDetails = segue.destinationViewController as? FullRequestFromMap

            // Option 1: You can directly assign it
            fullRequestDetails?.userNumberLabelText = "12"

            // Option 2: You can call your method
            fullRequestDetails?.changeEventDescr(self.descr, username: "adam", number: "12", created_at: self.created_at, photo: self.photo)
        }
    }
}