Array Index Out Of Range - Error when optional unbinding

问题

I have an entity called Settings with an attribute called backgroundColor of type Int, if it is 1 then the view controller will have a background of white if 0 then a background of dark grey.

But I am getting the following error when trying to open the view controller;

fatal error: Array Index out of range

For the following line in my function

    if settingsArray.count == 1 {
        setting = settingsArray[1]
    } else if settingsArray.count <= 0 {
      println("No settings in array")
    }

View Controller

var settingsArray: [Settings]! 
var setting: Settings!
var backgroundSetting: Bool = true

override func viewWillAppear(animated: Bool) {
    backgroundSettings()
}

override func viewDidLoad() {
    super.viewDidLoad()
    backgroundSettings()
}

// Function to fetch settings and change background
func backgroundSettings() {

    var error: NSError?
    let request = NSFetchRequest(entityName: "Settings")
    self.settingsArray = moc?.executeFetchRequest(request, error: &error) as! [Settings]

    if settingsArray.count == 1 {
        setting = settingsArray[1]
    } else if settingsArray.count <= 0 {
      println("No settings in array")
    }

    if setting != nil {

        if setting.backgroundColor == 1 {
            backgroundSetting = true
        } else if setting.backgroundColor == 0{
            backgroundSetting = false
        }

    }


    if backgroundSetting == true {
        self.view.backgroundColor = UIColor.whiteColor()
    } else if backgroundSetting == false {
        self.view.backgroundColor = UIColor.darkGrayColor()
    }

}

//Button to change the color and settings
@IBAction func backgroundColor(sender: AnyObject) {


    if setting != nil {

        if setting.backgroundColor == 1 {
            setting.backgroundColor = 0
        } else {
            setting.backgroundColor = 1
        }

        var error: NSError?
        moc?.save(&error)

    } else {

        println("No settings available")

        var settings = NSEntityDescription.insertNewObjectForEntityForName("Settings", inManagedObjectContext: moc!) as! Settings

        settings.backgroundColor = 1

        var error: NSError?
        moc?.save(&error)

    }

    backgroundSettings()

}

Any ideas where I may be going wrong ?

回答1:

ETA:

I have been having another think about this. I have left my old answer below so that the previous comments make sense.

In if let thisSetting = settingsArray?[0] … if settingsArray is nil then the right side is potentially effectively nil[0]. Therefore I believe that this may eliminate the crash:

// ...
if !settingsArray.isEmpty {
    if let thisSetting = settingsArray?[0] {
        setting = thisSetting
    }
}

settingsArray[0] being nil would I think then be a separate issue. Which I think would relate to the lifecycle.

Previous answer follows:

I believe that the problem may be being caused by you calling func backgroundSettings() from viewDidLoad() - i.e. too early in the View Controller lifecycle - and before the values have been initialized.

回答2:

In Swift (as in Objective C and many other languages), the indexes of arrays start at 0. (See this wikipedia article for a list on this.)

This means when you check if settingsArray.count == 1, there will be only (exactly) one item in your list. Since indexes start at 0, this item will be at index 0, hence the Error.

So either you check if settingsArray.count == 2 and leave setting = settingsArray[1], or you change to setting = settingsArray[0].

来源：https://stackoverflow.com/questions/31640075/array-index-out-of-range-error-when-optional-unbinding

标签

ios

arrays

swift

core-data

optional