与NSUserDefault一起使用字符串数组的最佳实践 [英] Best Practice using an Array of Strings with NSUserDefault

问题描述

将字符串数组保存到userDefaults时遇到了一些麻烦.我在类中声明了一个字符串数组，并带有一个属性观察器来同步到userDefaults.此外，我希望将数组限制为最多5个字符串.

I'm having a bit trouble saving an array of strings to userDefaults. I have an Array of strings declaired in a class, with a property observer ment to sync to userDefaults. Furthermore, I want the array to be limited to a maximum of 5 Strings.

let userDefaults = NSUserDefaults.standardUserDefaults()

var suggestions: [String]! {
    didSet {
        var arrayEnd = suggestions.count
        if arrayEnd >= 5 {
            arrayEnd = 4
        }
        let newArray = Array(suggestions[0...arrayEnd])
        userDefaults.setObject(newArray, forKey: "suggestions")
        userDefaults.synchronize()
    }
}

func getSuggestionData() {
    if userDefaults.valueForKey("suggestions") != nil {
        self.suggestions = userDefaults.valueForKey("suggestions") as? [String]
    }
}

override func viewDidLoad() {
    super.viewDidLoad()

    getSuggestionData()
    suggestions.insert("someString", atIndex: 0)
    }
}

当我运行它时，我得到:

When i run this i get:

致命错误:解开可选值时意外发现nil

fatal error: unexpectedly found nil while unwrapping an Optional value

我尝试在数组中插入新对象的行上的

. 我尝试按照此线程中的方法进行操作，但没有将任何内容保存到列表中.

on the line where I try to insert a new object to the array. I have tried following the approach in this thread, but it didn't save anything to the list.

我是新手，并且可选类型不是我的强项，也许有人知道这是什么问题?

I'm new to swift, and optional-types aren't my strong side, maybe someone know what's wrong?

推荐答案

由于从用户默认值读取可能会返回nil值，因此请使用可选绑定以确保安全:

As reading from user defaults could return nil values, use optional binding to be safe:

func getSuggestionData() {
  if let suggestionArray = userDefaults.objectForKey("suggestions") {
    self.suggestions = suggestionArray
  } else {
    self.suggestions = [String]()
  }
}

---

但是我建议使用具有定义的默认值的非可选变量.

---

But I'd recommend to use an non-optional variable with a defined default value.

在应用程序的AppDelegate中，覆盖init()或插入代码以注册键/值对.

In AppDelegate of your app, override init() or insert the code to register the key/value pair.

override init()
{
  let defaults = NSUserDefaults.standardUserDefaults()
  let defaultValue = ["suggestions" : [String]()];
  defaults.registerDefaults(defaultValue)
  super.init()
}

注册默认键和值是Apple在

Registering the default keys and values is the way Apple suggests in the documentation.

• 如果尚未写入任何值，则读取默认值(空数组).
• 如果有值，则读取实际值

代替变量suggestions的值观察器，实现一种方法来插入新值，删除最后一个值并将数组写入磁盘. 无需使用可选绑定，因为用户默认设置中的数组始终处于定义状态.

Instead of the value observer of the variable suggestions implement a method to insert a new value, delete the last one and write the array to disk. There is no need to use optional binding because the array in user defaults has always a defined state.

let userDefaults = NSUserDefaults.standardUserDefaults()
var suggestions = [String]()

func insertSuggestion(suggestion : String)
{
  if suggestions.count == 5 { suggestions.removeLast() }
  suggestions.insert(suggestion, atIndex: 0)
  userDefaults.setObject(suggestions, forKey: "suggestions")
  userDefaults.synchronize()
}

func getSuggestionData() {
  suggestions = userDefaults.objectForKey("suggestions") as! [String]
}

override func viewDidLoad() {
  super.viewDidLoad()
  getSuggestionData()
  insertSuggestion("someString")
}

旁注:
如果您不需要显式地使用键值编码方法，则永远不要使用valueForKey:而不是objectForKey:.

A side note:
never use valueForKey: rather than objectForKey: if you don't need explicitly the key-value-coding method.

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