Swift getter代码重复

Question

我正在尝试在swift中使用关键应用程序范围的用户设置,因此每次更改后都必须将它们保存到磁盘.有少量的这些变量,我对应用程序启动后从磁盘发生的第一次读取感到满意.

我有类似于这样的代码:

var _myEnumMember:MyEnum?
var myEnumMember:MyEnum {
    get {
        if let value = _myEnumMember { // in memory
            return value
        }
        var c:Cache = Cache()
        var storedValue:MyEnum? = c.get("SomeStorageKey");
        if let value = storedValue { // exists on disk
            self.myEnumMember = value // call setter to persist
            return self.myEnumMember // call getter again with value set
        }
        self.myEnumMember = .DefaultValue // assign via setter
        return self.rankingDuration // call getter after `set`
    }
    set (newValue){
        self._myEnumMember = newValue // assign to memory
        var c:Cache = Cache()
        c.put("SomeStorageKey", data: ser) // store in disk
    }

我有大约5-6个属性需要这样做 - 我不想一遍又一遍地重复自己 - 有没有办法干掉这个代码所以我不必在几个地方重复这个逻辑？

(注意:请问这里而不是CR.SE,因为我想解答如何在这些情况下干掉getter/setter而不是接受对特定代码段的批评)

Answer 1

我最近正在做类似的事情 - 这可能是你最好的选择.我使用它作为嵌套struct,但它并不严格需要嵌套.

首先,定义一个LocalCache将处理属性持久性的类型:

struct LocalCache {
    // set up keys as constants
    // these could live in your class instead
    static let EnumKey      = "EnumKey"
    static let IntKey       = "IntKey"
    static let StringKey    = "StringKey"

    // use a single cache variable, hopefully?
    var cache = Cache()

    // in-memory values go in a Dictionary
    var localValues: [String: Any] = [:]

    // fetch from local storage or from disk
    // note that the default value also sets the return type
    mutating func fetch<T>(key: String, defaultValue: T) -> T {
        if localValues[key] == nil {
            localValues[key] = cache.get(key) ?? defaultValue
        }
        return localValues[key]! as T
    }

    // save in both local storage and to disk
    mutating func store(key: String, _ value: Any) {
        localValues[key] = value
        cache.put(key, data: value)
    }
}

然后LocalCache在您的类中添加一个实例,您可以使用更简单的getter/setter.

class Test {
    // instance of the local cache 
    var localCache = LocalCache()

    var enumPropery: MyEnum {
        get { return localCache.fetch(LocalCache.EnumKey, defaultValue: MyEnum.DefaultValue) }
        set { localCache.store(LocalCache.EnumKey, newValue) }
    }

    var intProperty: Int {
        get { return localCache.fetch(LocalCache.IntKey, defaultValue: 0) }
        set { localCache.store(LocalCache.IntKey, newValue) }
    }

    var stringProperty: String {
        get { return localCache.fetch(LocalCache.StringKey, defaultValue: "---") }
        set { localCache.store(LocalCache.StringKey, newValue) }
    }
}