如何解决此构建问题 - 无法分配给属性:'date'是一个get get属性

你的日期实际上是一个computed property只有一个,getter 所以它只能被阅读,而不是写.

相比之下,你FullDate是一个stored property.

computed properties 通常用于隐藏其他类的功能或以方便的方式暴露隐藏的数据.

下面是SomeClass对象中的一些插图.

stored properties 只是普通的老房产

computed properties是getters也可能也是setters.一个很好的例子是height和width的CGRect,这些其实都是从CGRect.size而是作为实现convenience getters

property observers是stored properties更改值时执行的函数.didSet和willSet

access controlon get并set允许您stored properties根据访问权限创建只读或写.

class SomeClass {

    // just a variable
    var storedProperty : Int = 10

    // just a variable but the setter is hidden from other "files"
    private(set) var storedPropertyWithPartialAccessControl : Int = 0

    // only calculates when set
    var derivedProperty : Int = 0 {
        didSet {
            storedProperty = doStuffs(derivedProperty)
        }
    }

    // calculates every get and set
    var computedProperty : Int {
        get { // get is needed
            return doStuffs(storedProperty)
        }
        set(value) { // set is not needed, using only get gives you a read only property
            storedProperty = doReverseStuffs(value)
        }
    }

    // hidden functions that are used in the computed property
    private func doStuffs(value:Int) -> Int {
        return value * 10
    }

    private func doReverseStuffs(value:Int) -> Int {
        return value / 10
    }
}

因为NSDateFormatter效率非常低,所以在它中使用它是非常糟糕的computed property.每次获得都会创建一个.浪费了大量的时间和精力,因为结果值在大多数情况下可能是相同的.

最佳实践 :

使用computed properties到get stored properties和set做计算.

使用Singleton存储 NSDateFormatter

一个更好的方法:

class SomeEvent {

    // Store calculated results. 
    // This is private to make the class cleaner in use.
    private var storedDate : NSDate?

    private var storedFormattedDate : String = ""

    // get returns stored values without calculations
    // set updates both stored values after calculations
    var date : NSDate? {
        get {
            return storedDate
        }
        set(value) {
            storedFormattedDate = format(date: value)
            storedDate = value
        }
    }

    var formattedDate : String {
        get {
            return storedFormattedDate
        }
        set(value) {
            storedDate = readDateFrom(dateString: value)
            storedFormattedDate = format(date: storedDate)
        }
    }

    // hidden functions, again to make the class cleaner in use.
    private func format(date maybeDate:NSDate?) -> String {

        guard let date = maybeDate else {
            return ""
        }
        guard let dateFormatter = MyDateFormatter.shared().dateFormatter else {
            return ""
        }
        return dateFormatter.stringFromDate(date)

    }

    private func readDateFrom(dateString string:String) -> NSDate? {

        // get detector
        if let detector = MyDateFormatter.shared().dateReader {
            // results
            let matches = detector.matchesInString(string, options: [], range: NSRange(location: 0, length: string.characters.count))
            // first result
            if let match = matches.first, let date = match.date {
                return date
            }
        }

        // just format if there is no detector
        if let dateFormatter = MyDateFormatter.shared().dateFormatter {
            if let date = dateFormatter.dateFromString(string) {
                return date
            }
        }

        // everything failed
        return nil
    }
}

单身人士存储NSDateFormatter和NSDataDetector:

谷歌: Swift Singleton

class MyDateFormatter {

    private static var privateShared : MyDateFormatter?

    var dateFormatter : NSDateFormatter?
    var dateReader : NSDataDetector?

    class func shared() -> MyDateFormatter {
        guard let uwShared = privateShared else {
            privateShared = MyDateFormatter()
            return privateShared!
        }
        return uwShared
    }

    class func destroy() {
        privateShared = nil
    }

    private init() {
        dateReader = try? NSDataDetector(types: NSTextCheckingType.Date.rawValue)
        dateFormatter = NSDateFormatter()
        dateFormatter!.dateFormat = "EEEE MMM dd"
    }
}

测试:

let event = SomeEvent()
event.formattedDate = "tomorrow"
event.formattedDate // "Friday Dec 04"
event.formattedDate = "25/02/2010"
event.formattedDate // "Thursday Feb 25"
event.formattedDate = "Sunday Mar 13"
event.formattedDate // "Sunday Mar 13"