如何在Swift中创建非原子全局或静态变量

Question

我经常需要缓存一个值,以便在类的多个实例中重用它,例如:

class Session {
    typealias T = Session

    let chatId: Int64
    let userId: Int64
    var name = ""
    var url = ""

    // ------- Define schema helpers --------
    // The problem: access to them is dispatch_once-d.
    static let table = Table("Session")
    static let chatIdColumn = Expression<Int64>("ChatId")
    static let userIdColumn = Expression<Int64>("UserId")
    static let nameColumn = Expression<String>("Name")
    static let urlColumn = Expression<String>("Url")

    init() {
        ...
        // ------ They're used in constructor: ------
        guard let chat = db.pluckItem(T.table, T.chatIdColumn, chatId)
            else { return }

        userId = wish[T.userIdColumn]
        name = wish[T.nameColumn]
        url = wish[T.urlColumn]
    }

    func save() throws {
        // ------ They're also used when saving the object back to DB ------
        ...
        try db.upsert(T.table, T.wishIdColumn, wishId, setters:
            T.userIdColumn <- userId,
            T.nameColumn <- name,
            T.urlColumn <- url
    )

    func other() {
        // ------ Sometimes they're used in other methods of the class ------
    }
}

静态和全局变量使用dispatch_once进行原子初始化,因此访问它们会产生一些开销.(请参阅Jckarter对https://devforums.apple.com/thread/229436的回复)

有没有办法以非原子方式缓存值？在大多数情况下,我不需要线程安全,并且在类实现内部使用变量.

简单地声明非静态实例变量不是一种选择,因为计算的值可能很昂贵,或者在每个类实例中存储的值太大.

当然,我可以为这些共享属性创建一个单独的类,并在EACH对象的成员变量中存储对它的引用,但这会不必要地使实现复杂化并增加内存开销.

Answer 1

请阅读:https://mikeash.com/pyblog/friday-qa-2014-06-06-secrets-of-dispatch_once.html.

该文章深入介绍了dispatch_once其性能与非线程安全等效项的实现并进行了比较.结论是在对变量进行初始写入后,后续读取变量没有开销.

因此,我认为你应该把时间花在其他地方.:-)