ios - 在惰性变量中使用 dispatch_once 或静态变量

Question

来自Swift guide :

If a property marked with the lazy modifier is accessed by multiple threads simultaneously and the property has not yet been initialized, there is no guarantee that the property will be initialized only once.

所以，据我所知，使用

class SomeClass {
    lazy var someVar: SomeOtherClass = {
       return SomeOtherClass()
    }()
}

不是确保使用 SomeClass 实例的每个人都使用与 SomeClass().someVar 相同的 SomeOtherClass 实例的线程安全方法。

如果是这样，就是使用

class SomeClass {
    lazy var someVarr: SomeOtherClass = {
        var some: SomeOtherClass? = nil
        var token: dispatch_once_t = 0
        dispatch_once(&token) {
            some = SomeOtherClass()
        }
        return some!
    }()
}

或使用:

class SomeClass {
    lazy var someVarrr: SomeOtherClass = {
        struct Static {
            static let some = SomeOtherClass()
        }
        return Static.some
    }()
}

实例化 SomeOtherClass 的正确、线程安全的方法？

为了提供一些上下文:在 Objective-C 中，我使用了:

-(NSURLSession *)ephemeralURLSession {
    if (!_ephemeralURLSession) {
        static dispatch_once_t onceToken;
        dispatch_once(&onceToken, ^{
            NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration ephemeralSessionConfiguration];
            _ephemeralURLSession = [NSURLSession sessionWithConfiguration:configuration];
        });
    }
    return _ephemeralURLSession;
}

现在我想知道在 Swift 中实例化 NSURLSession 的正确方法是什么，例如:

lazy var ephemeralURLSession: NSURLSession = {
    var session: NSURLSession? = nil
    var token: dispatch_once_t = 0
    dispatch_once(&token) {
        let configuration: NSURLSessionConfiguration = NSURLSessionConfiguration.ephemeralSessionConfiguration()
        session = NSURLSession(configuration: configuration)
    }
    return session!
}()

Answer 1

你可以这样定义它:

static let ephemeralURLSession: NSURLSession = {
    let configuration = NSURLSessionConfiguration.ephemeralSessionConfiguration()
    return NSURLSession(configuration: configuration)
}()

这既是惰性的又是线程安全的。