Question

我将应用转换为swift3并遇到以下问题。

@objc required init(response: HTTPURLResponse, representation: [NSObject : AnyObject])
{
    if (representation.value(forKeyPath: "title") is String)    {
        self.title = **representation.value**(forKeyPath: "title") as! String
    }

我收到以下错误：

类型[NSObject：AnyObject]的值没有成员值。

在旧版本的代码中，我只使用AnyObject作为表示类型，但如果我这样做，我会在那里得到错误AnyObject is not a subtype of NSObject：

if (representation.value(forKeyPath: "foo") is String) {
    let elementObj = Element(response: response, representation:**representation.value(forKeyPath: "foo")**!)
}

Answer 1

您正在混合使用Objective-C和Swift样式。最好真正做出决定。

回到NSDictionary不是自动的。

考虑：

let y: [NSObject: AnyObject] = ["foo" as NSString: 3 as AnyObject] // this is awkward, mixing Swift Dictionary with explicit types yet using an Obj-C type inside
let z: NSDictionary = ["foo": 3]
(y as NSDictionary).value(forKeyPath: "foo") // 3
// y["foo"] // error, y's keys are explicitly typed as NSObject; reverse bridging String -> NSObject/NSString is not automatic
y["foo" as NSString] // 3
y["foo" as NSString] is Int // true
z["foo"] // Bridging here is automatic though because NSDictionary is untyped leaving compiler freedom to adapt your values
z["foo"] is Int // true
// y.value // error, not defined

// Easiest of all:
let ynot = ["foo": 3]
ynot["foo"] // Introductory swift, no casting needed
ynot["foo"] is Int // Error, type is known at compile time

参考：

https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/WorkingWithCocoaDataTypes.html

请注意明确使用'as'以使String返回NSString。桥接不是隐藏的，因为他们希望您使用值类型（String）而不是引用类型（NSString）。所以这有点麻烦。

值类型相对于引用类型的主要优点之一是他们可以更容易地推断您的代码。更多有关值类型的信息，请参阅Swift中的类和结构编程语言（Swift 3）和WWDC 2015会话414 Building 在Swift中使用值类型的更好的应用程序。

[NSObject：AnyObject]类型的字典没有成员＆＃34;值（forKeyPath：...）＆＃34;

1 个答案: