如何将NSDictionary转换为Swift Dictionary <String,NSObject>？

Question

我正在重写Swift中的Objective-C类来感受语言.在Objective-C中,我的课程包括以下方法:

- (id) initWithCoder:(NSCoder *)aDecoder
{
    return [self initWithActionName:[aDecoder decodeObjectOfClass:[NSString class] forKey:@"actionName"]
                            payload:[aDecoder decodeObjectOfClass:[NSDictionary class] forKey:@"payload"]
                          timestamp:[aDecoder decodeObjectOfClass:[NSDate class] forKey:@"timestamp"]];
}

在使用编译器进行一些试验和错误之后,我设法将其重写为:

convenience init(aDecoder: NSCoder) {
    let actionName = aDecoder.decodeObjectOfClass(NSString.self, forKey: "actionName") as NSString
    let payload = aDecoder.decodeObjectOfClass(NSDictionary.self, forKey: "payload") as NSDictionary
    let timestamp = aDecoder.decodeObjectOfClass(NSDate.self, forKey: "timestamp") as NSDate
    self.init(actionName: actionName, payload: payload, timestamp: timestamp)
}

但是,这仍然在最后一行给出了一个错误:"找不到init接受提供的参数的重载."方法签名init是

init(actionName: String, payload: Dictionary<String, NSObject>, timestamp: NSDate)

所以我认为问题是我试图传递NSDictionary而不是Dictionary<String, NSObject>.如何在这两种不完全等效的类型之间进行转换？我是否应该只使用NSDictionary所有必须与其他Objective-C组件交互的代码？

Answer 1

将字典解码为Dictionary<String, NSObject>而不是NSDictionary.

let payload = aDecoder.decodeObjectOfClass(NSDictionary.self, forKey: "payload") as! Dictionary<String, NSObject>

由于您使用Cocoa Touch进行序列化和反序列化,因此它们被序列化为NSDictionary,但您可以Dictionary<,>根据WWDC 2014 Intermediary Swift和Advanced Swift视频将其投射到Swift.

你也可以解码为NSDictionary然后显式地转换对象,如下所示:

self.init(actionName: actionName, payload: payload as! Dictionary<String, NSObject>, timestamp: timestamp)

基本上,你在这里玩协方差/逆变.