如何转换为String - 从Swift 1.1更新到Swift 1.2

Question

let version:String = NSBundle.mainBundle().objectForInfoDictionaryKey(kCFBundleVersionKey) as? String ?? "unknown"

我收到错误：'（String？，StringLiteralConvertible）'无法转换为'StringLiteralConvertible'

Answer 1

Xcode 7有更好的错误消息：

error: 'CFString!' is not implicitly convertible to 'String'; did you mean to use 'as' to explicitly convert?

并制作＆＃34;修复它＆＃34;插入as String的建议：

let version = NSBundle.mainBundle()
    .objectForInfoDictionaryKey(kCFBundleVersionKey as String)
    as? String ?? "unknown"

这与Swift 1.2（Xcode 6.4）和当前版本都进行了编译 Swift 2.0（Xcode 7）。

Answer 2

这个怎么样：

var version = "unknown"
if let key = NSBundle.mainBundle().objectForInfoDictionaryKey(kCFBundleVersionKey) as String {
    version = key
}