Swift 3 中的 UnsafeMutablePointer<CFTypeRef>

Question

我试图在我的钥匙串实用程序类中调用 SecItemCopyMatching 以便从钥匙串中获取数据，但在获取 result 参数 UnsafeMutablePointer<CFTypeRef?> 时遇到了问题。

最初的声明（在 Swift 2 中，在迁移到 Swift 3 之前）是

// query is a dictionary of [String : AnyObject]

var result: Data?
let status = withUnsafeMutablePointer(to: &result) {
    SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0))
}

但在 Swift 3 中，您现在需要调用 .withMemoryRebound 才能查看内存。根据 Xcode 告诉你的操作，我尝试了这个

var result: Data?
let status = withUnsafeMutablePointer(to: &result){
    $0.withMemoryRebound(to: Data.self, capacity: 1){
        SecItemCopyMatching(query as CFDictionary, UnsafePointer($0))
    }
}

但是这样做，我得到一个错误

无法将“UnsafePointer<_>”类型的值转换为预期的参数类型“UnsafeMutablePointer<CFTypeRef?>?”

所以，我尝试使用CFTypeRef 而不是Data

var result: CFTypeRef?
let status = withUnsafeMutablePointer(to: &result){
    $0.withMemoryRebound(to: CFTypeRef.self, capacity: 1){
        SecItemCopyMatching(query as CFDictionary, UnsafePointer($0))
    }
}

将UnsafePointer($0) 替换为简单的$0 会导致相同的错误消息。

如何获得UnsafeMutablePointer<CFTypeRef?> 用于从钥匙串中获取数据？

Answer 1

错误消息有些误导 - 实际问题是 result 必须是 AnyObject? - 不需要使用 withMemoryRebound。

var result: AnyObject?
let status = withUnsafeMutablePointer(to: &result){
    SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0))
}

按预期工作并从钥匙串中获得正确的结果 - 只需将其转换为 Data。其实withUnsafeMutablePointer doesn't even have to be used。

所以，我的新代码是

var query: [String : AnyObject] = [:]
//set up the query

var result: AnyObject?
let status = SecItemCopyMatching(query as CFDictionary, &result)

var data: Data?
if status == noErr{
    data = result as? Data
}

Answer 2

在您的情况下，您不需要使用withMemoryRebound 或withUnsafeMutablePointer(to:)。

相反，您可以使用

var result: AnyObject?
let status = SecItemCopyMatching(query as CFDictionary, &result)

if status == noErr, let data = result as? Data {
    //use data...
}

通常，当您需要将UnsafeMutablePointer<T> 传递给函数时，请声明T 类型的变量并将其作为inout 参数&variable 传递。在您的情况下，T 是 CFTypeRef?，而在 Swift 3 中，CFTypeRef 只是 AnyObject 的类型别名。

---

即使在 Swift 2.2 中，您也不需要使用 withUnsafeMutablePointer

var result: AnyObject?  
let status = SecItemCopyMatching(query, &result)