swift - 无法将类型 'UnsafePointer' 的值转换为预期参数类型 'UnsafePointer<_>'

Question

我正在尝试更新我在 http://mattg411.com/swift-coremidi-callbacks/ 找到的 CoreMidi 示例中的代码

代码是 Swift 3 之前的代码，所以我需要做一些调整。问题是我基本上从来不需要和不安全的指针和 friend 一起玩。所以我想我已经成功解决了一些问题，但仍然存在一个问题并给我带来了这个错误 Cannot convert value of type 'UnsafePointer<MIDINotification>' to expected argument type 'UnsafePointer<_>'给出此错误的代码是 ...UnsafePointer<MIDIObjectAddRemoveNotification>(message)

此方法的一部分:

func MIDIUtil_MIDINotifyProc(message: UnsafePointer<MIDINotification>, refCon: UnsafeMutableRawPointer) -> Void
    {
        let notification:MIDINotification = message.pointee

        if (notification.messageID == .msgObjectAdded || notification.messageID == .msgObjectRemoved)
        {
            let msgPtr:UnsafePointer<MIDIObjectAddRemoveNotification> = UnsafePointer<MIDIObjectAddRemoveNotification>(message)
            let changeMsg:MIDIObjectAddRemoveNotification = msgPtr.pointee
            let h:AnyObject = unbridgeMutable(ptr: refCon)
            let handler:MIDICallbackHandler = h as! MIDICallbackHandler
            handler.processMidiObjectChange(message: changeMsg)
        }
    }

编辑:我根据在网上找到的一些教程创建了一个小项目。包括来自 user28434 的修复

https://github.com/nissaba/Librarian

Answer 1

如果我正确理解代码，行

let msgPtr:UnsafePointer<MIDIObjectAddRemoveNotification> = UnsafePointer<MIDIObjectAddRemoveNotification>(message)

应将内存从 MIDINotification 重新绑定(bind)到 MIDIObjectAddRemoveNotification。

在 Swift 3.0+ 中，您应该使用 withMemoryRebound(to:capacity:_:) 来完成此操作.

类似这样的事情:

let msgPtr:UnsafePointer<MIDIObjectAddRemoveNotification> = message.withMemoryRebound(to: MIDIObjectAddRemoveNotification.self, capacity: 1) { (pointer) in
    return pointer
}

或者还有另一种方法:将 UnsafePointer 转换为 UnsafeRawPointer，然后 "assume memory bound" :

let msgPtr:UnsafePointer<MIDIObjectAddRemoveNotification> = UnsafeRawPointer(message).assumingMemoryBound(to: MIDIObjectAddRemoveNotification.self)