如何从Swift中的void UnsafePointer中提取数据？

Question

我在Objective-C（使用Audio Toolbox框架）中有以下代码，我正在尝试将其转换为Swift：

MusicTimeStamp timeStamp;
MusicEventType eventType;
const void *eventData;
UInt32 eventDataSize;

//MusicEventIteratorGetEventInfo is declared in Audio Toolbox framework as this:
//OSStatus MusicEventIteratorGetEventInfo ( MusicEventIterator inIterator, MusicTimeStamp *outTimeStamp, MusicEventType *outEventType, const void **outEventData, UInt32 *outEventDataSize );

MusicEventIteratorGetEventInfo(anIterator, &timeStamp, &eventType, &eventData, &eventDataSize);

switch (eventType){
    case kMusicEventType_MIDINoteMessage:
        //extract info from eventData assuming it's a MIDINoteMessage.
        UInt8 channel = ((MIDINoteMessage *) eventData)->channel
        UInt8 note = ((MIDINoteMessage *) eventData)->note
        UInt8 velocity = ((MIDINoteMessage *) eventData)->velocity
        Float32 duration = ((MIDINoteMessage *) eventData)->duration

    case kMusicEventType_MIDIChannelMessage:
        //extract info from eventData assuming it's a MIDIChannelMessage.

    //case ..., etc.
}

当在Swift中重写它时，我无法弄清楚如何从“const void * eventData”部分获取数据，尽管阅读了关于不安全指针的Apple文档（他们的例子似乎都与提取数据无关）以这种方式的不安全指针）。 这是我到目前为止所拥有的，但我在switch语句应该是什么方面迷失了：

var timeStamp: MusicTimeStamp = 0
var eventType: MusicEventType = 0
var eventData: UnsafeMutablePointer<UnsafePointer<Void>> = nil
var eventDataSize: UInt32 = 0

MusicEventIteratorGetEventInfo(iterator, &timeStamp, &eventType, eventData, &eventDataSize)

//what would simplest equivalent switch statement be?

任何帮助将不胜感激 - 谢谢。

Answer 1

只要你知道你在那个不安全的指针中找到了什么样的数据，你总是可以使用每个变量的各种构造函数来在指针之间进行转换。 如果你牢记这一点，你的例子是微不足道的翻译：

import AudioToolbox

var timeStamp : MusicTimeStamp = 0
var eventType : MusicEventType = 0
var eventData : UnsafePointer<Void> = nil
var eventDataSize : UInt32 = 0;

MusicEventIteratorGetEventInfo(anIterator, &timeStamp, &eventType, &eventData, &eventDataSize);

switch (eventType){
    case kMusicEventType_MIDINoteMessage:
        let data = UnsafePointer<MIDINoteMessage>(eventData)
        let channel = data.memory.channel
        let note = data.memory.note
        let velocity = data.memory.velocity
        let duration = data.memory.duration

    case kMusicEventType_MIDIChannelMessage:
        //extract info from eventData assuming it's a MIDIChannelMessage.

        //case ..., etc.
}