使用Swift CFunctionPointer传递一个回调到CoreMIDI API [英] Using Swift CFunctionPointer to pass a callback to CoreMIDI API

问题描述

这可能是目前实际上是不可能的，这将是不幸的。我试图调用CoreMIDI API来设置MIDI输入。这是我想在Swift中做的：

  var midiClient = MIDIClientRef（）
 var inputPort = MIDIEndpointRef （）
 var status：OSStatus 
 
 func readProc（packetList：UnsafePointer< MIDIPacketList>，readProcRefCon：UnsafeMutablePointer< Void>，srcConnRefCon：UnsafeMutablePointer< Void> Void {
} 
 
 status = MIDIClientCreate（MIDI client，nil，nil，& midiClient）; 
 
 status = MIDIDestinationCreate（midiClient，MIDI input，readProc，nil，& inputPort）;但是我得到这个错误：'（UnsafePointer，UnsafeMutablePointer，UnsafeMutablePointer） - > Void'不是可转换的到'MIDIReadProc'
 
 
 
 MIDIReadProc的typedef如下：
  typealias MIDIReadProc = CFunctionPointer<（（UnsafePointer< MIDIPacketList>，UnsafeMutablePointer< Void>，UnsafeMutablePointer< Void>） - > Void） 
  
有没有办法获得一个函数指针，让我的readProc方法传递给MIDIDestinationCreate API？ / p> 
 
解决方案
在Swift 2.0（目前处于测试阶段，作为Xcode 7测试版的一部分），处理函数指针的C API使用函数注释 @convention（c）的类型。你可以传递任何Swift函数，方法或闭包作为 @convention（c）函数类型 - 但只有当闭包符合C约定...。它无法从其周围范围捕获状态。
 
 
 
有关详细信息，请参阅类型属性。 
 > 
 
 
 
 
 
至于当前的测试版本... Swift 1.x没有办法转换Swift函数或闭包到C函数指针 -   CFunctionPointer 类型的唯一用途是将从（Obj）C API导入的函数指针传递给其他（Obj）C API。
 
 
 
您可以通过项目的桥接头在C代码中声明一个函数指针，然后使用Swift将其传递给CoreMIDI。但是，由于你要穿过桥梁，无论如何你可能会考虑你的项目的哪些部分最好保持在C和什么最好的接口是从这些部分到你的Swift代码是
 
It may be that this is actually not possible currently, which would be unfortunate. I'm trying to call the CoreMIDI API to set up a MIDI input. This is what I'm trying to do in Swift:
var midiClient = MIDIClientRef()
var inputPort = MIDIEndpointRef()
var status: OSStatus

func readProc(packetList: UnsafePointer<MIDIPacketList>, readProcRefCon: UnsafeMutablePointer<Void>, srcConnRefCon: UnsafeMutablePointer<Void>) -> Void {
}

status = MIDIClientCreate("MIDI client", nil, nil, &midiClient);

status = MIDIDestinationCreate(midiClient, "MIDI input", readProc, nil, &inputPort);
But I get this error: '(UnsafePointer, UnsafeMutablePointer, UnsafeMutablePointer) -> Void' is not convertible to 'MIDIReadProc'

MIDIReadProc's typedef is the following:
typealias MIDIReadProc = CFunctionPointer<((UnsafePointer<MIDIPacketList>, UnsafeMutablePointer<Void>, UnsafeMutablePointer<Void>) -> Void)>
Is there a way to get a function pointer for my readProc method to pass to the MIDIDestinationCreate API?
解决方案
In Swift 2.0 (currently in beta, as part of the Xcode 7 beta), C APIs that deal in function pointers use function types that are annotated @convention(c). You can pass any Swift function, method, or closure as a @convention(c) function type — but only if that closure conforms to C conventions... e.g. it can't capture state from its surrounding scope.

For details, see Type Attributes in The Swift Programming Language.



As for what's currently out of beta... Swift 1.x doesn't have a way to convert a Swift function or closure to a C function pointer -- the sole use of the CFunctionPointer type is to pass function pointers imported from (Obj)C APIs to other (Obj)C APIs.

You can declare a function pointer in C code that you expose to Swift via your project's bridging header, then use Swift to pass that to CoreMIDI. But since you're going to be reaching across a bridge anyway, you might instead think about which parts of your project are best to keep in C and what the best interface is from those parts to your Swift code is.

                        
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