Swift 2 - UnsafeMutablePointer<Void>反对 [英] Swift 2 - UnsafeMutablePointer&lt;Void&gt; to object

问题描述

如果我有这样的方法:

func someMethod(contextPtr: UnsafeMutablePointer<Void>)

如何从 contextPtr 获取对象?

func someMethod(contextPtr: UnsafeMutablePointer<Void>){    
    let object:MyObject = contextPtr.memory
}

给出:

'Void' 不能转换为 'MyObject'

'Void' is not convertible to 'MyObject'

秘诀是什么

更多细节:

我在这里实际做的是为 SCNetworkReachability 设置一个全局回调函数:

What I'm actually doing here is setting up a global callback function for SCNetworkReachability:

func callback(reachability:SCNetworkReachability, flags: SCNetworkReachabilityFlags, info: UnsafeMutablePointer<Void>) {

    let r:Reachability = info.memory
}

然后添加回调如下:

var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
var s = self
withUnsafeMutablePointer(&s) {
    context.info = UnsafeMutablePointer($0)
}
SCNetworkReachabilitySetCallback(reachability, callback, &context)

推荐答案

这应该有效:将对象指针作为不透明的非托管指针传递到回调:

This should work: pass the object pointer as an opaque unmanaged pointer to the callback:

context.info = UnsafeMutablePointer(Unmanaged.passUnretained(myObject).toOpaque())
SCNetworkReachabilitySetCallback(reachability, callback, &context) 

并通过以下方式在回调中检索:

and retrieve in the callback via:

func callback(reachability:SCNetworkReachability, flags: SCNetworkReachabilityFlags, info: UnsafeMutablePointer<Void>) {

    let myObject = Unmanaged<MyObject>.fromOpaque(COpaquePointer(info)).takeUnretainedValue()

}

当然这假设存在对对象的一些强引用只要安装了回调，这样对象就不会解除分配.

Of course this assumes that some strong reference to the object exists as long as the callback is installed, so that the object is not deallocated.

更新: 请注意，从对象指针到 void 指针的两种转换如果您愿意使用不安全"功能，则可以简化并返回:

Update: Note that both conversions from object pointer to void pointer and back can be simplified if you are willing to use "unsafe" functions:

context.info = unsafeAddressOf(myObject)
// ...
myObject = unsafeBitCast(info, MyObject.self)

生成的汇编代码——就我所见——完全相同.

The generated assembly code is – as far as I can see – identical.

更新 2:另见 如何将 self 转换为 UnsafeMutablePointer输入 swift 以获取更多信息关于桥接"和一些可以在这里使用的辅助函数.

Update 2: See also How to cast self to UnsafeMutablePointer<Void> type in swift for more information about the "bridging" and some helper functions which can be used here.

Swift 3 更新(Xcode 8 beta 6):

var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
context.info = UnsafeMutableRawPointer(Unmanaged.passUnretained(self).toOpaque())

// ...

func callback(reachability:SCNetworkReachability, flags: SCNetworkReachabilityFlags, info: UnsafeMutableRawPointer?) {
    if let info = info {
        let myObject = Unmanaged<MyObject>.fromOpaque(info).takeUnretainedValue()
        // ...
    }
}

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