有没有办法在低级 C 方法调用中使用 Swift 命令？

Question

我正在维护从AVCaptureSynchronizedData 获得的一组数据。我使用的一种方法是修改从AVCaptureSynchronizedData 获得的CVPixelBuffers。修改时。 CVPixelBuffer，我通过

创建了CVPixelBuffer 的副本

let status = CVPixelBufferCreateWithBytes(nil, scaleWidth, scaleHeight,
                                                  pixelFormat, destData,
                                                  destBytesPerRow, releaseCallback,
                                                  nil, nil, &dstPixelBuffer)

releaseCallBack 被引用为

let releaseCallback: CVPixelBufferReleaseBytesCallback = { _, ptr in
    if let ptr = ptr {
        free(UnsafeMutableRawPointer(mutating: ptr))
    }
}

但是，我希望能够跟上这被调用的次数。但是，如果我添加类似

var num:Int = 0
let releaseCallback: CVPixelBufferReleaseBytesCallback = { _, ptr in
    if let ptr = ptr {
        num += 1
        free(UnsafeMutableRawPointer(mutating: ptr))
    }
}

我得到了错误

不能从捕获上下文的闭包中形成 C 函数指针

不确定是否有可能，但能够记录指针被销毁的次数会非常有用

Answer 1

在闭包中引用实例变量num会捕获self，而这在纯C函数的回调中是不可能的。

与How to use instance method as callback for function which takes only func or literal closure 类似，您必须使用自定义指针（此处为：releaseRefCon）才能传递给回调：

let releaseCallback: CVPixelBufferReleaseBytesCallback = { releaseRefCon, ptr in
    let mySelf = Unmanaged<YourClass>.fromOpaque(releaseRefCon!).takeUnretainedValue()
    if let ptr = ptr {
        free(UnsafeMutableRawPointer(mutating: ptr))
        mySelf.num += 1
    }
}

let releaseRefCon = UnsafeMutableRawPointer(Unmanaged.passUnretained(self).toOpaque())
let status = CVPixelBufferCreateWithBytes(nil, scaleWidth, scaleHeight,
                                          pixelFormat, destData,
                                          destBytesPerRow,
                                          releaseCallback, releaseRefCon,
                                          nil, &dstPixelBuffer)

关于从实例指针（此处：self）到 void 指针并返回到实例指针（此处：mySelf）的转换，请参见 How to cast self to UnsafeMutablePointer<Void> type in swift。