swift @objc 和选择器：从选择器返回类型的编译器错误

Question

在 MacOS 10.14.6、Swift 和 XCode 11.0、VLCKit 3.3.0 上，我尝试在 class 中使用 @objc 函数创建回调。但是使用#selector 指向我的@objc func，编译器产生错误

这些是我在 class

中声明的 2 个函数

class VLCStreamProcessor {
// ...

@objc func lock_frame(
    opaque: UnsafeMutableRawPointer? ,
    planes: UnsafeMutablePointer<UnsafeMutableRawPointer?>?)
    -> UnsafeMutableRawPointer? 
{ // ...
    return UnsafeMutableRawPointer(user_data.pic)
}

@objc func unlock_frame(
    opaque: UnsafeMutableRawPointer?,
    picture: UnsafeMutableRawPointer?,
    planes: UnsafePointer<UnsafeMutableRawPointer?>?)
{ // ...
}

然后在另一个函数 (myVLCProcessing) 中，仍然在同一个 class 中，我准备回调

func myVLCProcessing() 
{ // ...

  libvlc_video_set_callbacks( 
    mplayerPtr, // type libvlc_media_player_t*
    #selector( lock_frame(opaque:planes:) ), // type libvlc_video_lock_cb <- 1st error
    #selector( unlock_frame(opaque:picture:planes: )), // type libvlc_video_unlock_cb <- 2nd error
    0,  // type libvlc_video_display_cb
    opaque // type void*
    ) 

  // ...
}

我在带有 #selector 的两行中收到编译器错误：第一：

Cannot convert value of type 'Selector' to expected argument type 'libvlc_video_lock_cb?' (aka 'Optional<@convention(c)(Optional<UnsafeMutableRawPointer>, Optional<UnsafeMutablePointer<Optional<UnsafeMutableRawPointer>>>) -> Optional<UnsafeMutableRawPointer>>')

第二个：

Cannot convert value of type 'Selector' to specified type 'libvlc_video_unlock_cb' (aka '@convention(c) (Optional<UnsafeMutableRawPointer>, Optional<UnsafeMutableRawPointer>, Optional<UnsafePointer<Optional<UnsafeMutableRawPointer>>>) -> ()')

来自 libvlc (libvlc_media_player.h)，2 个 C 函数应为：

typedef void *(*libvlc_video_lock_cb)(void *opaque, void **planes);

typedef void (*libvlc_video_unlock_cb)(void *opaque, void *picture,
                                   void *const *planes);

欢迎任何建议。

Answer 1

选择器与 C 函数不同。您需要提供闭包或函数指针。

只需尝试删除您提供的函数周围的#selector()。

Answer 2

只能将全局函数或闭包（不捕获上下文）传递给回调参数的 C 函数。与 How to use instance method as callback for function which takes only func or literal closure 类似，您可以通过将回调函数 self（指向实例的指针）“隧道”转换为 void 指针并返回。回调函数然后可以调用实例方法：

class VLCStreamProcessor {
    func lock_frame(
        planes: UnsafeMutablePointer<UnsafeMutableRawPointer?>?)
        -> UnsafeMutableRawPointer?
    { // ...
    }

    func unlock_frame(
        picture: UnsafeMutableRawPointer?,
        planes: UnsafePointer<UnsafeMutableRawPointer?>?)
    { // ...
    }

    func myVLCProcessing()  {
        // ...

        let opaque = UnsafeMutableRawPointer(Unmanaged.passUnretained(self).toOpaque())
        libvlc_video_set_callbacks(mplayerPtr, { (opaque, planes) -> UnsafeMutableRawPointer? in
            let mySelf = Unmanaged<VLCStreamProcessor>.fromOpaque(opaque!).takeUnretainedValue()
            return mySelf.lock_frame(planes: planes)
        }, { (opaque, picture, planes) in
            let mySelf = Unmanaged<VLCStreamProcessor>.fromOpaque(opaque!).takeUnretainedValue()
            mySelf.unlock_frame(picture: picture, planes: planes)
        }, nil, opaque)

    }
}

这里假设VLCStreamProcessor只要设置了libvlc回调就存在，否则必须保留指针。

swift @objc 和选择器：从选择器返回类型的编译器错误

swift @objc and selector : compiler error with type returned from selector

swift

xcode

macos

libvlc

vlckit