iOS Swift 将UnsafeMutableRawPoint

获取视频流(CMSampleBufferRef)sampleBuffer的地址，并将其转为uint8_t/UInt8格式，Objective-C实现方式如下：

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
    //获取buffer
    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
    //获取buffer地址并强转
    uint8_t *baseAddress = (uint8_t *)CVPixelBufferGetBaseAddress(imageBuffer);
}

但Swift直接这样写：

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        
    let imageBuffer: CVImageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)!
    let baseAddress: UInt8 = CVPixelBufferGetBaseAddress(imageBuffer)! as UInt8
}

编译会报错：

Cannot convert value of type 'UnsafeMutableRawPointer' to type 'UInt8' in coercion

需要多转一步才可以：

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        
        let imageBuffer: CVImageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)!
        
        let tempPointer: UnsafeMutableRawPointer = CVPixelBufferGetBaseAddress(imageBuffer)!
        let opaquePtr = OpaquePointer(tempPointer)
        let baseAddress = UnsafeMutablePointer<UInt8>(opaquePtr)
}

---

以下资料参考# 在 Swift 里使用指针

Swift 一共有 8 种指针类型。基础是不可变指针 UnsafePointer，对应的会有可变指针 UnsafeMutablePointer。另外还有可以塞数组的 Buffer 指针和完全不知道是什么内容的 Raw 指针，也即没有指定泛型。

四种带类型指针：
UnsafeMutablePointer
UnsafePointer
UnsafeMutableBufferPointer
UnsafeBufferPointer

四种不带类型的 RawPointer:
UnsafeMutableRawPointer
UnsafeRawPointer
UnsafeMutableRawBufferPointer
UnsafeRawBufferPointer

*Apple Developer 文档里有 C 指针和 Swift 指针的对应表：

C Syntax	Swift Syntax
const Type *	UnsafePointer
Type *	UnsafeMutablePointer
Type * const *	UnsafePointer
Type * __strong *	UnsafeMutablePointer
Type **	AutoreleasingUnsafeMutablePointer
const void *	UnsafeRawPointer
void *	UnsafeMutableRawPointer

从CoreVideo框架头文件可看出， Objective-C返回值 Swift返回值
CVPixelBufferGetBaseAddress返回值为void *，对应Swift为UnsafeMutableRawPointer，而UnsafeMutableRawPointer不带任何类型，必须为它指一个类型，就要用到OpaquePointer。