captureOutput未调用

Question

没有调用captureOutput，我已经全力以赴寻找解决方案了。 “Hello”字符串不会打印。任何人都可以了解它应该如何运作？我使用的是swift 3，我得到了几乎匹配的可选要求captureOutput（：didDrop：from）

import UIKit
import AVFoundation

class FirstViewController: UIViewController {
@IBOutlet weak var previewView: UIView!
@IBOutlet weak var captureImageView: UIImageView!

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    Extractor().initCapSession(preView: previewView)
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

@IBAction func didTakePhoto(_ sender: Any) {
}

}

class Extractor: NSObject, AVCaptureVideoDataOutputSampleBufferDelegate {

let session = AVCaptureSession()

override init(){
    super.init()
}


func initCapSession(preView: UIView) {

    session.sessionPreset = AVCaptureSessionPresetHigh
    let camera = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)

    do {
        let cameraInput = try AVCaptureDeviceInput(device: camera)
        let output = AVCaptureVideoDataOutput()
        output.alwaysDiscardsLateVideoFrames = true
        output.setSampleBufferDelegate(self, queue: DispatchQueue(label: "sample buffer"))
        session.addInput(cameraInput)
        session.addOutput(output)

        // preview layer
        let preview = AVCaptureVideoPreviewLayer(session: session)
        preview?.frame = preView.bounds
        preview?.videoGravity = AVLayerVideoGravityResizeAspectFill
        preView.layer.insertSublayer(preview!, at: 0)
    } catch {
        print(error)
    }

    session.startRunning()
}

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
    print("Hello")
}

}