相机预览层使按钮不可见

我正在尝试在摄像机预览上方添加一个按钮，但是运行程序时该按钮未显示（我有约束）。我调查了一下代码并尝试进行调试，但是我对swift和Xcode还是陌生的，而对于调试而言，我还是新的。我看到当我注释掉相机预览层时，该按钮出现了。谢谢！

import UIKit
import AVFoundation
import QuartzCore

class View1: UIViewController , AVCaptureVideoDataOutputSampleBufferDelegate{

let captureSession = AVCaptureSession()
var previewLayer:CALayer!
var captureDevice:AVCaptureDevice!

@IBOutlet weak var cameraView:UIView!

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    prepareCamera()
}

func prepareCamera() {
    captureSession.sessionPreset = AVCaptureSessionPreset1920x1080

    if let availableDevices = AVCaptureDeviceDiscoverySession(deviceTypes: [.builtInWideAngleCamera],                                                                                                          mediaType: AVMediaTypeVideo,                                                                                                             position: .back).devices {

        captureDevice = availableDevices.first
        beginSession()
    }
}

func beginSession() {
    do {
        let captureDeviceInput = try AVCaptureDeviceInput(device: captureDevice)
        captureSession.addInput(captureDeviceInput)
    } catch {
        print(error.localizedDescription)
        //Figure out what to do here 
 }
    if let previewLayer = AVCaptureVideoPreviewLayer(session: captureSession) {



        self.previewLayer = previewLayer
        self.view.layer.addSublayer(self.previewLayer)
        self.previewLayer.frame = self.view.layer.frame
        captureSession.startRunning()

        let dataOutput = AVCaptureVideoDataOutput()
        dataOutput.videoSettings = [(kCVPixelBufferPixelFormatTypeKey as NSString): NSNumber(value: kCVPixelFormatType_32BGRA)]

        dataOutput.alwaysDiscardsLateVideoFrames = true

        if captureSession.canAddOutput(dataOutput){

            captureSession.addOutput(dataOutput)
            captureSession.commitConfiguration()
        }

        let queue = DispatchQueue(label: "com.PhotoAllergy.captureQueue")
        dataOutput.setSampleBufferDelegate(self, queue: queue)
    }  
}

}