黄色警告：从“AVCaptureVideoPreviewLayer”到“AVCaptureVideoPreviewLayer”的条件转换总是成功

Question

I have this bit of code and it's throwing a yellow warning. I can't work out how to code it so the yellow warning goes away. Trying to clean up my code after converting from Swift-2 -> 3 -> 4.

if let previewLayer = AVCaptureVideoPreviewLayer(session: captureSession) as? AVCaptureVideoPreviewLayer {
    previewLayer.bounds = imageView.bounds
    previewLayer.position = CGPoint(x: 140, y: 140)
    previewLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
    imageView.layer.addSublayer(previewLayer)
    view.addSubview(imageView)    
}

Removing as? AVCaptureVideoPreviewLayer as? AVCaptureVideoPreviewLayer shows an error:

Initializer for conditional binding must have Optional type, not 'AVCaptureVideoPreviewLayer'

Answer 1

AVCaptureVideoPreviewLayer(session: captureSession) always returns an object of type AVCaptureVideoPreviewLayer , not of type AVCaptureVideoPreviewLayer? (ie the returned object is not an optional and so it can't be nil).

When you do if let , Swift expects an Optional type. Since no optional type is created, the error occurs.

My suggestion is to remove the if part and just have

let previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
previewLayer.bounds = imageView.bounds
previewLayer.position = CGPoint(x: 140, y: 140)
previewLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
imageView.layer.addSublayer(previewLayer)
view.addSubview(imageView) 

Hope this helps!