use of unresolved identifier - swift

Question

The input has been be defined as try AVCaptureDeviceInput(device: captureDevice) but it still says input is an unresolved identifier.
Please see my code below, I have tried multiple methods but not success.
import UIKit
import AVFoundation
class ViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
var captureSession: AVCaptureSession?
var videoPreviewLayer: AVCaptureVideoPreviewLayer?
var qrCodeFrameView: UIView?
override func viewDidLoad() {
super.viewDidLoad()
// Get an instance of AVCaptureDevice class to initialize a device object and provide the video as the media type parameter.
let captureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
// Get an instance of the AVCaptureDeviceInput class using the previous device object.
do {
let captureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
let input = try AVCaptureDeviceInput(device: captureDevice)
// Do the rest of your work...
} catch let error as NSError {
// Handle any errors
print(error)
}
// Initialize the captureSession object
captureSession = AVCaptureSession()
captureSession?.addInput(input as! AVCaptureInput)
// Set the input device on the capture session.
// Initialize a AVCaptureMetadataOutput object and set it as the output device to the capture session.
let captureMetadataOuput = AVCaptureMetadataOutput()
captureSession?.addOutput(captureMetadataOuput)
// Set delegate and use the default dispatch queue to execute the call back
captureMetadataOuput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
captureMetadataOuput.metadataObjectTypes = [AVMetadataObjectTypeQRCode]
// Initialize the video preview layer and add it as a sublayer to the viewPreview view's layer.
videoPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
videoPreviewLayer?.videoGravity = AVLayerVideoGravityResizeAspectFill
videoPreviewLayer?.frame = view.layer.bounds
view.layer.addSublayer(videoPreviewLayer!)
// Start video capture
captureSession?.startRunning()
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
How do I fix this?

Answer 1

As already explained in the other answers, your input
variable is limited to the scope of the do block.
An alternative solution – if you want to keep the do/catch blocks
smaller and localized – is to declare the variable outside of the block:
let captureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
let input: AVCaptureDeviceInput
do {
input = try AVCaptureDeviceInput(device: captureDevice)
} catch let error as NSError {
print(error)
return // Must return from method here ...
}
// `input` is defined and initialized now ...
captureSession = AVCaptureSession()
captureSession?.addInput(input)
// ...
Note that this requires that your return immediately in the error
case, as input would be undefined then.
Or, if the error message is not important, use try? in a guard
statement:
let captureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
guard let input = try? AVCaptureDeviceInput(device: captureDevice) else {
return
}
// `input` is defined and initialized now ...
captureSession = AVCaptureSession()
captureSession?.addInput(input)
// ...

Answer 2

It's a scope issue. Your captureDevice and input constants are only usable inside the do block. Update your code to something like this:
override func viewDidLoad() {
super.viewDidLoad()
// Get an instance of AVCaptureDevice class to initialize a device object and provide the video as the media type parameter.
let captureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
// Get an instance of the AVCaptureDeviceInput class using the previous device object.
do {
let captureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
let input = try AVCaptureDeviceInput(device: captureDevice)
// Initialize the captureSession object
captureSession = AVCaptureSession()
captureSession?.addInput(input as! AVCaptureInput)
// Set the input device on the capture session.
// Initialize a AVCaptureMetadataOutput object and set it as the output device to the capture session.
let captureMetadataOuput = AVCaptureMetadataOutput()
captureSession?.addOutput(captureMetadataOuput)
// Set delegate and use the default dispatch queue to execute the call back
captureMetadataOuput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
captureMetadataOuput.metadataObjectTypes = [AVMetadataObjectTypeQRCode]
// Initialize the video preview layer and add it as a sublayer to the viewPreview view's layer.
videoPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
videoPreviewLayer?.videoGravity = AVLayerVideoGravityResizeAspectFill
videoPreviewLayer?.frame = view.layer.bounds
view.layer.addSublayer(videoPreviewLayer!)
// Start video capture
captureSession?.startRunning()
} catch let error as NSError {
// Handle any errors
print(error)
}
}

Answer 3

input is in the scope of the do block, it's not visible outside.
Basically it's a very bad idea to just print the error and continue as if nothing happened. Always put the entire good code into the do block:
do {
let captureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
let input = try AVCaptureDeviceInput(device: captureDevice)
// Initialize the captureSession object
captureSession = AVCaptureSession()
captureSession?.addInput(input as! AVCaptureInput)
// Initialize a AVCaptureMetadataOutput object and set it as the output
...
// Start video capture
captureSession?.startRunning()
// Do the rest of your work...
} catch let error as NSError {
// Handle any errors
print(error)
}