预期返回“ UIImage”的函数中缺少返回

Question

I am a beginner of Swift.
Currently, I am using AVFoundation to create camera applications.
I am coding from Processing A to Processing B.
However, the following error was displayed.
Error:

Missing return in a function expected to return 'UIImage'.

I do not know how to use return in the switch statement.

//Processing A
func captureImage(_ sampleBuffer: CMSampleBuffer) -> UIImage {
    .....
    switch self.input.device.position {
        case .front:
            let resultImage = UIImage(cgImage: imageRef, scale: 1.0, orientation: UIImageOrientation.down)
            return resultImage
        case .back:
            let resultImage = UIImage(cgImage: imageRef, scale: 1.0, orientation: UIImageOrientation.up)
            return resultImage
        default:
            print("error")
    }
}

Answer 1

This error occurs when not all paths of your code return a value. If you say your method returns a UIImage , it must always do .

Let's take a closer look at the implementation of captureImage :

func captureImage(_ sampleBuffer: CMSampleBuffer) -> UIImage {
    .....
    switch self.input.device.position {
        case .front:
            let resultImage = UIImage(cgImage: imageRef, scale: 1.0, orientation: UIImageOrientation.down)
            return resultImage
        case .back:
            let resultImage = UIImage(cgImage: imageRef, scale: 1.0, orientation: UIImageOrientation.up)
            return resultImage
        default:
            print("error")
    }
 }

We can see that if input.device.position is .front or .back , the method returns a value. However, what if input.device.position is neither of those values? The method will just print "error" and return nothing. That's not acceptable is it?

You might say, " I'm sure that input.device.position can only be either front or back in this situation. It can't be anything else!" Well, the compiler isn't sure about that. It just sees that there are other possible values for input.device.position .

In this case, I suggest that you just do fatalError() when it's neither of those values. It will just crash your app. If you do this then the method doesn't need to return anything. The app is crashed after all.

Answer 2

In each case you need to return UIImage. If case is default just return a empty UIImage.

func captureImage(_ sampleBuffer: CMSampleBuffer) -> UIImage {
.....
switch self.input.device.position {
    case .front:
        let resultImage = UIImage(cgImage: imageRef, scale: 1.0, orientation: UIImageOrientation.down)
        return resultImage
    case .back:
        let resultImage = UIImage(cgImage: imageRef, scale: 1.0, orientation: UIImageOrientation.up)
        return resultImage
    default:
        print("error")
        return UIImage()
  }
}

Answer 3

See Sweepers answer for a solid explanation of why you're running into this error. However, I would simply return a blank UIImage inside your default case after printing 'error'. return UIImage()