Swift:无法在协议扩展中使用可变方法

我有4个视图控制器,需要根据用户屏幕宽度使用不同的图像。我试图保持代码干燥并使用协议扩展。

以下是我的方案:

import UIKit

enum ScreenWidths: CGFloat {
        case iPhone455s = 320.0
        case iPhone6 = 375.0
        case iPhone6Plus = 414.0
}

protocol ScreenSizeProtocolExt {
    mutating func setupBG() -> String    
}


extension ScreenSizeProtocolExt {
    mutating func setupBG() -> String {
        let imageName: String
        let userScreenWidth = UIScreen.mainScreen().bounds.width

        switch userScreenWidth {
        case ScreenSizeWidth.iPhone455s.rawValue:
            imageName = "imageA"
        case ScreenSizeWidth.iPhone6.rawValue:
            imageName = "imageB"
        case ScreenSizeWidth.iPhone6Plus.rawValue:
            imageName = "imageC"
        default:
            imageName = "imageAll"
        }

        return imageName
    }
}

现在我正在尝试使用它:

extension myViewController: ScreenSizeProtocolExt {

let imageToUse = setupBG()
// Here is get an error: 'Use of instance member 'setupBG' on type 'inout Self'; did you mean to use a value of type 'inout self' instead?
let image = UIImage(named: imageToUse)
imageView.image = image

}

如何使用协议扩展检测屏幕宽度并为我提供正确的 imageName 使用。