来自'UIImage'的沮丧？'UIImage'只展开选项

Question

我正在UIButton用图像创建一个

我为此编写了以下代码:

    let btnImg=UIButton.buttonWithType(UIButtonType.Custom) as UIButton
    let img  = UIImage(named: "turn_left") as UIImage
    btnImg.setTitle("Turn left", forState: UIControlState.Normal)
    btnImg.setImage(img, forState: UIControlState.Normal)
    btnImg.frame = CGRectMake(10, 150, 200, 45)
    self.view.addSubview(btnImg)

但我得到以下错误let img = UIImage(named: "turn_left") as UIImage:

Swift Compiler error:
 Downcast from 'UIImage?' to 'UIImage' only unwraps optionals; did you mean to use '!'?

Answer 1

正如错误所述你必须使用'!' ,

试试下面的代码,

   let img  = UIImage(named: "turn_left") as UIImage!  // implicitly unwrapped

要么

   let img : UIImage? = UIImage(named: "turn_left")  //optional

编辑

创建后,img您需要在使用前检查它是否为零.

Answer 2

您总是可以在'if let'中执行此操作,但请注意语法的差异,具体取决于swift的版本.

斯威夫特<2.0:

if let img = img as? UIImage {

Swift 2.0:

if let img = img as UIImage! {

注意感叹号的位置