图片大小跟 UIImageView 的大小是一样的

代码如下：

extension UIImage {
  func thumbnails(with options: SomeDrawingOptions) -> UIImage? {
    // ...

    UIGraphicsBeginImageContextWithOptions(targetRect.size, false, 0)
    defer {
      UIGraphicsEndImageContext()
    }
    let context = UIGraphicsGetCurrentContext()!
    
    UIColor.white.setFill()
    context.fill(targetRect)

    UIBezierPath(roundedRect: targetRect, cornerRadius: 8).addClip()

    // Drawing image
    draw(in: targetRect)
    
    // Drawing a transparent mask
    UIColor.black.withAlphaComponent(0.4).setFill()
    context.fill(targetRect)

    // Drawing bar
    UIColor.white.setFill()
    context.fill(someBarRect)

    return UIGraphicsGetImageFromCurrentImageContext()
  }
}

截图：