此thread中的每个解决方案都成功地调整了图像的大小 * 但是 * 调整了大小的图像有背景（黑色或白色取决于“UIGraphicsBeginImageContextWithOptions（）”中的“不透明”bool设置为什么），即使原始图像没有。有没有一种方法可以在不添加背景的情况下调整图像的大小？
例如，公认的解决方案：

func resizeImage(image: UIImage, targetSize: CGSize) -> UIImage? {
    let size = image.size

    let widthRatio  = targetSize.width  / size.width
    let heightRatio = targetSize.height / size.height

    // Figure out what our orientation is, and use that to form the rectangle
    var newSize: CGSize
    if(widthRatio > heightRatio) {
        newSize = CGSize(width: size.width * heightRatio, height: size.height * heightRatio)
    } else {
        newSize = CGSize(width: size.width * widthRatio, height: size.height * widthRatio)
    }

    // This is the rect that we've calculated out and this is what is actually used below
    let rect = CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height)

    // Actually do the resizing to the rect using the ImageContext stuff
    UIGraphicsBeginImageContextWithOptions(newSize, false, 1.0)
    image.draw(in: rect)
    let newImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    return newImage
}

original image
resized image