解决UIImage显示方向和内存方向不一致的问题

iOS中的UIImage中有imageOrientation属性，该属性决定了UIImage在手机上显示时的方向。如果imageOrientation的值为left或right，那么显示出来的图像和实际图像在内存中的存储就存在90度的旋转问题。比如显示出来的图片是720*1280的，实际上该图在内存中是按1280*720存储的。
在stackoverflow上折腾了一番，找到了下面靠谱的解决办法。

extension UIImage {
    func fixImageOrientation() -> UIImage? {

        guard let cgImage = self.cgImage else {
            return nil
        }

        if self.imageOrientation == UIImageOrientation.up {
            return self
        }

        let width  = self.size.width
        let height = self.size.height

        var transform = CGAffineTransform.identity

        switch self.imageOrientation {
        case .down, .downMirrored:
            transform = transform.translatedBy(x: width, y: height)
            transform = transform.rotated(by: CGFloat.pi)

        case .left, .leftMirrored:
            transform = transform.translatedBy(x: width, y: 0)
            transform = transform.rotated(by: 0.5*CGFloat.pi)

        case .right, .rightMirrored:
            transform = transform.translatedBy(x: 0, y: height)
            transform = transform.rotated(by: -0.5*CGFloat.pi)

        case .up, .upMirrored:
            break
        }

        switch self.imageOrientation {
        case .upMirrored, .downMirrored:
            transform = transform.translatedBy(x: width, y: 0)
            transform = transform.scaledBy(x: -1, y: 1)

        case .leftMirrored, .rightMirrored:
            transform = transform.translatedBy(x: height, y: 0)
            transform = transform.scaledBy(x: -1, y: 1)

        default:
            break;
        }

        // Now we draw the underlying CGImage into a new context, applying the transform
        // calculated above.
        guard let colorSpace = cgImage.colorSpace else {
            return nil
        }

        guard let context = CGContext(
            data: nil,
            width: Int(width),
            height: Int(height),
            bitsPerComponent: cgImage.bitsPerComponent,
            bytesPerRow: 0,
            space: colorSpace,
            bitmapInfo: UInt32(cgImage.bitmapInfo.rawValue)
            ) else {
                return nil
        }

        context.concatenate(transform);

        switch self.imageOrientation {

        case .left, .leftMirrored, .right, .rightMirrored:
            // Grr...
            context.draw(cgImage, in: CGRect(x: 0, y: 0, width: height, height: width))

        default:
            context.draw(cgImage, in: CGRect(x: 0, y: 0, width: width, height: height))
        }

        // And now we just create a new UIImage from the drawing context
        guard let newCGImg = context.makeImage() else {
            return nil
        }

        let img = UIImage(cgImage: newCGImg)

        return img;
    }
 }

上面的代码能根据UIImage中的imageOrientation去翻转图像的实际存储布局。

如果想任意角度旋转图像，比如来个30度？下面的函数可以实现。

func imageRotatedByDegrees(oldImage: UIImage, deg degrees: CGFloat) -> UIImage {
    //Calculate the size of the rotated view's containing box for our drawing space

    var rotatedViewBox: UIView = UIView(frame: CGRect(x: 0, y: 0, width: oldImage.size.width, height: oldImage.size.height))

    print(oldImage.size.height)
    if (oldImage.imageOrientation == .right)
    {
        rotatedViewBox = UIView(frame: CGRect(x: 0, y: 0, width: oldImage.size.height, height: oldImage.size.width))
    }

    let t: CGAffineTransform = CGAffineTransform(rotationAngle: degrees * CGFloat.pi / 180)
    rotatedViewBox.transform = t
    let rotatedSize: CGSize = rotatedViewBox.frame.size
    //Create the bitmap context
    UIGraphicsBeginImageContext(rotatedSize)
    let bitmap: CGContext = UIGraphicsGetCurrentContext()!
    //Move the origin to the middle of the image so we will rotate and scale around the center.
    bitmap.translateBy(x: rotatedSize.width / 2, y: rotatedSize.height / 2)
    //Rotate the image context
    bitmap.rotate(by: (degrees * CGFloat.pi / 180))
    //Now, draw the rotated/scaled image into the context
    bitmap.scaleBy(x: 1.0, y: -1.0)
    bitmap.draw(oldImage.cgImage!, in: CGRect(x: -oldImage.size.height / 2, y: -oldImage.size.width / 2, width: oldImage.size.height, height: oldImage.size.width))
    let newImage: UIImage = UIGraphicsGetImageFromCurrentImageContext()!
    UIGraphicsEndImageContext()
    return newImage
}