How to crop an image with a selectable area in swift 4 or later?

问题

I need some help with a function that I'd like to implement in my app. I have a view with an image view with content mode in Aspect Fit. When I get an image from my library I would like to crop an area with an adjustable rectangle creating a new image. I've looked for some exemple or online tutorial but I did not succeed. Can anyone help me with that? Here are the images from my View.

回答1:

The simple solution is to just render the image view within a particular CGRect:

func snapshot(in imageView: UIImageView, rect: CGRect) -> UIImage {
    return UIGraphicsImageRenderer(bounds: rect).image { _ in
        imageView.drawHierarchy(in: imageView.bounds, afterScreenUpdates: true)
    }
}

The limitation of that approach is that if the image is a considerably higher resolution than the image view could render (as is often the case when we use “aspect scale fit”), you’ll lose this additional precision.

If you want to preserve the resolution, you should convert the CGRect to coordinates with the image, in this case, assuming “aspect scale fit” (namely, centered and scaled so the whole image is shown):

func snapshot(in imageView: UIImageView, rect: CGRect) -> UIImage {
    assert(imageView.contentMode == .scaleAspectFit)

    let image = imageView.image!

    // figure out what the scale is

    let imageRatio = imageView.bounds.width / imageView.bounds.height
    let imageViewRatio = image.size.width / image.size.height

    let scale: CGFloat
    if imageRatio > imageViewRatio {
        scale = image.size.height / imageView.bounds.height
    } else {
        scale = image.size.width / imageView.bounds.width
    }

    // convert the `rect` into coordinates within the image, itself

    let size = rect.size * scale
    let origin = CGPoint(x: image.size.width  / 2 - (imageView.bounds.midX - rect.minX) * scale,
                         y: image.size.height / 2 - (imageView.bounds.midY - rect.minY) * scale)
    let scaledRect = CGRect(origin: origin, size: size)

    // now render the image and grab the appropriate rectangle within
    // the image’s coordinate system

    let format = UIGraphicsImageRendererFormat()
    format.scale = image.scale
    format.opaque = false

    return UIGraphicsImageRenderer(bounds: scaledRect, format: format).image { _ in
        image.draw(at: .zero)
    }
}

Using this extension:

extension CGSize {
    static func * (lhs: CGSize, rhs: CGFloat) -> CGSize {
        return CGSize(width: lhs.width * rhs, height: lhs.height * rhs)
    }
}

That yields: