UIImageView subclass that works with Auto Layout to express its desired aspect ratio

ScaleAspectFitImageView.swift

import UIKit

// known-good: Xcode 8.2.1

/**
 
 UIImageView subclass which works with Auto Layout to try 
 to maintain the same aspect ratio as the image it displays.
 
 This is unlike the usual behavior of UIImageView, where the 
 scaleAspectFit content mode only affects what the view displays 
 and not the size it prefers, and so it does not play 
 well with AL constraints. In particular, UIImageView.intrinsicContentSize 
 always returns each of the intrinsic size dimensions of the image 
 itself, not a size that adjusts to reflect constraints on the
 view. So if you constrain the width of a UIImageView, for example,
 the view's intrinsic content size still declares a preferred
 height based on the image's intrinsic height, rather than the 
 displayed height produced by the scaleAspectFit content mode.
 
 In contrast, this subclass has a few notable properties:
 
 - If you externally constraint one dimension, its internal constraints 
   will then adjust the other dimension so it holds the image's aspect 
   ratio.

 - Uses a low layout priority to do this. So if you externally 
   require it to have an incorrect aspect ratio, you do not get conflicts.

 - Still uses the scaleAspectFit content mode internally, so if a 
   client requires an incorrect aspect, you still get scaleAspectFit 
   behavior to determining what is displayed within whatever 
   dimensionsare finally used.

 - It is a subclass of UIImageView and supports all of UIImageView's 
   initializers, so it is a drop-in substitute.

 */
public class ScaleAspectFitImageView : UIImageView
{
  /// constraint to maintain same aspect ratio as the image
  private var aspectRatioConstraint:NSLayoutConstraint? = nil
  
  required public init?(coder aDecoder: NSCoder)
  {
    super.init(coder:aDecoder)
    self.setup()
  }
  
  public override init(frame:CGRect)
  {
    super.init(frame:frame)
    self.setup()
  }
  
  public override init(image: UIImage!)
  {
    super.init(image:image)
    self.setup()
  }
  
  public override init(image: UIImage!, highlightedImage: UIImage?)
  {
    super.init(image:image,highlightedImage:highlightedImage)
    self.setup()
  }
  
  override public var image: UIImage? {
    didSet {
      self.updateAspectRatioConstraint()
    }
  }
  
  private func setup()
  {
    self.contentMode = .scaleAspectFit
    self.updateAspectRatioConstraint()
  }
  
  /// Removes any pre-existing aspect ratio constraint, and adds a new one based on the current image
  private func updateAspectRatioConstraint()
  {
    // remove any existing aspect ratio constraint
    if let c = self.aspectRatioConstraint {
      self.removeConstraint(c)
    }
    self.aspectRatioConstraint = nil
    
    if let imageSize = image?.size, imageSize.height != 0
    {
      let aspectRatio = imageSize.width / imageSize.height
      let c = NSLayoutConstraint(item: self, attribute: .width,
                                 relatedBy: .equal,
                                 toItem: self, attribute: .height,
                                 multiplier: aspectRatio, constant: 0)
      // a priority above fitting size level and below low
      c.priority = (UILayoutPriorityDefaultLow + UILayoutPriorityFittingSizeLevel) / 2.0
      self.addConstraint(c)
      self.aspectRatioConstraint = c
    }
  }
}