With Auto Layout, how do I make a UIImageView's size dynamic depending on the image?

The image view's intrinsic size is already dependent on the size of the image. Your assumptions (and constraints are correct).

However, if you've set up your image view in interface builder and have not provided it with an image, then the layout system (interface builder) won't know how big your image view is supposed to be at compile time. Your layout is ambiguous because your image view could be many sizes. This is what throws the errors.

Once you set your image view's image property, then the image view's intrinsic size is defined by the size of the image. If you're setting the view at runtime, then you can do exactly what Anna mentioned and provide interface builder with a "placeholder" intrinsic size in the property inspector of the image view. This tells interface builder, "use this size for now, I'll give you a real size later". The placeholder constraints are ignored at runtime.

Your other option is to assign the image to the image view in interface builder directly (but I assume your images are dynamic, so this won't work for you).

Here is the essential problem: the only way in which UIImageView interacts with Auto Layout is via its intrinsicContentSize property. That property provides the intrinsic size of the image itself, and nothing more. This creates three situations, with different solutions.

case 1: view with image's size

In the first case, if you place an UIImageView in a context where external Auto Layout constraints affect only its position, then the view's intrinsicContentSize will declare it wants to be the size of its image, and Auto Layout will automatically resize the view to equal the size of its image. This is sometimes exactly what you want, and then life is good!

case 2: fully constrained view size, preserving image's aspect ratio

At other times, you're in a different case. You know exactly the size you want the image view to take, but you also want it to preserve the aspect ratio of the displayed image. In this case, you can use Auto Layout to constrain the size of the image, while setting the old contentMode property to .scaleAspectFit on the image view. That property will cause the image view to rescale the displayed image, adding padding as needed. And if this is what you want, life is good!

case 3: partially constrained view size, adapting to image aspect ratio

But often times, you're in the tricky third case. You want to use Auto Layout to partially determine the size of the view, while allowing the image's aspect ratio to determine the other part. For instance, you might want to use Auto Layout constraints to determine one dimension of the size (like the width, in a vertical timeline scroll), while relying on the view to tell Auto Layout that it wants a height matching the image's aspect ratio (so the scrollable items are not too short or too tall).

You cannot configure an ordinary image view to do this because because an image view only communicates its intrinsicContentSize. So if you put an image view in a context where Auto Layout constrains one dimension (for instance, constraining it to a short width), then the image view does not tell Auto Layout it wants its height rescaled in tandem. It continues to report its intrinsic content size, with the unmodified height of the image itself.

In order to configure the image view to tell Auto Layout that it wants to take on the aspect ratio of the image it contains, you need to add a new constraint to that effect. Furthermore, you will need to update that constraint whenever the image itself is updated. You can build a UIImageView subclass that does this, so the behavior is automatic. Here's an example:

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
    }
  }
}

More comment is available at this gist

For @algal case 3, all I had to do was

class ScaledHeightImageView: UIImageView {

    override var intrinsicContentSize: CGSize {

        if let myImage = self.image {
            let myImageWidth = myImage.size.width
            let myImageHeight = myImage.size.height
            let myViewWidth = self.frame.size.width

            let ratio = myViewWidth/myImageWidth
            let scaledHeight = myImageHeight * ratio

            return CGSize(width: myViewWidth, height: scaledHeight)
        }
        return CGSize(width: -1.0, height: -1.0)
    }
}

This scales the height component of the intrinsicContentSize. (-1.0, -1.0) is returned when there is no image because this is the default behavior in the superclass.

Also, I did not need to set UITableViewAutomaticDimension for the table with the cell containing the image view, and the cell still sizes automatically. The image view's content mode is Aspect Fit.