使用自动布局，如何根据图像动态调整 UIImageView 的大小？

Question

I want my UIImageView to grow or shrink depending on the size of what the actual image it's displaying is. But I want it to stay vertically centered and 10pts from the leading edge of the superview.

However, if I set these two constraints it complains I haven't set up enough constraints to satisfy Auto Layout. To me it seems to perfectly describe what I want. What am I missing?

Answer 1

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).

Answer 2

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

Answer 3

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 .

Answer 4

Here is an AspectFit UIImageView -derived implementation that works with Auto Layout when only one dimension is constrained. The other one will be set automatically. It will keep image aspect ratio and doesn't add any margins around the image.

It's tweaked Objective-C implementation of @algal idea with the following differences:

• Expression priority = (UILayoutPriorityDefaultLow + UILayoutPriorityFittingSizeLevel) / 2.0 which evaluates to 150 isn't enough to beat the priority of 1000 of the default image content size constraint. So aspect constraint priority was increased to 1000 as well.
• it removes\\re-adds the aspect ratio constraint only if necessary. Keep in mind that this is heavy operation so there is no need to do so if aspect ratio is the same. Moreover, image setter could be called multiple times, so it's a good idea to not cause extra layout calculations here.
• not sure why we need to override required public init?(coder aDecoder: NSCoder) and public override init(frame:CGRect) , so these two are taken out. They aren't overridden by UIImageView , that's why there is no point of adding the aspect constraint until an image is set.

AspectKeepUIImageView.h :

NS_ASSUME_NONNULL_BEGIN

@interface AspectKeepUIImageView : UIImageView

- (instancetype)initWithImage:(nullable UIImage *)image;
- (instancetype)initWithImage:(nullable UIImage *)image highlightedImage:(nullable UIImage *)highlightedImage;

@end

NS_ASSUME_NONNULL_END

AspectKeepUIImageView.m :

#import "AspectKeepUIImageView.h"

@implementation AspectKeepUIImageView
{
    NSLayoutConstraint *_aspectContraint;
}

- (instancetype)initWithImage:(nullable UIImage *)image
{
    self = [super initWithImage:image];

    [self initInternal];

    return self;
}

- (instancetype)initWithImage:(nullable UIImage *)image highlightedImage:(nullable UIImage *)highlightedImage
{
    self = [super initWithImage:image highlightedImage:highlightedImage];

    [self initInternal];

    return self;
}

- (void)initInternal
{
    self.contentMode = UIViewContentModeScaleAspectFit;
    [self updateAspectConstraint];
}

- (void)setImage:(UIImage *)image
{
    [super setImage:image];
    [self updateAspectConstraint];
}

- (void)updateAspectConstraint
{
    CGSize imageSize = self.image.size;
    CGFloat aspectRatio = imageSize.height > 0.0f
        ? imageSize.width / imageSize.height
        : 0.0f;
    if (_aspectContraint.multiplier != aspectRatio)
    {
        [self removeConstraint:_aspectContraint];

        _aspectContraint =
        [NSLayoutConstraint constraintWithItem:self
                                     attribute:NSLayoutAttributeWidth
                                     relatedBy:NSLayoutRelationEqual
                                        toItem:self
                                     attribute:NSLayoutAttributeHeight
                                    multiplier:aspectRatio
                                      constant:0.f];

        _aspectContraint.priority = UILayoutPriorityRequired;

        [self addConstraint:_aspectContraint];
    }
}

@end

Answer 5

I used @algal's excellent answer (case #3) and improved it for my use case.

I wanted to be able to set min and max ratio constraints . So if I have a minimum ratio constraint of 1.0 and set a picture that is higher than wide, the size should be square. Also it should fill the UIImageView in those cases.

This works even in Interface Builder. Just add a UIImageView , set RatioBasedImageView as custom class in the identity inspector and set the max and min aspect ratios in the attributes inspector.

Here's my code.

@IBDesignable
public class RatioBasedImageView : UIImageView {
    /// constraint to maintain same aspect ratio as the image
    private var aspectRatioConstraint:NSLayoutConstraint? = nil

    // This makes it use the correct size in Interface Builder
    public override func prepareForInterfaceBuilder() {
        invalidateIntrinsicContentSize()
    }

    @IBInspectable
    var maxAspectRatio: CGFloat = 999 {
        didSet {
            updateAspectRatioConstraint()
        }
    }

    @IBInspectable
    var minAspectRatio: CGFloat = 0 {
        didSet {
            updateAspectRatioConstraint()
        }
    }


    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.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 constraint = self.aspectRatioConstraint {
            self.removeConstraint(constraint)
        }
        self.aspectRatioConstraint = nil

        if let imageSize = image?.size, imageSize.height != 0 {
            var aspectRatio = imageSize.width / imageSize.height            
            aspectRatio = max(minAspectRatio, aspectRatio)
            aspectRatio = min(maxAspectRatio, aspectRatio)

            let constraint = NSLayoutConstraint(item: self, attribute: .width,
                                       relatedBy: .equal,
                                       toItem: self, attribute: .height,
                                       multiplier: aspectRatio, constant: 0)
            constraint.priority = .required
            self.addConstraint(constraint)
            self.aspectRatioConstraint = constraint
        }
    }
}

Answer 6

I usually add no image height and width constraints with value 0 and priority less than UIImageView 's contentCompressionResistancePriority , which by default is 750 . These two constraints will activate only when there is no image, and will take care of ambiguous constraints warnings both at run time and compile time.

Pretty straight forward to add these in IB. Programatically, it should look something like below.

let noImageWidthConstraint = imageview.widthAnchor.constraint(equalToConstant: 0)
noImageWidthConstraint.priority = UILayoutPriority(rawValue: 100)
noImageWidthConstraint.isActive = true

let noImageHeightConstraint = imageview.heightAnchor.constraint(equalToConstant: 0)
noImageHeightConstraint.priority = UILayoutPriority(rawValue: 100)
noImageHeightConstraint.isActive = true

Answer 7

If anyone comes here wanting a simple copy+paste for Xamarin heres @algal's code (with some of @alexander's changes) converted to .Net

Also I've changed it to constrain the Height based on the Width + Aspect Ratio as that fit all of my use cases.

/// <summary>
///  UIImage view that will set the height correct based on the current
///  width and the aspect ratio of the image.
///  https://stackoverflow.com/a/42654375/9829321
/// </summary>
public class UIAspectFitImageView : UIImageView
{
    private NSLayoutConstraint _heightConstraint;

    public override UIImage Image
    {
        get => base.Image;
        set
        {
            base.Image = value;
            UpdateAspectRatioConstraint();
        }
    }

    public UIAspectFitImageView()
        => Setup();

    public UIAspectFitImageView(UIImage image) : base(image)
        => Setup();

    public UIAspectFitImageView(NSCoder coder) : base(coder)
        => Setup();

    public UIAspectFitImageView(CGRect frame) : base(frame)
        => Setup();

    public UIAspectFitImageView(UIImage image, UIImage highlightedImage) : base(image, highlightedImage)
        => Setup();

    private void Setup()
    {
        ContentMode = UIViewContentMode.ScaleAspectFit;
        UpdateAspectRatioConstraint();
    }

    private void UpdateAspectRatioConstraint()
    {
        if (Image != null && Image.Size.Width != 0)
        {
            var aspectRatio = Image.Size.Height / Image.Size.Width;

            if (_heightConstraint?.Multiplier != aspectRatio)
            {
                if (_heightConstraint != null)
                {
                    RemoveConstraint(_heightConstraint);
                    _heightConstraint.Dispose();
                    _heightConstraint = null;
                }

                _heightConstraint = HeightAnchor.ConstraintEqualTo(WidthAnchor, aspectRatio);
                _heightConstraint.Priority = 1000;
                _heightConstraint.Active = true;
                AddConstraint(_heightConstraint);
            }
        }
    }
}

Answer 8

If you read the docs on UIImageView they say:

Setting the image property does not change the size of a UIImageView. Call sizeToFit to adjust the size of the view to match the image.