UIImageView which constraints its own height based on its layouted width

LKImageView.m

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

    if (self.lk_heightConstrainedToAspectWidth && self.image) {
        CGFloat imageAspectRatio = self.image.size.height / self.image.size.width;
        // If we haven't constrained ourself, or the constraint needs updating
        if (!self.aspectRatioHeightConstraint || self.aspectRatioHeightConstraint.multiplier != imageAspectRatio) {
            [self setNeedsUpdateConstraints];
        }
    }
}

- (void) updateConstraints
{
    if (self.lk_heightConstrainedToAspectWidth && self.image) {
        CGFloat aspectRatioToSet = self.image.size.height / self.image.size.width;
        if (!self.aspectRatioHeightConstraint || self.aspectRatioHeightConstraint.multiplier != aspectRatioToSet) {
            // We haven't constrained ourself, or the constraint needs updating

            // Remove the outdated constraint
            if (self.aspectRatioHeightConstraint) {
                [self removeConstraint:self.aspectRatioHeightConstraint];
                self.aspectRatioHeightConstraint = nil;
            }

            // Create a new constraint based upon the new aspect ratio
            self.aspectRatioHeightConstraint = [NSLayoutConstraint constraintWithItem:self
                                                                            attribute:NSLayoutAttributeHeight
                                                                            relatedBy:NSLayoutRelationEqual
                                                                               toItem:self
                                                                            attribute:NSLayoutAttributeWidth
                                                                           multiplier:aspectRatioToSet
                                                                             constant:0.0];
            [self addConstraint:self.aspectRatioHeightConstraint];
        }

    } else if (self.aspectRatioHeightConstraint) {
        [self removeConstraint:self.aspectRatioHeightConstraint];
        self.aspectRatioHeightConstraint = nil;
    }

    [super updateConstraints];
}

// In case this is dynamically changed at runtime, update our constraints
- (void) setLk_heightConstrainedToAspectWidth:(BOOL)lk_heightConstrainedToAspectWidth
{
    if (_lk_heightConstrainedToAspectWidth != lk_heightConstrainedToAspectWidth) {
        _lk_heightConstrainedToAspectWidth = lk_heightConstrainedToAspectWidth;
        if (self.image != nil) {
            [self setNeedsUpdateConstraints];
        }
    }
}

After talking to @Eridius on iOS Folks slack, he suggested setting up an "aspect ratio" constraint (height to width) that just gets set whenever the image is updated. Doing that next...