NSTextAttachment that scales and aligns image

ImageAttachment

//  Created by Tony Mann on 3/22/15.
//  Copyright (c) 2015 7Actions. All rights reserved.
//
//  Adapted from http://ossh.com.au/design-and-technology/software-development/implementing-rich-text-with-images-on-os-x-and-ios/

import UIKit

class ImageAttachment: NSTextAttachment {
    var verticalOffset: CGFloat = 0.0
    
    // To vertically center the image, pass in the font descender as the vertical offset.
    // We cannot get this info from the text container since it is sometimes nil when `attachmentBoundsForTextContainer`
    // is called.
    
    convenience init(_ image: UIImage, verticalOffset: CGFloat = 0.0) {
        self.init()
        self.image = image
        self.verticalOffset = verticalOffset
    }
    
    override func attachmentBoundsForTextContainer(textContainer: NSTextContainer, proposedLineFragment lineFrag: CGRect, glyphPosition position: CGPoint, characterIndex charIndex: Int) -> CGRect {
        let height = lineFrag.size.height
        var scale: CGFloat = 1.0;
        let imageSize = image!.size
        
        if (height < imageSize.height) {
            scale = height / imageSize.height
        }
        
        return CGRect(x: 0, y: verticalOffset, width: imageSize.width * scale, height: imageSize.height * scale)
    }
}

If you're working on an objc project:

@interface ImageAttachment : NSTextAttachment

@property (nonatomic, assign) CGFloat verticalOffset;

- (instancetype)initWithImage:(UIImage *)image verticalOffset:(CGFloat)offset;

@end

@implementation ImageAttachment

- (instancetype)initWithImage:(UIImage *)image verticalOffset:(CGFloat)offset {
    self = [super init];
    if (self) {
        self.image = image;
        self.verticalOffset = offset;
    }
    return self;
}

- (CGRect)attachmentBoundsForTextContainer:(NSTextContainer *)textContainer proposedLineFragment:(CGRect)lineFrag glyphPosition:(CGPoint)position characterIndex:(NSUInteger)charIndex {
    CGFloat height = lineFrag.size.height;
    CGFloat scale = 1.0;
    CGSize imageSize = self.image.size;
    
    if (height < imageSize.height) {
        scale = height / imageSize.height;
    }
    
    return CGRectMake(0, _verticalOffset, imageSize.width * scale, imageSize.height * scale);
}

@end