Load remote images in NSTextAttachment.

async_attachment.swift

/*
 *
 * If you're using this in UITextView, setting isSelectable or isEditable properties to false will cause the image to not load,
 * instead set dataDetectorTypes to an empty array if you don't want user to interact with the text view.
 *
*/

import UIKit

class AsyncTextAttachment: NSTextAttachment {

    private let url: URL
    private let range: ClosedRange<Int>
    private(set) var isDownloaded = false
    private let placeholderImage: UIImage
    private let textContainer: NSTextContainer

    init(url: URL, size: CGSize, range: ClosedRange<Int>, placeholderImage: UIImage, textContainer: NSTextContainer) {
        self.url = url
        self.range = range
        self.textContainer = textContainer
        self.placeholderImage = placeholderImage

        super.init(data: nil, ofType: nil)

        self.bounds = CGRect(origin: .zero, size: size)

        loadImage()
    }

    required init?(coder: NSCoder) {
        fatalError()
    }

    override func image(forBounds imageBounds: CGRect, textContainer: NSTextContainer?, characterIndex charIndex: Int) -> UIImage? {
        if let image = image {
            return image
        } else {
            return placeholderImage
        }
    }

    private func loadImage() {
      
        // load image here
            
        // on success
      
        isDownloaded = true
        image = loadedImage
      
        // on failure
      
        isDownloaded = false
        image = placeholderImage

        textContainer.layoutManager?.invalidateLayout(forCharacterRange: NSRange(range), actualCharacterRange: nil)
    }
}