Tusker/Tusker/Views/BaseEmojiLabel.swift

//
//  BaseEmojiLabel.swift
//  Tusker
//
//  Created by Shadowfacts on 10/18/20.
//  Copyright © 2020 Shadowfacts. All rights reserved.
//

import UIKit
import Pachyderm

private let emojiRegex = try! NSRegularExpression(pattern: ":(\\w+):", options: [])

protocol BaseEmojiLabel: AnyObject {
    var emojiIdentifier: String? { get set }
    var emojiRequests: [ImageCache.Request] { get set }
    var emojiFont: UIFont { get }
    var emojiTextColor: UIColor { get }
}

extension BaseEmojiLabel {
    func replaceEmojis(in string: String, emojis: [Emoji], identifier: String, completion: @escaping (NSAttributedString) -> Void) {
        let matches = emojiRegex.matches(in: string, options: [], range: NSRange(location: 0, length: string.utf16.count))
        guard !matches.isEmpty else {
            completion(NSAttributedString(string: string))
            return
        }
        
        let emojiImages = MultiThreadDictionary<String, UIImage>(name: "BaseEmojiLabel Emoji Images")
        var foundEmojis = false
        
        let group = DispatchGroup()
        
        for emoji in emojis {
            // only make requests for emojis that are present in the text to avoid making unnecessary network requests
            guard matches.contains(where: { (match) in
                let matchShortcode = (string as NSString).substring(with: match.range(at: 1))
                return emoji.shortcode == matchShortcode
            }) else {
                continue
            }
            
            foundEmojis = true
            
            group.enter()
            let request = ImageCache.emojis.get(emoji.url) { (_, image) in
                defer { group.leave() }
                guard let image = image,
                      let transformedImage = ImageGrayscalifier.convertIfNecessary(url: emoji.url, image: image) else {
                    return
                }
                emojiImages[emoji.shortcode] = transformedImage
            }
            if let request = request {
                emojiRequests.append(request)
            }
        }
        
        guard foundEmojis else {
            completion(NSAttributedString(string: string))
            return
        }
        
        group.notify(queue: .main) { [weak self] in
            // if e.g. the account changes before all emojis are loaded, don't bother trying to set them
            guard let self = self, self.emojiIdentifier == identifier else { return }
            
            let mutAttrString = NSMutableAttributedString(string: string)
            // replaces the emojis starting from the end of the string as to not alter the indices of preceeding emojis
            for match in matches.reversed() {
                let shortcode = (string as NSString).substring(with: match.range(at: 1))
                guard let emojiImage = emojiImages[shortcode] else {
                    continue
                }
                
                let attachment = NSTextAttachment(emojiImage: emojiImage, in: self.emojiFont, with: self.emojiTextColor)
                let attachmentStr = NSAttributedString(attachment: attachment)
                mutAttrString.replaceCharacters(in: match.range, with: attachmentStr)
            }
            
            completion(mutAttrString)
        }
    }
}