Improve emoji loading behavior

Use transparent placeholders to prevent wrong initial layout when some
or all emojis aren't cached.

Tusker/Extensions/NSTextAttachment+Emoji.swift

@@ -17,13 +17,21 @@ extension NSTextAttachment {
         let defaultScale: CGFloat = 1.4
         imageSizeMatchingFontSize = CGSize(width: imageSizeMatchingFontSize.width * defaultScale, height: imageSizeMatchingFontSize.height * defaultScale)
         
-        UIGraphicsBeginImageContextWithOptions(imageSizeMatchingFontSize, false, 0.0)
+        let attachmentImage = UIGraphicsImageRenderer(size: imageSizeMatchingFontSize).image { (_) in
             textColor.set()
             image.draw(in: CGRect(origin: .zero, size: imageSizeMatchingFontSize))
-        let attachmentImage = UIGraphicsGetImageFromCurrentImageContext()
-        UIGraphicsEndImageContext()
+        }
         
         self.init()
         self.image = attachmentImage
     }
+    
+    convenience init(emojiPlaceholderIn font: UIFont) {
+        let adjustedCapHeight = font.capHeight - 1
+        // assumes emoji are mostly square
+        let size = CGSize(width: adjustedCapHeight, height: adjustedCapHeight)
+        let image = UIGraphicsImageRenderer(size: size).image { (_) in }
+        self.init()
+        self.image = image
+    }
 }

Tusker/Views/BaseEmojiLabel.swift

@@ -19,19 +19,28 @@ protocol BaseEmojiLabel: AnyObject {
 }
 
 extension BaseEmojiLabel {
-    func replaceEmojis(in attributedString: NSAttributedString, emojis: [Emoji], identifier: String?, completion: @escaping (_ attributedString: NSAttributedString, _ didReplaceEmojis: Bool) -> Void) {
+    func replaceEmojis(in attributedString: NSAttributedString, emojis: [Emoji], identifier: String?, setAttributedString: @escaping (_ attributedString: NSAttributedString, _ didReplaceEmojis: Bool) -> Void) {
+        // blergh
+        precondition(Thread.isMainThread)
+        
+        emojiRequests.forEach { $0.cancel() }
+        emojiRequests = []
+        
         guard !emojis.isEmpty else {
-            completion(attributedString, false)
+            setAttributedString(attributedString, false)
             return
         }
         
         let matches = emojiRegex.matches(in: attributedString.string, options: [], range: NSRange(location: 0, length: attributedString.length))
         guard !matches.isEmpty else {
-            completion(attributedString, false)
+            setAttributedString(attributedString, false)
             return
         }
         
-        let emojiImages = MultiThreadDictionary<String, UIImage>(name: "BaseEmojiLabel Emoji Images")
+        // not using a MultiThreadDictionary so that cached images can be added immediately
+        // without jumping through various queues so that we can use them immediately
+        // in building either the final string or the string with placeholders
+        var emojiImages: [String: UIImage] = [:]
         var foundEmojis = false
         
         let group = DispatchGroup()
@@ -47,47 +56,83 @@ extension BaseEmojiLabel {
             
             foundEmojis = true
             
+            if let image = ImageCache.emojis.get(emoji.url)?.image {
+                // if the image is cached, add it immediately
+                emojiImages[emoji.shortcode] = image
+            } else {
+                // otherwise, perform the network request
+                
                 group.enter()
+                // todo: ImageCache.emojis.get here will re-check the memory and disk caches, there should be another method to force-refetch
                 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 {
+                              group.leave()
                               return
                     }
+                    // sync back to the main thread to add the dictionary
+                    // todo: using the main thread for this isn't great
+                    DispatchQueue.main.async {
                         emojiImages[emoji.shortcode] = transformedImage
+                        group.leave()
+                    }
                 }
                 if let request = request {
                     emojiRequests.append(request)
                 }
             }
+        }
         
         guard foundEmojis else {
-            completion(attributedString, false)
+            setAttributedString(attributedString, false)
             return
         }
         
+        func buildStringWithEmojisReplaced(usePlaceholders: Bool) -> NSAttributedString {
+            let mutAttrString = NSMutableAttributedString(attributedString: attributedString)
+            
+            // 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 = (attributedString.string as NSString).substring(with: match.range(at: 1))
+                let attachment: NSTextAttachment
+                
+                if let emojiImage = emojiImages[shortcode] {
+                    attachment = NSTextAttachment(emojiImage: emojiImage, in: self.emojiFont, with: self.emojiTextColor)
+                } else if usePlaceholders {
+                    attachment = NSTextAttachment(emojiPlaceholderIn: self.emojiFont)
+                } else {
+                    continue
+                }
+                
+                let attachmentStr = NSAttributedString(attachment: attachment)
+                mutAttrString.replaceCharacters(in: match.range, with: attachmentStr)
+            }
+            
+            return mutAttrString
+        }
+        
+        if emojiRequests.isEmpty {
+            // waiting on the group when there are only cached emojis results in a bad first layout,
+            // because it would still wait until the next runloop iteration
+            // but, since there were no requests, we can generate the correct string immediately
+            let replacedString = buildStringWithEmojisReplaced(usePlaceholders: false)
+            setAttributedString(replacedString, true)
+        } else {
+            // if we did find emojis and there are pending network requests,
+            // set an attributed strings with placeholders for non-loaded emojis to avoid layout shift
+            setAttributedString(buildStringWithEmojisReplaced(usePlaceholders: true), true)
+            
             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(attributedString: attributedString)
-            // 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 = (attributedString.string as NSString).substring(with: match.range(at: 1))
-                guard let emojiImage = emojiImages[shortcode] else {
-                    continue
+                let replacedString = buildStringWithEmojisReplaced(usePlaceholders: false)
+                setAttributedString(replacedString, true)
             }
-                
-                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, true)
         }
     }
     
-    func replaceEmojis(in string: String, emojis: [Emoji], identifier: String?, completion: @escaping (_ attributedString: NSAttributedString, _ didReplaceEmojis: Bool) -> Void) {
-        replaceEmojis(in: NSAttributedString(string: string), emojis: emojis, identifier: identifier, completion: completion)
+    func replaceEmojis(in string: String, emojis: [Emoji], identifier: String?, setAttributedString: @escaping (_ attributedString: NSAttributedString, _ didReplaceEmojis: Bool) -> Void) {
+        replaceEmojis(in: NSAttributedString(string: string), emojis: emojis, identifier: identifier, setAttributedString: setAttributedString)
     }
 }