Improve performance when displaying posts with many custom emojis

Closes #204

Tusker/Extensions/NSTextAttachment+Emoji.swift

@@ -22,8 +22,7 @@ extension NSTextAttachment {
             image.draw(in: CGRect(origin: .zero, size: imageSizeMatchingFontSize))
         }
         
-        self.init()
-        self.image = attachmentImage
+        self.init(image: attachmentImage)
     }
     
     convenience init(emojiPlaceholderIn font: UIFont) {
@@ -31,7 +30,6 @@ extension NSTextAttachment {
         // assumes emoji are mostly square
         let size = CGSize(width: adjustedCapHeight, height: adjustedCapHeight)
         let image = UIGraphicsImageRenderer(size: size).image { (_) in }
-        self.init()
-        self.image = image
+        self.init(image: image)
     }
 }

Tusker/Views/BaseEmojiLabel.swift

@@ -86,20 +86,24 @@ extension BaseEmojiLabel {
         func buildStringWithEmojisReplaced(usePlaceholders: Bool) -> NSAttributedString {
             let mutAttrString = NSMutableAttributedString(attributedString: attributedString)
             
-            // lock once for the entire loop, rather than lock/unlocking for each iteration to do the lookup
             // OSAllocatedUnfairLock.withLock expects a @Sendable closure, so this warns about captures of non-sendable types (attribute dstrings, text checking results)
             // even though the closures is invoked on the same thread that withLock is called, so it's unclear why it needs to be @Sendable (FB11494878)
             // so, just ignore the warnings
-            emojiImages.withLock { emojiImages in
+            let emojiAttachments = emojiImages.withLock {
+                $0.mapValues { image in
+                    NSTextAttachment(emojiImage: image, in: self.emojiFont, with: self.emojiTextColor)
+                }
+            }
+            let placeholder = usePlaceholders ? NSTextAttachment(emojiPlaceholderIn: self.emojiFont) : nil
+            
             // 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)
+                if let emoji = emojiAttachments[shortcode] {
+                    attachment = emoji
                 } else if usePlaceholders {
-                        attachment = NSTextAttachment(emojiPlaceholderIn: self.emojiFont)
+                    attachment = placeholder!
                 } else {
                     continue
                 }
@@ -107,7 +111,6 @@ extension BaseEmojiLabel {
                 let attachmentStr = NSAttributedString(attachment: attachment)
                 mutAttrString.replaceCharacters(in: match.range, with: attachmentStr)
             }
-            }
             
             return mutAttrString
         }