Wrap selected text in a markdown link when pasting a URL over it

Pasting a single link on top of a selection in the message composer now
produces a markdown link, e.g. selecting "Google" and pasting
"google.com" yields "[Google](google.com)". Non-link clipboard
contents and pastes without a selection keep their existing behaviour.

ElementX/Sources/Screens/RoomScreen/ComposerToolbar/View/MessageComposerTextField.swift

@@ -283,6 +283,13 @@ private class ElementTextView: UITextView, PillAttachmentViewProviderDelegate {
     }
     
     override func paste(_ sender: Any?) {
+        // When pasting a link over a selection, wrap the selection in a markdown link.
+        if selectedRange.length > 0, let link = UIPasteboard.general.pastedLink {
+            let selectedText = (attributedText.string as NSString).substring(with: selectedRange)
+            insertText("[\(selectedText)](\(link))")
+            return
+        }
+        
         let providers = UIPasteboard.general.itemProviders
         
         // Use the default behavior if there are any unsupported providers
@@ -331,6 +338,25 @@ private class ElementTextView: UITextView, PillAttachmentViewProviderDelegate {
     }
 }
 
+private extension UIPasteboard {
+    /// The pasteboard's string contents when they consist of a single link and nothing else.
+    var pastedLink: String? {
+        guard let string = string?.trimmingCharacters(in: .whitespacesAndNewlines), !string.isEmpty,
+              let detector = try? NSDataDetector(types: NSTextCheckingResult.CheckingType.link.rawValue) else {
+            return nil
+        }
+        
+        let range = NSRange(string.startIndex..., in: string)
+        let matches = detector.matches(in: string, range: range)
+        
+        guard matches.count == 1, matches[0].range == range else {
+            return nil
+        }
+        
+        return string
+    }
+}
+
 struct MessageComposerTextField_Previews: PreviewProvider, TestablePreview {
     static var previews: some View {
         VStack(spacing: 16) {