launch_url [nfc]: Move launchUrl logic to a new file

- Move the existing _launchUrl logic from content.dart to a new file named launch_url.dart.
- Split function into pieces to handle realm-based and non-realm URLs.
- Refactor error handling into a private  function.

Author: VatsalBhesaniya

lib/widgets/content.dart

@@ -1,22 +1,17 @@
-import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
-import 'package:flutter/services.dart';
 import 'package:html/dom.dart' as dom;
 import 'package:intl/intl.dart';
 import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 
 import '../api/core.dart';
 import '../api/model/model.dart';
 import '../model/avatar_url.dart';
-import '../model/binding.dart';
 import '../model/content.dart';
-import '../model/internal_link.dart';
 import 'code_block.dart';
-import 'dialog.dart';
 import 'icons.dart';
+import 'launch_url.dart';
 import 'lightbox.dart';
-import 'message_list.dart';
 import 'store.dart';
 import 'text.dart';
 
@@ -525,7 +520,7 @@ class _BlockInlineContainerState extends State<_BlockInlineContainer> {
 
   void _prepareRecognizers() {
     _recognizers.addEntries(widget.links.map((node) => MapEntry(node,
-      TapGestureRecognizer()..onTap = () => _launchUrl(context, node.url))));
+      TapGestureRecognizer()..onTap = () => launchUrlWithRealm(context, node.url))));
   }
 
   void _disposeRecognizers() {
@@ -891,52 +886,6 @@ class GlobalTime extends StatelessWidget {
   }
 }
 
-void _launchUrl(BuildContext context, String urlString) async {
-  Future<void> showError(BuildContext context, String? message) {
-    return showErrorDialog(context: context,
-      title: 'Unable to open link',
-      message: [
-        'Link could not be opened: $urlString',
-        if (message != null) message,
-      ].join("\n\n"));
-  }
-
-  final store = PerAccountStoreWidget.of(context);
-  final url = store.tryResolveUrl(urlString);
-  if (url == null) { // TODO(log)
-    await showError(context, null);
-    return;
-  }
-
-  final internalNarrow = parseInternalLink(url, store);
-  if (internalNarrow != null) {
-    Navigator.push(context,
-      MessageListPage.buildRoute(context: context,
-        narrow: internalNarrow));
-    return;
-  }
-
-  bool launched = false;
-  String? errorMessage;
-  try {
-    launched = await ZulipBinding.instance.launchUrl(url,
-      mode: switch (defaultTargetPlatform) {
-        // On iOS we prefer LaunchMode.externalApplication because (for
-        // HTTP URLs) LaunchMode.platformDefault uses SFSafariViewController,
-        // which gives an awkward UX as described here:
-        //  https://chat.zulip.org/#narrow/stream/48-mobile/topic/in-app.20browser/near/1169118
-        TargetPlatform.iOS => UrlLaunchMode.externalApplication,
-        _ => UrlLaunchMode.platformDefault,
-      });
-  } on PlatformException catch (e) {
-    errorMessage = e.message;
-  }
-  if (!launched) { // TODO(log)
-    if (!context.mounted) return;
-    await showError(context, errorMessage);
-  }
-}
-
 /// Like [Image.network], but includes [authHeader] if [src] is on-realm.
 ///
 /// Use this to present image content in the ambient realm: avatars, images in

lib/widgets/launch_url.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+import '../model/binding.dart';
+import '../model/internal_link.dart';
+import 'dialog.dart';
+import 'message_list.dart';
+import 'store.dart';
+
+/// Handles showing an error dialog with a customizable message.
+Future<void> _showError(BuildContext context, String? message, String urlString) {
+  return showErrorDialog(
+    context: context,
+    title: 'Unable to open link',
+    message: [
+      'Link could not be opened: $urlString',
+      if (message != null) message,
+    ].join("\n\n"));
+}
+
+/// Launches a URL without considering a realm base URL.
+void launchUrlWithoutRealm(BuildContext context, Uri url) async {
+  bool launched = false;
+  String? errorMessage;
+  try {
+    launched = await ZulipBinding.instance.launchUrl(url,
+      mode: switch (defaultTargetPlatform) {
+        // On iOS we prefer LaunchMode.externalApplication because (for
+        // HTTP URLs) LaunchMode.platformDefault uses SFSafariViewController,
+        // which gives an awkward UX as described here:
+        //  https://chat.zulip.org/#narrow/stream/48-mobile/topic/in-app.20browser/near/1169118
+        TargetPlatform.iOS => UrlLaunchMode.externalApplication,
+        _ => UrlLaunchMode.platformDefault,
+      });
+  } on PlatformException catch (e) {
+    errorMessage = e.message;
+  }
+  if (!launched) {
+    if (!context.mounted) return;
+    await _showError(context, errorMessage, url.toString());
+  }
+}
+
+/// Launches a URL considering a realm base URL.
+void launchUrlWithRealm(BuildContext context, String urlString) async {
+  final store = PerAccountStoreWidget.of(context);
+  final url = store.tryResolveUrl(urlString);
+  if (url == null) { // TODO(log)
+    await _showError(context, null, urlString);
+    return;
+  }
+
+  final internalNarrow = parseInternalLink(url, store);
+  if (internalNarrow != null) {
+    Navigator.push(context, MessageListPage.buildRoute(context: context, narrow: internalNarrow));
+    return;
+  }
+
+  launchUrlWithoutRealm(context, url);
+}