Moved App Sources into separate files

lib/app_sources/gitlab.dart

@@ -0,0 +1,63 @@
+import 'package:html/parser.dart';
+import 'package:http/http.dart';
+import 'package:obtainium/app_sources/github.dart';
+import 'package:obtainium/providers/source_provider.dart';
+
+class GitLab implements AppSource {
+  @override
+  late String host = 'gitlab.com';
+
+  @override
+  String standardizeURL(String url) {
+    RegExp standardUrlRegEx = RegExp('^https?://$host/[^/]+/[^/]+');
+    RegExpMatch? match = standardUrlRegEx.firstMatch(url.toLowerCase());
+    if (match == null) {
+      throw notValidURL;
+    }
+    return url.substring(0, match.end);
+  }
+
+  @override
+  Future<APKDetails> getLatestAPKDetails(String standardUrl) async {
+    Response res = await get(Uri.parse('$standardUrl/-/tags?format=atom'));
+    if (res.statusCode == 200) {
+      var standardUri = Uri.parse(standardUrl);
+      var parsedHtml = parse(res.body);
+      var entry = parsedHtml.querySelector('entry');
+      var entryContent =
+          parse(parseFragment(entry?.querySelector('content')!.innerHtml).text);
+      var apkUrlList = [
+        ...getLinksFromParsedHTML(
+            entryContent,
+            RegExp(
+                '^${escapeRegEx(standardUri.path)}/uploads/[^/]+/[^/]+\\.apk\$',
+                caseSensitive: false),
+            standardUri.origin),
+        // GitLab releases may contain links to externally hosted APKs
+        ...getLinksFromParsedHTML(entryContent,
+                RegExp('/[^/]+\\.apk\$', caseSensitive: false), '')
+            .where((element) => Uri.parse(element).host != '')
+            .toList()
+      ];
+      if (apkUrlList.isEmpty) {
+        throw noAPKFound;
+      }
+
+      var entryId = entry?.querySelector('id')?.innerHtml;
+      var version =
+          entryId == null ? null : Uri.parse(entryId).pathSegments.last;
+      if (version == null) {
+        throw couldNotFindLatestVersion;
+      }
+      return APKDetails(version, apkUrlList);
+    } else {
+      throw couldNotFindReleases;
+    }
+  }
+
+  @override
+  AppNames getAppNames(String standardUrl) {
+    // Same as GitHub
+    return GitHub().getAppNames(standardUrl);
+  }
+}