feat: implement web service

Signed-off-by: The one with the braid <info@braid.business>

lib/pkpass_web_wervice/web_service.dart

@@ -0,0 +1,69 @@
+import 'dart:typed_data';
+
+import 'package:http/http.dart';
+
+import 'package:pkpass/pkpass.dart';
+import 'package:pkpass/pkpass_web_wervice/utils/http_date.dart';
+import 'web_service_error.dart';
+
+/// PassKit WebService implementation
+///
+/// A Representational State Transfer (REST)–style web service protocol is used
+/// to communicate with your server about changes to passes, and to fetch the
+/// latest version of a pass when it has changed.
+///
+/// https://developer.apple.com/library/archive/documentation/PassKit/Reference/PassKit_WebService/WebService.html#//apple_ref/doc/uid/TP40011988
+class PkPassWebService {
+  static const _apiVersion = 'v1';
+  static Client? _client;
+
+  /// The [PassMetadata] to check for updates.
+  final PassMetadata metadata;
+
+  /// An optional [Client] used for any http requests
+  final Client? client;
+
+  const PkPassWebService(this.metadata, {this.client});
+
+  PassWebService get webService {
+    final service = metadata.webService;
+    if (service == null) noWebServiceProvided();
+    return service;
+  }
+
+  Client get httpClient {
+    final client = this.client;
+    if (client != null) return client;
+    return _client ??= Client();
+  }
+
+  /// Getting the Latest Version of a Pass
+  ///
+  /// Requests the latest version of the current PkPass file
+  ///
+  /// [modifiedSince] should be provided in order to support "304 Not Modified"
+  Future<Uint8List?> getLatestVersion([DateTime? modifiedSince]) async {
+    final identifier = metadata.passTypeIdentifier;
+    final serial = metadata.serialNumber;
+    final endpoint = '/$_apiVersion/passes/$identifier/$serial';
+
+    final response = await httpClient.get(
+      Uri.parse(webService.webServiceURL.toString() + endpoint),
+      headers: {
+        if (modifiedSince != null)
+          'If-Modified-Since': HttpDate.format(modifiedSince),
+        'Authorization': 'ApplePass ${webService.authenticationToken}',
+      },
+    );
+    switch (response.statusCode) {
+      case 200:
+        return response.bodyBytes;
+      case 304:
+        return null;
+      default:
+        throw WebServiceResponseError(response);
+    }
+  }
+
+  Never noWebServiceProvided() => throw WebServiceUnavailable();
+}