feat: Implementing lazy parsing for date releated headers and updatin…

…g tests

lib/src/headers/headers.dart

@@ -96,11 +96,18 @@ abstract base class Headers {
 
   /// Define header properties
 
-  /// Date-related headers
-  final DateTime? date;
-  final DateTime? expires;
-  final DateTime? ifModifiedSince;
-  final DateTime? lastModified;
+  /// Date-related headers`
+  final _LazyInit<DateTime?> _date;
+  DateTime? get date => _date.value;
+
+  final _LazyInit<DateTime?> _expires;
+  DateTime? get expires => _expires.value;
+
+  final _LazyInit<DateTime?> _ifModifiedSince;
+  DateTime? get ifModifiedSince => _ifModifiedSince.value;
+
+  final _LazyInit<DateTime?> _lastModified;
+  DateTime? get lastModified => _lastModified.value;
 
   /// General Headers
   final Uri? origin;
@@ -264,10 +271,10 @@ abstract base class Headers {
 
   Headers._({
     // Date-related headers
-    this.date,
-    this.expires,
-    this.ifModifiedSince,
-    this.lastModified,
+    required _LazyInit<DateTime?> date,
+    required _LazyInit<DateTime?> expires,
+    required _LazyInit<DateTime?> ifModifiedSince,
+    required _LazyInit<DateTime?> lastModified,
 
     // General Headers
     this.origin,
@@ -340,7 +347,11 @@ abstract base class Headers {
     this.crossOriginEmbedderPolicy,
     this.crossOriginOpenerPolicy,
     required this.failedHeadersToParse,
-  }) : custom = custom ?? CustomHeaders.empty();
+  })  : _date = date,
+        _expires = expires,
+        _ifModifiedSince = ifModifiedSince,
+        _lastModified = lastModified,
+        custom = custom ?? CustomHeaders.empty();
 
   /// Create a new request headers instance from a Dart IO request
   factory Headers.fromHttpRequest(
@@ -368,21 +379,29 @@ abstract base class Headers {
 
     return _HeadersImpl(
       // Date-related headers
-      date: dartIOHeaders.parseSingleValue(
-        dateHeader,
-        onParse: parseDate,
-      ),
-      expires: dartIOHeaders.parseSingleValue(
-        expiresHeader,
-        onParse: parseDate,
-      ),
-      ifModifiedSince: dartIOHeaders.parseSingleValue(
-        ifModifiedSinceHeader,
-        onParse: parseDate,
-      ),
-      lastModified: dartIOHeaders.parseSingleValue(
-        lastModifiedHeader,
-        onParse: parseDate,
+      date: _LazyInit.lazy(
+        init: () => dartIOHeaders.parseSingleValue(
+          dateHeader,
+          onParse: parseDate,
+        ),
+      ),
+      expires: _LazyInit.lazy(
+        init: () => dartIOHeaders.parseSingleValue(
+          expiresHeader,
+          onParse: parseDate,
+        ),
+      ),
+      ifModifiedSince: _LazyInit.lazy(
+        init: () => dartIOHeaders.parseSingleValue(
+          ifModifiedSinceHeader,
+          onParse: parseDate,
+        ),
+      ),
+      lastModified: _LazyInit.lazy(
+        init: () => dartIOHeaders.parseSingleValue(
+          lastModifiedHeader,
+          onParse: parseDate,
+        ),
       ),
 
       // General Headers
@@ -687,8 +706,10 @@ abstract base class Headers {
   }) {
     return _HeadersImpl(
       // Date-related headers
-      date: date,
-      ifModifiedSince: ifModifiedSince,
+      date: _LazyInit.value(value: date),
+      ifModifiedSince: _LazyInit.value(value: ifModifiedSince),
+      expires: _LazyInit.nullValue(),
+      lastModified: _LazyInit.nullValue(),
 
       // Request Headers
       xPoweredBy: xPoweredBy,
@@ -782,9 +803,12 @@ abstract base class Headers {
     CrossOriginOpenerPolicyHeader? crossOriginOpenerPolicy,
   }) {
     return _HeadersImpl(
-      date: date ?? DateTime.now(),
-      expires: expires,
-      lastModified: lastModified,
+      // Date-related headers
+      date: _LazyInit.value(value: date ?? DateTime.now()),
+      expires: _LazyInit.value(value: expires),
+      lastModified: _LazyInit.value(value: lastModified),
+      ifModifiedSince: _LazyInit.nullValue(),
+
       origin: origin,
       server: server,
       via: via,
@@ -920,10 +944,10 @@ abstract base class Headers {
 final class _HeadersImpl extends Headers {
   _HeadersImpl({
     /// Date-related headers
-    super.date,
-    super.expires,
-    super.ifModifiedSince,
-    super.lastModified,
+    required super.date,
+    required super.expires,
+    required super.ifModifiedSince,
+    required super.lastModified,
 
     /// General Headers
     super.origin,
@@ -1077,12 +1101,15 @@ final class _HeadersImpl extends Headers {
     Object? crossOriginOpenerPolicy = _Undefined,
   }) {
     return _HeadersImpl(
-      date: date is DateTime? ? date : this.date,
-      expires: expires is DateTime? ? expires : this.expires,
-      ifModifiedSince:
-          ifModifiedSince is DateTime? ? ifModifiedSince : this.ifModifiedSince,
-      lastModified:
-          lastModified is DateTime? ? lastModified : this.lastModified,
+      date: date is DateTime? ? _LazyInit.value(value: date) : _date,
+      expires:
+          expires is DateTime? ? _LazyInit.value(value: expires) : _expires,
+      ifModifiedSince: ifModifiedSince is DateTime?
+          ? _LazyInit.value(value: ifModifiedSince)
+          : _ifModifiedSince,
+      lastModified: lastModified is DateTime?
+          ? _LazyInit.value(value: lastModified)
+          : _lastModified,
 
       /// General Headers
       origin: origin is Uri? ? origin : this.origin,
@@ -1478,3 +1505,51 @@ final class _HeadersImpl extends Headers {
 }
 
 class _Undefined {}
+
+typedef _LazyInitializer<T> = T Function();
+
+class _LazyInit<T> {
+  final _LazyInitializer<T>? _init;
+  bool _isInitialized = false;
+  T? _value;
+
+  _LazyInit._({
+    _LazyInitializer<T>? init,
+    T? value,
+    bool isInitialized = false,
+  })  : _init = init,
+        _value = value,
+        _isInitialized = isInitialized;
+
+  factory _LazyInit.value({
+    required T value,
+  }) {
+    return _LazyInit._(
+      value: value,
+      isInitialized: true,
+    );
+  }
+
+  factory _LazyInit.lazy({
+    required _LazyInitializer<T> init,
+  }) {
+    return _LazyInit._(
+      init: init,
+      isInitialized: false,
+    );
+  }
+
+  factory _LazyInit.nullValue() {
+    return _LazyInit._(
+      isInitialized: true,
+    );
+  }
+
+  T? get value {
+    if (!_isInitialized) {
+      _value = _init?.call();
+      _isInitialized = true;
+    }
+    return _value;
+  }
+}

test/headers/basic/date_header_test.dart

@@ -72,6 +72,21 @@ void main() {
       },
     );
 
+    test(
+      'when a Date header with an invalid date format is passed '
+      'then the server does not respond with a bad request if the headers '
+      'is not actually used',
+      () async {
+        Headers headers = await getServerRequestHeaders(
+          server: server,
+          headers: {'date': 'invalid-date-format'},
+          echoHeaders: false,
+        );
+
+        expect(headers, isNotNull);
+      },
+    );
+
     test(
       'when a valid Date header is passed then it should parse the date correctly',
       () async {

test/headers/basic/expires_header_test.dart

@@ -72,6 +72,21 @@ void main() {
       },
     );
 
+    test(
+      'when an Expires header with an invalid date format is passed '
+      'then the server does not respond with a bad request if the headers '
+      'is not actually used',
+      () async {
+        Headers headers = await getServerRequestHeaders(
+          server: server,
+          headers: {'expires': 'invalid-date-format'},
+          echoHeaders: false,
+        );
+
+        expect(headers, isNotNull);
+      },
+    );
+
     test(
       'when a valid Expires header is passed then it should parse the date correctly',
       () async {

test/headers/basic/if_modified_since_header_test.dart

@@ -72,6 +72,21 @@ void main() {
       },
     );
 
+    test(
+      'when an If-Modified-Since header with an invalid date format is passed '
+      'then the server does not respond with a bad request if the headers '
+      'is not actually used',
+      () async {
+        Headers headers = await getServerRequestHeaders(
+          server: server,
+          headers: {'if-modified-since': 'invalid-date-format'},
+          echoHeaders: false,
+        );
+
+        expect(headers, isNotNull);
+      },
+    );
+
     test(
       'when a valid If-Modified-Since header is passed then it should parse the '
       'date correctly',

test/headers/basic/last_modified_header.dart

@@ -72,6 +72,21 @@ void main() {
       },
     );
 
+    test(
+      'when a Last-Modified header with an invalid date format is passed '
+      'then the server does not respond with a bad request if the headers '
+      'is not actually used',
+      () async {
+        Headers headers = await getServerRequestHeaders(
+          server: server,
+          headers: {'last-modified': 'invalid-date-format'},
+          echoHeaders: false,
+        );
+
+        expect(headers, isNotNull);
+      },
+    );
+
     test(
       'when a valid Last-Modified header is passed then it should parse the '
       'date correctly',

test/headers/headers_test_utils.dart

@@ -17,13 +17,17 @@ class BadRequestException implements Exception {
 Future<Headers> getServerRequestHeaders({
   required RelicServer server,
   required Map<String, String> headers,
+  // Whether to echo the headers back to the client.
+  bool echoHeaders = true,
 }) async {
   Headers? parsedHeaders;
 
   server.mountAndStart(
     (Request request) {
       parsedHeaders = request.headers;
-      return Response.ok();
+      return Response.ok(
+        headers: echoHeaders ? parsedHeaders : null,
+      );
     },
   );