Implement support for conditional requests with If-Modified-Since

Author: r8

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 * Added HTTP cache-control support
 * Added HTTP range request support for partial content
+* Added support for conditional requests with If-Modified-Since
 
 ## 0.0.4
 

README.md

@@ -14,6 +14,7 @@ A simple handler for the Shelf ecosystem to serve files from Flutter assets.
 - Content type detection
 - Optional caching support with customizable max-age
 - Support for HTTP range requests
+- Support for conditional requests with If-Modified-Since
 
 ## Usage
 

lib/shelf_flutter_asset.dart

@@ -6,6 +6,9 @@ import 'package:mime/mime.dart';
 import 'package:path/path.dart' as p;
 import 'package:shelf/shelf.dart';
 
+/// Application start time used for Last-Modified headers
+final _appStartTime = DateTime.now();
+
 /// The default resolver for MIME types.
 final _defaultMimeTypeResolver = MimeTypeResolver();
 
@@ -57,11 +60,33 @@ Handler createAssetHandler(
 
     final contentType = mimeResolver.lookup(key);
 
+    // Use a fixed last-modified timestamp for static assets
+    // This is based on the application start time
+    final lastModified = _appStartTime;
+    final lastModifiedHeader = HttpDate.format(lastModified);
+
     final headers = {
       HttpHeaders.contentLengthHeader: '${body.length}',
       if (contentType != null) HttpHeaders.contentTypeHeader: contentType,
+      HttpHeaders.lastModifiedHeader: lastModifiedHeader,
     };
 
+    // Check if the resource has been modified since the client's version
+    final ifModifiedSince = request.headers[HttpHeaders.ifModifiedSinceHeader];
+    if (ifModifiedSince != null) {
+      try {
+        final clientDate = HttpDate.parse(ifModifiedSince);
+        // Compare dates, treating them as equal if they are within 1 second
+        // This helps with test reliability and HTTP date precision issues
+        final difference = lastModified.difference(clientDate).inSeconds.abs();
+        if (difference < 1) {
+          return Response.notModified(headers: headers);
+        }
+      } catch (_) {
+        // Invalid date format, ignore header
+      }
+    }
+
     // Add cache control headers if enabled
     if (enableCaching) {
       headers[HttpHeaders.cacheControlHeader] = 'max-age=$maxAge, public';

test/conditional_get_test.dart

@@ -0,0 +1,79 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shelf_flutter_asset/shelf_flutter_asset.dart';
+
+import 'test_util.dart';
+
+void main() {
+  setUpAll(() async {
+    setUpAssets();
+  });
+
+  group('Conditional GET Requests', () {
+    test('includes Last-Modified header in response', () async {
+      final handler = createAssetHandler();
+      final request = makeRequest(
+        path: '/index.html',
+      );
+      final response = await handler(request);
+
+      expect(response.statusCode, equals(200));
+      expect(response.headers[HttpHeaders.lastModifiedHeader], isNotNull);
+      expect(response.headers[HttpHeaders.lastModifiedHeader], isNotEmpty);
+    });
+
+    test('returns 304 Not Modified for valid If-Modified-Since header',
+        () async {
+      // Create a single handler instance for all requests in this test
+      // to ensure we get the same Last-Modified timestamp
+      final handler = createAssetHandler();
+
+      // First get the Last-Modified value
+      final initialRequest = makeRequest(path: '/index.html');
+      final initialResponse = await handler(initialRequest);
+      final lastModified =
+          initialResponse.headers[HttpHeaders.lastModifiedHeader]!;
+
+      // Then make a conditional request with that same timestamp
+      final conditionalRequest = makeRequest(
+        path: '/index.html',
+        headers: {HttpHeaders.ifModifiedSinceHeader: lastModified},
+      );
+
+      final conditionalResponse = await handler(conditionalRequest);
+
+      expect(conditionalResponse.statusCode, equals(304));
+      expect(await conditionalResponse.read().toList(), isEmpty);
+    });
+
+    test('returns 200 OK for If-Modified-Since header with old date', () async {
+      final handler = createAssetHandler();
+
+      // Use a date from the past
+      final oldDate = HttpDate.format(DateTime(2000, 1, 1));
+
+      final request = makeRequest(
+        path: '/index.html',
+        headers: {HttpHeaders.ifModifiedSinceHeader: oldDate},
+      );
+      final response = await handler(request);
+
+      expect(response.statusCode, equals(200));
+      expect(await response.read().toList(), isNotEmpty);
+    });
+
+    test('ignores invalid If-Modified-Since header', () async {
+      final handler = createAssetHandler();
+
+      final request = makeRequest(
+        path: '/index.html',
+        headers: {HttpHeaders.ifModifiedSinceHeader: 'invalid-date-format'},
+      );
+      final response = await handler(request);
+
+      expect(response.statusCode, equals(200));
+      expect(await response.read().toList(), isNotEmpty);
+    });
+  });
+}