app_dart/lib/src/request_handling/cache_request_handler.dart

// Copyright 2019 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:async';
import 'dart:typed_data';

import 'package:meta/meta.dart';

import '../request_handling/request_handler.dart';
import '../service/cache_service.dart';
import 'body.dart';

/// A [RequestHandler] for serving cached responses.
///
/// High traffic endpoints that have responses that do not change
/// based on request are good for caching. Additionally, saves
/// reading from Datastore which is expensive both timewise and monetarily.
@immutable
class CacheRequestHandler<T extends Body> extends RequestHandler<T> {
  /// Creates a new [CacheRequestHandler].
  const CacheRequestHandler({
    required this.delegate,
    required super.config,
    required this.cache,
    this.ttl = const Duration(minutes: 1),
  });

  /// [RequestHandler] to fallback on for cache misses.
  final RequestHandler<T> delegate;

  final CacheService cache;

  /// The time to live for the response stored in the cache.
  final Duration ttl;

  @visibleForTesting
  static const String responseSubcacheName = 'response';

  @visibleForTesting
  static const String flushCacheQueryParam = 'flushCache';

  /// Services a cached request.
  ///
  /// Given the query param [flushCacheQueryParam]=true, it will purge the
  /// response from the cache before getting it to set the cached response
  /// to the latest information.
  @override
  Future<T> get() async {
    final String responseKey = '${request!.uri.path}:${request!.uri.query}';

    if (request!.uri.queryParameters[flushCacheQueryParam] == 'true') {
      await cache.purge(responseSubcacheName, responseKey);
    }

    final Uint8List? cachedResponse = await cache.getOrCreateWithLocking(
      responseSubcacheName,
      responseKey,
      createFn: () => getBodyBytesFromDelegate(delegate),
      ttl: ttl,
    );

    return Body.forStream(Stream<Uint8List?>.value(cachedResponse)) as T;
  }

  /// Get a Uint8List that contains the bytes of the response from [delegate]
  /// so it can be stored in [cache].
  Future<Uint8List> getBodyBytesFromDelegate(RequestHandler<T> delegate) async {
    final Body body = await delegate.get();

    // Body only offers getting a Stream<Uint8List> since it just sends
    // the data out usually to a client. In this case, we want to store
    // the bytes in the cache which requires several conversions to get a
    // Uint8List that contains the bytes of the response.
    final List<int> rawBytes = await body.serialize().expand<int>((Uint8List? chunk) => chunk!).toList();
    return Uint8List.fromList(rawBytes);
  }
}