New configuration respectMaxAgeZero

If a server sends "Cache-Control: max-age=0" this might be a
misconfiguration on the server; in this case we should ignore it (the
behaviour up to now).
It might also mean, that the content potentially already expired; for
this case this commit provides a new configuration parameter to respect
this response.

Author: aytchell

flutter_cache_manager/README.md

@@ -60,7 +60,7 @@ you call this method with other height/width parameters.
 When your files are stored on Firebase Storage you can use [flutter_cache_manager_firebase](https://pub.dev/packages/flutter_cache_manager_firebase).
 
 ## Customize
-The cache manager is customizable by creating a new CacheManager. It is very important to not create more than 1
+The cache manager is customizable by creating a new CacheManager. It is very important to not create more than one
  CacheManager instance with the same key as these bite each other. In the example down here the manager is created as a 
  Singleton, but you could also use for example Provider to Provide a CacheManager on the top level of your app.
 Below is an example with other settings for the maximum age of files, maximum number of objects
@@ -85,6 +85,7 @@ class CustomCacheManager {
 - [How are the cache files stored?](#how-are-the-cache-files-stored)
 - [When are the cached files updated?](#when-are-the-cached-files-updated)
 - [When are cached files removed?](#when-are-cached-files-removed)
+- [Why are cached files kept even though the server sends max-age=0?](#why-are-cached-files-kept-even-though-the-server-sends-max-age0)
 
 
 ### How are the cache files stored?
@@ -113,6 +114,18 @@ The cache knows when files have been used latest. When cleaning the cache (which
 deletes files when there are too many, ordered by last use, and when files just haven't been used for longer than
 the stale period.
 
+### Why are cached files kept even though the server sends max-age=0?
+
+The case when a web server responds with `Cache-Control: max-age=0` is kind of an edge case.
+It could either mean, that the content expires really fast (then `Cache-Control: no-cache` might
+be a better response) or it could be that the server has some misconfiguration in place.
+
+There where some confusions among users of this library because of the second case (see also
+[this issue](https://github.com/Baseflow/flutter_cache_manager/issues/283) so as a default
+behaviour this library ignores `max-age=0`.
+
+If you want to treat `max-age=0` the same as `no-cache`, then set the `Config` parameter
+`respectMaxAgeZero` to `true`.
 
 ## Breaking changes in v2
 - There is no longer a need to extend on BaseCacheManager, you can directly call the constructor. The BaseCacheManager

flutter_cache_manager/lib/src/config/_config_io.dart

@@ -11,11 +11,13 @@ class Config implements def.Config {
     CacheInfoRepository? repo,
     FileSystem? fileSystem,
     FileService? fileService,
+    bool? respectMaxAgeZero,
   })  : stalePeriod = stalePeriod ?? const Duration(days: 30),
         maxNrOfCacheObjects = maxNrOfCacheObjects ?? 200,
         repo = repo ?? _createRepo(cacheKey),
         fileSystem = fileSystem ?? IOFileSystem(cacheKey),
-        fileService = fileService ?? HttpFileService();
+        fileService = fileService ??
+            HttpFileService(respectMaxAgeZero: respectMaxAgeZero ?? false);
 
   @override
   final CacheInfoRepository repo;

flutter_cache_manager/lib/src/config/_config_unsupported.dart

@@ -18,6 +18,8 @@ class Config implements def.Config {
     FileSystem? fileSystem,
     //ignore: avoid_unused_constructor_parameters
     FileService? fileService,
+    //ignore: avoid_unused_constructor_parameters
+    bool? respectMaxAgeZero,
   }) {
     throw UnsupportedError('Platform is not supported');
   }

flutter_cache_manager/lib/src/config/_config_web.dart

@@ -12,11 +12,13 @@ class Config implements def.Config {
     CacheInfoRepository? repo,
     FileSystem? fileSystem,
     FileService? fileService,
+    bool? respectMaxAgeZero,
   })  : stalePeriod = stalePeriod ?? const Duration(days: 30),
         maxNrOfCacheObjects = maxNrOfCacheObjects ?? 200,
         repo = repo ?? NonStoringObjectProvider(),
         fileSystem = fileSystem ?? MemoryCacheSystem(),
-        fileService = fileService ?? HttpFileService();
+        fileService = fileService ??
+            HttpFileService(respectMaxAgeZero: respectMaxAgeZero ?? false);
 
   @override
   final CacheInfoRepository repo;

flutter_cache_manager/lib/src/config/config.dart

@@ -20,13 +20,18 @@ abstract class Config {
   /// [JsonCacheInfoRepository].
   /// The [fileSystem] defines where the cached files are stored and the
   /// [fileService] defines where files are fetched, for example online.
+  /// If an http response contains a cache-control header with max-age=0 this
+  /// might mean that the content expires really fast or it could be a
+  /// misconfiguration of the server. With [respectMaxAgeZero] one can
+  /// control the lib's behaviour in this case.
   factory Config(
     String cacheKey, {
     Duration stalePeriod,
     int maxNrOfCacheObjects,
     CacheInfoRepository repo,
     FileSystem fileSystem,
     FileService fileService,
+    bool respectMaxAgeZero,
   }) = impl.Config;
 
   String get cacheKey;

flutter_cache_manager/lib/src/web/file_service.dart

@@ -23,9 +23,11 @@ abstract class FileService {
 /// [WebHelper]. One can easily adapt it to use dio or any other http client.
 class HttpFileService extends FileService {
   final http.Client _httpClient;
+  final bool _respectMaxAgeZero;
 
-  HttpFileService({http.Client? httpClient})
-      : _httpClient = httpClient ?? http.Client();
+  HttpFileService({http.Client? httpClient, bool? respectMaxAgeZero})
+      : _httpClient = httpClient ?? http.Client(),
+        _respectMaxAgeZero = respectMaxAgeZero ?? false;
 
   @override
   Future<FileServiceResponse> get(String url,
@@ -36,7 +38,7 @@ class HttpFileService extends FileService {
     }
     final httpResponse = await _httpClient.send(req);
 
-    return HttpGetResponse(httpResponse);
+    return HttpGetResponse(httpResponse, _respectMaxAgeZero);
   }
 }
 
@@ -64,12 +66,14 @@ abstract class FileServiceResponse {
 
 /// Basic implementation of a [FileServiceResponse] for http requests.
 class HttpGetResponse implements FileServiceResponse {
-  HttpGetResponse(this._response);
+  HttpGetResponse(this._response, this._respectMaxAgeZero);
 
   final DateTime _receivedTime = clock.now();
 
   final http.StreamedResponse _response;
 
+  final bool _respectMaxAgeZero;
+
   @override
   int get statusCode => _response.statusCode;
 
@@ -98,7 +102,7 @@ class HttpGetResponse implements FileServiceResponse {
         if (sanitizedSetting.startsWith('max-age=')) {
           final validSeconds =
               int.tryParse(sanitizedSetting.split('=')[1]) ?? 0;
-          if (validSeconds > 0) {
+          if (validSeconds > 0 || _respectMaxAgeZero) {
             ageDuration = Duration(seconds: validSeconds);
           }
         }

flutter_cache_manager/lib/src/web/web_helper.dart

@@ -18,9 +18,8 @@ const statusCodesNewFile = [HttpStatus.ok, HttpStatus.accepted];
 const statusCodesFileNotChanged = [HttpStatus.notModified];
 
 class WebHelper {
-  WebHelper(this._store, FileService? fileFetcher)
-      : _memCache = {},
-        fileFetcher = fileFetcher ?? HttpFileService();
+  WebHelper(this._store, this.fileFetcher)
+      : _memCache = {};
 
   final CacheStore _store;
   @visibleForTesting