flutter_cache_store

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flutter_cache_store

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More configurable cache manager for Flutter

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# flutter_cache_store

A flexible cache manager for Flutter.

This package is highly inspired by [flutter_cache_manager](https://pub.dartlang.org/packages/flutter_cache_manager). Can be easily switched to each other.

## Quick Start

```dart
import 'package:flutter_cache_store/flutter_cache_store.dart';

void demo(String url) async {
  final store = await CacheStore.getInstance();
  final file = await store.getFile(url);
  // do something with file...
}
```

---

## APIs

### CacheStore

```dart
void api() async {
  // get store instance
  CacheStore store = await CacheStore.getInstance(
    namespace: 'unique_name', // default: null - valid filename used as unique id
    policy: LeastFrequentlyUsedPolicy(), // default: null - will use `LessRecentlyUsedPolicy()`
    clearNow: true, // default: false - whether to clean up immediately
    fetch: myFetch, // default: null - a shortcut of `CacheStore.fetch`
  );

// You can change custom fetch method at anytime.
  // Set it to `null` will simply use `http.get`
  store.fetch = myFetch;

// fetch a file from an URL and cache it
  File file = await store.getFile(
    'url', // GET method
    key: null, // use custom string instead of URL
    headers: {}, // same as http.get
    fetch: myFetch, // Optional: CustomFunction for making custom request
    // Optional: Map<String, dynamic> any custom you want to pass to your custom fetch function.
    custom: {'method': 'POST', 'body': 'test'},
    flushCache: false, // whether to re-download the file
  );

// flush specific files by keys
  await store.flush([
    'key', // key (default is the URL) passed to `getFile`
  ]);

// remove all cached files
  await store.clearAll();
}

// Custom fetch function.
// A demo of how you can achieve a fetch supporting POST with body
Future<Response> myFetch(url,
    {Map<String, String> headers, Map<String, dynamic> custom}) {
  final data = custom ?? {};
  switch (data['method'] ?? '') {
    case 'POST':
      {
        return post(url, headers: headers, body: data['body']);
      }
    default:
      return get(url, headers: headers);
  }
}
```

### Cache File Structure

By default, the cached files will be saved under `$TEMP/cache_store`. `$TEMP` is generated by [path_provider](https://pub.dartlang.org/packages/path_provider). The default temp filenames are timestamp-based 11 chars.

You can customize your own file structure under `$TEMP/cache_store` by overriding Policy. There is an example:

> Let's suppose your are developing a reader for novels, and your app will cache chapters of books. Your API returns some IDs of books and chapters, and an ID only contains letters and digits.

```dart
// Extends a Policy class and override `generateFilename`
class LRUCachePolicy extends LessRecentlyUsedPolicy {
  LRUCachePolicy({int maxCount}) : super(maxCount: maxCount);

@override
  String generateFilename({final String key, final String url}) =>
      key; // use key as the filename
}

void customizedCacheFileStructure() async {
  // get store instance
  CacheStore store = await CacheStore.getInstance(
    policy: LRUCachePolicy(maxCount: 4096),
    namespace: 'my_store',
  );

// fetch a file from an URL and cache it
  String bookId = 'book123';
  String chapterId = 'ch42';
  String chapterUrl = 'https://example.com/book123/ch42';
  File file = await store.getFile(
    chapterUrl,
    key: '$bookId/$chapterId', // use IDs as path and filename
  );

// Your file will be cached as `$TEMP/cache_store__my_store/book123/ch42`
}
```

---

## Cache Policy

- `LessRecentlyUsedPolicy`

LRU policy. Less Recently Used files will be removed when reached `maxCount`. Each time you access a file will update its used timestamp.

```dart
  new LessRecentlyUsedPolicy(
    maxCount: 999,
  );
  ```

- `LeastFrequentlyUsedPolicy`

LFU policy. Least Frequently Used files will be removed when reached `maxCount`. Each time you access a file will increase its hit count. After `hitAge` time the hit will expire. It will fallback to LRU policy if files have same hit count.

```dart
  new LeastFrequentlyUsedPolicy(
    maxCount: 999,
    hitAge: Duration(days: 30),
  );
  ```

- `CacheControlPolicy`

`Cache-Control` header policy. This policy generally follows `max-age=<seconds>` or `s-maxage=<seconds>` rules of http response header [Cache-Control](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control). If the max-age-seconds not been set, it will use `minAge` unless you set it to `null`. The age will not be longer than `maxAge`.

```dart
  new CacheControlPolicy(
    maxCount: 999,
    minAge: Duration(seconds: 30), // nullable
    maxAge: Duration(days: 30), // nullable
  );
  ```

- `FifoPolicy`

First-In, First-Out policy, super simple and maybe for example only.

```dart
  new FifoPolicy(
    maxCount: 999,
  );
  ```

### Performance Warning

- The implementation maintains all key-item in memory to improve the speed. So `maxCount` must between 1 and 100000 (100k) due to the cost of RAM and file system.

- Currently, all the policies simply sort all items to expire files. It may hit performance due to `O(N*logN)` complexity.

> Will switch to priority queue which has `O(N*logK)` while `K` usually is a very small number.

### How to implement your own policy

The interface is a simple abstract class. You only have to implement a few methods.

```dart
abstract class CacheStorePolicy {
  // IT'S THE ONLY METHOD YOU HAVE TO IMPLEMENT.
  // `store` will invoke this method from time to time.
  // Make sure return all expired items at once.
  // then `store` will manage to remove the cached files.
  // you also have to save your data if need to persist some data.
  Future<Iterable<CacheItem>> cleanup(Iterable<CacheItem> allItems);

// will be invoked when store.clearAll called.
  Future<void> clearAll(Iterable<CacheItem> allItems) async {}

// will invoke only once when the `store` is created and load saved data.
  // you need to load persistent data and restore items' payload.
  // only returned items will be cached. others will be recycled later.
  Future<Iterable<CacheItem>> restore(List<CacheItem> allItems) async => allItems;

// event when a new `CacheItem` has been added to the cache.
  // you may need to attach a `CacheItemPayload` instance to it.
  Future<void> onAdded(final CacheItem addedItem) async {}

// event when an item just been accessed.
  // you may need to attach or update item's payload.
  Future<void> onAccessed(final CacheItem accessedItem, bool flushed) async {}

// event when a request just finished.
  // the response headers will be passed as well.
  Future<void> onDownloaded(final CacheItem item, final Map<String, String> headers) async {}

// event when `store.flush` has called
  Future<void> onFlushed(final Iterable<CacheItem> flushedItems) async {}

// filename (including path) relative to `CacheItem.rootPath`.
  // usually ignore this unless need a better files structure.
  // you must provide valid filenames.
  String generateFilename({final String key, final String url}) =>
      Utils.genName(); // timestamp based random filename
}
```

- Tips

> You don't have to implement all of the `onAdded`, `onAccessed` and `onDownloaded`.

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