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  1. aa7ca6f Update README.md (#8) by David Morgan · 7 weeks ago master
  2. 6b5e963 Pool.withResource() should take () -> FutureOr<T>. (#7) by Natalie Weizenbaum · 3 months ago 1.3.1
  3. b107b82 Add a Pool.done getter. (#5) by Natalie Weizenbaum · 6 months ago 1.3.0
  4. cab46fe Fix a new strong-mode error. by Natalie Weizenbaum · 1 year, 4 months ago 1.2.4
  5. 91ea1d1 Don't use async/await for Pool.withResource(). by Natalie Weizenbaum · 1 year, 5 months ago 1.2.3

The pool package exposes a Pool class which makes it easy to manage a limited pool of resources.

The easiest way to use a pool is by calling withResource. This runs a callback and returns its result, but only once there aren't too many other callbacks currently running.

// Create a Pool that will only allocate 10 resources at once. After 30 seconds
// of inactivity with all resources checked out, the pool will throw an error.
final pool = new Pool(10, timeout: new Duration(seconds: 30));

Future<String> readFile(String path) {
  // Since the call to [File.readAsString] is within [withResource], no more
  // than ten files will be open at once.
  return pool.withResource(() => new File(path).readAsString());
}

For more fine-grained control, the user can also explicitly request generic PoolResource objects that can later be released back into the pool. This is what withResource does under the covers: requests a resource, then releases it once the callback completes.

Pool ensures that only a limited number of resources are allocated at once. It‘s the caller’s responsibility to ensure that the corresponding physical resource is only consumed when a PoolResource is allocated.

class PooledFile implements RandomAccessFile {
  final RandomAccessFile _file;
  final PoolResource _resource;

  static Future<PooledFile> open(String path) {
    return pool.request().then((resource) {
      return new File(path).open().then((file) {
        return new PooledFile._(file, resource);
      });
    });
  }

  PooledFile(this._file, this._resource);

  // ...

  Future<RandomAccessFile> close() {
    return _file.close.then((_) {
      _resource.release();
      return this;
    });
  }
}