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generating_iterable.dart
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generating_iterable.dart
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// Copyright 2013 Google Inc. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
import 'dart:collection';
Iterable generate(Function() initial, Function(dynamic) next) =>
GeneratingIterable(initial, next);
/// An Iterable whose first value is the result of [_initial] and whose
/// subsequent values are generated by passing the current value to the [_next]
/// function.
///
/// The class is useful for creating lazy iterables from object hierarchies and
/// graphs.
///
/// The initial value and [_next] function are required to generate a sequence
/// that eventually terminates, otherwise calling methods that expect a finite
/// sequence, like `length` or `last`, will cause an infinite loop.
///
/// Example:
///
/// class Node {
/// Node parent;
///
/// /// An iterable of node and all ancestors up to the root.
/// Iterable<Node> ancestors =
/// GeneratingIterable<Node>(() => this, (n) => n.parent);
///
/// /// An iterable of the root and the path of nodes to this. The
/// /// reverse of ancestors.
/// Iterable<Node> path = ancestors.toList().reversed();
/// }
///
class GeneratingIterable<T> extends IterableBase<T> {
GeneratingIterable(this._initial, this._next);
final T Function() _initial;
final T? Function(T value) _next;
@override
Iterator<T> get iterator => _GeneratingIterator(_initial(), _next);
}
class _GeneratingIterator<T> implements Iterator<T> {
_GeneratingIterator(this.object, this.next);
final T? Function(T value) next;
T? object;
bool started = false;
@override
T get current {
final cur = started ? object : null;
return cur!;
}
@override
bool moveNext() {
final obj = object;
if (obj == null) return false;
if (started) {
object = next(obj);
} else {
started = true;
}
return object != null;
}
}