sdk_nnbd/lib/core/iterator.dart

// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

part of dart.core;

/**
 * An interface for getting items, one at a time, from an object.
 *
 * The for-in construct transparently uses `Iterator` to test for the end
 * of the iteration, and to get each item (or _element_).
 *
 * If the object iterated over is changed during the iteration, the
 * behavior is unspecified.
 *
 * The `Iterator` is initially positioned before the first element.
 * Before accessing the first element the iterator must thus be advanced using
 * [moveNext] to point to the first element.
 * If no element is left, then [moveNext] returns false,
 * and all further calls to [moveNext] will also return false.
 *
 * The [current] value must not be accessed before calling [moveNext]
 * or after a call to [moveNext] has returned false.
 *
 * A typical usage of an Iterator looks as follows:
 *
 *     var it = obj.iterator;
 *     while (it.moveNext()) {
 *       use(it.current);
 *     }
 *
 * **See also:**
 * [Iteration](http://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html#iteration)
 * in the [library tour](http://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html)
 */
abstract class Iterator<E> {
  /**
   * Advances the iterator to the next element of the iteration.
   *
   * Should be called before reading [current].
   * It the call to `moveNext` returns `true`,
   * then [current] will contain the next element of the iteration
   * until `moveNext` is called again.
   * If the call returns `false`, there are no further elements
   * and [current] should not be used any more.
   *
   * It is safe to call [moveNext] after it has already returned `false`,
   * but it must keep returning `false` and not have any other effect.
   *
   * A call to `moveNext` may throw for various reasons,
   * including a concurrent change to an underlying collection.
   * If that happens, the iterator may be in an inconsistent
   * state, and any further behavior of the iterator is unspecified,
   * including the effect of reading [current].
   */
  bool moveNext();

  /**
   * Returns the current element.
   *
   * If the iterator has not yet been moved to the first element
   * ([moveNext] has not been called yet),
   * or if the iterator has been moved past the last element of the [Iterable]
   * ([moveNext] has returned false),
   * then [current] is unspecified.
   * An [Iterator] may either throw or return an iterator specific default value
   * in that case.
   *
   * The `current` getter should keep its value until the next call to
   * [moveNext], even if an underlying collection changes.
   * After a successful call to `moveNext`, the user doesn't need to cache
   * the current value, but can keep reading it from the iterator.
   */
  E get current;
}