# Iterable<E> class

A collection of values, or "elements", that can be accessed sequentially.

The elements of the iterable are accessed by getting an Iterator using the iterator getter, and using it to step through the values. Stepping with the iterator is done by calling Iterator.moveNext, and if the call returns `true`, the iterator has now moved to the next element, which is then available as Iterator.current. If the call returns `false`, there are no more elements, and `iterator.current` returns `null`.

You can create more than one iterator from the same `Iterable`. Each time `iterator` is read, it returns a new iterator, and different iterators can be stepped through independently, each giving access to all the elements of the iterable. The iterators of the same iterable should provide the same values in the same order (unless the underlying collection is modified between the iterations, which some collections allow).

You can also iterate over the elements of an `Iterable` using the for-in loop construct, which uses the `iterator` getter behind the scenes. For example, you can iterate over all of the keys of a Map, because `Map` keys are iterable.

    Map kidsBooks = {'Matilda': 'Roald Dahl',
                     'Green Eggs and Ham': 'Dr Seuss',
                     'Where the Wild Things Are': 'Maurice Sendak'};
    for (var book in kidsBooks.keys) {
      print('$book was written by ${kidsBooks[book]}');
    }

The List and Set classes are both `Iterable`, as are most classes in the `dart:collection` library.

Some Iterable collections can be modified. Adding an element to a `List` or `Set` will change which elements it contains, and adding a new key to a `Map` changes the elements of Map.keys. Iterators created after the change will provide the new elements, and may or may not preserve the order of existing elements (for example, a HashSet may completely change its order when a single element is added).

Changing a collection while it is being iterated is generally not allowed. Doing so will break the iteration, which is typically signalled by throwing a ConcurrentModificationError the next time Iterator.moveNext is called. The current value of Iterator.current getter should not be affected by the change in the collection, the `current` value was set by the previous call to Iterator.moveNext.

Some iterables compute their elements dynamically every time they are iterated, like the one returned by Iterable.generate or the iterable returned by a `sync*` generator function. If the computation doesn't depend on other objects that may change, then the generated sequence should be the same one every time it's iterated.

The members of `Iterable`, other than `iterator` itself, work by looking at the elements of the iterable. This can be implemented by running through the iterator, but some classes may have more efficient ways of finding the result (like last or length on a List, or contains on a Set).

The methods that return another `Iterable` (like map and where) are all lazy - they will iterate the original (as necessary) every time the returned iterable is iterated, and not before.

Since an iterable may be iterated more than once, it's not recommended to have detectable side-effects in the iterator. For methods like map and where, the returned iterable will execute the argument function on every iteration, so those functions should also not have side effects.

Implementers
  - DoubleLinkedQueue
  - IterableBase
  - IterableMixin
  - LinkedList
  - Runes

## Constructors

Iterable()
  const
Iterable.empty()
  Creates an empty iterable. [...]
  const
  factory
Iterable.generate(int count, [ E generator(int index) ])
  Creates an `Iterable` which generates its elements dynamically. [...]
  factory

## Properties

first → E
  Returns the first element. [...]
  read-only
isEmpty → bool
  Returns `true` if there are no elements in this collection. [...]
  read-only
isNotEmpty → bool
  Returns true if there is at least one element in this collection. [...]
  read-only
iterator → Iterator<E>
  Returns a new `Iterator` that allows iterating the elements of this `Iterable`. [...]
  read-only
last → E
  Returns the last element. [...]
  read-only
length → int
  Returns the number of elements in this. [...]
  read-only
single → E
  Checks that this iterable has only one element, and returns that element. [...]
  read-only
hashCode → int
  The hash code for this object. [...]
  read-only, inherited
runtimeType → Type
  A representation of the runtime type of the object.
  read-only, inherited

## Methods

any(bool test(E element)) → bool
  Checks whether any element of this iterable satisfies `test`. [...]
cast<R>() → Iterable<R>
  Provides a view of this iterable as an iterable of `R` instances. [...]
contains(Object element) → bool
  Returns true if the collection contains an element equal to `element`. [...]
elementAt(int index) → E
  Returns the `index`th element. [...]
every(bool test(E element)) → bool
  Checks whether every element of this iterable satisfies `test`. [...]
expand<T>(Iterable<T> f(E element)) → Iterable<T>
  Expands each element of this Iterable into zero or more elements. [...]
firstWhere(bool test(E element), { E orElse() }) → E
  Returns the first element that satisfies the given predicate `test`. [...]
fold<T>(T initialValue, T combine(T previousValue, E element)) → T
  Reduces a collection to a single value by iteratively combining each element of the collection with an existing value [...]
followedBy(Iterable<E> other) → Iterable<E>
  Returns the lazy concatentation of this iterable and `other`. [...]
forEach(void f(E element)) → void
  Applies the function `f` to each element of this collection in iteration order.
join([String separator = "" ]) → String
  Converts each element to a String and concatenates the strings. [...]
lastWhere(bool test(E element), { E orElse() }) → E
  Returns the last element that satisfies the given predicate `test`. [...]
map<T>(T f(E e)) → Iterable<T>
  Returns a new lazy Iterable with elements that are created by calling `f` on each element of this `Iterable` in iteration order. [...]
reduce(E combine(E value, E element)) → E
  Reduces a collection to a single value by iteratively combining elements of the collection using the provided function. [...]
singleWhere(bool test(E element), { E orElse() }) → E
  Returns the single element that satisfies `test`. [...]
skip(int count) → Iterable<E>
  Returns an Iterable that provides all but the first `count` elements. [...]
skipWhile(bool test(E value)) → Iterable<E>
  Returns an `Iterable` that skips leading elements while `test` is satisfied. [...]
take(int count) → Iterable<E>
  Returns a lazy iterable of the `count` first elements of this iterable. [...]
takeWhile(bool test(E value)) → Iterable<E>
  Returns a lazy iterable of the leading elements satisfying `test`. [...]
toList({bool growable: true }) → List<E>
  Creates a List containing the elements of this Iterable. [...]
toSet() → Set<E>
  Creates a Set containing the same elements as this iterable. [...]
toString() → String
  Returns a string representation of (some of) the elements of `this`. [...]
  override
where(bool test(E element)) → Iterable<E>
  Returns a new lazy Iterable with all elements that satisfy the predicate `test`. [...]
whereType<T>() → Iterable<T>
  Returns a new lazy Iterable with all elements that have type `T`. [...]
noSuchMethod(Invocation invocation) → dynamic
  Invoked when a non-existent method or property is accessed. [...]
  inherited

## Operators

operator ==(dynamic other) → bool
  The equality operator. [...]
  inherited

## Static Methods

castFrom<S, T>(Iterable<S> source) → Iterable<T>
  Adapts `source` to be an `Iterable<T>`. [...]