A type that provides sequential, iterated access to its elements.
- Xcode 8.0+
- Swift Standard Library
A sequence is a list of values that you can step through one at a time. The most common way to iterate over the elements of a sequence is to use a
While seemingly simple, this capability gives you access to a large number of operations that you can perform on any sequence. As an example, to check whether a sequence includes a particular value, you can test each value sequentially until you’ve found a match or reached the end of the sequence. This example checks to see whether a particular insect is in an array.
Sequence protocol provides default implementations for many common operations that depend on sequential access to a sequence’s values. For clearer, more concise code, the example above could use the array’s
contains(_:) method, which every sequence inherits from
Sequence, instead of iterating manually:
Sequence protocol makes no requirement on conforming types regarding whether they will be destructively consumed by iteration. As a consequence, don’t assume that multiple
in loops on a sequence will either resume iteration or restart from the beginning:
In this case, you cannot assume either that a sequence will be consumable and will resume iteration, or that a sequence is a collection and will restart iteration from the first element. A conforming sequence that is not a collection is allowed to produce an arbitrary sequence of elements in the second
To establish that a type you’ve created supports nondestructive iteration, add conformance to the
Conforming to the Sequence Protocol
Making your own custom types conform to
Sequence enables many useful operations, like
in looping and the
contains method, without much effort. To add
Sequence conformance to your own custom type, add a
make method that returns an iterator.
Alternatively, if your type can act as its own iterator, implementing the requirements of the
Iterator protocol and declaring conformance to both
Iterator are sufficient.
Here’s a definition of a
Countdown sequence that serves as its own iterator. The
make method is provided as a default implementation.
A sequence should provide its iterator in O(1). The
Sequence protocol makes no other requirements about element access, so routines that traverse a sequence should be considered O(n) unless documented otherwise.