swift-mirror/stdlib/public/core/StringCharacterView.swift

//===--- StringCharacterView.swift - String's Collection of Characters ----===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2015 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
//  String is-not-a SequenceType or CollectionType, but it exposes a
//  collection of characters.
//
//===----------------------------------------------------------------------===//

extension String {
  /// A `String`'s collection of `Character`s ([extended grapheme
  /// clusters](http://www.unicode.org/glossary/#extended_grapheme_cluster))
  /// elements.
  public struct CharacterView {
    internal var _core: _StringCore

    /// Create a view of the `Character`s in `text`
    public init(_ text: String) {
      self._core = s._core
    }
    
    public // @testable
    init(_ _core: _StringCore) {
      self._core = _core
    }
  }

  /// A collection of `Characters` representing the `String`'s
  /// [extended grapheme
  /// clusters](http://www.unicode.org/glossary/#extended_grapheme_cluster)).
  public var characters: CharacterView {
    return CharacterView(self)
  }

  /// Efficiently mutate `self` by applying `body` to its `characters`.
  ///
  /// - warning: Do not rely on anything about `self` (the `String`
  ///   that is the target of this method) during the execution of
  ///   `body`: it may not appear to have its correct value.  Instead,
  ///   use only the `String.CharacterView` argument to `body`.
  public mutating func withMutableCharacters<R>(body: (inout CharacterView)->R) -> R {
    // Naively mutating self.characters forces multiple references to
    // exist at the point of mutation. Instead, temporarily move the
    // core of this string into a CharacterView.
    var tmp = CharacterView("")
    swap(&_core, &tmp._core)
    let r = body(&tmp)
    swap(&_core, &tmp._core)
    return r
  }
}