协议

协议-Sequence

public protocol Sequence {
  /// A type representing the sequence's elements.
  associatedtype Element

  /// A type that provides the sequence's iteration interface and
  /// encapsulates its iteration state.
  associatedtype Iterator: IteratorProtocol where Iterator.Element == Element

  /// A type that represents a subsequence of some of the sequence's elements.
  // associatedtype SubSequence: Sequence = AnySequence<Element>
  //   where Element == SubSequence.Element,
  //         SubSequence.SubSequence == SubSequence
  // typealias SubSequence = AnySequence<Element>

  /// Returns an iterator over the elements of this sequence.
  __consuming func makeIterator() -> Iterator
  // 省略...
}

协议-Collection

通过下面源码可以知道，Collection继承了Sequence协议

public protocol Collection: Sequence {
  // FIXME: ideally this would be in MigrationSupport.swift, but it needs
  // to be on the protocol instead of as an extension
  @available(*, deprecated/*, obsoleted: 5.0*/, message: "all index distances are now of type Int")
  typealias IndexDistance = Int  

  // FIXME: Associated type inference requires this.
  override associatedtype Element

  // 省略...
}

协议-MutableCollection

public protocol MutableCollection: Collection
where SubSequence: MutableCollection
{
  // FIXME: Associated type inference requires these.
  override associatedtype Element
  override associatedtype Index
  override associatedtype SubSequence

  /// Accesses the element at the specified position.
  ///
  /// For example, you can replace an element of an array by using its
  /// subscript.
  ///
  ///     var streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
  ///     streets[1] = "Butler"
  ///     print(streets[1])
  ///     // Prints "Butler"
  ///
  /// You can subscript a collection with any valid index other than the
  /// collection's end index. The end index refers to the position one
  /// past the last element of a collection, so it doesn't correspond with an
  /// element.
  ///
  /// - Parameter position: The position of the element to access. `position`
  ///   must be a valid index of the collection that is not equal to the
  ///   `endIndex` property.
  ///
  /// - Complexity: O(1)
  @_borrowed
  override subscript(position: Index) -> Element { get set }

Dictionary遵守Sequence

Dictionary结构体遵守了Sequence协议

extension Dictionary: Sequence {
  /// Returns an iterator over the dictionary's key-value pairs.
  ///
  /// Iterating over a dictionary yields the key-value pairs as two-element
  /// tuples. You can decompose the tuple in a `for`-`in` loop, which calls
  /// `makeIterator()` behind the scenes, or when calling the iterator's
  /// `next()` method directly.
  ///
  ///     let hues = ["Heliotrope": 296, "Coral": 16, "Aquamarine": 156]
  ///     for (name, hueValue) in hues {
  ///         print("The hue of \(name) is \(hueValue).")
  ///     }
  ///     // Prints "The hue of Heliotrope is 296."
  ///     // Prints "The hue of Coral is 16."
  ///     // Prints "The hue of Aquamarine is 156."
  ///
  /// - Returns: An iterator over the dictionary with elements of type
  ///   `(key: Key, value: Value)`.
  @inlinable
  @inline(__always)
  public __consuming func makeIterator() -> Iterator {
    return _variant.makeIterator()
  }
}

Array遵守MutableCollection

Array结构体遵守的协议RandomAccessCollection、MutableCollection最终都是Collection。

extension Array: RandomAccessCollection, MutableCollection {
  /// The index type for arrays, `Int`.
  public typealias Index = Int

  /// The type that represents the indices that are valid for subscripting an
  /// array, in ascending order.
  public typealias Indices = Range<Int>

  /// The type that allows iteration over an array's elements.
  public typealias Iterator = IndexingIterator<Array>

  /// The position of the first element in a nonempty array.
  ///
  /// For an instance of `Array`, `startIndex` is always zero. If the array
  /// is empty, `startIndex` is equal to `endIndex`.
  @inlinable
  public var startIndex: Int {
    return 0
  }