總結一下：

1.實例的比較：判斷兩個實例值是否相同 Equatable

Equatable協定：必須實作==方法，根據比較雙方的資料型別來選擇所對應的==方法。

struct Games {
    var winCount: Int
    var loseCount: Int
}
 
let g1 = Games(winCount: 2, loseCount: 2)
let g2 = Games(winCount: 3, loseCount: 1)
 
//g1 == g2 错误：自定义类型判断是否相等需要遵守协议Equatable，并实现相应方法
 
extension Games: Equatable {}  //此协议只声明了一个方法，方法的实现可以在外面
 
//==为方法名，因为此协议方法是通过==来调用
func ==(b1: Games, b2: Games) -> Bool {
   
    return b1.winCount == b2.winCount && b1.loseCount == b2.loseCount
}

2.比較兩個實例的大小 Comparable

Comparable協定：繼承自Equatable必須實作Equatable中的==方法，還必須實作

extension Games: Comparable {
 
    //注：对Games数据类型的==方法已经在上面实现，这里可以不用再写。
   
    //协议的实现方法可以写在内部，类型方法在结构体中需要加static
    static func <(b1: Games, b2: Games) -> Bool {
       
        let gScore1 = b1.winCount - b1.loseCount
        let gScore2 = b2.winCount - b2.loseCount
       
        return gScore1 < gScore2
    }
}

3.為什麼標準庫協議沒有在協議中用關鍵字 @objc optional 表明其它的是可選方法，簽署協議後也不用去實現？

//庫中的Comparable 協定。

public protocol Comparable : Equatable {

    /// Returns a Boolean value indicating whether the value of the first
    /// argument is less than that of the second argument.
    ///
    /// This function is the only requirement of the `Comparable` protocol. The
    /// remainder of the relational operator functions are implemented by the
    /// standard library for any type that conforms to `Comparable`.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func <(lhs: Self, rhs: Self) -> Bool

    /// Returns a Boolean value indicating whether the value of the first
    /// argument is less than or equal to that of the second argument.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func <=(lhs: Self, rhs: Self) -> Bool

    /// Returns a Boolean value indicating whether the value of the first
    /// argument is greater than or equal to that of the second argument.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func >=(lhs: Self, rhs: Self) -> Bool

    /// Returns a Boolean value indicating whether the value of the first
    /// argument is greater than that of the second argument.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func >(lhs: Self, rhs: Self) -> Bool
}

Havthgem 的回答: Swift 靜態函式庫對於剩餘的比較計算是有預設實作的，基於你自訂的 == 和

/// Instances of conforming types can be compared using relational
/// operators, which define a [strict total order](http://en.wikipedia.org/wiki/Total_order#Strict_total_order).
///
/// A type conforming to `Comparable` need only supply the `<` and
/// `==` operators; default implementations of `<=`, `>`, `>=`, and
/// `!=` are supplied by the standard library:
///
///     struct Singular : Comparable {}
///     func ==(x: Singular, y: Singular) -> Bool { return true }
///     func <(x: Singular, y: Singular) -> Bool { return false }
///
/// **Axioms**, in addition to those of `Equatable`:
///
/// - `x == y` implies `x <= y`, `x >= y`, `!(x < y)`, and `!(x > y)`
/// - `x < y` implies `x <= y` and `y > x`
/// - `x > y` implies `x >= y` and `y < x`
/// - `x <= y` implies `y >= x`
/// - `x >= y` implies `y <= x`

以上是來自 Comparable 協議的註釋，如果還看不懂的話，就回覆吧。