swift自定义运算符

参考：https://www.jianshu.com/p/4f025476701a

//运算符函数要声明在文件的最外层，不能在类里

//声明•的precedencegroup名：PointMultiplicationPrecedence
precedencegroup PointMultiplicationPrecedence {
    associativity: left//左结合
    higherThan: AdditionPrecedence//优先级：比加法高
    lowerThan: MultiplicationPrecedence//优先级：比乘法低
}

infix operator •: PointMultiplicationPrecedence

/// Point的数量积（Option+8）
///
/// - Parameters:
///   - left: One Point
///   - right: Other Point
/// - Returns: (left.x * right.x +  left.y + right.y)
func •(left: CGPoint, right: CGPoint) -> Double {
    return Double(left.x * right.x +  left.y * right.y)
}