计算具有特定属性值的数组中的项目数

I have a Person() class:

我有一个Person（）类：

class Person : NSObject {

    var firstName : String
    var lastName : String
    var imageFor : UIImage?
    var isManager : Bool?

    init (firstName : String, lastName: String, isManager : Bool) {
        self.firstName = firstName
        self.lastName = lastName
        self.isManager = isManager
    }
}
cla