组合模式允许将对象组合成树形结构来表现”整体/部分”的层次结构。组合能让客户以一致的方式处理个别对象以及对象组合。

组合模式牺牲单一责任设计原则换取透明性(transparency), 让组件的接口同时包含一些管理子节点和叶子节点的操作，使用者就可以将组合和叶子结点一视同仁, 也就是说, 一个元素究竟是组合还是叶子节点，对使用者是透明的.

组合模式中涉及到的角色:

Component: 是组合中的对象声明接口，在适当的情况下，实现所有类共有接口的默认行为
Leaf: 叶子结点对象，没有子结点
Composite: 存储Composite和Leaf两种类型的子部件，实现与子部件有关操作，如增加(add)和删除(remove)等

在以下情形时，考虑使用组合模式:

想获取对象抽象的树形表示
想让客户端同意处理组合结构中的所有对象

UML类图

代码示例


/*
  在公司结构中，一个大部门可以直接包含部分直属员工，也可以包含其他子部门
  子部门中同样可以包含直属员工和更细分的子部门
  这就形成了一个树形结构
 */


/// 员工类和部门类都需要实现EmployeeComponent接口，调用某些非共有操作需要抛出异常
/// 这样如果员工类和部门类如果不支持某个操作，那它们就不需要做任何事情
enum EmployeeError: Error {
    case UnSupportedOperation
}

/// 抽象类，实现了员工类和部门类的所有方法
class EmployeeComponent  {
    /// 组合方法: 新增，删除，获取组件
    func add(component: EmployeeComponent) throws -> Void {
        throw EmployeeError.UnSupportedOperation
    }
    func remove(component: EmployeeComponent) throws -> Void {
        throw EmployeeError.UnSupportedOperation
    }
    func getChild(index: Int) throws -> EmployeeComponent {
        throw EmployeeError.UnSupportedOperation
    }
    
    /// 操作方法
    func getName() throws -> String {
        throw EmployeeError.UnSupportedOperation
    }
    func getNumber() throws -> Int {
        throw EmployeeError.UnSupportedOperation
    }
    func introduce() throws -> Void {
        throw EmployeeError.UnSupportedOperation
    }
}


/// 员工类，包含姓名和工号两个属性
class Employee: EmployeeComponent {
    var name: String
    var number: Int
    init(name: String, number: Int) {
        self.name = name
        self.number = number
    }
    
    /// 覆盖EmployeeComponent的部分方法
    override func getName() throws -> String {
        return name
    }
    override func getNumber() throws -> Int {
        return number
    }
    override func introduce() throws {
        print("Hello, My name is " + name + "My job number is \(number)")
    }
}

/// 部门类
class Department: EmployeeComponent {
    
    /// 部门名称
    var name: String
    
    /// 部门可以有任意数量的子部门和直属员工
    var employees = NSMutableArray()
    
    init(name: String) {
        self.name = name
    }
    
    /// 覆盖EmployeeComponent的部分操作方法
    override func getName() throws -> String {
        return name;
    }
    //使用迭代器模式打印出部门所有子信息
    override func introduce() throws {
        print("This is \(name) department")
        let iterator = employees.objectEnumerator()
        var obj = iterator.nextObject()
        while obj != nil {
            try (obj as! EmployeeComponent).introduce()
            obj = iterator.nextObject()
        }
        
    }
    
    /// 覆盖EmployeeComponent的组件方法
    override func add(component: EmployeeComponent) throws {
        employees.add(component)
    }
    override func remove(component: EmployeeComponent) throws {
        employees.remove(component)
    }
    override func getChild(index: Int) throws -> EmployeeComponent {
        guard index < employees.count && index >= 0 else {
            throw EmployeeError.UnSupportedOperation
        }
        return employees[index] as! EmployeeComponent
    }
    
}

客户端调用:

//公司，CEO
let company = Department(name: "devzhang.cn")
let ceo = Employee(name: "zhang", number: 1)
try company.add(component: ceo)

//市场部
let marketing = Department(name: "Marketing Department")
let marketingEmployee1 = Employee(name: "张三", number: 2)
let marketingEmployee2 = Employee(name: "李四", number: 3)
try? marketing.add(component: marketingEmployee1)
try? marketing.add(component: marketingEmployee2)
try? marketing.introduce()

//研发部
let develop = Department(name: "Develop Department")
let developEmployee1 = Employee(name: "赵六", number: 4)
let developEmployee2 = Employee(name: "孙七", number: 5)
try? develop.add(component: developEmployee1)
try? develop.add(component: developEmployee2)
try? develop.introduce()


/// 异常处理
do {
    try developEmployee1.add(component: developEmployee2)
}catch {
    print(error)
}

输出结果:

This is Marketing Department department
Hello, My name is 张三My job number is 2
Hello, My name is 李四My job number is 3
This is Develop Department department
Hello, My name is 赵六My job number is 4
Hello, My name is 孙七My job number is 5
UnSupportedOperation

做点有意思的事情

组合模式入门

UML类图

代码示例