責任鏈

在物件導向的設計中，責任鏈模式是一種由 command 物件和一系列 processing 物件組成的設計模式。每個 processing 物件都包含定義它可以處理的命令物件型別的邏輯; 其餘的傳遞給鏈中的下一個 processing 物件。還有一種機制可以將新的 processing 物件新增到該鏈的末尾。維基百科

建立構成責任鏈的類。

首先，我們為所有 processing 物件建立一個介面。

protocol PurchasePower {  
var allowable : Float { get }
  var role : String { get }
  var successor : PurchasePower? { get set }
}

extension PurchasePower {
  func process(request : PurchaseRequest){
    if request.amount < self.allowable {
      print(self.role + " will approve $ \(request.amount) for \(request.purpose)")
    } else if successor != nil {
      successor?.process(request: request)
    }
  }
}

然後我們建立 command 物件。

struct PurchaseRequest {
  var amount : Float
  var purpose : String
}

最後，建立構成責任鏈的物件。

class ManagerPower : PurchasePower {
  var allowable: Float = 20
  var role : String = "Manager"
  var successor: PurchasePower?
}

class DirectorPower : PurchasePower {
  var allowable: Float = 100
  var role = "Director"
  var successor: PurchasePower?
}

class PresidentPower : PurchasePower {
  var allowable: Float = 5000
  var role = "President"
  var successor: PurchasePower?
}

啟動並將它連結在一起：

let manager = ManagerPower()
let director = DirectorPower()
let president = PresidentPower()

manager.successor = director
director.successor = president

這裡連結物件的機制是屬性訪問

建立執行它的請求：

manager.process(request: PurchaseRequest(amount: 2, purpose: "buying a pen"))  // Manager will approve $ 2.0 for buying a pen
manager.process(request: PurchaseRequest(amount: 90, purpose: "buying a printer")) // Director will approve $ 90.0 for buying a printer

manager.process(request: PurchaseRequest(amount: 2000, purpose: "invest in stock")) // President will approve $ 2000.0 for invest in stock