Swift委托作为函数参数

Question

I created my protocol as below:

import Foundation
protocol ITcpCLient: class {
    func OnMessageReceived(_ message: String);
}

The class using protocol as below:

import Foundation

class tcpConnection {
    var tcpClientdelegate: ITcpCLient?

    init(client: ITcpCLient) {
        self.tcpClientdelegate? = client

        if self.tcpClientdelegate == nil {
            print("tcpClient Delegate is nil!")
        }
    }

    func trigger() {
        tcpClientdelegate?.OnMessageReceived("From Trigger")
    }

}

My ViewController Class is below:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        print("viewDidLoad!")
        let myTcpConnection = tcpConnection(client: self)
        myTcpConnection.trigger()
    }


}


extension ViewController: ITcpCLient {

    func OnMessageReceived(_ message: String) {
        print("onMessageReceived")
        print(message)
    }

}

The output is: tcpClient Delegate is nil! If I create delegate without question mark, the code working as expected.But when I use optional type, I cant assign viewcontroller class as a delegate.

Answer 1

Since self.tcpClientdelegate is nil then appending ? operator will cause whole statement not to be run => which causes that delegate to be un-assinged = nil

self.tcpClientdelegate? = client

so replace with

self.tcpClientdelegate = client

Answer 2

The problem lies in the init :

self.tcpClientdelegate? = client

The postfix ? operator will not carry out whatever operation you specified if its operand is nil .

Here, self.tcpClientdelegate is nil, so the value is not assigned.

From the Swift Language Reference :

Optional-chaining expressions must appear within a postfix expression, and they cause the postfix expression to be evaluated in a special way. If the value of the optional-chaining expression is nil, all of the other operations in the postfix expression are ignored and the entire postfix expression evaluates to nil.

Just assign it normally to fix the problem, because you don't care about whether self.tcpClientdelegate was nil or not:

self.tcpClientdelegate = client