Swift can't call protocol method via delegate

问题:

I have two classes. One class is named ViewController and the other class is named TabView.

My goal is to call a function changeTab() which is inside the TabView class from the ViewController.

Somehow I am having trouble with it because everytime my delegate is nil.

Here is my code for ViewController:

protocol TabViewProtocol: class {
    func changeTab() 
}

class ViewController: NSViewController {
    // delegate
    weak var delegateCustom : TabViewProtocol?

    override func viewDidLoad() {
        print(delegateCustom) // outputs "nil"
    }

    buttonClickFunction() {
        print(delegateCustom) // outputs "nil"
        delegateCustom?.changeTab() // doesn't work
    }
}

Here is my code for TabView:

class TabView: NSTabViewController, TabViewProtocol {

    let myVC = ViewController()

    override func viewDidLoad() {
        super.viewDidLoad()
        myVC.delegateCustom = self
    }

    func changeTab() {
        print("test succeed")
    }
}

Can someone explain me what I am doing wrong? - I am new to delegates and protocols...

回答1:

You are using the delegate pattern wrongly. It is hard to tell which controller you want to define the protocol for and which one you want to adopt it - but here is one possible way.

// 1. Define your protocol in the same class file as delegate property.
protocol TabViewProtocol: class {
    func changeTab() 
}

// 2. Define your delegate property
class ViewController: NSViewController {
    // delegate
    weak var delegateCustom : TabViewProtocol?

    override func viewDidLoad() {
        // It should be nil as you have not set the delegate yet.
        print(delegateCustom) // outputs "nil"
    }

    func buttonClickFunction() {
        print(delegateCustom) // outputs "nil"
        delegateCustom?.changeTab() // doesn't work
    }
}

// 3. In the class that will use the protocol add it to the class definition statement

class TabView: NSTabViewController, TabViewProtocol {

    let myVC = ViewController()

    override func viewDidLoad() {
        super.viewDidLoad()
        myVC.delegateCustom = self

        // Should output a value now
        print(myVC.delegateCustom) // outputs "self"
    }

    func changeTab() {
        print("test succeed")
    }
}

回答2:

you are creating a new instance in this line:

let myVC = ViewController()

you should get existing instance of your ViewController.then set

myVC.delegateCustom = self