Unable to call another class method using delegate in swift

Question

I'm trying to call method of Class B from class A on the button tap event. But it does not work and below is my code.

// Viewcontroller
class ViewController: UIViewController {
   @IBAction func btnClicked(_ sender: Any) {
        var objA = A()
        objA.delegate?.TestA()
    }
}

// ClassA.swift
protocol TestA {
    func TestA()
}

class A {
    var delegate: TestA?
}


// ClassB.swift
class B : TestA {
    func TestA() {
        print(" Function A from b")
    }
}

When tapping a button, function TestA() does not invoke.

I even tried the code below, but it also didn't work:

var objB = B()
var objA = A()
objA.delegate = objB

You have not assigned anything to the delegate property of objA. Its nil. Thats why nothing is happening. Create object of class B. Assign it to delegate of class A object. — Muhammad Zohaib Ehsan
– Muhammad Zohaib Ehsan, Commented Feb 13, 2019 at 7:47
This answer can help you stackoverflow.com/a/40503024/2370587 — Maximelc
– Maximelc, Commented Feb 13, 2019 at 8:59
On my machine, it works (if you set the delegate to objB). Will btnClicked be called at all? — Andreas Oetjen
– Andreas Oetjen, Commented Feb 13, 2019 at 9:05

Sandeep Bhandari · Accepted Answer · 2019-02-13 08:35:19Z

1

Because you instantiate instance of Class A using

    var objA = A()

Clearly you haven't initialised delegate property in A because its optional its default value is nil

Now when you call

    objA.delegate?.TestA()

delegate is nil hence function TestA will not be called

Suggestion

Always use camelCasing for declaring names of functions. So TestA() is incorrect rather use testA()

EDIT 1:

Tested this

@IBAction func btnClicked(_ sender: Any) {
    let objA = A()
    let objB = B()
    objA.delegate = objB
    objA.delegate?.TestA()
}

This is working fine what is the issue?

edited Feb 13, 2019 at 8:35

answered Feb 13, 2019 at 7:48

Sandeep Bhandari

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2 Comments

Sandeep Bhandari Over a year ago

@manisha-upadhyay: Thank you for accepting the answer, what was the issue ?

Pankaj Kulkarni Over a year ago

The EDIT 1 above will work but does not benefit from defining the protocol TestA. This will work even when class B is not conforming to protocol TestA. See my answer bellow for explanation.

Daniel · Accepted Answer · 2019-02-13 07:48:40Z

1

The objA.delegate is never assigned to an object, so it has an initial value of nil. The ? operator avoids calling a function on a nil object.

answered Feb 13, 2019 at 7:48

Daniel

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Comments

Pankaj Kulkarni · Accepted Answer · 2019-02-14 06:02:19Z

The answer by Sandeep Bhandari is right.

Some information for better understanding of Protocol and Delegates. TestA is a protocol and an optional var delegate is defined in class A. This setup is right. The idea behind this setup is that any user of class A, in this case class B which conforms to protocol TestA gets a callback from class A. You need to call the delegate.testA() function from within class A. The current implementation of ViewController is not at all benefiting from defining Protocol and Delegates.

To achieve proper usage, the class A cab be modified as follows:

protocol TestA {
    func testA()
}

class A {
        var delegate: TestA?
        func process() {
            // Do something and call delegate function to report it.
            delegate?.testA()
        }
    }

And modify ViewController as follows (copied class B for completeness):

class ViewController: UIViewController {
    @IBAction func btnClicked(_ sender: Any) {
        var objA = A()
        var objB = B()
        objA.delegate = objB
        objA.process()
    }
}

// ClassB.swift
class B : TestA {

    func TestA() {
        print(" Function A from b")
    }
}

Now function implemented in class B to conform to protocol TestA will be called when process() function on objA of class A is called. This is better use of Protocol and Delegate. Hope this helps.

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