CODEWITHSUNDEEP
iosswiftdelegates
Ian Holtham
Ian Holtham

Reputation: 41

iOS Swift Delegate Syntax

I'm new to iOS and Swift. I'm having a problem understanding the syntax used in Protocol methods used in Delegates. As an example, the following two methods used in the UIPickerView:

func numberOfComponentsInPickerView(pickerView: UIPickerView) -> Int {
    return 1
}

func pickerView(pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
    return count
}

The first method is fine but the syntax of the second method confuses me. The format of the second parameter is baffling, as far as I understand it this should be an Int called "component", so why does the action name "numberOfRowsInComponent" precede it?

Also why are the delegate methods all called "pickerView", are they all just overloads?

Any guidance would be appreciated.

Upvotes: 3

Views: 252

Answers (2)

Kirsteins
Kirsteins

Reputation: 27335

numberOfRowsInComponent component first name is external name, but second one is internal. numberOfRowsInComponent is used when you call the method, but component is used in you method implementation.

Also why are the delegate methods all called "pickerView", are they all just overloads?

They are not exactly overloads as they have different signature names. For example

pickerView:numberOfRowsInComponent:
pickerView:widthForComponent:
// etc

It is method overload only if method signatures match, but have different parameter count or types.

Upvotes: 1

Azat
Azat

Reputation: 6795

The full method name is pickerView:numberOfRowsInComponent but the component is the parameter name that will actually contain passed value

Read about External Parameter Names

Sometimes it’s useful to name each parameter when you call a function, to indicate the purpose of each argument you pass to the function.

If you want users of your function to provide parameter names when they call your function, define an external parameter name for each parameter, in addition to the local parameter name. You write an external parameter name before the local parameter name it supports, separated by a space:

func someFunction(externalParameterName localParameterName: Int) {
    // function body goes here, and can use localParameterName
    // to refer to the argument value for that parameter
}

Upvotes: 2

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