What is the difference between the designated and convenience initializer?

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Main Source: 🔗 Ace the iOS Interview

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TL/DR

Every class requires a designated initializer as it’s responsible for initializing stored properties and calling the superclass’s init().

A convenience initializer can be thought of as a wrapper around the designated initializer. They allow you to create a simpler initialization option for your class that either provides defaults for certain initialization parameters or helps transform some input into the exact format the designated initializer needs.

In Swift, we can create a convenience initializer by placing the keyword convenience before the init. A class can have any number of convenience initializers. These initializers can even call other convenience initializers in turn, but eventually they’ll need to call the designated initializer:

class Money: NSObject {
    let value: Int
    let currencyCode: String

    init(value: Int, currencyCode: String) {
        self.value = value
        self.currencyCode = currencyCode
        super.init()
    }

    convenience init?(value: String, currencyCode:String) {
        guard let numericValue =Int(value) else {
            return nil
        }

        self.init(value: numericValue, currencyCode:currencyCode)
    }
}

In this example, the designated initializer expects value to be an Int. So, we can create a convenience initializer that helps us handle scenarios when we might have a String as input instead.

Notice that the convenience initializer is eventually calling the designated initializer.