Convenience Init

Init

init is a keyword in Swift. It it shorthand for “initialization.”

It is used to designate a special type of instance method in classes, structs, and enums.

This special instance method has one goal: to make the instance ready to use.

We can say the init method “initializes an instance of a class/struct/enum”.

In its most basic form, an init method looks like this:

func init() { ... }

Within the init method, you setup everything you need to setup before this instance is ready to be used.

In this example, we have a class with a stored property. Inside the init we need to set the value for the stored property, so an instance of this class is “ready to use.”

class CraftBeer {
	let name: String

	init() {
		name = "Super Hoppy IPA"
	}
}

Designated Init

The main initializer for a class is called a designated initializer. This designated initializer does all of the setup necessary to make an instance of this class ready to be used.

Classes usually have only one of these, but they can have more.

Every class must have at least one designated initializer.

In the example CraftBeer, the init we have already seen is considered the designated initializer:

class CraftBeer {
	let name: String

	// Designated initializer
	init() {
		name = "Super Hoppy IPA"
	}
}

Convenience Init

convenience is a modifier in Swift that we can add before our init method.

convenience init() { ... }

A convenience init is not the main init for a class.

Its goal is to do some custom setup, and then call the designated initto finish the job.

init() { ... }

convenience init() {
	// Perform some custom setup

	// Call the designated init
	self.init()
} 

The goal of a convenience init is to provide a more “convenient” way to initialize a class. This is useful when the designated init is large and complex.

Imagine in our app that uses the class CraftBeer, a user can add a new beer to a list of beers. Each beer has a name, brewery, beer style, and alcohol percentage:

class CraftBeer {
	let name: String
	let brewery: String
	let style: String
	let alcoholPercentage: Double?

	// Designated init
	init(name: String, brewery: String, style: String, alcoholPercentage: Double?) {
		self.name = name
		self.brewery = brewery
		self.style = style
		self.alcoholPercentage = alcoholPercentage
	}
}

Sometimes the user knows all of the information about the CraftBeer and sometimes, the user knows only the name of the beer. We want the user to be able to save this new beer even if s/he only knows the name.

To do this, we create a convenience init with only one argument: name. Then we set default values for the rest of the properties, and call the designated init to finish the initialization:

class CraftBeer {
	let name: String
	let brewery: String
	let style: String
	let alcoholPercentage: Double?

	// Designated init
	init(name: String, brewery: String, style: String, alcoholPercentage: Double?) {
		self.name = name
		self.brewery = brewery
		self.style = style
		self.alcoholPercentage = alcoholPercentage
	}

	convenience init(name: String) {
		self.init(name: name, brewery: "Unknown", style: "Unknown", alcoholPercentage: nil))
	}
}

Later, the user can go back and add the rest of the information about this beer.

Read more

Official Swift Documentation