NEW! Master Swift design patterns with my latest book! >>

What are convenience initializers?

Paul Hudson       @twostraws

Designated initializers are the default way of creating new instances of a type. There are others, known as convenience initializers, that are there to help you accomplish common tasks more easily, but those are in addition to your designated initializers rather than a replacement.

For example, you might have a Polygon class that stores sets of points to be drawn later on, like this:

class Polygon {
    var points: [CGPoint]

    init(points: [CGPoint]) {
        self.points = points
    }
}

Now, if that were just a struct you could go ahead and add other initializers. But as it’s a class – where the rules for initialization are quite complex – you could add a convenience initializer that sets up squares of a specific length, like this:

convenience init(squareWithLength length: CGFloat) {
    let points = [
        CGPoint(x: 0, y: 0),
        CGPoint(x: length, y: 0),
        CGPoint(x: length, y: length),
        CGPoint(x: 0, y: length),
    ]

    self.init(points: points)
}

Note how the convenience initializer ends by calling the designated initializer – this is a requirement, and means that your convenience initializers are only responsible for setting up the parts that are unique to them rather than doing everything.

Available from iOS 8.0 – learn more in my book Swift Design Patterns

Did this solution work for you? Please pass it on!

Other people are reading…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Swift on the server is here

Get ahead of the game and learn server-side Swift with my latest book – build real-world projects while you learn!

MASTER SWIFT NOW
Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Practical iOS 11 Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let me know!

Click here to visit the Hacking with Swift store >>