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What are convenience initializers?

Written by 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

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