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What are designated initializers?

Swift version: 5.2

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 create 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
    }
}

That initializer is your designated initializer: one that will set up all properties fully in a default way. You could add convenience initializers on top to perform certain tasks – creating squares or triangles would make sense in this scenario – but those must always end by calling a designated initializer.

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

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