Swift: Making a Protocol

Sunday, March 22, 2015

Overview

In the MVC design pattern, the model does not have access to the controller. But it has to let the controller know somehow that its data has changed. To do that it uses a protocol. A protocol is basically an interface in Java. Here’s how you would use it in swift.

Content

Protocol

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// can only be implemented by a class
protocol FaceViewDataSource: class {
    func smilinessForFaceView(sender: FaceView) -> Double
}

View

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class FaceView: UIView {
    ...

    // has to be weak... if this points to the controller
    // and the controller points to this model, they're keeping each
    // other in memory.
    weak var dataSource: FaceViewDataSource?
    override func drawRect(rect: CGRect){
        let smiliness = dataSource?.smilinessForFaceView(self) ?? 0.0
    }
    ...
}

Controller/Model

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class HappinessController: UIViewController, FaceViewDataSource {
    // outlet to the UIView inside the controller
    @IBOutlet weak var faceView: FaceView! {
        didSet {
            faceView.dataSource = self
        }
    }

    var happiness: Int = 75 { // Model
        didSet {
            // 0 <= happiness <= 100
            happiness = min(max(happiness, 0), 100)
            // says that the view needs to be redrawn
            faceView.setNeedsDisplay()
        }
    }
    func smilinessForFaceView(sender: FaceView) -> Double? {
        return Double(75)
    }
}

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