picking.swift

struct RectangleConfig {
    var width: Int = 0
    var height: Int = 0
}

class ConfigView: NSView {
    // Models
    let config = MutableProperty<RectangleConfig?>(nil)

    var width: SignalProducer<Int?, NoError>!
    
    func awakeFromNib() {

        // Extract config.width to self.width

        // self.width should change when config.width changes, or when
        // config changes

        // self.width is Optional<Int> because config is Optional<RectangleConfig>
        // and so if there is no config, then there is no width

        self.width = config.producer.map { $0?.width }
    }
}