Dynamic attributes (getters/setters) in Swift on data models

Question

I'm trying to use Swift (which I'm very new at) to create a clean interface for my data models. These models are backed by Firebase (though this is ultimately irrelevant to the actual question). What I want to avoid is writing tons of setter/accessor boiler plate code over and over.

For example:

class MyData {
  let connection: Firebase!

  private var _name: String? // internal storage, like an iVar in ObjC

  init(connection: Firebase!) {
    self.connection = connection
    self.connection.observeEventType(.Value, withBlock: { snapshot in
      _name = snapshot["name"]
    }
  }

  var name: {
    get {
      return _name
    }
    set(name) {
      // When the client sets the name, write it to Firebase
      _name = name
      self.connection.childByAppendingPath("name").setValue(name)
    }
  }
}

I'm sure I'm making a lot of mistakes in there. The idea is that the data is first loaded from the server when the instance is instantiated. Subsequently, we could call my_data_instance.name to get that name, or my_data_instance.name = "foo" and the name would be automatically written to the server.

This requires ~10 lines of code for a single attribute (of which there will be many). Nuts! There must be a better way!

---

EDIT: to be clear, I want to obviate the need to write as MUCH boiler plate code as possible. Consider a library like Mantle , where merely defining a @property is sufficient to do everything you want. In my opinion, anything more than one single line of code to say I have an attribute called "name", handle it via Firebase is overly verbose.

Answer 1

You can use Key-Value Observing to monitor changes in your properties. More info in Adopting Cocoa Design Patterns in Swift .

import Foundation

private var KVOContext = 0

// Your class must inherit from NSObject
class MyData : NSObject {
    private let propertiesToObserve = ["name", "location"]

    dynamic var name: String
    dynamic var location: String

    init(name: String, location: String) {
        self.name = name
        self.location = location
        super.init()

        // Add the properties that you want to observe
        for property in self.propertiesToObserve {
            self.addObserver(self, forKeyPath: property, options: [.New, .Old], context: &KVOContext)
        }
    }

    // This method is called whenever an observed property is changed
    override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
        if let property = keyPath,
            newValue = change![NSKeyValueChangeNewKey],
            oldValue = change![NSKeyValueChangeOldKey] {
                print("\(property) changed from \(oldValue) to \(newValue)")
                // If oldValue != newValue, write back to Firebase
        }
    }

    // Remove self as observer of self
    deinit {
        for property in self.propertiesToObserve {
            self.removeObserver(self, forKeyPath: property)
        }
    }
}

let data = MyData(name: "John", location: "Chicago")
data.name = "David"         // print: name changed from John to David
data.location = "New York"  // print: location changed from Chicago to New York

Answer 2

Swift provides that functionality called property observer

var name: String {
   didSet {
     self.connection.childByAppendingPath("name").setValue(name)
   }
}

There is a second observer willSet which is called before the value is changed.

Note (from the documentation):

When you assign a default value to a stored property, or set its initial value within an initializer, the value of that property is set directly, without calling any property observers.