idiomatic property changed notification in scala? - scala

Question

I'm trying to find a cleaner alternative (that is idiomatic to Scala) to the kind of thing you see with data-binding in WPF/silverlight data-binding - that is, implementing INotifyPropertyChanged. First, some background:
In .Net WPF or silverlight applications, you have the concept of two-way data-binding (that is, binding the value of some element of the UI to a .net property of the DataContext in such a way that changes to the UI element affect the property, and vise versa. One way to enable this is to implement the INotifyPropertyChanged interface in your DataContext. Unfortunately, this introduces a lot of boilerplate code for any property you add to the "ModelView" type. Here is how it might look in Scala:
trait IDrawable extends INotifyPropertyChanged
{
protected var drawOrder : Int = 0
def DrawOrder : Int = drawOrder
def DrawOrder_=(value : Int) {
if(drawOrder != value) {
drawOrder = value
OnPropertyChanged("DrawOrder")
}
}
protected var visible : Boolean = true
def Visible : Boolean = visible
def Visible_=(value: Boolean) = {
if(visible != value) {
visible = value
OnPropertyChanged("Visible")
}
}
def Mutate() : Unit = {
if(Visible) {
DrawOrder += 1 // Should trigger the PropertyChanged "Event" of INotifyPropertyChanged trait
}
}
}
For the sake of space, let's assume the INotifyPropertyChanged type is a trait that manages a list of callbacks of type (AnyRef, String) => Unit, and that OnPropertyChanged is a method that invokes all those callbacks, passing "this" as the AnyRef, and the passed-in String). This would just be an event in C#.
You can immediately see the problem: that's a ton of boilerplate code for just two properties. I've always wanted to write something like this instead:
trait IDrawable
{
val Visible = new ObservableProperty[Boolean]('Visible, true)
val DrawOrder = new ObservableProperty[Int]('DrawOrder, 0)
def Mutate() : Unit = {
if(Visible) {
DrawOrder += 1 // Should trigger the PropertyChanged "Event" of ObservableProperty class
}
}
}
I know that I can easily write it like this, if ObservableProperty[T] has Value/Value_= methods (this is the method I'm using now):
trait IDrawable {
// on a side note, is there some way to get a Symbol representing the Visible field
// on the following line, instead of hard-coding it in the ObservableProperty
// constructor?
val Visible = new ObservableProperty[Boolean]('Visible, true)
val DrawOrder = new ObservableProperty[Int]('DrawOrder, 0)
def Mutate() : Unit = {
if(Visible.Value) {
DrawOrder.Value += 1
}
}
}
// given this implementation of ObservableProperty[T] in my library
// note: IEvent, Event, and EventArgs are classes in my library for
// handling lists of callbacks - they work similarly to events in C#
class PropertyChangedEventArgs(val PropertyName: Symbol) extends EventArgs("")
class ObservableProperty[T](val PropertyName: Symbol, private var value: T) {
protected val propertyChanged = new Event[PropertyChangedEventArgs]
def PropertyChanged: IEvent[PropertyChangedEventArgs] = propertyChanged
def Value = value;
def Value_=(value: T) {
if(this.value != value) {
this.value = value
propertyChanged(this, new PropertyChangedEventArgs(PropertyName))
}
}
}
But is there any way to implement the first version using implicits or some other feature/idiom of Scala to make ObservableProperty instances function as if they were regular "properties" in scala, without needing to call the Value methods? The only other thing I can think of is something like this, which is more verbose than either of the above two versions, but is still less verbose than the original:
trait IDrawable {
private val visible = new ObservableProperty[Boolean]('Visible, false)
def Visible = visible.Value
def Visible_=(value: Boolean): Unit = { visible.Value = value }
private val drawOrder = new ObservableProperty[Int]('DrawOrder, 0)
def DrawOrder = drawOrder.Value
def DrawOrder_=(value: Int): Unit = { drawOrder.Value = value }
def Mutate() : Unit = {
if(Visible) {
DrawOrder += 1
}
}
}

Answer 1

I couldn't claim that this is a canonical property change framework in Scala, but I've used a class like this before:
abstract class Notifier[T,U](t0: T) {
import java.util.concurrent.atomic.AtomicReference
import scala.actors.OutputChannel
type OCUT = OutputChannel[(U,AtomicReference[T])]
val data = new AtomicReference[T](t0)
def id: U
protected var callbacks = Nil:List[T => Unit]
protected var listeners = Nil:List[OCUT]
def apply() = data.get
def update(t: T) {
val told = data.getAndSet(t)
if (t != told) {
callbacks.foreach(_(t))
listeners.foreach(_ ! (id,data))
}
}
def attend(f: T=>Unit) { callbacks ::= f }
def attend(oc: OCUT) { listeners ::= oc }
def ignore(f: T=>Unit) { callbacks = callbacks.filter(_ != f) }
def ignore(oc: OCUT) { listeners = listeners.filter(_ != oc) }
}
The motivation for creating this class was that I wanted a flexible thread-safe way to react to changes, which this provides (as it delivers both callbacks and can push messages to actors).
It seems to me--unless I'm misunderstanding exactly what you want because I haven't had occasion to learn the WPF/Silverlight stuff--that this can implement everything you want and more.
For example,
class IDrawable extends SomethingWithOnPropertyChanged {
val drawOrder = new Notifier[Int,Symbol](0) { def id = 'DrawOrder }
val visible = new Notifier[Boolean,Symbol](false) { def id = 'Visible }
drawOrder.attend((i:Int) => OnPropertyChanged(drawOrder.id))
def mutate {
if (visible()) drawOrder() += 1
}
}
should be roughly equivalent to what you want. (Again, I'm not sure how flexible you want this to be; you could create a set of symbol -> notifier mappings that you would look up with an apply method so the target would have an easier time of doing something when it gets the DrawOrder symbol.)
The only significant difference from your usage is that the Notifier uses its apply/update methods to save boilerplate; you don't have to write def x and def x_= methods every time, but you do have to use () for access.