Wednesday, July 15, 2009

Functional Paradigms on Android using Scala

I am slowly plugging away on my little side project. One of the fun things about "pimping" an API, is you can tweak the design of the API. For example, I was modifying TextView, a class used for creating text input boxes of various sorts in Android. One common use case in Android is to create an auto-complete text box. To do this, you need to subclass TextView. Then you can override various lifecycle methods like onEditorAction, onKeyUp, etc. However, I thought that it might be nice to do something like this:

val textView = new TextView
textView.keyUp = (code:Int, event:KeyEvent) => {
// do stuff to your TextView

As Debasish pointed out to me, this is a lot like you would do in JavaScript. Is it better? It's a little easier in certain cases. If you just need to change the behavior of a single TextView in a very specific way, then this would seem like the way to go. Subclassing TextView just to override a single method and then using that subclass exactly once definitely seems like overkill. Now if you really wanted to create a new kind of TextView that you could use in multiple places, then subclassing makes sense.

One thing worth mentioning is how this kind of thing is handled in Cocoa on the iPhone. The Cocoa way would be to create an untyped delegate. In the delegate you could override the lifecycle methods that you were interested in. It's untyped, so you have to know the right names. You still wind up defining a new class, even though you usually only want to customize the behavior of one particular instance. Also, it's a little clunky as the TextView instance would have to be passed in to each of the delegate methods, so that your delegate could actually do something to the TextView. I find this a little better than having to subclass as you do in Android, but once again I think using Scala can yield some nice results.

There was another interesting case that popped up while sugaring TextView. I wanted to sugar the setCompoundDrawablesWithIntrinsicBounds methods. This method is overloaded, taking either four integers or four Drawables. Here is what I first thought of doing:

def compoundDrawablesWithIntrinsicBounds_=(bounds:Tuple4[Int,Int,Int,Int]) {...}
def compoundDrawablesWithIntrinsicBounds_=(bounds:Tuple4[Drawable,Drawable,Drawable,Drawable]) {...}
// this allows
myTextView.compoundDrawablesWithIntrinsicBounds = (1,2,3,4)
// or
myTextView.compoundDrawablesWithIntrinsicBounds = (d1,d2,d3,d4)

Of course this won't work. The type parameters of Tuple4 are going to be erased by the compiler, and the two methods are indistinguishable. However, The Amazing Mr. Spiewak gave me a solution:

def compoundDrawablesWithIntrinsicBounds_=[T <% Either[Int, Drawable]](bounds:Tuple4[T,T,T,T]){
bounds._1 match {
case Left => baseTextView.setCompoundDrawablesWithIntrinsicBounds(bounds._1.left.get, bounds._2.left.get,
bounds._3.left.get, bounds._4.left.get)
case Right => baseTextView.setCompoundDrawablesWithIntrinsicBounds(bounds._1.right.get, bounds._2.right.get,
bounds._3.right.get, bounds._4.right.get)

In addition to this, you need implicits to convert an Int to a Left and a Drawable to a Right. The API looks a lot different, but it allows exactly the same usage that I wanted. Another case of how you need some non-trivial Scala (though I wouldn't be surprised if there is a more elegant way than the above) to create an API that is super simple to use.

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