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This is getting off-topic, but it's something I actually know a tiny bit
about.
The compiler is complaining (I think) because you're not actually
overriding buttonPressed. This can happen when type inference goes a
little astray, especially with methods that return Unit. Naming
buttonPressed's param 'b' shouldn't be a problem. I'd try explicitly
specifying the return type, like
def pressedDo (aB: PushButton, action: Unit) = {
aB.getButtonPressListeners.add(new ButtonPressListener {
override def buttonPressed(b: PushButton): Unit = action
})
}
There are also inference rule that take effect regarding the = in
def foo(...) = { }
vs def foo(...) { }
The latter (I believe) is assumed to return Unit, but I always add the =
just to be sure. (/Programming Scala/ by Odersky, Venners, and Spoon
explains things like this very, very well.)
Additionally, you probably want something like this:
def pressedDo (aB: PushButton, action: => Unit) = {
aB.getButtonPressListeners.add(new ButtonPressListener {
override def buttonPressed(b: PushButton) = action
})
}
Note the new type of the 'action' param. It's now "a function that
takes no params and returns Unit", instead of an actual value of type
Unit. This is an easy way to have lazily-evaluated parameters.
Due to some neat syntactic sugar for writing anonymous functions of this
type, if you split the parameter list for pressedDo, like
def pressedDo (aB: PushButton)(action: => Unit) = {
aB.getButtonPressListeners.add(new ButtonPressListener {
override def buttonPressed(b: PushButton) = action
})
}
You could call it like:
pressedDo(someButton) {
val foo = new Foo
someOtherThing.doStuff(foo)
//etc
}
Here the stuff in the braces becomes an anonymous function that gets
passed to pressedDo.
It's also possible to use implicit conversions (the perhaps
tastelessly-named "Pimp-My-Library" pattern[1]) to "add" the pressedDo
method to the Button class. Well, kind of, not really to the Button
class, but combined with the above technique, you could make a call like
val someButton: Button = ...
someButton pressed {
val foo = new Foo
someOtherThing.doStuff(foo)
}
which is starting to look smooth. Implicits can get pretty black-magic
though, so use them with care and restraint if you do.
[1]: http://scala.sygneca.com/patterns/pimp-my-library
Bojan Vučinić wrote:
> Hi All,
>
> If you are using Scala for your code then you are obliged to write listeners
> in the following way:
>
>
> printButton.getButtonPressListeners.add(new ButtonPressListener {
> override def buttonPressed(b: Button) {
> print
> }
> })
>
>
> I've tried to simplify this by defining the following method:
>
> def pressedDo (aB: PushButton, action: Unit) = {
> aB.getButtonPressListeners.add(new ButtonPressListener {
> override def buttonPressed(b: PushButton) { action }})
> }
>
>
> and than invoking the method in this way (example)
>
> pressedDo(printButton, print)
>
>
> however, this results in a compiler error:
>
>
> error: object creation impossible, since method buttonPressed in trait
> ButtonPressListener of type (x$1: org.apache.pivot.wtk.Button)Unit is not
> defined
> aB.getButtonPressListeners.add(new ButtonPressListener {
>
>
> I'm not a programming language specialist, and in addition a Scala novice,
> but I'm puzzled why putting boilerplate code in a separate method is not
> working??
>
> Best regards,
> Bojan
>
>
> Dr. Bojan Vučinić
> President, Ma-CAD<http://www.ma-cad.com/>
> [email protected]<mailto:[email protected]>
> IM: bvucinic (Skype)
> http://fr.linkedin.com/in/bvucinic
>
> Maintenance Concept Adjustment & Design 31, rue Chanzy
> Paris, Ile de France 75011 France
> Work: +33 (6) 14 15 36 70
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>
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