Scala reflection: How to pass an object's method as parameter to another method - scala

Question

let say I have 2 classes:
class A {
def sayHello(name: String) {
println("Hi " + name)
}
}
class B {
var methodMaps = Map[String, String => Unit]()
def registerMethod(methodName: String, method: String => Unit) {
methodMaps += (methodName -> method)
}
}
Okay, normally, I will call something like:
val b = new B
val a = new A
b.registerMethod("sayHello", a.sayHello)
But now I want to put the information into a config file, for example:
<method class="A" name="sayHello" />
Now, in the code it need to be something like this:
val b = new B
val className = readFromConfig()
val methodName = readFromConfig()
val aInstance = createInstanceFromReflection(className)
b.registerMethod(methodName, ...)
The problem is I don't know how to get the a.sayHello to pass to registerMethod, I can get the MethodMirror for sayHello, but how can I pass it to the registerMethod?
Thanks.

Answer 1

I guess that in your case, you need plain old reflection. From your example, in the expression b.registerMethod("sayHello", a.sayHello), the Scala compiler will lift the a.sayHello into a function.
When you are using dynamic class data from a file, the compiler can't help you; meaning that you need to do the work yourself. Following your own example, we should have:
val className = readFromConfig()
val methodName = readFromConfig()
val clazz = Class.forName(className)
val method = clazz.getMethods.find(x=>x.getName == "methodName" && x.getParameterTypes().length==1)
val aInstance = clazz.newInstance()
def invoke1[T,U](obj:Any, method:Method)(param:T):U = method.invoke(obj,Seq(param.asInstanceOf[java.lang.Object]):_*).asInstanceOf[U]
Now you should be able to register such construct in your map:
registerMethod(methodName, invoke1(aInstance,method) _ )
(*) This is not tested, but should be in the right direction, we use a similar construct in some part of our system.