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Behaviours

A behaviour is what an instance can do: a name, arguments, an optional return type and a body. Behaviours are declared in a model or in a concept. The examples use a model Talker and, for creation and events, the Library model of the previous pages.

Declaring a behaviour​

public String greet() {
	return "hello";
}

public String greet(String who) {
	return "hello " + parameters.who;
}

public remember(String text) {
	last = parameters.text;
}

A behaviour may start with a visibility, then a return type, then its name and its arguments. remember has no return type: it acts on the instance and gives nothing back. A script calls a behaviour on an instance:

assert talker.greet() == "hello";
assert talker.greet("Ann") == "hello Ann";
talker.remember("x");
assert talker.last == "x";

Two behaviours may have the same name when their arguments differ, as the two greet do.

Parameters​

A behaviour reads its arguments through parameters: parameters.who. There can be several, of any type:

public int add(int a, int b) {
	return parameters.a + parameters.b;
}
assert talker.add(2, 3) == 5;

A bare parameter name, who instead of parameters.who, is not an error: it gives null.

Conditions and recursion​

The body may hold conditions and return statements, and a behaviour may call itself:

public int fact(int n) {
	if (parameters.n <= 1) {
		return 1;
	}
	return parameters.n * this.fact(parameters.n - 1);
}

public String pick(int n) {
	if (parameters.n > 0) {
		return "positive";
	}
	else {
		return "not positive";
	}
}
assert talker.pick(2) == "positive";
assert talker.pick(0) == "not positive";
assert talker.fact(5) == 120;

Calling a behaviour from a behaviour​

A behaviour of the same instance is called on this. A bare call does not resolve, and gives null without an error:

public String twin() {
	return "twin";
}

public String bareCall() {
	return twin();
}

public String thisCall() {
	return this.twin();
}
assert pen.thisCall() == "twin";
assert pen.bareCall() == null;

This is the same rule as for parameters, and it also applies inside a model: a behaviour of the model calls another one of the model with this.chorus(), not chorus().

Creating instances​

A create behaviour, a creation scheme, runs when an instance is created with new. Inside a behaviour of a model, new Shelf(…) creates an instance of the concept Shelf in the model instance; inside a behaviour of a concept, the new instance is contained in the current one.

A concept may have several creation schemes. At most one is anonymous; the others are named, with create::name, and are chosen by the name in the new:

create(required String label, int capacity=10) {
	label = parameters.label;
	capacity = parameters.capacity;
}

create::withDefaultCapacity(String label) {
	label = parameters.label;
	capacity = 10;
}
public Shelf newShelf(String label) {
	Shelf shelf = new Shelf(parameters.label, 20);
	fire new ShelfCreated(parameters.label);
	return shelf;
}

public Shelf newSmallShelf(String label) {
	return new Shelf::withDefaultCapacity(parameters.label);
}

A parameter may declare a default value, as capacity=10 does, but a call must still give every argument: new Shelf(parameters.label) does not resolve. When a shorter call is wanted, declare a named creation scheme, as withDefaultCapacity does.

A creation scheme reaches the one of its parent concept with super(…), or super.name(…) for a named one (see Declaring models and concepts).

A model is instantiated from a script with new, and given its name with with:

talker = new Talker() with (name="talker");

Deleting instances​

The delete instruction deletes an instance. It runs the delete behaviour of its concept, if it has one:

public removeNote(Note note) {
	delete parameters.note;
}

public concept Note {

	String text;

	create(String text) {
		text = parameters.text;
	}

	delete() {
		container.last = "note deleted";
	}
}
note = talker.addNote("milk");
talker.removeNote(note);
assert talker.last == "note deleted";

Deleting an instance does not remove it from the properties that hold it: take it out of them first, as Shelf.removeBook does with books.remove(parameters.book).

Reacting to events​

A behaviour can be triggered by an event instead of being called. An event is declared with event, has properties and a creation scheme like a concept, and is raised with fire. A listen behaviour runs when an event of that type is fired by the source it names:

public Shelf newShelf(String label) {
	Shelf shelf = new Shelf(parameters.label, 20);
	fire new ShelfCreated(parameters.label);
	return shelf;
}

listen ShelfCreated from this {
	lastCreatedShelf = evt.label;
}

event ShelfCreated {
	String label;
	create(String label) {
		label = parameters.label;
	}
}
fiction = library.newShelf("Fiction");
assert library.lastCreatedShelf == "Fiction";

Inside a listen, evt is the event that was fired.

Behaviours offered by technology adapters​

A behaviour can also be declared with with and a kind offered by a technology adapter: name(arguments) with Kind(parameters) { … }. Such behaviours are how a model hooks into what happens in a federated resource, for instance when an element is added to it. What each adapter offers is described with the technology adapters.

What you have seen​

A behaviour has a name, arguments read through parameters, and a body; two behaviours can share a name when their arguments differ. Behaviours of the same instance are called on this. Creation and deletion are behaviours too, create and delete, and a model reacts to events with listen. Three silent traps to keep in mind: a bare parameter name, a bare call of a behaviour, and a creation call that omits an argument with a default value.

The models and scripts on this page (Talker.fml, NameLookup.fml and Library.fml) are tests of the platform, in the openflexo-core repository.