Have been seeing the term "Expando" used recently with Dart. Sounds interesting. The API did not provide much of a clue to me.
An example or two could be most helpful!
(Not sure if this is related, but I am most anxious for a way to add methods (getters) and/or variables to a class. Hoping this might be a key to solving this problem. (hint: I am using the Nosuchmethod method now and want to be able to return the value of the unfound method.))
Thanks in advance,
_swarmii
Expandos allow you to associate objects to other objects. One very useful example of this is an HTML DOM element, which cannot itself be sub-classed. Let's make a top-level expando to add some functionality to an element - in this case a Function signature given in the typedef statement:
typedef CustomFunction(int foo, String bar);
Expando<CustomFunction> domFunctionExpando = new Expando<CustomFunction>();
Now to use it:
main(){
// Assumes dart:html is imported
final myElement = new DivElement();
// Use the expando on our DOM element.
domFunctionExpando[myElement] = someFunc;
// Now that we've "attached" the function to our object,
// we can call it like so:
domFunctionExpando[myElement](42, 'expandos are cool');
}
void someFunc(int foo, String bar){
print('Hello. $foo $bar');
}
Just to clarify the difference between expando and maps: as reported in the groups, expando has weak references.
This means that a key can be garbage collected even if it's still present in the expando (as long as there are no other references to it).
For all other intents and purposes it's a map.
I played with it a little bit. Here's what I've got.
import 'dart:html';
const String cHidden = 'hidden';
class ExpandoElement {
static final Expando<ExpandoElement> expando =
new Expando<ExpandoElement>("ExpandoElement.expando");
final Element element;
const ExpandoElement._expand(this.element);
static Element expand(Element element) {
if (expando[element] == null)
expando[element] = new ExpandoElement._expand(element);
return element;
}
// bool get hidden => element.hidden; // commented out to test noSuchMethod()
void set hidden(bool hidden) {
if (element.hidden = hidden)
element.classes.add(cHidden);
else
element.classes.remove(cHidden);
}
noSuchMethod(InvocationMirror invocation) => invocation.invokeOn(element);
}
final Expando<ExpandoElement> x = ExpandoElement.expando;
Element xquery(String selector) => ExpandoElement.expand(query(selector));
final Element input = xquery('#input');
void main() {
input.classes.remove(cHidden);
assert(!input.classes.contains(cHidden));
input.hidden = true;
assert(x[input].hidden); // Dart Editor warning here, but it's still true
assert(!input.classes.contains(cHidden)); // no effect
input.hidden = false;
assert(!x[input].hidden); // same warning, but we'll get input.hidden via noSuchMethod()
assert(!input.classes.contains(cHidden));
x[input].hidden = true;
assert(input.hidden); // set by the setter of ExpandoElement.hidden
assert(input.classes.contains(cHidden)); // added by the setter
assert(x[input].hidden);
assert(x[input].classes.contains(cHidden)); // this is input.classes
x[input].hidden = false;
assert(!input.hidden); // set by the setter
assert(!input.classes.contains(cHidden)); // removed by the setter
assert(!x[input].hidden);
assert(!x[input].classes.contains(cHidden));
// confused?
assert(input is Element);
assert(x[input] is! Element); // is not
assert(x[input] is ExpandoElement);
assert(x is Expando<ExpandoElement>);
}