What is the Dart “Expando” feature about, what does it do?

Question

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 he

Answer 1

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 domFunctionExpando = new Expando();

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');
}