dartexpandonosuchmethod

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

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

Solution

  • 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');
}