onEvent(obj, eventName, callback)

Register an event handler on the object obj to trigger when the event eventName is dispatched. obj must implement @@can.onEvent or .addEventListener() to be compatible with can-reflect.onKeyValue. The function passed as callback will receive the event descriptor as the first argument, and any data passed to the event dispatch as subsequent arguments.

var obj = new DefineMap({ foo: "bar" });
canReflect.onEvent(obj, "foo", function(ev, newVal, oldVal) {
    console.log("foo is now", newVal, ", was", oldVal);
});

canEvent.dispatch.call(obj, "foo", ["baz", "quux"]);  // -> logs "foo is now baz , was quux"