onKeyValue(obj, key, handler, [queueName])

Register a handler on the Map-like object obj to trigger when the property key key changes. obj must implement @@can.onKeyValue to be compatible with can-reflect.onKeyValue. The function passed as handler will receive the new value of the property as the first argument, and the previous value of the property as the second argument.

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

obj.foo = "baz";  // -> logs "foo is now baz , was bar"