On Mon, 19 Jul 2010, Greg Brown wrote:
What about properties of properties of objects---like
${obj.prop.subprop}? If the "obj.prop" value changes, do the binding
listeners get notified?
No, because those listeners have indicated that the property they care
about is "subprop". They don't necessarily know or care that "prop" is
referred to by another object ("obj", in this case).
I can't think of a case off the top of my head where you would actually
run into this situation in the current implementation. If you can, it
would help illustrate the problem.
I think the current implementation is valid. But just for comparison, it
would be useful to know what Flex does in this case.
You have the flex-ibility (sorry, couldn't help myself) to define a "site"
and a "chain" when creating bindings in code. The "site" is the root of
the bound hierarchy. The "chain" is the sequence of properties for each
of which a listener is registered. If any property in the chain changes,
the bound listener will receive a notification if the last property in the
chain does in fact change.
But bindings are rarely done in code and are rarely thought out that
thoroughly. By default, in a root-property sequence
"object.prop1.prop2.prop3..." any change in any of prop1, prop2, prop3,
etc. will notify a listener. I think I can give a simple example of why
this is desirable.
Suppose a component binds one of its properties to the expression
"person.name.first". If first changes from "John" to "Jane" the listener
will receive a notification of a change in "person.name.first" from "John"
to "Jane". If name changes from {first: "John", last: "Doe"} to {first:
"Jane", last: "Doe"}, shouldn't the listener know the first name changed?
Strictly speaking, the original name's "first" property didn't
change---it's still "John".
Similarly, if person is reassigned from person1 to person2 and they have
different first names, etc. If this person is a data model backing a
form, and the person is changed wholesale, any "first name"
labels/inputs/validators/parsers/what-have-you need to know.
Cheers,
Michael
