Question

So assume I have a class that has an init method that does something like… grabs some data off the net in xml format and parses it to initialize some of its properties. My concern is how should I handle the case where the network is down or the xml data my object receives is bad?

Normally in C i would use return values to indicate an error and what kind and then that would get propagated back till I could report it to the user. I don’t really think that will work in this situation.

Answer 1

Use asynchronous network requests.

1. Create the UI and show it with
   either dummy replacement for the
   actual values (like pictures) or no
   data (for example empty table).

2. Then create and send the request for
   data and register handler that gets
   called with data.

3. When you receive data your handler
   gets called with them.

4. You parse the data and update the
   UI. In case of data being invalid
   you can now update UI to inform the
   user.

5. You can use timeouts to cancel
   requests in case of network problem
   and functions not returning with
   data within specific time.

There was an example in last year Stanford’s CS193p class (iPhone programming but the same applies to desktop apps) with showing empty user interface and updating it when data coming back. You can probably find references to it on net or otherwise there’ll be new example this year.