I always thought our Activities are way too fat, meaning we've put too many
things into activities without any structure.
The major problems with this are:
- They have too many fields, which are not final and not even orthogonal.
This makes them very hard to understand/maintain. Changing one tiny bit
can always cause unanticipated side-effects.
- Very hard, or almost impossible to test.
I really think we should break them into independent and self-contained
subcomponents which can be tested separately.
Introducing MessageListXLStateManager, which manages the current account,
mailbox and message, and show/hide/update fragments accordingly
for MessageListXL.
With this class, MessageListXL will be able to switch accounts/mailboxes/
messages by just calling the methods such as selectAccount(), without
worrying about when to show/hide what fragment and how to initialize them.
(In other words, MessageListXLStateManager encapsulates the two-pane screen
transition. It's not intended to be reused for the phone UI.)
I didn't make it a nested class in MessageListXL, because nested classes can't
have real private members (private member are accessible from outer classes and
even brother classes!!), and I wanted it to be really self-contained anyway.
Change-Id: I1c121e99e30f12cc118e1c35abc9b30f49939a4a
02fa70aa18