[Dev] About Item.checkItem() and tests

[Andi Vajda]

For a long time there has been a method defined on Item called check() that 
will perform a number of repository consistency checks and report failures in 
the log.

That functionality is even available from the Chandler UI's Test menu with the 
'Check Repository' menu item. It will recursively walk the UI repository view, 
loading *all* items and calling check() on them.

This method has been invaluable to me for ensuring that a real Chandler 
repository is still consistent after some use. No amount of unit test writing 
can replace such a real life test.

I just added another method, called Item.checkItem(), that is meant to be 
overriden by developers to write additional checks to verify that the 
semantics of a given item are still valid within their domain of use. When 
Item.check() succeeds, it calls checkItem() before returning.

For a hypothetical example, I'd expect a calendar event item to have an 
'endDate' that is after its 'startDate'. It'd be pretty simple to override 
checkItem() on that class and verify this constraint.

When overriding Item.checkItem(), please don't forget to call super() and 
combine its return value with yours, ie, if super() returned False, return 
False even if all your checks succeeded. When you find a failure, log it, but 
try to continue running all checks in the method unless they depend on each 
other, then return False. If all checks passed, return True.

Unless there is a bug triggered by the checkItem() code itself, checkItem() 
returns False in case of error and True otherwise. This should make it 
convenient to use in unit tests as well...

Andi..