It allows to cache on 4 levels:
a) tree: for all instances/domains runining on a certain source path
b) instance: for all sessions on a given instance
c) session: for all requests of a session, same as egw_session::appsession()
d) request: just for this request (same as using a static variable)
There's a get, a set and a unset method for each level: eg. getTree()
or setInstance(), as well as a variant allowing to specify the level as first
parameter: eg. unsetCache()
getXXX($app,$location,$callback=null,array $callback_params,$expiration=0)
has three optional parameters allowing to specify:
3. a callback if requested data is not yes stored. In that case the
callback is called and it's value is stored in the cache AND retured
4. parameters to pass to the callback as array, see call_user_func_array
5. an expiration time in seconds to specify how long data should be cached,
default 0 means infinit (this time is not garantied and not
supported for all levels!)
Data is stored under an application name and a location, like
egw_session::appsession().
In fact data stored at cache level egw_cache::SESSION, is stored in
the same way as egw_session::appsession() so both methods can be used
with each other.
The $app parameter should be either the app or the class name, which
both are unique.
The tree and instance wide cache uses a certain provider class, to
store the data eg. in memcached or if there's nothing else configured
in the filesystem (eGW's temp_dir).
