| A MemoClosure represents a value that has not yet been evaluated: the value is represented
by an expression, together with saved values of all the context variables that the
expression depends on.
The MemoClosure is designed for use when the value is only read several times. The
value is saved on the first evaluation and remembered for later use.
The MemoClosure maintains a reservoir containing those items in the value that have
already been read. When a new iterator is requested to read the value, this iterator
first examines and returns any items already placed in the reservoir by previous
users of the MemoClosure. When the reservoir is exhausted, it then uses an underlying
Input Iterator to read further values of the underlying expression. If the value is
not read to completion (for example, if the first user did exists($expr), then the
Input Iterator is left positioned where this user abandoned it. The next user will read
any values left in the reservoir by the first user, and then pick up iterating the
base expression where the first user left off. Eventually, all the values of the
expression will find their way into the reservoir, and future users simply iterate
over the reservoir contents. Alternatively, of course, the values may be left unread.
Delayed evaluation is used only for expressions with a static type that allows
more than one item, so the evaluateItem() method will not normally be used, but it is
supported for completeness.
The expression may depend on local variables and on the context item; these values
are held in the saved XPathContext object that is kept as part of the Closure, and they
will always be read from that object. The expression may also depend on global variables;
these are unchanging, so they can be read from the Bindery in the normal way. Expressions
that depend on other contextual information, for example the values of position(), last(),
current(), current-group(), should not be evaluated using this mechanism: they should
always be evaluated eagerly. This means that the Closure does not need to keep a copy
of these context variables.
|
