net.sf.saxon.value

Class MemoClosure

public class MemoClosure extends Closure

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.

Nested Class Summary
classMemoClosure.ProgressiveIterator
A ProgressiveIterator starts by reading any items already held in the reservoir; when the reservoir is exhausted, it reads further items from the inputIterator, copying them into the reservoir as they are read.
Field Summary
protected static intEMPTY
protected intstate
Constructor Summary
MemoClosure()
Constructor should not be called directly, instances should be made using the make() method.
Method Summary
intgetLength()
Get the length of the sequence
booleanisFullyRead()
Determine whether the contents of the MemoClosure have been fully read
ItemitemAt(int n)
Get the n'th item in the sequence (starting from 0).
SequenceIteratoriterate()
Evaluate the expression in a given context to return an iterator over a sequence
Valuematerialize()
Return a value containing all the items in the sequence returned by this SequenceIterator
voidprocess(XPathContext context)
Process the expression by writing the value to the current Receiver

Field Detail

EMPTY

protected static final int EMPTY

state

protected int state

Constructor Detail

MemoClosure

public MemoClosure()
Constructor should not be called directly, instances should be made using the make() method.

Method Detail

getLength

public int getLength()
Get the length of the sequence

isFullyRead

public boolean isFullyRead()
Determine whether the contents of the MemoClosure have been fully read

Returns: true if the contents have been fully read

itemAt

public Item itemAt(int n)
Get the n'th item in the sequence (starting from 0). This is defined for all SequenceValues, but its real benefits come for a SequenceValue stored extensionally

iterate

public SequenceIterator iterate()
Evaluate the expression in a given context to return an iterator over a sequence

materialize

public Value materialize()
Return a value containing all the items in the sequence returned by this SequenceIterator

Returns: the corresponding value

process

public void process(XPathContext context)
Process the expression by writing the value to the current Receiver

Parameters: context The dynamic context, giving access to the current node, the current variables, etc.