uk.co.strangeskies.expression

Class ActiveExpression<T>

java.lang.Object

uk.co.strangeskies.expression.ActiveExpression<T>

Type Parameters:

T - The type of the value of this expression

All Implemented Interfaces:

Expression<T>

Direct Known Subclasses:

DependentExpression, IdentityExpression

public abstract class ActiveExpression<T>
extends java.lang.Object
implements Expression<T>

An abstract class to help designing mutable expression, implementing a simple observer list, locking mechanism, and update mechanism.

This implementation is appropriate for expressions which store their own mutable state which can be changed independently of dependent expressions. For expressions which do not store such state it should be more efficient to extend PassiveExpression.

Constructor Summary

Constructors

Constructor and Description
ActiveExpression()

Method Summary

Modifier and Type	Method and Description
protected boolean	beginWrite()
protected void	cancelChange()
protected HotObservable<Expression<? extends T>>	createObservable()
protected boolean	endWrite()
protected void	fireChange()
T	getValue() This should always return the correct current value for this Expression.
protected abstract T	getValueImpl(boolean dirty) Implementing classes should compute the value of the Expression here.
Observable<Expression<? extends T>>	invalidations()
protected boolean	isChanging()
protected void	write(java.lang.Runnable runnable)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface uk.co.strangeskies.expression.Expression

anonymize, decoupleValue, immutable, over, over

Constructor Detail

ActiveExpression

public ActiveExpression()

Method Detail

invalidations

public Observable<Expression<? extends T>> invalidations()

Specified by:

invalidations in interface Expression<T>

createObservable

protected HotObservable<Expression<? extends T>> createObservable()

cancelChange

protected void cancelChange()

isChanging

protected boolean isChanging()

write

protected void write(java.lang.Runnable runnable)

beginWrite

protected boolean beginWrite()

endWrite

protected boolean endWrite()

fireChange

protected void fireChange()

getValue

public T getValue()

Description copied from interface: Expression

This should always return the correct current value for this Expression. Be careful to remember that the reference returned should not be able to mutate this Expression, i.e. it should be either an immutable class, a const reference, or a copy of the underlying value. This is important, but conversely it does not mean that the return value can necessarily be relied upon not to not mutate when this expression is updated, unless a read lock is held.

Once a value has been returned, it is up to the implementing Expression as to whether the value will be reliable such that it will remain the same even if the conceptual value of this expression subsequently changes, or whether it will update automatically with the expression. Please only rely on either behavior if it is explicitly documented, otherwise use Expression.decoupleValue() if you need a persistent reference which is safe to mutate and/or safe from external mutation.

The observers should only ever be notified of an update from the thread which has the write lock on an Expression, and Expressions should be careful to only notify observers when they are in a state where their value can be fetched. Immediate fetch is discouraged, though. Expressions should generally update lazily, not eagerly.

Specified by:

getValue in interface Expression<T>

Returns:

The fully evaluated value of this Expression at the time of method invocation.

getValueImpl

protected abstract T getValueImpl(boolean dirty)

Implementing classes should compute the value of the Expression here. Read lock is guaranteed to be obtained. This method should never be invoked manually.

Parameters:

dirty - Whether the expression has been mutated since this method was last invoked.

Returns:

The value of this Expression.