/*
* Copyright 2000-2009 JetBrains s.r.o.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.intellij.psi.util;
import org.jetbrains.annotations.NotNull;
/**
* A wrapper object that holds a computation ({@link #getValueProvider()}) and caches the result of the computation.<p/>
*
* The recommended way of creation is to use one of {@link com.intellij.psi.util.CachedValuesManager} methods, e.g.
* {@link com.intellij.psi.util.CachedValuesManager#getCachedValue(com.intellij.psi.PsiElement, CachedValueProvider)}
*
* When {@link #getValue()} is invoked the first time, the computation is run and its result is returned and remembered internally.
* In subsequent invocations, the result will be reused to avoid running the same code again and again.<p/>
*
* The computation will be re-run in the following circumstances:
* <ol>
* <li/>Garbage collector collects the result cached internally (it's kept via a {@link java.lang.ref.SoftReference}).
* <li/>IDEA determines that cached value is outdated because some its dependencies are changed. See
* {@link com.intellij.psi.util.CachedValueProvider.Result#getDependencyItems()}
* </ol>
*
* The implementation is thread-safe but not atomic, i.e. if several threads request the cached value simultaneously, the computation may
* be run concurrently on more than one thread.
*
* @param <T> The type of the computation result.
*
* @see com.intellij.psi.util.CachedValueProvider
* @see com.intellij.psi.util.CachedValuesManager
*/
public interface CachedValue<T> {
/**
* @return cached value if it's already computed and not outdated, newly computed value otherwise
*/
T getValue();
/**
* @return the object calculating the value to cache
*/
@NotNull
CachedValueProvider<T> getValueProvider();
/**
* @return whether there is a cached result inside this object and it's not outdated
*/
boolean hasUpToDateValue();
}