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Diffstat (limited to 'src/com/android/contacts/common/util/SingletonHolder.java')
-rw-r--r-- | src/com/android/contacts/common/util/SingletonHolder.java | 93 |
1 files changed, 93 insertions, 0 deletions
diff --git a/src/com/android/contacts/common/util/SingletonHolder.java b/src/com/android/contacts/common/util/SingletonHolder.java new file mode 100644 index 00000000..c1a183d4 --- /dev/null +++ b/src/com/android/contacts/common/util/SingletonHolder.java @@ -0,0 +1,93 @@ +/* + * Copyright (C) 2016 The CyanogenMod Project + * + * 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.android.contacts.common.util; + +/** + * Encapsulates a threadsafe singleton pattern. + * + * This class is designed to be used as a public constant, living within a class that has a private constructor. + * It defines a {@link #create(I)} method that will only ever be called once, upon the first call of {@link #get(I)}. + * That method is responsible for creating the actual singleton instance, and that instance will be returned for all + * future calls of {@link #get(I)}. + * + * Example: + * <code> + * public class FooSingleton { + * public static final SingletonHolder<FooSingleton, ParamObject> HOLDER = + * new SingletonHolder<FooSingleton, ParamObject>() { + * @Override + * protected FooSingleton create(ParamObject param) { + * return new FooSingleton(param); + * } + * }; + * + * private FooSingleton(ParamObject param) { + * + * } + * } + * + * // somewhere else + * FooSingleton.HOLDER.get(params).doStuff(); + * </code> + * @param <E> The type of the class to hold as a singleton. + * @param <I> A parameter object to use during creation of the singleton object. + */ +public abstract class SingletonHolder<E, I> { + E mInstance; + final Object LOCK = new Object(); + + public E get(I initializer) { + if (null == mInstance) { + synchronized (LOCK) { + if (null == mInstance) { + mInstance = create(initializer); + } + } + } + + return mInstance; + } + + protected abstract E create(I initializer); + + /** + * Specialized version of {@link SingletonHolder} which will keep a count of referring + * objects, and clear the instance when all references have been removed. + * + * @param <E> The type of the class to hold as a singleton. + * @param <I> A parameter object to use during creation of the singleton object. + */ + public static abstract class RefCountedSingletonHolder<E, I> extends SingletonHolder<E, I> { + private int mRefCount; + + public E get(I initializer) { + synchronized (LOCK) { + mRefCount++; + } + return super.get(initializer); + } + + public void release() { + synchronized (LOCK) { + if(--mRefCount == 0) { + destroy(mInstance); + mInstance = null; + } + } + } + + protected abstract void destroy(E instance); + } +} |