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/*
* Copyright (C) 2016 The Android Open Source 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.apkzlib.utils;
import com.android.annotations.NonNull;
import java.util.function.Supplier;
/**
* Supplier that will cache a computed value and always supply the same value. It can be used to
* lazily compute data. For example:
* <pre>
* CachedSupplier<Integer> value = new CachedSupplier<>(() -> {
* Integer result;
* // Do some expensive computation.
* return result;
* });
*
* if (a) {
* // We need the result of the expensive computation.
* Integer r = value.get();
* }
*
* if (b) {
* // We also need the result of the expensive computation.
* Integer r = value.get();
* }
*
* // If neither a nor b are true, we avoid doing the computation at all.
* </pre>
*/
public class CachedSupplier<T> {
/**
* The cached data, {@code null} if computation resulted in {@code null}.
*/
private T cached;
/**
* Is the current data in {@link #cached} valid?
*/
private boolean valid;
/**
* Actual supplier of data, if computation is needed.
*/
@NonNull
private final Supplier<T> supplier;
/**
* Creates a new supplier.
*/
public CachedSupplier(@NonNull Supplier<T> supplier) {
valid = false;
this.supplier = supplier;
}
/**
* Obtains the value.
*
* @return the value, either cached (if one exists) or computed
*/
public synchronized T get() {
if (!valid) {
cached = supplier.get();
valid = true;
}
return cached;
}
/**
* Resets the cache forcing a {@code get()} on the supplier next time {@link #get()} is invoked.
*/
public synchronized void reset() {
cached = null;
valid = false;
}
/**
* In some cases, we may be able to precompute the cache value (or load it from somewhere we
* had previously stored it). This method allows the cache value to be loaded.
*
* <p>If this method is invoked, then an invocation of {@link #get()} will not trigger an
* invocation of the supplier provided in the constructor.
*
* @param t the new cache contents; will replace any currently cache content, if one exists
*/
public synchronized void precomputed(T t) {
cached = t;
valid = true;
}
/**
* Checks if the contents of the cache are valid.
*
* @return are there valid contents in the cache?
*/
public synchronized boolean isValid() {
return valid;
}
}
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