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/*
* Copyright (C) 2015 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.messaging.util;
import java.util.ArrayList;
import java.util.List;
/**
* Provides a generic and loose-coupled framework to execute one primary and multiple fallback
* strategies for solving a given task.<p>
* Basically, what would have been a nasty try-catch that's hard to separate and maintain:
* <pre><code>
* try {
* // doSomething() that may fail.
* } catch (Exception ex) {
* try {
* // fallback1() that may fail.
* } catch (Exception ex2) {
* try {
* // fallback2() that may fail.
* } catch (Exception ex3) {
* // ...
* }
* }
* }
* </code></pre>
* Now becomes:<br>
* <pre><code>
* FallbackStrategies
* .startWith(something)
* .thenTry(fallback1)
* .thenTry(fallback2)
* .execute();
* </code></pre>
*/
public class FallbackStrategies<Input, Output> {
public interface Strategy<Input, Output> {
Output execute(Input params) throws Exception;
}
private final List<Strategy<Input, Output>> mChainedStrategies;
private FallbackStrategies(final Strategy<Input, Output> primaryStrategy) {
mChainedStrategies = new ArrayList<Strategy<Input, Output>>();
mChainedStrategies.add(primaryStrategy);
}
public static <Input, Output> FallbackStrategies<Input, Output> startWith(
final Strategy<Input, Output> primaryStrategy) {
return new FallbackStrategies<Input, Output>(primaryStrategy);
}
public FallbackStrategies<Input, Output> thenTry(final Strategy<Input, Output> strategy) {
Assert.isFalse(mChainedStrategies.isEmpty());
mChainedStrategies.add(strategy);
return this;
}
public Output execute(final Input params) {
final int count = mChainedStrategies.size();
for (int i = 0; i < count; i++) {
final Strategy<Input, Output> strategy = mChainedStrategies.get(i);
try {
// If succeeds, this will directly return.
return strategy.execute(params);
} catch (Exception ex) {
LogUtil.e(LogUtil.BUGLE_TAG, "Exceptions occured when executing strategy " +
strategy + (i < count - 1 ?
", attempting fallback " + mChainedStrategies.get(i + 1) :
", and running out of fallbacks."), ex);
// This will fall through and continue with the next strategy (if any).
}
}
// Running out of strategies, return null.
// TODO: Should this accept user-defined fallback value other than null?
return null;
}
}
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