BaseCancelable.java

/*
 * Copyright (C) 2009 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.yixia.zi.widget.cropimage;

import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;

/**
 * An abstract class for the interface <code>Cancelable</code>. Subclass can
 * simply override the <code>execute()</code> function to provide an
 * implementation of <code>Cancelable</code>.
 */
public abstract class BaseCancelable<T> implements Cancelable<T> {

	/**
	 * The state of the task, possible transitions are:
	 * 
	 * <pre>
	 *     INITIAL -> CANCELED
	 *     EXECUTING -> COMPLETE, CANCELING, ERROR, CANCELED
	 *     CANCELING -> CANCELED
	 * </pre>
	 * 
	 * When the task stop, it must be end with one of the following states:
	 * COMPLETE, CANCELED, or ERROR;
	 */
	private static final int STATE_INITIAL = (1 << 0);
	private static final int STATE_EXECUTING = (1 << 1);
	private static final int STATE_CANCELING = (1 << 2);
	private static final int STATE_CANCELED = (1 << 3);
	private static final int STATE_ERROR = (1 << 4);
	private static final int STATE_COMPLETE = (1 << 5);

	private int mState = STATE_INITIAL;

	private Throwable mError;
	private T mResult;
	private Cancelable<?> mCurrentTask;
	private Thread mThread;

	protected abstract T execute() throws Exception;

	protected synchronized void interruptNow() {
		if (isInStates(STATE_CANCELING | STATE_EXECUTING)) {
			mThread.interrupt();
		}
	}

	/**
	 * Frees the result (which is not null) when the task has been canceled.
	 */
	protected void freeCanceledResult(T result) {
		// Do nothing by default;
	}

	private boolean isInStates(int states) {
		return (states & mState) != 0;
	}

	private T handleTerminalStates() throws ExecutionException {
		if (mState == STATE_CANCELED) {
			throw new CancellationException();
		}
		if (mState == STATE_ERROR) {
			throw new ExecutionException(mError);
		}
		if (mState == STATE_COMPLETE)
			return mResult;
		throw new IllegalStateException();
	}

	public synchronized void await() throws InterruptedException {
		while (!isInStates(STATE_COMPLETE | STATE_CANCELED | STATE_ERROR)) {
			wait();
		}
	}

	public final T get() throws InterruptedException, ExecutionException {
		synchronized (this) {
			if (mState != STATE_INITIAL) {
				await();
				return handleTerminalStates();
			}
			mThread = Thread.currentThread();
			mState = STATE_EXECUTING;
		}
		try {
			mResult = execute();
		} catch (CancellationException e) {
			mState = STATE_CANCELED;
		} catch (InterruptedException e) {
			mState = STATE_CANCELED;
		} catch (Throwable error) {
			synchronized (this) {
				if (mState != STATE_CANCELING) {
					mError = error;
					mState = STATE_ERROR;
				}
			}
		}
		synchronized (this) {
			if (mState == STATE_CANCELING)
				mState = STATE_CANCELED;
			if (mState == STATE_EXECUTING)
				mState = STATE_COMPLETE;
			notifyAll();
			if (mState == STATE_CANCELED && mResult != null) {
				freeCanceledResult(mResult);
			}
			return handleTerminalStates();
		}
	}

	/**
	 * Requests the task to be canceled.
	 * 
	 * @return true if the task is running and has not been canceled; false
	 * otherwise
	 */

	public synchronized boolean requestCancel() {
		if (mState == STATE_INITIAL) {
			mState = STATE_CANCELED;
			notifyAll();
			return false;
		}
		if (mState == STATE_EXECUTING) {
			if (mCurrentTask != null)
				mCurrentTask.requestCancel();
			mState = STATE_CANCELING;
			return true;
		}
		return false;
	}

	/**
	 * Whether the task's has been requested for cancel.
	 */
	protected synchronized boolean isCanceling() {
		return mState == STATE_CANCELING;
	}

	/**
	 * Runs a <code>Cancelable</code> subtask. This method is helpful, if the
	 * task can be composed of several cancelable tasks. By using this function,
	 * it will pass <code>requestCancel</code> message to those subtasks.
	 * 
	 * @param <T> the return type of the sub task
	 * @param cancelable the sub task
	 * @return the result of the subtask
	 */
	@SuppressWarnings("hiding")
	protected <T> T runSubTask(Cancelable<T> cancelable) throws InterruptedException, ExecutionException {
		synchronized (this) {
			if (mCurrentTask != null) {
				throw new IllegalStateException("cannot two subtasks at the same time");
			}
			if (mState == STATE_CANCELING)
				throw new CancellationException();
			mCurrentTask = cancelable;
		}
		try {
			return cancelable.get();
		} finally {
			synchronized (this) {
				mCurrentTask = null;
			}
		}
	}

}