Android之AsyncTask源碼解析

標籤：android   asynctask   源碼   解析   

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http://blog.csdn.net/as02446418/article/details/47684415

注釋都寫在代碼裡了，直接看代碼吧：

/* * Copyright (C) 2008 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 android.os;import java.util.concurrent.ThreadPoolExecutor;import java.util.concurrent.TimeUnit;import java.util.concurrent.BlockingQueue;import java.util.concurrent.LinkedBlockingQueue;import java.util.concurrent.ThreadFactory;import java.util.concurrent.Callable;import java.util.concurrent.FutureTask;import java.util.concurrent.ExecutionException;import java.util.concurrent.TimeoutException;import java.util.concurrent.CancellationException;import java.util.concurrent.atomic.AtomicInteger;/** * AsyncTask能夠使得操作UI線程更加容易，AsyncTask可以使得在耗時操作得到結果更新UI線程而不依靠Handler等</p> * * An asyntask能夠在後台進行耗時計算操作然後把結果反饋給UI線程，整個操作是非同步，非同步任務總共有三個類屬性Params, Progress and Result, * 和4個步驟, 分別是begin,doInBackground,processProgress and end. * * AsyncTask 至少需要重寫一個方法——doInBackground,最經常用的還有onPostExecute. * 其中三個類屬性 *    1.Params , 當任務開始時需要傳給任務的參數 *    2.Progress , 當後台計算時發布的即時進度 *    3.Result , 後台計算的結果 * 使用AsyncTask需要滿足的條件： *    1.任務必須建立在UI線程中 *    2.使用execute方法必須在UI線程中 *    3.不要顯示地調用onPreExecute，onPostExecute,doInBackground，onProgressUpdate *    4.任務只能exceute一次，不然會拋異常 */public abstract class AsyncTask<Params, Progress, Result> {    private static final String LOG_TAG = "AsyncTask";    private static final int CORE_POOL_SIZE = 5;    private static final int MAXIMUM_POOL_SIZE = 128;    private static final int KEEP_ALIVE = 1;    //阻塞隊列    private static final BlockingQueue<Runnable> sWorkQueue =            new LinkedBlockingQueue<Runnable>(10);    private static final ThreadFactory sThreadFactory = new ThreadFactory() {        //原子interger計數變數        private final AtomicInteger mCount = new AtomicInteger(1);        //對線程數量進行計數        public Thread newThread(Runnable r) {            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());        }    };    //定義了線程池    private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,            MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);    private static final int MESSAGE_POST_RESULT = 0x1;    private static final int MESSAGE_POST_PROGRESS = 0x2;    private static final int MESSAGE_POST_CANCEL = 0x3;    //繼承的handler類，內部自己實現的handler    private static final InternalHandler sHandler = new InternalHandler();    private final WorkerRunnable<Params, Result> mWorker;    //FutureTask容器，存放結果類型    private final FutureTask<Result> mFuture;    private volatile Status mStatus = Status.PENDING;    /**     * 表明任務目前狀態的枚舉，且任務只能處於一種狀態     */    public enum Status {        /**         * 掛起狀態，還沒開始任務         */        PENDING,        /**        *運行狀態，表明狀態進行中        */        RUNNING,        /**         * 結束狀態，表明狀態已完成         */        FINISHED,    }    /**     * 建立AsyncTask，再次聲明：只能在UI線程中建立     */    public AsyncTask() {        mWorker = new WorkerRunnable<Params, Result>() {            public Result call() throws Exception {                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);                return doInBackground(mParams);            }        };        mFuture = new FutureTask<Result>(mWorker) {            @Override            protected void done() {                Message message;                Result result = null;                try {                    result = get();                } catch (InterruptedException e) {                    android.util.Log.w(LOG_TAG, e);                } catch (ExecutionException e) {                    throw new RuntimeException("An error occured while executing doInBackground()",                            e.getCause());                } catch (CancellationException e) {                    message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,                            new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));                    message.sendToTarget();                    return;                } catch (Throwable t) {                    throw new RuntimeException("An error occured while executing "                            + "doInBackground()", t);                }                message = sHandler.obtainMessage(MESSAGE_POST_RESULT,                        new AsyncTaskResult<Result>(AsyncTask.this, result));                message.sendToTarget();            }        };    }    /**     * 獲得任務目前狀態     *     * @return The current status.     */    public final Status getStatus() {        return mStatus;    }    /**     *這個就是doInBackground的抽象方法了     *     * @see #onPreExecute()     * @see #onPostExecute     * @see #publishProgress     */    protected abstract Result doInBackground(Params... params);    /**     * 在doInBackground之前執行     *     * @see #onPostExecute     * @see #doInBackground     */    protected void onPreExecute() {    }    /**     * 在doInBackground之後執行，如果任務被取消或者拋異常則返回null     *     * @see #onPreExecute     * @see #doInBackground     */    @SuppressWarnings({"UnusedDeclaration"})    protected void onPostExecute(Result result) {    }    /**     * 在publishProgress調用後執行，其中參數的values是傳給publishProgress的     *     * @see #publishProgress     * @see #doInBackground     */    @SuppressWarnings({"UnusedDeclaration"})    protected void onProgressUpdate(Progress... values) {    }    /**     * 在cancel()被調用後執行的方法     *     * @see #cancel(boolean)     * @see #isCancelled()     */    protected void onCancelled() {    }    /**     * 如果在正常結束之前取消任務了則返回true，否則為false     */    public final boolean isCancelled() {        return mFuture.isCancelled();    }    /**     * 試圖取消執行的任務，途中可能會失敗如果任務已經完成了，或者已經被取消了，或者其他原因，     * 如果這個方法在任務啟動之前調用則任務不會啟動，如果在啟動之後調用，     * 傳入的參數mayInterruptIfRunning決定了是不是要在任務啟動並執行時候終止任務     * @see #isCancelled()     * @see #onCancelled()     */    public final boolean cancel(boolean mayInterruptIfRunning) {        return mFuture.cancel(mayInterruptIfRunning);    }    /**     * 等待直到結果完成的時候則返回結果     *     * @return The computed result.     *     * @throws CancellationException If the computation was cancelled.     * @throws ExecutionException If the computation threw an exception.     * @throws InterruptedException If the current thread was interrupted     *         while waiting.     */    public final Result get() throws InterruptedException, ExecutionException {        return mFuture.get();    }    /**     * 等到timeout結束時就檢查結果，不管任務有沒有完成，通常放到耗時任務時     *     * @param timeout Time to wait before cancelling the operation.     * @param unit The time unit for the timeout.     *     * @return The computed result.     *     * @throws CancellationException If the computation was cancelled.     * @throws ExecutionException If the computation threw an exception.     * @throws InterruptedException If the current thread was interrupted     *         while waiting.     * @throws TimeoutException If the wait timed out.     */    public final Result get(long timeout, TimeUnit unit) throws InterruptedException,            ExecutionException, TimeoutException {        return mFuture.get(timeout, unit);    }    /**     * 返回AsyncTask<Params, Progress, Result>對象，需要在UI線程中執行     *     *     * @param params The parameters of the task.     *     * @return This instance of AsyncTask.     *     * @throws IllegalStateException If {@link #getStatus()} returns either     *         {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.     */    public final AsyncTask<Params, Progress, Result> execute(Params... params) {        if (mStatus != Status.PENDING) {            switch (mStatus) {                case RUNNING:                    throw new IllegalStateException("Cannot execute task:"                            + " the task is already running.");                case FINISHED:                    throw new IllegalStateException("Cannot execute task:"                            + " the task has already been executed "                            + "(a task can be executed only once)");            }        }        mStatus = Status.RUNNING;        onPreExecute();        mWorker.mParams = params;        sExecutor.execute(mFuture);        return this;    }    /**     * 這個方法會被doInBackground調用並表示當前已完成的狀態     *     * @see #onProgressUpdate     * @see #doInBackground     */    protected final void publishProgress(Progress... values) {        sHandler.obtainMessage(MESSAGE_POST_PROGRESS,                new AsyncTaskResult<Progress>(this, values)).sendToTarget();    }    /**    * 任務任務時候內部自動調用此方法    */    private void finish(Result result) {        if (isCancelled()) result = null;        onPostExecute(result);        mStatus = Status.FINISHED;    }    /**    * 這個是最重要的內部實現的handler了，根據不同的msg進行分發不同操作    */    private static class InternalHandler extends Handler {        @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})        @Override        public void handleMessage(Message msg) {            AsyncTaskResult result = (AsyncTaskResult) msg.obj;            switch (msg.what) {                case MESSAGE_POST_RESULT:                    // There is only one result                    result.mTask.finish(result.mData[0]);                    break;                case MESSAGE_POST_PROGRESS:                    result.mTask.onProgressUpdate(result.mData);                    break;                case MESSAGE_POST_CANCEL:                    result.mTask.onCancelled();                    break;            }        }    }    private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {        Params[] mParams;    }    @SuppressWarnings({"RawUseOfParameterizedType"})    private static class AsyncTaskResult<Data> {        final AsyncTask mTask;        final Data[] mData;        AsyncTaskResult(AsyncTask task, Data... data) {            mTask = task;            mData = data;        }    }}

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Android之AsyncTask源碼解析