AsyncTask使用总结

概述

AsyncTask是由Android封装的一个轻量级异步抽象类，可以在线程池中执行后台任务，然后把执行的进度和最终结果传递给主线程并在主线程中更新UI。AsynTask的源码如下：

public abstract class AsyncTask<Params, Progress, Result> {
    private static final String LOG_TAG = "AsyncTask";
    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
    private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
    private static final int KEEP_ALIVE_SECONDS = 30;

    private static final ThreadFactory sThreadFactory = new ThreadFactory() {
        private final AtomicInteger mCount = new AtomicInteger(1);

        public Thread newThread(Runnable r) {
            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
        }
    };

    private static final BlockingQueue<Runnable> sPoolWorkQueue = new LinkedBlockingQueue<Runnable>(128);

    public static final Executor THREAD_POOL_EXECUTOR;

    static {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
                sPoolWorkQueue, sThreadFactory);
        threadPoolExecutor.allowCoreThreadTimeOut(true);
        THREAD_POOL_EXECUTOR = threadPoolExecutor;
    }


    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
    private static final int MESSAGE_POST_RESULT = 0x1;
    private static final int MESSAGE_POST_PROGRESS = 0x2;
    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
    private static InternalHandler sHandler;
    private final WorkerRunnable<Params, Result> mWorker;
    private final FutureTask<Result> mFuture;
    private volatile Status mStatus = Status.PENDING;
    private final AtomicBoolean mCancelled = new AtomicBoolean();
    private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
    private static class SerialExecutor implements Executor {
        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
        Runnable mActive;

        public synchronized void execute(final Runnable r) {
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
                        r.run();
                    } finally {
                        scheduleNext();
                    }
                }
            });
            if (mActive == null) {
                scheduleNext();
            }
        }

        protected synchronized void scheduleNext() {
            if ((mActive = mTasks.poll()) != null) {
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }


    public enum Status {
        PENDING,
        RUNNING,
        FINISHED,
    }

    private static Handler getHandler() {
        synchronized (AsyncTask.class) {
            if (sHandler == null) {
                sHandler = new InternalHandler();
            }
            return sHandler;
        }
    }

    public static void setDefaultExecutor(Executor exec) {
        sDefaultExecutor = exec;
    }

    public AsyncTask() {
        mWorker = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);
                Result result = null;
                try {
                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    //noinspection unchecked
                    result = doInBackground(mParams);
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
                    postResult(result);
                }
                return result;
            }
        };

        mFuture = new FutureTask<Result>(mWorker) {
            @Override
            protected void done() {
                try {
                    postResultIfNotInvoked(get());
                } catch (InterruptedException e) {
                    android.util.Log.w(LOG_TAG, e);
                } catch (ExecutionException e) {
                    throw new RuntimeException("An error occurred while executing doInBackground()",
                            e.getCause());
                } catch (CancellationException e) {
                    postResultIfNotInvoked(null);
                }
            }
        };
    }

    private void postResultIfNotInvoked(Result result) {
        final boolean wasTaskInvoked = mTaskInvoked.get();
        if (!wasTaskInvoked) {
            postResult(result);
        }
    }

    private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

    public final Status getStatus() {
        return mStatus;
    }

    @WorkerThread
    protected abstract Result doInBackground(Params... params);

    @MainThread
    protected void onPreExecute() {
    }

    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onPostExecute(Result result) {
    }

    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onProgressUpdate(Progress... values) {
    }

    @SuppressWarnings({"UnusedParameters"})
    @MainThread
    protected void onCancelled(Result result) {
        onCancelled();
    }    

    @MainThread
    protected void onCancelled() {
    }

    public final boolean isCancelled() {
        return mCancelled.get();
    }

    public final boolean cancel(boolean mayInterruptIfRunning) {
        mCancelled.set(true);
        return mFuture.cancel(mayInterruptIfRunning);
    }

    public final Result get() throws InterruptedException, ExecutionException {
        return mFuture.get();
    }

    public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
            ExecutionException, TimeoutException {
        return mFuture.get(timeout, unit);
    }

    @MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }

    @MainThread
    public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
                                                                       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;
        exec.execute(mFuture);

        return this;
    }

    @MainThread
    public static void execute(Runnable runnable) {
        sDefaultExecutor.execute(runnable);
    }

    @WorkerThread
    protected final void publishProgress(Progress... values) {
        if (!isCancelled()) {
            getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

    private void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }

    private static class InternalHandler extends Handler {
        public InternalHandler() {
            super(Looper.getMainLooper());
        }

        @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;
            }
        }
    }

    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;
        }
    }
}

从源码中可以看出，其内部封装了两个线程池(SerialExecutor和THREAD_POOL_EXECUTOR)和一个Handler(InternalHandler)。其中SerialExecutor线程池用于任务的排队，让需要执行的多个耗时任务，按顺序排列，THREAD_POOL_EXECUTOR线程池才真正地执行任务，InternalHandler用于从工作线程切换到主线程。

重点方法说明

onPreExecute()
该方法在主线程执行，调用顺序在后台任务开始执行之前。因此，用于进行一些界面上的初始化操作，比如显示一个进度条对话框等。

doInBackground(Params…)
该方法在子线程中运行，程序在这里去处理所有的耗时任务。任务一旦完成就可以通过return语句来将任务的执行结果进行返回，如果AsyncTask的第三个泛型参数指定的是void，就可以不返回任务执行结果。

onProgressUpdate(Progress…)
该方法在主线程执行，publishProgress(Progress…)调用时就很快会被调用，方法中携带的参数就是在后台任务中传递过来的。在这个方法中可以对UI进行操作，在主线程中进行，利用参数中的数值就可以对界面元素进行相应的更新。可以在这个方法内显示后台线程的执行进度，比如文件下载时的进度等。

onPostExecute(Result)
该方法在主线程执行，当doInBackground(Params…)执行完毕并通过return语句进行返回时，这个方法就很快会被调用。可以利用返回的数据来进行一些UI操作。

总之，上述方法的执行顺序为：onPreExecute() –> doInBackground() –> publishProgress() –> onProgressUpdate() –> onPostExecute()

应用举例

class downLoadTask extends AsyncTask<String, String, File> {
        private InputStream mIn;
        private FileOutputStream mOut;

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected File doInBackground(String... params) {
            URL url = null;
            try {
                url = new URL(params[0]);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                conn.connect();
                long fileOfLength = conn.getContentLength();
                long totalLength = 0;
                int length;
                mIn = InputStreamGet(String.valueOf(url));
                mOut = new FileOutputStream(mFile);
                byte[] buff = new byte[1024 * 1024];
                while ((length = mIn.read(buff)) > 0) {
                    totalLength += length;
                    String str1 = "" + (int) ((totalLength * 100) / fileOfLength);
                    publishProgress(str1);
                    mOut.write(buff, 0, length);
                }
                mIn.close();
                mOut.close();
                mOut.flush();
            } catch (Exception e) {
                e.printStackTrace();
            }
            return mFile;
        }

        @Override
        protected void onProgressUpdate(String... values) {
            super.onProgressUpdate(values);
            mFileInfo.setVisibility(View.GONE);
            mFileProgress.setVisibility(View.VISIBLE);
            mFileProgress.setProgress(Integer.parseInt(values[0]));
        }

        @Override
        protected void onPostExecute(File f) {
            super.onPostExecute(f);
            if (mDownListener != null) {
                mDownListener.downloadSuccess(f);
            }
        }
    }