* <p>AsyncTask is designed to be a helper class around {@link Thread} and {@link Handler} * and does not constitute a generic threading framework. AsyncTasks should ideally be * used for short operations (a few seconds at the most.) If you need to keep threads * running for long periods of time, it is highly recommended you use the various APIs * provided by the <code>java.util.concurrent</code> package such as {@link Executor}, * {@link ThreadPoolExecutor} and {@link FutureTask}.</p> AsyncTask 应该被用来处理短时间(几秒)的操作。如果想要让线程长时间的运行,强烈推荐使用 java.util.concurrent 包下面的工具。
* <p>AsyncTask must be subclassed to be used. The subclass will override at least * one method ({@link #doInBackground}), and most often will override a * second one ({@link #onPostExecute}.)</p> AsyncTask 是抽象类,子类需要实现其抽象方法,实例化后才能使用,下面是一个使用例子: * <p>Here is an example of subclassing:</p> * <pre class="prettyprint"> * private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> { * protected Long doInBackground(URL... urls) { * int count = urls.length; * long totalSize = 0; * for (int i = 0; i < count; i++) { * totalSize += Downloader.downloadFile(urls[i]); * publishProgress((int) ((i / (float) count) * 100)); * // Escape early if cancel() is called * if (isCancelled()) break; * } * return totalSize; * } * * protected void onProgressUpdate(Integer... progress) { * setProgressPercent(progress[0]); * } * * protected void onPostExecute(Long result) { * showDialog("Downloaded " + result + " bytes"); * } * }
* <h2>AsyncTask‘s generic types</h2> * <p>The three types used by an asynchronous task are the following:</p> * <ol> * <li><code>Params</code>, the type of the parameters sent to the task upon * execution.</li> * <li><code>Progress</code>, the type of the progress units published during * the background computation.</li> * <li><code>Result</code>, the type of the result of the background * computation.</li> * </ol> * <p>Not all types are always used by an asynchronous task. To mark a type as unused, * simply use the type {@link Void}:</p> * <pre> * private class MyTask extends AsyncTask<Void, Void, Void> { ... } * </pre> AsyncTask 的三个泛型参数: Params --- Task 执行时需要的参数类型 Progress --- Task 执行时的进行类型 Result --- Task 执行完成之后返回的结果类型
* <h2>The 4 steps</h2> * <p>When an asynchronous task is executed, the task goes through 4 steps:</p> * <ol> * <li>{@link #onPreExecute()}, invoked on the UI thread before the task * is executed. This step is normally used to setup the task, for instance by * showing a progress bar in the user interface.</li> * <li>{@link #doInBackground}, invoked on the background thread * immediately after {@link #onPreExecute()} finishes executing. This step is used * to perform background computation that can take a long time. The parameters * of the asynchronous task are passed to this step. The result of the computation must * be returned by this step and will be passed back to the last step. This step * can also use {@link #publishProgress} to publish one or more units * of progress. These values are published on the UI thread, in the * {@link #onProgressUpdate} step.</li> * <li>{@link #onProgressUpdate}, invoked on the UI thread after a * call to {@link #publishProgress}. The timing of the execution is * undefined. This method is used to display any form of progress in the user * interface while the background computation is still executing. For instance, * it can be used to animate a progress bar or show logs in a text field.</li> * <li>{@link #onPostExecute}, invoked on the UI thread after the background * computation finishes. The result of the background computation is passed to * this step as a parameter.</li> * </ol> AsyncTask 执行的时候,下面四个方法将会顺序执行: onPreExecute() 在主线程执行,在Task执行前调用。 doInBackground() 在后台线程执行,onPreExecute() 执行完之后,该方法立即执行。 onProgressUpdate() 在主线程执行,可用来展示 Task 的进度。 onPostExecute() 在主线程执行,Task 执行完之后,该方法执行。
* <h2>Cancelling a task</h2> * <p>A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking * this method will cause subsequent calls to {@link #isCancelled()} to return true. * After invoking this method, {@link #onCancelled(Object)}, instead of * {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])} * returns. To ensure that a task is cancelled as quickly as possible, you should always * check the return value of {@link #isCancelled()} periodically from * {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)</p> 取消任务,调用 cancel() 方法,调用该方法之后 isCancel() 方法会返回 true。 并且,onPostExecute() 方法不会执行,onCancelled() 会执行。 为了保证 Task 尽快取消,你应该在 doInBackground() 中随时检查 isCancelled() 的返回值。
AsyncTask 的构造函数
/** * Creates a new asynchronous task. This constructor must be invoked on the UI thread. */ public AsyncTask() { mWorker = new WorkerRunnable<Params, Result>() { public Result call() throws Exception { mTaskInvoked.set(true); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked return postResult(doInBackground(mParams)); } }; 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 occured while executing doInBackground()", e.getCause()); } catch (CancellationException e) { postResultIfNotInvoked(null); } } }; }
在构造函数中,实例化了两个变量
mWorker 就是一个 Callable 的实现类。
mFuture 需要看一下 FutureTask 的源码。
下面看最主要的方法:
public final AsyncTask<Params, Progress, Result> execute(Params... params) { return executeOnExecutor(sDefaultExecutor, params); }
该方法调用了下面的方法:
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; }
executeOnExecutor 方法可以指定 Executor 来执行多线程服务。
当我们调用了 execute 方法的时候,从代码中可以看出 onPreExecute() 会先执行。
然后是 exec.execute(mFuture) 执行,由于 mFuture 也实现了 Runnable 对象,并且还实现了 Callable 对象。
即 mFuture 的 run() 方法被调用:
public void run() { if (state != NEW || !UNSAFE.compareAndSwapObject(this, runnerOffset, null, Thread.currentThread())) return; try { Callable<V> c = callable; if (c != null && state == NEW) { V result; boolean ran; try { result = c.call(); ran = true; } catch (Throwable ex) { result = null; ran = false; setException(ex); } if (ran) set(result); } } finally { // runner must be non-null until state is settled to // prevent concurrent calls to run() runner = null; // state must be re-read after nulling runner to prevent // leaked interrupts int s = state; if (s >= INTERRUPTING) handlePossibleCancellationInterrupt(s); } }
其中,调用了 callable 的 call() 方法。由构造函数可知,mWorker 的 call() 方法被调用。
即执行了 doInBackground() 方法。然后,run() 方法的最后,会调用到 mFuture 的 done() 方法(省略一连串的调用)。
即 finish() 方法被调用(省略一连串的调用)。
private void finish(Result result) { if (isCancelled()) { onCancelled(result); } else { onPostExecute(result); } mStatus = Status.FINISHED; }
可以看出,onPostExecute() 会被调用.....
onProgressUpdate() 由下面的方法触发:
/** * This method can be invoked from {@link #doInBackground} to * publish updates on the UI thread while the background computation is * still running. Each call to this method will trigger the execution of * {@link #onProgressUpdate} on the UI thread. * * {@link #onProgressUpdate} will not be called if the task has been * canceled. * * @param values The progress values to update the UI with. * * @see #onProgressUpdate * @see #doInBackground */ protected final void publishProgress(Progress... values) { if (!isCancelled()) { sHandler.obtainMessage(MESSAGE_POST_PROGRESS, new AsyncTaskResult<Progress>(this, values)).sendToTarget(); } }
PS:序列执行器
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); } } }
时间: 2024-10-18 11:36:37