編輯:關於Android編程
Android AsyncTask實現機制
示例代碼:
public final AsyncTask<Params, Progress, Result> execute(Params... params) { return executeOnExecutor(sDefaultExecutor, params); } 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; }
execute先調用onPreExecute()(可見,onPreExecute是自動調用的)然後調用exec.execute(mFuture)
public interface Executor { void execute(Runnable command); }
這是一個接口,具體實現在
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); } } }
從上面可知,AsyncTask執行過程如下:先執行onPreExecute,然後交給SerialExecutor執行。在SerialExecutor中,先把Runnable添加到mTasks中。
如果沒有Runnable正在執行,那麼就調用SerialExecutor的scheduleNext。同時當一個Runnable執行完以後,繼續執行下一個任務
AsyncTask中有兩個線程池,THREAD_POOL_EXECUTOR和SERIAL_EXECUTOR,以及一個Handler–InternalHandler
/** * An {@link Executor} that can be used to execute tasks in parallel. */ public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory); /** * An {@link Executor} that executes tasks one at a time in serial * order. This serialization is global to a particular process. */ public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); private static InternalHandler sHandler;
SERIAL_EXECUTOR用於任務的排列,THREAD_POOL_EXECUTOR真正執行線程,InternalHandler用於線程切換
先看構造函數
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); } } }; }
看到了熟悉的doInBackground了吧,然後調用postResult
private Result postResult(Result result) { @SuppressWarnings("unchecked") Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult<Result>(this, result)); message.sendToTarget(); return result; }
主線程中創建InternalHandler並發送MESSAGE_POST_RESULT消息,然後調用finish函數
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 void finish(Result result) { if (isCancelled()) { onCancelled(result); } else { onPostExecute(result); } mStatus = Status.FINISHED; }
finish中調用onPostExecute。
AsyncTask工作流程:new MyThread().execute(1);
先構造函數,然後execute
構造函數只是准備了mWorker和mFuture這兩個變量
execute中調用onPreExecute,然後exec.execute(mFuture),其中響應了call函數,call中調用doInBackground,然後將結果傳給Handler然後finish掉,finish函數調用onPostExecute
你可能會奇怪,為什麼沒有onProgressUpdate,有注解可以解釋
/** * Runs on the UI thread after {@link #publishProgress} is invoked. * The specified values are the values passed to {@link #publishProgress}. * * @param values The values indicating progress. * * @see #publishProgress * @see #doInBackground */ @SuppressWarnings({"UnusedDeclaration"}) protected void onProgressUpdate(Progress... values) { }
也就是說必須調用publishProgress才會自動調用onProgressUpdate。
那如何調用publishProgress呢?
/** * Override this method to perform a computation on a background thread. The * specified parameters are the parameters passed to {@link #execute} * by the caller of this task. * * This method can call {@link #publishProgress} to publish updates * on the UI thread. * * @param params The parameters of the task. * * @return A result, defined by the subclass of this task. * * @see #onPreExecute() * @see #onPostExecute * @see #publishProgress */ protected abstract Result doInBackground(Params... params);
doInBackground說的很明確,在doInBackground函數裡面顯示調用publishProgress即可。
publishProgress源碼:
protected final void publishProgress(Progress... values) { if (!isCancelled()) { getHandler().obtainMessage(MESSAGE_POST_PROGRESS, new AsyncTaskResult<Progress>(this, values)).sendToTarget(); } } 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: //****************************************在這裡調用 result.mTask.onProgressUpdate(result.mData); break; } } }
感謝閱讀,希望能幫助到大家,謝謝大家對本站的支持!
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