簡介
AsyncTask是一個輕量級的異步任務類,它可以在線程池中執行後臺任務,然後把執行的進度和結果傳遞給主線程並且在主線程中更新UI。
需要注意的是
- AsyncTask 不可重複執行。
- AsyncTask的構造和運行都需要在主線程中處理。
- 不適合執行特別耗時的任務
- 在目前的版本中,如果不特別處理,所有的AsyncTask將採用串行執行。
使用
這裏直接上代碼
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_async_task);
new MyAsyncTask().execute("233333");
}
public static class MyAsyncTask extends AsyncTask<String, Integer, String> {
private static final String TAG = "MyAsyncTask";
@Override
protected void onPreExecute() {
super.onPreExecute();
Log.d(TAG, "onPreExecute: ");
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
Log.d(TAG, "onPostExecute: "+s);
}
@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
Log.d(TAG, "onProgressUpdate: "+values[0]);
}
@Override
protected String doInBackground(String... strings) {
Log.d(TAG, "doInBackground: "+strings.length);
for (int i=0;i<100;i++){
publishProgress(i);
try {
Thread.sleep(20);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
return strings[0];
}
}
源碼閱讀
閱讀AsyncTask的源碼,我們從excute方法入手
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;
}
上面的代碼很明顯的可以看出,只有AsyncTask沒有執行過的時候,才能執行,否則的話,就會拋出異常。
在執行以後,首先將狀態更改爲運行,然後執行了onPreExecute()方法,然後將參數保存到了mWorker,隨後運行了語句exec.execute(mFuture);,而這裏的exec,我們通過上面的代碼很明顯就可以看到是上面的sDefaultExecutor。
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
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);
}
}
}
這裏可以看到AsyncTask內部實現了一個SerialExecutor,而這個類的功能非常簡單,就是構造一個Runnable方法,將原來的run方法包裹起來,然後判斷當前線程池是否有Runnable方法正在運行,如果沒有的話,就手動去調用THREAD_POOL_EXECUTOR 線程池。正是由於這種處理方法,如果第一個AsyncTask正在執行,當第二個AsyncTask進來的時候,只會將新構造的Runnable方法保存進SerialExecutor,而不會直接將其添加THREAD_POOL_EXECUTOR線程池。
那麼這個THREAD_POOL_EXECUTOR又是何方神聖呢?
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final int KEEP_ALIVE = 1;
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
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());
}
};
public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
這裏可以看到THREAD_POOL_EXECUTOR線程池是一個核心線程爲 CPU_COUNT + 1,線程池最大線程數爲 CPU_COUNT * 2 + 1;,任務隊列最大爲128的線程池。
有人可能在這裏就要犯迷糊了,不是說AsyncTask是串行運行嘛。爲啥這裏需要建這麼大一個線程池,應該一個SingleThreadExecute就夠用了啊。你說的沒錯,這是正常情況。這不是我們AsyncTask不是還提供了併發執行的方法executeOnExecutor嘛。這裏的線程池多餘了這麼多的資源,就是爲了這個準備的。
好了,這裏我們把exec搞清楚了,接下來我們看**exec.execute(mFuture);**中的mFuture
private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
/**
* 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
Result result = doInBackground(mParams);
Binder.flushPendingCommands();
return postResult(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);
}
}
};
}
這裏的FutureTask的就是進化版的runnable,實現了Runnable接口和Future接口。一旦線程開始運行,就會執行call中的方法,然後就會執行doInBackground方法。
這裏的doInBackground方法,就是我們在AsyncTask中實現的doInBackground方法。之後線程就會阻塞在這裏,而我們在doInBackground中,如果想更新進度,都會去調用AsyncTask的publishProgress方法。所以我們來看下publishProgress的實現。
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}
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;
}
}
}
這裏的邏輯就非常簡單了,就是使用了handler然後在ui線程中調用onProgressUpdate去更新進度。
當doInBackground執行完畢之後,會返回一個result.
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
然後又會調用postResult,將最後的result通過handler發送到finish方法。所以finish方法也是在UI線程中執行的。
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
而根據代碼,finish方法,如果爲正常結束的話,又會調用onPostExecute方法,所以onPostExecute也是在UI線程中執行的。
好了,正常的邏輯,我們都已經走完了。搞定睡覺!