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AsyncTask机制

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AsyncTask机制

AsyncTask可以让我们更容易地使用UI线程。它允许执行后台操作,并把结果发布到UI线程上,而不需要操作线程或Handler。AsyncTask被设计成一个和Thread、Handler相关的一个帮助类。AsyncTask用于短时(最多是几秒)的操作。

AsyncTask使用需要注意以下几点:

  • AsyncTask类必须在UI线程上加载。AsyncTask必须在UI线程实例化。execute()方法也必须在UI线程调用。

  • 不要手动调用onPreExecute()、onPostExecute()、doInBackground()、onProgressUpdate()方法。

  • 每个AsyncTask实例只能调用一次execute,如果再次调用,则会抛出异常。

AsyncTask首次引入时,AsyncTask中的任务是串行的。从Android1.6之后,AsyncTask被设计成并行的。从Android3.0后,AsyncTask被重新设计成串行。如果在3.0后的版本需要并行,则可以调用AsyncTask的executeOnExecutor(java.util.concurrent.Executor, Object[])方法,手动传入Executor。

在AsyncTask类加载时,会初始化ThreadPoolExecutor:

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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;
}

其中,核心线程数,最小为2个,最大为4个:

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private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));

最大线程数CPU数量*2+1:

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private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;

KeepAlive时间为30s:

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private static final int KEEP_ALIVE_SECONDS = 30;

任务队列最大是128:

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

AsyncTask的基本使用:

1、定义一个类,继承自AsyncTask,根据需要重写doInBackground()、onProgressUpdate()、onPostExecute()方法,一般doInBackground()、onPostExecute()方法是需要重写的,在这里实现自己的业务。doInBackground()方法运行在子线程中。onProgressUpdate()和onPostExecute()运行在UI线程。

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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");
}
}

2、创建DownloadFilesTask的实例,并执行execute()方法:

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new DownloadFilesTask().execute(url1, url2, url3);

下面,从源码角度来分析下AsyncTask的原理。

AsyncTask的执行入口是execute方法:

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

execute()方法必须在UI线程调用。在方法内部调用了executeOnExecutor()方法。

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  @MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
//检查AsyncTask状态,不是未执行状态(如任务正在运行或已完成),则会抛出相应异常
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)");
}
}

//将状态置为RUNNING
mStatus = Status.RUNNING;

onPreExecute();

mWorker.mParams = params;
exec.execute(mFuture);

return this;
}

executeOnExecutor()方法也必须在UI线程调用。在方法开始时,会检查AsyncTask状态,不是未执行状态(如任务正在运行或已完成),则会抛出相应异常。然后,将任务状态置为RUNNING状态,调用onPreExecute()方法,这个方法需要自己重写,可以做一些UI提示。然后,将参数设置为mWorker,调用Executor的execute()方法。

如果使用默认的Executor,则为串行。

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

接下来,看看sDefaultExecutor的定义:

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private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

而SERIAL_EXECUTOR的具体实现如下:

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   public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

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);
}
}
}

mWorker的定义:

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mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
//将线程设置为后台线程
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
//调用doInBackground方法
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
//发送结果
postResult(result);
}
return result;
}
};

当执行execute()方法,会调用mWorker的call()方法,在此方法中,会将线程设置为后台线程,然后调用doInBackground()方法,并在执行完成后调用postResult()方法。在doInBackground()方法中,可以调用publishProgress()方法,将进度信息发送到UI线程中。

postResult()方法:

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

发送一个Message到Handler中.

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

@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;
}
}
}

在Handler的handleMessage()方法中处理消息。如果已经执行完成,则会调用AsyncTask的finish()方法,如果是更新进度,则会调用AsyncTask的onProgressUpdate()方法:

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    private void finish(Result result) {
if (isCancelled()) {
//如果是取消任务,则回调onCancelled()方法。
onCancelled(result);
} else {
//回调onPostExecute()方法
onPostExecute(result);
}
//设置状态为FINISHED
mStatus = Status.FINISHED;
}

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

publishProgress()方法:

@WorkerThread
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
文章作者: milovetingting
文章链接: http://www.milovetingting.cn/2019/12/31/Android/AsyncTask%E6%9C%BA%E5%88%B6/
版权声明: 本博客所有文章除特别声明外,均采用 CC BY-NC-SA 4.0 许可协议。转载请注明来自 milovetingting

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