1. Params
这三个参数的用途如下：
1. Params
在执行AsyncTask时需要传入的参数，可用于在后台任务中使用。
2. Progress
后台任务执行时，如果需要在界面上显示当前的进度，则使用这里指定的泛型作为进度单位。
3. Result
当任务执行完毕后，如果需要对结果进行返回，则使用这里指定的泛型作为返回值类型。
Android SDK中给出的一个AsyncTask用法如下：
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");
}
}
new DownloadFilesTask().execute(url1, url2, url3);AsyncTask的源码
当初使用AsyncTask的时候，一直不明白onProcessUpdate为何会定期被调用。后来在工作中要实现一个定时更新任务进度的功能，我们用的是一个Handler给自己发送消息来进行定时刷新。现在研究了一下AsyncTask的实现原理，发现它也是使用Handler给自己发送消息的原理来实现定时刷新。当我选用的是Android 4.4的源码，若大家能搞懂AsyncTask的实现原理，也能自己写一个简单的AsyncTask。
首先看看构造函数：注意，构造函数一定要在UI中调用。
/**
* 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);
}
}
};
}这个函数初试化了两个变量：mWorker和mFuture。mWorker是一个Callable对象，mFuture是一个实现了Runnable和Future的对象。启动AsyncTask调用的是execute(Params) public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}该方法调用了executeOnExecutor()方法，代码如下：
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;
}该方法先设置运行状态，然后调用onPreExecute，最后调用exec.execute(mFuture).传入的exec是sDefaultExecutor, sDefaultExecutor是一个SerialExecutor对象，源代码如下：
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);
}
}
}
当调用sDefaultExecutor.execute(runnable),该方法用一个ArrayDeque来保存runnable，runnable会在合适的时候被调用。第一次运行mActivity等于null了，于是会调用scheduleNext()方法。在这个方法中会从队列的头部取值，并赋值给mActive对象，然后调用THREAD_POOL_EXECUTOR去执行取出的取出的Runnable对象。这里使用了一个try finally代码块，并在finally中调用了scheduleNext()方法，每次当一个任务执行完毕后，下一个任务才会得到执行，SerialExecutor模仿的是单一线程池的效果，如果我们快速地启动了很多任务，同一时刻只会有一个线程正在执行，其余的均处于等待状态。
sDefaultExecutor.execute(Runnable)传入的对象是mFuture，mFuture是一个FutureTask对象，其构造函数如下所示：
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}由AsyncTask的构造函数可知，FutureTask中的callable是mWorkder对象。FutureTask的run()代码如下所示： 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);
}
}因此，最终会调用mWorker的call()。mWorker的call()位于AsyncTask的构造函数中，大家可以发现其先调用doInBackgroud(Params),然后调用postResult().postResult(Result)的源代码如下：
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}sHandler是一个InternalHandler，代码如下：
private static class InternalHandler extends Handler {
@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;
}
}
}
当传入的是一条MESSAGE_POST_RESULT消息，就会去执行AsyncTask.finish()方法，如果这是一条MESSAGE_POST_PROGRESS消息，就会去执行onProgressUpdate()方法。当我们在doInBackgroud(Params)中利用for循环来调用publishProgress(Progress... values)，就会模拟出 protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
它首先会判断线程是否已经结束，若没结束，则sHandler给自己发送一个消息，消息的类型为MESSAGE_POST_PROGRESS，sHandler.handleMessage(Message)则会调用onProgressUpdate(Progress... values)。 private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}postResult(Result)已经在前面有过讲解，在此不再赘述。源码分析Android AsyncTask,布布扣,bubuko.com
原文地址：http://blog.csdn.net/bigconvience/article/details/27093683
