1.
http://developer.android.com/reference/android/os/AsyncTask.html
2.
http://blog.csdn.net/pi9nc/article/details/12622797
3.
http://www.tuicool.com/articles/VnMf2i3
下面详细解析Asynctask的源码流程。我们在阅读源码的时候需要知道的重要点是：输入输出（参数）、以及程序入口（什么地方进入调用）。
/**
* 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.参数为传给execute的方法参数 看方法名称此方法在非主线程中调用
*
* This method can call {@link #publishProgress} to publish updates
* on the UI thread.
*
* @param params The parameters of the task. 任务参数如不同的URL等
*
* @return A result, defined by the subclass of this task. 结果
*
*/
protected abstract Result doInBackground(Params... params);
/**
* Runs on the UI thread before {@link #doInBackground}.
* 运行在UI线程 在调用doInBackground
*/
protected void onPreExecute() {
}
/**
* <p>Runs on the UI thread after {@link #doInBackground}. The
* specified result is the value returned by {@link #doInBackground}.</p>
*
* <p>This method won't be invoked if the task was cancelled.</p>
*
* @param result The result of the operation computed by {@link #doInBackground}.
*/
@SuppressWarnings({"UnusedDeclaration"})
protected void onPostExecute(Result result) {
}
/**
* 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.泛型参数为进度条
*
*/
@SuppressWarnings({"UnusedDeclaration"})
protected void onProgressUpdate(Progress... values) {
}
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}一句话又开始跳转： 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;
}3-13行是检查任务启动是否合理 17行调用：onPreExecute();明显该方法在主线程调用。19行更新mWorker参数（实现此为callable接口的对象 后面会提到）。20行为
exec.execute(mFuture);看又开始跳转。exec为传输的参数：
sDefaultExecutor故接着跳入：
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);
}
}
}mTask为ArrayDeque为Runnable 数组双端队列。但同时存在线程不安全性
故方法名前添加synchronized关键字。mTask.offer 每添加一个元素，就把元素加到数组的尾部。这里每次回去创建一个线程。在线程run方法中执行r.run。重点来了r是之前提到的mFutrue。查找下源码的该成员变量。 private final FutureTask<Result> mFuture;
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);
}
}
};
}
我们先看FutrueTask是个什么东东
如果对多线程很熟悉的童鞋不会陌生。FutureTask是一个RunnableFuture<V>，而RunnableFuture实现了Runnbale又实现了Futrue<V>这两个接口。public class FutureTask<V> implements RunnableFuture<V>
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}此处源码码mFuture = new FutureTask<Result>(mWorker)
调用的是
public FutureTask(Callable<V> callable)
mWorker实现callable方法。具体mFuture如何调用callable方法参见：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));
}
};有人会奇怪 mWork怎么会实现了callable方法？看此处：private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}callable接口与runnable很相似。不过最大好处在于callable接口的call方法有返回值。回到mWorker的call方法中。返回了postResult(doInBackground(mParams))
这里先是调用了我们的doInBackground(mParams)显然此处在非主线程 故可以处理耗时任务 最后通过postResult返回结果 private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}我们发现熟悉的了 通过sHandler来处理消息。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;
}
}
}
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}这里判断任务 是否被cancelled 否的话调用onPostExecute(result); 是的话调用 onCancelled(result);并更新状态。此处完成。 回到handleMessage MESSAGE_POST_PROGRESS是什么谁发出的呢？protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}那什么时候调用的呢？使用过AsyncTask的童鞋会记得。我们会在doInbackgroud方法中调用 publishProgress（）方法。整个asyncTask方法阅读完成。但有些坑。比如为什么采用线程池+FutureTask方式。又比如public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);此处线程池的个数与之前3.0 版本AsyncTask设置不一样。版权声明：本文为博主原创文章，未经博主允许不得转载。
原文地址：http://blog.csdn.net/nothingl3/article/details/48086269
