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前言：
/**
AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers.
AsyncTask is designed to be a helper class around {@link Thread} and {@link Handler} and does not constitute a generic threading framework. AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs
provided by the java.util.concurrent package such as {@link Executor},{@link ThreadPoolExecutor} and {@link FutureTask}.
An asynchronous task is defined by a computation that runs on a background thread and whose result is published on the UI thread. An asynchronous task is defined by 3 generic types, called Params, Progress and Result,and 4 steps, calledonPreExecute, doInBackground, onProgressUpdate and onPostExecute.
For more information about using tasks and threads, read the
* Processes and
* Threads developer guide.
AsyncTask must be subclassed to be used. The subclass will override at least one method ({@link #doInBackground}), and most often will override a second one ({@link #onPostExecute}.)
Here is an example of subclassing:
* 译文：异步任务必须被子类继承并且必须实现至少一种方法（doInBackground）并且大多数将会覆盖第二种方法（onPostExecute）这里有一个例子如下：
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");
}
}
Once created, a task is executed very simply:
new DownloadFilesTask().execute(url1, url2, url3);
*
The three types used by an asynchronous task are the following:
Params, the type of the parameters sent to the task upon execution.Progress, the type of the progress units published during the background computation.Result, the type of the result of the background computation.Not all types are always used by an asynchronous task. To mark a type as unused,simply use the type {@link Void}
译文：不是所有类型都被一个异步任务使用。要标记为未使用的类型，只需使用该类型
* private class MyTask extends AsyncTask<Void, Void, Void> { … }
*
When an asynchronous task is executed, the task goes through 4 steps:
译文：当一个异步任务被执行时，任务就通过4个步骤来完成A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking this method will cause subsequent calls to {@link #isCancelled()} to return true. After invoking this method, {@link #onCancelled(Object)}, instead of {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])} returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value of {@link #isCancelled()} periodically from{@link #doInBackground(Object[])}, if possible (inside a loop for instance.)
There are a few threading rules that must be followed for this class to work properly:
AsyncTask guarantees that all callback calls are synchronized in such a way that the following operations are safe without explicit synchronizations.
When first introduced, AsyncTasks were executed serially on a single background thread. Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are executed on a single thread to avoid common application errors caused by parallel execution.
If you truly want parallel execution, you can invoke{@link #executeOnExecutor(java.util.concurrent.Executor, Object[])} with
{@link #THREAD_POOL_EXECUTOR}.
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原文地址：http://blog.csdn.net/u013200864/article/details/46801887
