AsyncTask

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Class Overview

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 aroundThreadandHandlerand 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 thejava.util.concurrentpackage such asExecutor,ThreadPoolExecutorandFutureTask.

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, calledParams,ProgressandResult, and 4 steps, calledonPreExecute,doInBackground,onProgressUpdateandonPostExecute.

Usage

AsyncTask must be subclassed to be used. The subclass will override at least one method (doInBackground(Params...)), and most often will override a second one (onPostExecute(Result).)

Here is an example of subclassing:

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);  

AsyncTask‘s generic types

The three types used by an asynchronous task are the following:

1. Params, the type of the parameters sent to the task upon execution.
2. Progress, the type of the progress units published during the background computation.
3. 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 typeVoid:

 private class MyTask extends AsyncTask<Void, Void, Void> { ... }  

The 4 steps

When an asynchronous task is executed, the task goes through 4 steps:

1. onPreExecute(), invoked on the UI thread before the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface.
2. doInBackground(Params...), invoked on the background thread immediately afteronPreExecute()finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also usepublishProgress(Progress...)to publish one or more units of progress. These values are published on the UI thread, in theonProgressUpdate(Progress...)step.
3. onProgressUpdate(Progress...), invoked on the UI thread after a call topublishProgress(Progress...). The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field.
4. onPostExecute(Result), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

Cancelling a task

A task can be cancelled at any time by invokingcancel(boolean). Invoking this method will cause subsequent calls toisCancelled()to return true. After invoking this method,onCancelled(Object), instead ofonPostExecute(Object)will be invoked afterdoInBackground(Object[])returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value ofisCancelled()periodically fromdoInBackground(Object[]), if possible (inside a loop for instance.)

Threading rules

There are a few threading rules that must be followed for this class to work properly:

• The AsyncTask class must be loaded on the UI thread. This is done automatically as ofJELLY_BEAN.
• The task instance must be created on the UI thread.
• execute(Params...)must be invoked on the UI thread.
• Do not callonPreExecute(),onPostExecute(Result),doInBackground(Params...),onProgressUpdate(Progress...)manually.
• The task can be executed only once (an exception will be thrown if a second execution is attempted.)

Memory observability

AsyncTask guarantees that all callback calls are synchronized in such a way that the following operations are safe without explicit synchronizations.

• Set member fields in the constructor oronPreExecute(), and refer to them indoInBackground(Params...).
• Set member fields indoInBackground(Params...), and refer to them inonProgressUpdate(Progress...)andonPostExecute(Result).

Order of execution

When first introduced, AsyncTasks were executed serially on a single background thread. Starting withDONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting withHONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution.

If you truly want parallel execution, you can invokeexecuteOnExecutor(java.util.concurrent.Executor, Object[])withTHREAD_POOL_EXECUTOR.

AsyncTask

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原文地址：http://my.oschina.net/u/588339/blog/402188