Android API之android.os.AsyncTask

android.os.AsyncTask<Params, Progress, Result>

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.

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, called begin, doInBackground, processProgress and end.

要点：AsyncTask允许执行后台操作。无需线程、句柄，AsyncTask就可以将结果发布到UI线程。

Usage

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

要点：AsyncTask必须被扩展。子类至少重写doInBackground方法，通常还会重写onPostExcute方法。

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));
         }
         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 type Void:

并不是所有类型都用到，没用到的可以标记成Void。

 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 immediately after 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, invoked on the background thread immediately after onPreExecute() 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 use publishProgress to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate step.
3. onProgressUpdate, invoked on the UI thread after a call to publishProgress. 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, invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

Threading rules

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

• The task instance must be created on the UI thread.
• execute must be invoked on the UI thread.
• Do not call onPreExecute(), onPostExecute, doInBackground, onProgressUpdate manually.
• The task can be executed only once (an exception will be thrown if a second execution is attempted.)