15AsyncTask

Class Overview

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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在后台执行操作，将结果返回给UI线程。

AsyncTask is designed to be a helper class around Thread and 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 asExecutor, ThreadPoolExecutor and FutureTask.

AsyncTask 适用于很短的操作（最多几秒钟），如果想要让线程运行很长时间，需要使用并发包下的一些类。

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, Progress and Result, and 4 steps, called onPreExecute, doInBackground, onProgressUpdate and onPostExecute.

AsyncTask 的使用包括三个参数，四个步骤。

Developer Guides

For more information about using tasks and threads, read the Processes and Threads developer guide.

Usage

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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

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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:

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

The 4 steps

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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 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(Progress...) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Progress...) step.
3. onProgressUpdate(Progress...), invoked on the UI thread after a call to publishProgress(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

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

Threading rules

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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 of JELLY_BEAN.
• The task instance must be created on the UI thread.
• execute(Params...) must be invoked on the UI thread.
• Do not call onPreExecute(), 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

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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 or onPreExecute(), and refer to them in doInBackground(Params...).
• Set member fields in doInBackground(Params...), and refer to them in onProgressUpdate(Progress...) and onPostExecute(Result).

Order of execution

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When first introduced, AsyncTasks were executed serially on a single background thread. Starting with DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with 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 executeOnExecutor(java.util.concurrent.Executor, Object[]) with THREAD_POOL_EXECUTOR.