AsyncTask 学习笔记

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 可以以适当且容易的方式去使用UI线程。该类容许你不用操做线程和处理器就能够去去执行后台操做和返回结果给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 as Executor, ThreadPoolExecutor and FutureTask.

AsyncTask 被设计成助手类而不失一个通用线程框架，AsyncTasks在短操做（最多几秒钟）使用是理想的，若是你须要保持线程运行很长一段时间，强烈建议你使用各类java.util.concurrent包中提供的api，如Executor，ThreadPoolExecutor、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, called onPreExecute, doInBackground, onProgressUpdate and onPostExecute.

异步任务被定义为一个运行在后台线程的计算，其计算结果返回到UI线程上。异步任务被定义为3泛型类型。称为Params，Progress和Result，和四步，称为onPreExecute，doInBackground，onProgressUpdate和onPostExecute

AsyncTask's generic types

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

所使用的三种类型的异步任务以下：

1. Params, the type of the parameters sent to the task upon execution.

   Params，这参数类型在任务执行时被发送

2. Progress, the type of the progress units published during the background computation.

   Progress，这参数类型直到后台计算完成被发送

3. Result, the type of the result of the background computation.

   Result，这result参数类型是后台的计算结果

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

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

当一个异步任务被执行，这任务会通过四个步骤：

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.

   onPreExecute（）方法在任务被执行前被UI线程调用，一般在这里设置一些任务，如实例化一个展现的进度条

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.

   doInBackground（Params。。。）方法在onPreExecute（）方法执行完成后当即被调用。
   

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

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 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 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.)