AsyncTask 學習筆記

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 可以以適當且容易的方式去使用UI線程。該類容許你不用操做線程和處理器就能夠去去執行後臺操做和返回結果給UI線程

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 被設計成助手類而不失一個通用線程框架，AsyncTasks在短操做（最多幾秒鐘）使用是理想的，若是你須要保持線程運行很長一段時間，強烈建議你使用各類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.

異步任務被定義爲一個運行在後臺線程的計算，其計算結果返回到UI線程上。異步任務被定義爲3泛型類型。稱爲Params，Progress和Result，和四步，稱爲onPreExecute，doInBackground，onProgressUpdate和onPostExecute

AsyncTask's generic types

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

所使用的三種類型的異步任務以下：

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

   Params，這參數類型在任務執行時被髮送

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

   Progress，這參數類型直到後臺計算完成被髮送

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

   Result，這result參數類型是後臺的計算結果

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:

當一個異步任務被執行，這任務會通過四個步驟：

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（）方法在任務被執行前被UI線程調用，一般在這裏設置一些任務，如實例化一個展現的進度條

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（）方法執行完成後當即被調用。
   

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.)