【Android】使用AsyncTask来处理一些简单的需要后台处理的动作
[0]首先让我们看看官网上是怎么解释AsyncTask的：
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, calledParams,ProgressandResult, and 4 steps, calledonPreExecute,doInBackground,onProgressUpdateandonPostExecute.
简单的来说AsyncTask就是一个简单的用来避免UI阻塞的，运行在background的工具线程类。我们不需要人工来处理这个线程的生命周期，而是按照一定的规则来执行的。
这个Task有3个参数，分别是传入的参数（Params），执行过程中产生的参数(Process)，与最后任务结束返回的结果(Result)。如果有不需要的参数，可以用Void来替代。
有4个步骤，分别是：
onPreExecute():用来处理任务执行前需要做的初始化
doInBackground():真正开始在后台执行操作的步骤（这个是必须override的步骤），在这个步骤中可以使用publishProgress(progress……)来提供用于与UI交互显示信息
onProgressUpdate():用来执行过程中即时显示处理进度的函数（获取到doInBackground里面传递过来的参数而进行显示）
onPostExecute();任务执行结束后做的事情
[1]下面是一个简单的AsyncTask范例：
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"); } }
要让一个Task开始执行，只需要像下面这样：
new DownloadFilesTask().execute(url1, url2, url3);
[2]那么如何取消一个AsyncTask呢？
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.)
我们可以在任何时候call cancel(boolean)的方法来取消一个Task，如果呼叫到这个方法会导致之后呼叫isCancelled()返回true.那么如果这样的话，之后会用onCancelled(Object)来替代onPostExecute(Object)的执行。
为了确保这个Task能够尽快被取消，我们需要在doInBackground(Object[])执行的时候去checkisCancelled()
- The task instance must be created on the UI thread.（这个task必须在UI thread中创建）
-
execute(Params...)must be invoked on the UI thread.（必须在UI thread中叫起执行task） - Do not call
onPreExecute(),onPostExecute(Result),doInBackground(Params...),onProgressUpdate(Progress...)manually.（不要手动去呼叫那4个方法） - The task can be executed only once (an exception will be thrown if a second execution is attempted.) （这个task必须是单次执行的，不要在这个任务没有结束前再次呼叫）
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 indoInBackground(Params...). - Set member fields in
doInBackground(Params...), and refer to them inonProgressUpdate(Progress...)andonPostExecute(Result).