AsyncTask

AsyncTask

AsyncTask的设计目标是比较短的操作（最多几秒），如果希望执行更长时间，使用 java.util.concurrent 中的 Executor, ThreadPoolExecutor, FutureTask更合适。

Asyntask的泛型实现：

• Params：发送给Asyntask的执行参数类型
• Progress：the type of the progress units published during the background computation
• Result: background计算阶段的结果类型

Asyntask执行中的四步：

• onPreExecute() : 任务执行前执行，一般用途例如显示加载框，进度条
• doInBackground(Params...) : 执行后台任务（途中可更新进度条）
• onProgressUpdate(Progress...) : 更新进度条
• onPostExecute(Result) :

注意点

• Asyntask实例必须在UI线程中简历
• execute（Params...）必须在UI线程中调用。
  new DownloadFilesTask().execute(urlStr);
• 不要手动调用onPreExecute(), onPostExecute()..等方法
• The task can be executed only once(an exception will be thrown if a second execution is attempted)

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)