Android AsyncTask 示例及说明

Question

我想在我的应用程序中使用 AsyncTask，但我无法找到一个简单解释事情工作原理的代码 fragment 。我只是想要一些东西来帮助我快速恢复速度，而不必涉水the documentation或再次进行大量问答。

Answer 1

AsyncTask 是在 Android 中实现并行性的最简单方法之一，无需处理线程等更复杂的方法。尽管它提供了与 UI 线程的基本并行度，但它不应用于更长的操作(例如，不超过 2 秒)。

AsyncTask 有四个方法

• onPreExecute()
• doInBackground()
• onProgressUpdate()
• onPostExecute()

其中 doInBackground() 是最重要的，因为它是执行后台计算的地方。

代码:

这是一个带有解释的骨架代码大纲:

public class AsyncTaskTestActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);  

        // This starts the AsyncTask
        // Doesn't need to be in onCreate()
        new MyTask().execute("my string parameter");
    }

    // Here is the AsyncTask class:
    //
    // AsyncTask<Params, Progress, Result>.
    //    Params – the type (Object/primitive) you pass to the AsyncTask from .execute() 
    //    Progress – the type that gets passed to onProgressUpdate()
    //    Result – the type returns from doInBackground()
    // Any of them can be String, Integer, Void, etc. 

    private class MyTask extends AsyncTask<String, Integer, String> {

        // Runs in UI before background thread is called
        @Override
        protected void onPreExecute() {
            super.onPreExecute();

            // Do something like display a progress bar
        }

        // This is run in a background thread
        @Override
        protected String doInBackground(String... params) {
            // get the string from params, which is an array
            String myString = params[0];

            // Do something that takes a long time, for example:
            for (int i = 0; i <= 100; i++) {

                // Do things

                // Call this to update your progress
                publishProgress(i);
            }

            return "this string is passed to onPostExecute";
        }

        // This is called from background thread but runs in UI
        @Override
        protected void onProgressUpdate(Integer... values) {
            super.onProgressUpdate(values);

            // Do things like update the progress bar
        }

        // This runs in UI when background thread finishes
        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);

            // Do things like hide the progress bar or change a TextView
        }
    }
}

流程图:

这是一个图表，可以帮助解释所有参数和类型的去向:

其他有用的链接:

• What arguments are passed into AsyncTask<arg1, arg2, arg3>?
• Slidenerd Android AsyncTask Tutorial: Android Tutorial For Beginners
• Understanding AsyncTask – Once and Forever
• Dealing with AsyncTask and Screen Orientation
• How to pass multiple parameters to AsynkTask
• how to pass in two different data types to AsyncTask, Android