Android AsyncTask 示例及说明
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Android AsyncTask example and explanation
提问by Suragch
I want to use an AsyncTaskin my app, but I am having trouble finding a code snippet with a simple explanation of how things work. I just want something to help me get back up to speed quickly without having to wade through the documentationor lots of Q&As again.
我想AsyncTask在我的应用程序中使用 ，但我无法找到一个代码片段，其中包含对事物如何工作的简单解释。我只是想要一些东西来帮助我快速恢复速度，而不必再次浏览文档或大量问答。
回答by Suragch
AsyncTaskis one of the easiest ways to implement parallelism in Android without having to deal with more complex methods like Threads. Though it offers a basic level of parallelism with the UI thread, it should not be used for longer operations (of, say, not more than 2 seconds).
AsyncTask是在 Android 中实现并行性的最简单方法之一，而无需处理更复杂的方法，如线程。尽管它提供了与 UI 线程的基本并行级别，但不应将其用于较长的操作（例如，不超过 2 秒）。
AsyncTask has four methods
AsyncTask 有四种方法
onPreExecute()doInBackground()onProgressUpdate()onPostExecute()
onPreExecute()doInBackground()onProgressUpdate()onPostExecute()
where doInBackground()is the most important as it is where background computations are performed.
哪里doInBackground()是最重要的，因为它是执行后台计算的地方。
Code:
代码：
Here is a skeletal code outline with explanations:
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
}
}
}
Flow Diagram:
流程图：
Here is a diagram to help explain where all the parameters and types are going:
这是一个图表，可帮助解释所有参数和类型的去向：
Other helpful links:
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
