Android AsyncTask示例教程
今天我们要研究Android AsyncTask。我们将开发一个Android示例应用程序，在后台执行一个抽象的AsyncTask。
Android 异步任务
Android AsyncTask是由Android提供的一个抽象类，它使我们可以在后台执行重任务，同时保持UI线程的轻量化，从而使应用程序更加响应。Android应用在启动时只运行在一个线程上。由于这种单线程模型，执行需要较长时间来获取响应的任务可能会使应用程序无法响应。为了避免这种情况，我们使用android AsyncTask在专用线程上执行重任务，并将结果传递回UI线程。因此，在Android应用中使用AsyncTask始终使UI线程保持响应。下面定义了在android AsyncTask类中使用的基本方法：
- doInBackground() : This method contains the code which needs to be executed in background. In this method we can send results multiple times to the UI thread by publishProgress() method. To notify that the background processing has been completed we just need to use the return statements
- onPreExecute() : This method contains the code which is executed before the background processing starts
- onPostExecute() : This method is called after doInBackground method completes processing. Result from doInBackground is passed to this method
- onProgressUpdate() : This method receives progress updates from doInBackground method, which is published via publishProgress method, and this method can use this progress update to update the UI thread
在Android的AsyncTask类中，有以下三种通用类型。
- Params : The type of the parameters sent to the task upon execution
- Progress : The type of the progress units published during the background computation
- Result : The type of the result of the background computation
安卓AsyncTask示例
要启动一个AsyncTask，在MainActivity类中必须包含以下代码段：
MyTask myTask = new MyTask();
myTask.execute();
在上面的代码片段中，我们使用了一个扩展了AsyncTask类的示例类名，并使用execute方法来启动后台线程。注意：
- The AsyncTask instance must be created and invoked in the UI thread.
- The methods overridden in the AsyncTask class should never be called. They’re called automatically
- AsyncTask can be called only once. Executing it again will throw an exception
在这个教程中，我们将实现一个AsyncTask，它能让进程根据用户设置的时间段进入睡眠状态。
安卓异步任务项目结构
安卓异步任务示例代码
xml布局在activity_main.xml中定义，如下所示：activity_main.xml
<RelativeLayout xmlns:android="https://schemas.android.com/apk/res/android"
xmlns:tools="https://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity" >
<TextView
android:id="@+id/tv_time"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textSize="10pt"
android:textColor="#444444"
android:layout_alignParentLeft="true"
android:layout_marginRight="9dip"
android:layout_marginTop="20dip"
android:layout_marginLeft="10dip"
android:text="Sleep time in Seconds:"/>
<EditText
android:id="@+id/in_time"
android:layout_width="150dip"
android:layout_height="wrap_content"
android:background="@android:drawable/editbox_background"
android:layout_toRightOf="@id/tv_time"
android:layout_alignTop="@id/tv_time"
android:inputType="number"
/>
<Button
android:id="@+id/btn_run"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Run Async task"
android:layout_below="@+id/in_time"
android:layout_centerHorizontal="true"
android:layout_marginTop="64dp" />
<TextView
android:id="@+id/tv_result"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textSize="7pt"
android:layout_below="@+id/btn_run"
android:layout_centerHorizontal="true" />
</RelativeLayout>
在上述布局中，我们使用了预定义的可绘制对象作为EditText的边框。下面是MainActivity.java的定义。
package com.Olivia.asynctask;
import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
private Button button;
private EditText time;
private TextView finalResult;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
time = (EditText) findViewById(R.id.in_time);
button = (Button) findViewById(R.id.btn_run);
finalResult = (TextView) findViewById(R.id.tv_result);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
AsyncTaskRunner runner = new AsyncTaskRunner();
String sleepTime = time.getText().toString();
runner.execute(sleepTime);
}
});
}
private class AsyncTaskRunner extends AsyncTask<String, String, String> {
private String resp;
ProgressDialog progressDialog;
@Override
protected String doInBackground(String... params) {
publishProgress("Sleeping..."); // Calls onProgressUpdate()
try {
int time = Integer.parseInt(params[0])*1000;
Thread.sleep(time);
resp = "Slept for " + params[0] + " seconds";
} catch (InterruptedException e) {
e.printStackTrace();
resp = e.getMessage();
} catch (Exception e) {
e.printStackTrace();
resp = e.getMessage();
}
return resp;
}
@Override
protected void onPostExecute(String result) {
// execution of result of Long time consuming operation
progressDialog.dismiss();
finalResult.setText(result);
}
@Override
protected void onPreExecute() {
progressDialog = ProgressDialog.show(MainActivity.this,
"ProgressDialog",
"Wait for "+time.getText().toString()+ " seconds");
}
@Override
protected void onProgressUpdate(String... text) {
finalResult.setText(text[0]);
}
}
}
在以上代码中，我们使用AsyncTaskRunner类来执行AsyncTask操作。以秒为单位的时间作为参数传递给该类，并且对于给定的时间量显示一个ProgressDialog。下面的图片是项目生成的输出，用户设定的时间为5秒。这就结束了本教程。您可以从下面的链接下载最终的Android AsyncTask项目。
下载Android AsyncTask示例项目
