AsyncTask异步任务使用详细实例(一)



MainActivity如下:

package com.example.asynctasktest;import java.io.ByteArrayOutputStream;import java.io.InputStream;import org.apache.http.HttpEntity;import org.apache.http.HttpResponse;import org.apache.http.HttpStatus;import org.apache.http.client.HttpClient;import org.apache.http.client.methods.HttpGet;import org.apache.http.impl.client.DefaultHttpClient;import android.app.Activity;import android.os.AsyncTask;import android.os.Bundle;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import android.widget.ProgressBar;import android.widget.TextView;import android.widget.Toast;public class MainActivity extends Activity {private Button satrtButton;private Button cancelButton;private ProgressBar progressBar;private TextView textView;private DownLoaderAsyncTask downLoaderAsyncTask;    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        initView();    }public void initView() {satrtButton=(Button) findViewById(R.id.startButton);cancelButton=(Button) findViewById(R.id.cancelButton);satrtButton.setOnClickListener(new ButtonOnClickListener());cancelButton.setOnClickListener(new ButtonOnClickListener());progressBar=(ProgressBar) findViewById(R.id.progressBar);textView=(TextView) findViewById(R.id.textView);}   private class ButtonOnClickListener implements OnClickListener{public void onClick(View v) {switch (v.getId()) {case R.id.startButton://注意://1 每次需new一个实例,新建的任务只能执行一次,否则会出现异常//2 异步任务的实例必须在UI线程中创建//3 execute()方法必须在UI线程中调用。downLoaderAsyncTask=new DownLoaderAsyncTask();downLoaderAsyncTask.execute("http://www.baidu.com");break;case R.id.cancelButton://取消一个正在执行的任务,onCancelled()方法将会被调用                   downLoaderAsyncTask.cancel(true);break;default:break;}}      }   //构造函数AsyncTask<Params, Progress, Result>参数说明:    //Params   启动任务执行的输入参数   //Progress 后台任务执行的进度   //Result   后台计算结果的类型   private class DownLoaderAsyncTask extends AsyncTask<String, Integer, String>{//onPreExecute()方法用于在执行异步任务前,主线程做一些准备工作   @Overrideprotected void onPreExecute() {super.onPreExecute();textView.setText("调用onPreExecute()方法--->准备开始执行异步任务");System.out.println("调用onPreExecute()方法--->准备开始执行异步任务");}//doInBackground()方法用于在执行异步任务,不可以更改主线程中UI @Overrideprotected String doInBackground(String... params) {   System.out.println("调用doInBackground()方法--->开始执行异步任务");try {HttpClient client = new DefaultHttpClient();HttpGet get = new HttpGet(params[0]);HttpResponse response = client.execute(get);if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {HttpEntity entity = response.getEntity();InputStream is = entity.getContent();long total = entity.getContentLength();ByteArrayOutputStream bos = new ByteArrayOutputStream();byte[] buffer = new byte[1024];int count = 0;int length = -1;while ((length = is.read(buffer)) != -1) {bos.write(buffer, 0, length);count += length;//publishProgress()为AsyncTask类中的方法//常在doInBackground()中调用此方法//用于通知主线程,后台任务的执行情况.//此时会触发AsyncTask中的onProgressUpdate()方法publishProgress((int) ((count / (float) total) * 100));//为了演示进度,休眠1000毫秒Thread.sleep(1000);}return new String(bos.toByteArray(), "UTF-8");}} catch (Exception e) {return null;}return null;}//onPostExecute()方法用于异步任务执行完成后,在主线程中执行的操作@Overrideprotected void onPostExecute(String result) {super.onPostExecute(result);Toast.makeText(getApplicationContext(), "调用onPostExecute()方法--->异步任务执行完毕", 0).show();//textView显示网络请求结果textView.setText(result);System.out.println("调用onPostExecute()方法--->异步任务执行完毕");}   //onProgressUpdate()方法用于更新异步执行中,在主线程中处理异步任务的执行信息   @Overrideprotected void onProgressUpdate(Integer... values) {super.onProgressUpdate(values);//更改进度条progressBar.setProgress(values[0]);//更改TextViewtextView.setText("已经加载"+values[0]+"%");}//onCancelled()方法用于异步任务被取消时,在主线程中执行相关的操作 @Overrideprotected void onCancelled() {super.onCancelled();//更改进度条进度为0progressBar.setProgress(0);//更改TextViewtextView.setText("调用onCancelled()方法--->异步任务被取消");System.out.println("调用onCancelled()方法--->异步任务被取消");}   }}

main.xml如下:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical" >    <Button        android:id="@+id/startButton"        android:layout_width="fill_parent"        android:layout_height="wrap_content"        android:text="开始异步任务" />    <Button        android:id="@+id/cancelButton"        android:layout_width="fill_parent"        android:layout_height="wrap_content"        android:text="取消异步任务" />    <ProgressBar        android:id="@+id/progressBar"        style="?android:attr/progressBarStyleHorizontal"        android:layout_width="fill_parent"        android:layout_height="wrap_content"        android:max="100"        android:progress="0" />    <ScrollView        android:id="@+id/scrollView"        android:layout_width="fill_parent"        android:layout_height="wrap_content" >        <TextView            android:id="@+id/textView"            android:layout_width="fill_parent"            android:layout_height="wrap_content"            android:text="test test" />    </ScrollView></LinearLayout>

重要参考资料:

http://blog.csdn.net/liuhe688/article/details/6532519

Thank you very much