Android中AsyncTask使用实例，,MainActivity

Android中AsyncTask使用实例，,MainActivity

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       @Override    protected 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     @Override    protected 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    @Override    protected 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       @Override    protected void onProgressUpdate(Integer... values) {        super.onProgressUpdate(values);        //更改进度条        progressBar.setProgress(values[0]);        //更改TextView        textView.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     @Override    protected void onCancelled() {        super.onCancelled();        //更改进度条进度为0        progressBar.setProgress(0);        //更改TextView        textView.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:```xml<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>```