Android 线程操作之AsyncTask

/**
 *Layout XML
 **/
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <Button
        android:id="@+id/execute"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="execute"/>
    <Button
        android:id="@+id/cancel"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:enabled="false"
        android:text="cancel"/>
    <ProgressBar
        android:id="@+id/progress_bar"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:progress="0"
        android:max="100"
        style="?android:attr/progressBarStyleHorizontal"/>
    <ScrollView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content">
        <TextView
            android:id="@+id/text_view"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"/>
    </ScrollView>
</LinearLayout>

/**
 *AndroidManifest.xml
 **/
<uses-permission android:name="android.permission.INTERNET"/>

/**
 *Activity code
 **/
package com.scott.async;

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.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;

public class MainActivity extends Activity {
    
  private static final String TAG = "ASYNC_TASK";
    
  private Button execute;
  private Button cancel;
  private ProgressBar progressBar;
  private TextView textView;
    
  private MyTask mTask;

  @Override
  public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        execute = (Button) findViewById(R.id.execute);
        execute.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //注意每次需new一个实例,新建的任务只能执行一次,否则会出现异常
                mTask = new MyTask();
                mTask.execute("http://www.baidu.com");
                
                execute.setEnabled(false);
                cancel.setEnabled(true);
            }
        });
        cancel = (Button) findViewById(R.id.cancel);
        cancel.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //取消一个正在执行的任务,onCancelled方法将会被调用
                mTask.cancel(true);
            }
        });
        progressBar = (ProgressBar) findViewById(R.id.progress_bar);
        textView = (TextView) findViewById(R.id.text_view);
        
    }
    
    private class MyTask extends AsyncTask<String, Integer, String> {
        //onPreExecute方法用于在执行后台任务前做一些UI操作
        @Override
        protected void onPreExecute() {
            Log.i(TAG, "onPreExecute() called");
            textView.setText("loading...");
        }
        
        //doInBackground方法内部执行后台任务,不可在此方法内修改UI
        @Override
        protected String doInBackground(String... params) {
            Log.i(TAG, "doInBackground(Params... params) called");
            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 baos = new ByteArrayOutputStream();
                    byte[] buf = new byte[1024];
                    int count = 0;
                    int length = -1;
                    while ((length = is.read(buf)) != -1) {
                        baos.write(buf, 0, length);
                        count += length;
                        //调用publishProgress公布进度,最后onProgressUpdate方法将被执行
                        publishProgress((int) ((count / (float) total) * 100));
                        //为了演示进度,休眠500毫秒
                        Thread.sleep(500);
                    }
                    return new String(baos.toByteArray(), "gb2312");
                }
            } catch (Exception e) {
                Log.e(TAG, e.getMessage());
            }
            return null;
        }
        
        //onProgressUpdate方法用于更新进度信息
        @Override
        protected void onProgressUpdate(Integer... progresses) {
            Log.i(TAG, "onProgressUpdate(Progress... progresses) called");
            progressBar.setProgress(progresses[0]);
            textView.setText("loading..." + progresses[0] + "%");
        }
        
        //onPostExecute方法用于在执行完后台任务后更新UI,显示结果
        @Override
        protected void onPostExecute(String result) {
            Log.i(TAG, "onPostExecute(Result result) called");
            textView.setText(result);
            
            execute.setEnabled(true);
            cancel.setEnabled(false);
        }
        
        //onCancelled方法用于在取消执行中的任务时更改UI
        @Override
        protected void onCancelled() {
            Log.i(TAG, "onCancelled() called");
            textView.setText("cancelled");
            progressBar.setProgress(0);
            
            execute.setEnabled(true);
            cancel.setEnabled(false);
        }
    }
}