Android AsyncTask实例

package com.example.asynctask;

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 com.example.androidtestproject.R;

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.csdn.net");

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
if ((mTask != null)) {
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];
long count = 0;
int length = -1;
Log.i(TAG, "total=" + total);
while ((length = is.read(buf)) != -1) {
if (isCancelled()) {
Log.i(TAG, "isCancelled=true");
return null; // don't forget to terminate this method
}

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50毫秒
Thread.sleep(50);
}

return new String(baos.toByteArray(), "utf-8");
}
} catch (Exception e) {
Log.e(TAG, "exception");
}

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() {
// TODO Auto-generated method stub
Log.i(TAG, "onCancelled called");
textView.append(", Cancelled");

execute.setEnabled(true);
cancel.setEnabled(false);
}
}

@Override
protected void onDestroy() {
// TODO Auto-generated method stub
super.onDestroy();
Log.i(TAG, "onDestroy called");
// 当Activity Destory时判断任务是否还在运行，还在运行的话则取消任务运行，否则Activity Destory掉后mTask还在后台运行，直到结束。
if ((mTask != null) && (mTask.getStatus() == AsyncTask.Status.RUNNING)) {
Log.i(TAG, "mTask.cancel");
mTask.cancel(true);
}
}

}

long total = entity.getContentLength();