Android中AsyncTask使用實例

MainActivity如下:

[java]
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一個實例,新建的任務只能執行一次,否則會出現異常 
                //2 異步任務的實例必須在UI線程中創建 
                //3 execute()方法必須在UI線程中調用。 
                downLoaderAsyncTask=new DownLoaderAsyncTask(); 
                downLoaderAsyncTask.execute("http://www.baidu.com"); 
                break; 
            case R.id.cancelButton: 
                //取消一個正在執行的任務,onCancelled()方法將會被調用    
                downLoaderAsyncTask.cancel(true); 
                break; 
            default: 
                break; 
            } 
        } 
        
   } 
   //構造函數AsyncTask<Params, Progress, Result>參數說明:  
   //Params   啟動任務執行的輸入參數 
   //Progress 後台任務執行的進度 
   //Result   後台計算結果的類型 
   private class DownLoaderAsyncTask extends AsyncTask<String, Integer, String>{ 
    //onPreExecute()方法用於在執行異步任務前,主線程做一些准備工作    
    @Override 
    protected void onPreExecute() { 
        super.onPreExecute(); 
        textView.setText("調用onPreExecute()方法--->准備開始執行異步任務"); 
        System.out.println("調用onPreExecute()方法--->准備開始執行異步任務"); 
    } 
     
    //doInBackground()方法用於在執行異步任務,不可以更改主線程中UI  
    @Override 
    protected String doInBackground(String... params) { 
       System.out.println("調用doInBackground()方法--->開始執行異步任務"); 
        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()為AsyncTask類中的方法 
                    //常在doInBackground()中調用此方法 
                    //用於通知主線程,後台任務的執行情況. 
                    //此時會觸發AsyncTask中的onProgressUpdate()方法 
                    publishProgress((int) ((count / (float) total) * 100)); 
                    //為了演示進度,休眠1000毫秒 
                    Thread.sleep(1000); 
                } 
                return new String(bos.toByteArray(), "UTF-8"); 
            } 
        } catch (Exception e) { 
            return null; 
        } 
        return null; 
    } 
     
    //onPostExecute()方法用於異步任務執行完成後,在主線程中執行的操作 
    @Override 
    protected void onPostExecute(String result) { 
        super.onPostExecute(result);     
        Toast.makeText(getApplicationContext(), "調用onPostExecute()方法--->異步任務執行完畢", 0).show(); 
        //textView顯示網絡請求結果 
        textView.setText(result); 
        System.out.println("調用onPostExecute()方法--->異步任務執行完畢"); 
    } 
        
    //onProgressUpdate()方法用於更新異步執行中,在主線程中處理異步任務的執行信息    
    @Override 
    protected void onProgressUpdate(Integer... values) { 
        super.onProgressUpdate(values); 
        //更改進度條 
        progressBar.setProgress(values[0]); 
        //更改TextView 
        textView.setText("已經加載"+values[0]+"%"); 
    } 
     
    //onCancelled()方法用於異步任務被取消時,在主線程中執行相關的操作  
    @Override 
    protected void onCancelled() { 
        super.onCancelled(); 
        //更改進度條進度為0 
        progressBar.setProgress(0); 
        //更改TextView 
        textView.setText("調用onCancelled()方法--->異步任務被取消"); 
        System.out.println("調用onCancelled()方法--->異步任務被取消"); 
    } 
   } 
} 

main.xml如下:

[html] 
<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="開始異步任務" /> 
 
    <Button 
        android:id="@+id/cancelButton" 
        android:layout_width="fill_parent" 
        android:layout_height="wrap_content" 
        android:text="取消異步任務" /> 
 
    <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>