【Android】AsyncTask实现异步处理

AsyncTask提供了简单易用的方式，执行后台操作并更新UI。

AsyncTask的3个泛型

• Param  传入数据类型
• Progress  更新UI数据类型
• Result  处理结果类型

AsyncTask的4个步骤

1. onPreExecute  执行前的操作
2. doInBackGround  后台执行的操作
3. onProgressUpdate  更新UI操作
4. onPostExecute  执行后的操作

示例代码：

 

下载远端资源，需要INTERNET权限
将资源写入到SD，需要WRITE_EXTERNAL_STORAGE权限
在AndroidManifest.xml中进行如下配置

 

    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

功能：

异步下载，并根据下载进步更新ProgresBar，下载完成后显示图像

 

package lizhen.download;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;

import android.os.AsyncTask;

public class DownloadAsyncTask extends AsyncTask<String, Integer, Boolean>{

	protected int size;
	protected String errorMessage;

	@Override
	protected Boolean doInBackground(String... params) {
		String url = params[0]; //資源地址
		String path = params[1]; //目標路徑
		try {
			InputStream source = requestInputStream(url); //請求源文件InputStream
			/*
			 * 創建文件寫入數據，并更新進度
			 * */
			fileWrite(source, path);

		} catch (ClientProtocolException e) {
			errorMessage = e.getMessage();
			cancel(true);
			return false;
		} catch (IOException e) {
			errorMessage = e.getMessage();
			cancel(true);
			return false;
		}
		return true;
	}
	
	/**
	 * 文件寫入
	 * @param in 數據源輸入流
	 * @param path 文件路徑
	 * @param listener 下載進度監聽器
	 * */
	private void fileWrite(InputStream in, String path) throws IOException {
		File file = createFile(path);
		FileOutputStream fileOutputStream = new FileOutputStream(file);
		byte buffer[] = new byte[1024];
		int progress = 0;
        int readBytes = 0;
        while ((readBytes = in.read(buffer)) != -1) {
        	progress += readBytes; 
            fileOutputStream.write(buffer, 0, readBytes);
            publishProgress(progress);
        }
        in.close();
        fileOutputStream.close();
	}
	
	/**
	 * 根據資源URL地址取得資源輸入流
	 * @param url URL地址
	 * @return 資源輸入流
	 * @throws ClientProtocolException
	 * @throws IOException
	 * */
	private InputStream requestInputStream(String url) throws ClientProtocolException, IOException {
		InputStream result = null;
		HttpGet httpGet = new HttpGet(url);
		HttpClient httpClient = new DefaultHttpClient();
		HttpResponse httpResponse = httpClient.execute(httpGet);
		int httpStatusCode = httpResponse.getStatusLine().getStatusCode();
		if(httpStatusCode == HttpStatus.SC_OK) {
			HttpEntity httpEntity = httpResponse.getEntity();
			size = (int) httpEntity.getContentLength();
			result = httpEntity.getContent();
		}
		return result;
	}
	
	/**
	 * 根據文件路徑創建文件
	 * @param path 文件路徑
	 * @return 文件File實例
	 * @return IOException
	 * */
	private File createFile(String path) throws IOException {
		File file = new File(path);
		file.createNewFile();
		return file;
	}
	
	/**
	 * 返回錯誤信息
	 * @return 錯誤信息
	 * */
	public String getErrorString() {
		return this.errorMessage;
	}
	
}

 

package lizhen.download;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;

public class Download extends Activity { 
    
	private Button startButton;
	private ProgressBar downloadProgressBar;
	private TextView progressTextView;
	private ImageView downloadImageView;
	
	private final String source = "http://192.168.211.86/comic/test.jpg"; //源文件地址
	private final String path = Environment.getExternalStorageDirectory().toString()+"/test.jpg"; //目標文件地址
	
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.download);
        
        startButton = (Button) findViewById(R.id.download_StartButton);
        downloadProgressBar = (ProgressBar) findViewById(R.id.download_ProgressBar);
        progressTextView = (TextView) findViewById(R.id.download_ProgressTextView);
        downloadImageView = (ImageView) findViewById(R.id.download_ImageView);
        
        startButton.setOnClickListener(new View.OnClickListener() {
			
			@Override
			public void onClick(View v) {
				new DownloadTask().execute(source, path); 
			}
		});
    }
    
    private class DownloadTask extends DownloadAsyncTask {

		@Override
		protected void onProgressUpdate(Integer... values) {
			super.onProgressUpdate(values);
			int progress = values[0];
			/*
			 * 更新進度條和下載百分率
			 * */
			downloadProgressBar.setMax(size);
			downloadProgressBar.setProgress(progress);
			int percentage = progress*100/size;
			progressTextView.setText("已完成"+percentage+"%");
		}

		@Override
		protected void onPostExecute(Boolean result) {
			super.onPostExecute(result);
			if(result) {
				Bitmap bitmap = BitmapFactory.decodeFile(path);
				downloadImageView.setImageBitmap(bitmap);
			} else {
				Toast.makeText(Download.this, "Error: "+errorMessage, 1000).show();
			}
		}    	
    }
}

 

运行结果：