Android - AsyncTask

在處理秏時的Android程序時，User可能感覺到系統畫面停頓，感到不協調，甚至發生ANR的錯誤。因此，該耗時程序應另指派一執行緒負責維護運作進行。
PS. UI Thread 若執行5秒以上的工作會拋出ANR的錯誤     

     
AsyncTask執行上的重要方法

(1) onPreExecute( )                               =>    任務執行前呼叫此方法      

(2) doInBackground(Params... )           =>    執行任務工作

(3) onProgressUpdate(Progress... )      =>    顯示任務執行進度
(4) onPostExecute(Result)                    =>    任務執行完成會呼叫此方法        

AsyncTask定義了三種泛型
AsyncTask<Params, Progress, Result>
(1) Params：啟動任務執行的輸入參數，設定於 execute() 的參數型別
(2) Progress：幕後工作執行的百分比
(3) Result：後台執行任務最終返回的結果，設定於 onPostExecute() 的參數型別

實作方法
package com.test;

import java.io.ByteArrayOutputStream;
import java.io.InputStream;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
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.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;

public class AsyncTeskProgress extends Activity
{
private Button Button01;
private ImageView imageView01;
private String imgURL = "http://vincentjava.skyhostsite.0lx.net/img/white.png";

    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
     
        Button01 = (Button) findViewById(R.id.Button01);
        imageView01 = (ImageView) findViewById(R.id.imageView01);
Button01.setOnClickListener(new AsyncBtnOnClickListener());
    }
 
    private class AsyncBtnOnClickListener implements OnClickListener
{
@Override
public void onClick(View view)
{
imageView01.setVisibility(View.VISIBLE);
new AsyncTaskLoadImageProgress().execute(imgURL);
}

private class AsyncTaskLoadImageProgress extends AsyncTask<String,Integer, Drawable>
{
ProgressDialog progressDialog;

@Override
protected void onPreExecute()
{
progressDialog = new ProgressDialog(AsyncTeskProgress.this);
progressDialog.setTitle("ProgressDialog");
progressDialog.setMessage("Wait!");
progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
progressDialog.setCancelable(false);
progressDialog.show();
}

@Override
protected Drawable doInBackground(String... args)
{
try
{
// 1.取得 HttpEntity 實體
HttpEntity entity = getHttpEntityByURL(args[0]);
long length = entity.getContentLength();
InputStream is = entity.getContent();

// 2.資料讀取與接收
ByteArrayOutputStream baos = new ByteArrayOutputStream();
byte[] buf = new byte[128];
int ch = -1;
int count = 0;

while ((ch = is.read(buf)) != -1)
{
baos.write(buf, 0, ch);
count += ch;

if (length > 0)
{
publishProgress((int) ((count / (float) length) * 100));
}
}

// 3.資料轉換 byte[]  -->  Bitmap  -->  Drawable
byte[] b = baos.toByteArray();
Bitmap bmp = BitmapFactory.decodeByteArray(b, 0, b.length);
Drawable drawable = new BitmapDrawable(bmp);

return drawable;

}

catch (Exception e)
{
e.printStackTrace();
}

return null;
}

@Override
protected void onProgressUpdate(Integer... values)
{
// 更新進度
progressDialog.setProgress(values[0]);
}

@Override
protected void onPostExecute(Drawable resultImage)
{
progressDialog.dismiss();

if (resultImage != null)
{
imageView01.setImageDrawable(resultImage);
imageView01.setVisibility(View.VISIBLE);
}
}
}
}

private HttpEntity getHttpEntityByURL(String url) throws Exception
{
HttpClient client = new DefaultHttpClient();
HttpGet get = new HttpGet(url);
HttpResponse response = client.execute(get);
HttpEntity entity = response.getEntity();
return entity;
}
}

執行結果如下：