android 異步任務AsyncTask

異步任務AsyncTask

AsyncTask主要用來更新UI線程，比較耗時的操作可以在AsyncTask中使用。

AsyncTask是個抽象類，使用時需要繼承這個類，然後調用execute()方法。注意繼承時需要設定三個泛型Params，Progress和Result的類型，如AsyncTask：

Params是指調用execute()方法時傳入的參數類型和doInBackgound()的參數類型一樣Progress是指更新進度時傳遞的參數類型，即於publishProgress()和onProgressUpdate()的參數類型一樣Result是指doInBackground()的返回值類型 上面的說明涉及到幾個方法：
• doInBackgound() 這個方法是繼承AsyncTask必須要實現的，運行於後台，耗時的操作可以在這裡做
  
• publishProgress() 更新進度，給onProgressUpdate()傳遞進度參數（在UI主進程中，執行這個方法）
  
• onProgressUpdate() 在publishProgress()調用完被調用，更新進度

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  異步任務所使用的三種類型:

  1. Params ,發送到任務的類型參數 執行。
  2. Progress進步的類型單位期間公布 後台計算。
  3. Result,背景的結果的類型 計算。

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     使用AsyncTask只要如下三步：
     1、創建AsyncTask的類，並為三個泛型參數指定類型。如果某個泛型參數不需要指定類型，就可將他指定為void

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     package com.example.asynctestdemo;
     
     import java.io.ByteArrayOutputStream;
     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.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;
     
     public class MainActivity extends Activity {
     	private Button bt_execute;
     	private Button bt_cancel;
     	private ProgressBar progressBar;
     	private TextView textView;
     	MyTask myTask;
     
     	protected void onCreate(Bundle savedInstanceState) {
     		super.onCreate(savedInstanceState);
     		setContentView(R.layout.activity_main);
     		bt_execute = (Button) findViewById(R.id.bt_execute);
     		bt_cancel = (Button) findViewById(R.id.bt_cancel);
     		progressBar = (ProgressBar) findViewById(R.id.progressBar);
     		textView = (TextView) findViewById(R.id.text_view);
     
     		bt_execute.setOnClickListener(new OnClickListener() {
     
     			public void onClick(View v) {
     				// 實例化異步任務MyTask類
     				myTask = new MyTask();
     				myTask.execute("http://www.baidu.com");
     			}
     		});
     
     		bt_cancel.setOnClickListener(new OnClickListener() {
     
     			public void onClick(View v) {
     				myTask.cancel(true);
     			}
     		});
     
     	}
     
     	class MyTask extends AsyncTask {
     
     		// onpreExecute()方法用於，在執行後台任務前，做一些UI操作
     		protected void onPreExecute() {
     			textView.setText("Loading......");
     		}
     
     		// doInBackground()方法，內部執行後台耗時任務操作，但不是不能在此方法內修改UI
     		protected String doInBackground(String... params) {
     			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();
     					System.out.println(total);
     					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;
     						// 調用publishProgress()方法公布進度，最後onProgressUpdata方法將被執行
     						publishProgress((int) (count/length));
     						//為了更明顯的展現效果,每次暫停0.5s
     						Thread.sleep(500);
     					}
     					return new String(baos.toByteArray(), "utf-8");
     				}
     			} catch (ClientProtocolException e) {
     				e.printStackTrace();
     			} catch (IOException e) {
     				e.printStackTrace();
     			} catch (InterruptedException e) {
     				e.printStackTrace();
     			}
     			return null;
     		}
     
     		// onProgressUpdata方法，用於更新進度信息，例如執行進度條程序
     		protected void onProgressUpdate(Integer... values) {
     			progressBar.setProgress(values[0]);
     			textView.setText("Loading......" + values[0]);
     		}
     
     		// onPostExecute方法，用於在執行完後台任務後，更新ＵＩ，顯示結果
     		protected void onPostExecute(String result) {
     			textView.setText(result);
     		}
     
     		// onCacelled方法，用於在取消執行中的任務時，更改ＵＩ
     		protected void onCancelled() {
     			textView.setText("cancelled");
     			progressBar.setProgress(0);
     		}
     
     	}
     }