android 非同步任務AsyncTask

android 非同步任務AsyncTask

非同步任務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是個抽象類別，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

  非同步任務所使用的三種類型:

  1. Params ,發送到任務的型別參數 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。
     
     
     
     
     
     使用AsyncTask只要如下三步：
     1、建立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) {// 執行個體化非同步任務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，在執行背景工作前，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UIprotected 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0.5sThread.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，用於更新進度資訊，例如執行進度條程式protected void onProgressUpdate(Integer... values) {progressBar.setProgress(values[0]);textView.setText("Loading......" + values[0]);}// onPostExecute方法，用於在執行完背景工作後，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ＵＩprotected void onCancelled() {textView.setText("cancelled");progressBar.setProgress(0);}}}