前台进程是用户当前正在使用的进程。只有一些前台进程可以在任何 时候都存在。他们是最后一个被结束的,当内存低到根本连他们都不能运 行的时候。一般来说, 在这种情况下,设备会进行内存调度,中止一些 前台进程来保持对用户交互的响应。

 1 import android.app.Activity;
 2 import android.os.Bundle;
 3 import android.os.Handler;
 4 import android.os.Message;
 5 import android.view.View;
 6 import android.widget.TextView;
 7
 8 public class UpdateUIActivity extends Activity {
 9     private TextView mTextView;
10
11     @Override
12     protected void onCreate(Bundle savedInstanceState) {
13         super.onCreate(savedInstanceState);
14         setContentView(R.layout.activity_update_ui);
15         mTextView = (TextView) findViewById(R.id.textView1);
16     }
17
18     public class NoHandler implements Runnable {
19         @Override
20         public void run() {
21             try {
22                 mTextView.setText("No Handler");
23             } catch (Exception e) {
24                 e.printStackTrace();
25             }
26         }
27     }
28
29     // 不使用Handler
30     public void onClick_NoHandler(View view) {
31         // mHandler.post(new NoHandler());
32         Thread thread = new Thread(new NoHandler());
33         thread.start();
34     }
35
36     // 使用Handler
37     public void onClick_Handler(View view) {
38         Thread thread = new Thread(new MyHandler(mHandler));
39         thread.start();
40     }
41
42     public class MyHandler implements Runnable {
43         private Handler mHandler;
44
45         public MyHandler(Handler handler) {
46             mHandler = handler;
47         }
48
49         @Override
50         public void run() {
51             Message message = new Message();
52             message.obj = "Handler";
53             mHandler.sendMessage(message);
54         }
55
56     }
57
58     Handler mHandler = new Handler() {
59         @Override
60         public void handleMessage(Message msg) {
61             String str = String.valueOf(msg.obj);
62             mTextView.setText(str);
63         }
64     };
65
66 }

 1 import android.app.Activity;
 2 import android.os.AsyncTask;
 3 import android.os.Bundle;
 4 import android.view.View;
 5 import android.widget.ProgressBar;
 6 import android.widget.TextView;
 7 import android.widget.Toast;
 8
 9 public class AsyncTaskActivity extends Activity {
10     private ProgressBar mProgressBar;
11     private TextView mTextView;
12     private ProgressAsyncTask mProgressAsyncTask;
13
14     @Override
15     protected void onCreate(Bundle savedInstanceState) {
16         super.onCreate(savedInstanceState);
17         setContentView(R.layout.activity_async_task);
18         mProgressBar = (ProgressBar) findViewById(R.id.progressBar1);
19         mTextView = (TextView) findViewById(R.id.textView1);
20     }
21
22     private class ProgressAsyncTask extends AsyncTask<String, Integer, Integer> {
23         private int mCount;
24         @Override
25         protected Integer doInBackground(String... params) {
26             mProgressBar.setMax(params.length);
27             mCount = params.length;
28
29             for (int i = 0; i < mCount; i++) {
30                 publishProgress(i + 1);
31                 if (isCancelled())
32                     break;
33                 try {
34                     Thread.sleep(2000);
35                 } catch (Exception e) {
36
37                 }
38             }
39             return params.length;
40         }
41
42         @Override
43         protected void onPostExecute(Integer result) {
44             super.onPostExecute(result);
45             Toast.makeText(AsyncTaskActivity.this, "任务完成，共处理" + result + "个值.",
46                     Toast.LENGTH_LONG).show();
47         }
48
49         @Override
50         protected void onProgressUpdate(Integer... values) {
51             super.onProgressUpdate(values);
52             mProgressBar.setProgress(values[0]);
53             mTextView.setText(100 * values[0] / mCount + "%");
54         }
55
56         @Override
57         protected void onCancelled() {
58             Toast.makeText(AsyncTaskActivity.this, "任务已取消", Toast.LENGTH_LONG)
59                     .show();
60
61             super.onCancelled();
62         }
63
64     }
65
66     public void onClick_Start(View view) {
67         mProgressAsyncTask = new ProgressAsyncTask();
68         mProgressAsyncTask.execute("str1", "str2", "str3", "str4", "str5");
69     }
70
71     public void onClick_Cancel(View view) {
72         mProgressAsyncTask.cancel(true);
73     }
74 }