Android Handler Message 通信機制

Android UI線程更新並不是線程安全的，並且必須在UI線程中進程UI更新操作，以下面一個錯誤為例：

1. public class MainActivity extends Activity {
3. private String title;
4. private Button btn;
5. private final Handler handler = new Handler(){
6. @Override
7. public void handleMessage(Message msg) {
8. setTitle(title);
9. }
10. };
12. @Override
13. public void onCreate(Bundle savedInstanceState) {
14. super.onCreate(savedInstanceState);
15. setContentView(R.layout.main);
16. btn = (Button) findViewById(R.id.time);
17. btn.setOnClickListener(new OnClickListener() {
18. @Override
19. public void onClick(View arg0) {
20. Timer timer = new Timer();
21. timer.schedule(new MyTask(), 1000);
22. }
23. });
24. }
27. class MyTask extends TimerTask{
29. Message msg;
30. Bundle bundle;
31. SimpleDateFormat sdf;
33. public MyTask(){
34. Message msg = new Message();
35. Bundle bundle = new Bundle();
36. sdf = new SimpleDateFormat();
37. }
39. @Override
40. public void run() {
41. String date = sdf.format(new Date());
42. //在子線程中更新UI
43. setTitle("當前時間 ："+date);
44. }
45. }
46. }

上面的例子會報“Only the original thread that created a view hierarchy can touch its views. ”

正確的寫法應該是在子線程中組裝數據，然後通過handler發送消息給UI線程，最後在UI線程中更新UI：

1. public class MainActivity extends Activity {
3. private Button btn;
4. private final Handler handler = new Handler(){
6. @Override
7. public void handleMessage(Message msg) {
8. //接收消息
9. switch (msg.what) {
10. case 110:
11. Bundle bundle = msg.getData();
12. String date = bundle.getString("date");
13. //更新UI
14. setTitle(date);
15. break;
16. }
17. }
18. };
20. @Override
21. public void onCreate(Bundle savedInstanceState) {
22. super.onCreate(savedInstanceState);
23. setContentView(R.layout.main);
24. btn = (Button) findViewById(R.id.time);
25. btn.setOnClickListener(new OnClickListener() {
26. @Override
27. public void onClick(View arg0) {
28. Timer timer = new Timer();
29. //設置每秒執行一次
30. timer.schedule(new MyTask(),0,1000);
31. }
32. });
33. }
36. class MyTask extends TimerTask{
38. Message msg;
39. Bundle bundle;
40. SimpleDateFormat sdf;
42. public MyTask(){
43. msg = new Message();
44. bundle = new Bundle();
45. sdf = new SimpleDateFormat("yyyy年MM月dd : HH時mm分ss");
46. }
48. @Override
49. public void run() {
50. //組裝數據
51. String date = sdf.format(new Date());
52. msg.what = 110;
53. bundle.putString("date", date);
54. msg.setData(bundle);
55. //發送消息
56. handler.sendMessage(msg);
57. }
58. }
59. }