直接在UI线程中开启子线程来更新TextView显示的内容，运行程序我们会发现，如下错 误：android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.翻译过来就是：只有创建这个控件的线程才能去更新该控件的内容。
所有的UI线程要去负责View的创建并且维护它，例如更新冒个TextView的显示，都必须在主线程中去做，我们不能直接在UI线程中去创建子线程，要利用消息机制：handler，如下就是handler的简单工作原理图：
01 | public class HandlerTestActivity extends Activity { |
02 | private TextView tv; |
03 | private static final int UPDATE = 0; |
04 | private Handler handler = new Handler() { |
05 | |
06 | @Override |
07 | public void handleMessage(Message msg) { |
08 | // TODO 接收消息并且去更新UI线程上的控件内容 |
09 | if (msg.what == UPDATE) { |
10 | // Bundle b = msg.getData(); |
11 | // tv.setText(b.getString("num")); |
12 | tv.setText(String.valueOf(msg.obj)); |
13 | } |
14 | super.handleMessage(msg); |
15 | } |
16 | }; |
17 | |
18 | /** Called when the activity is first created. */ |
19 | @Override |
20 | public void onCreate(Bundle savedInstanceState) { |
21 | super.onCreate(savedInstanceState); |
22 | setContentView(R.layout.main); |
23 | tv = (TextView) findViewById(R.id.tv); |
24 | |
25 | new Thread() { |
26 | @Override |
27 | public void run() { |
28 | // TODO 子线程中通过handler发送消息给handler接收，由handler去更新TextView的值 |
29 | try { |
30 | for (int i = 0; i < 100; i++) { |
31 | Thread.sleep(500); |
32 | Message msg = new Message(); |
33 | msg.what = UPDATE; |
34 | // Bundle b = new Bundle(); |
35 | // b.putString("num", "更新后的值：" + i); |
36 | // msg.setData(b); |
37 | msg.obj = "更新后的值：" + i; |
38 | handler.sendMessage(msg); |
39 | } |
40 | } catch (InterruptedException e) { |
41 | e.printStackTrace(); |
42 | } |
43 | } |
44 | }.start(); |
45 | } |
46 | |
47 | } |
我们就通过Handler机制来处理了子线程去更新UI线程控件问题，Andrid开发中要经常用到这种机制。