android Handler(一)

这几天一直在看Handler的用法，今天来总结下：

你或许会遇见android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

中文意思就是只有创建这个控件的线程才能去更新该控件的内容。

为什么会这样？这就要说到android保护机制了，为了不让UI线程（主线程）阻塞，android会要求你去建议一个新的线程去操作那些费时间的操作，而且UI的更新必需在主线程中更新，这是我们就可以用Handler来实现了。

先来个简单的例子：

该例子的目的是联网取得数据后在UI中显示数据。

public class MainActivity extends Activity {

	public TextView text;
	private HttpParams httpParams;
	private HttpClient httpClient;
	private Handler handler;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		System.out.println("creat中" + Thread.currentThread().getName());
		handler = new myHandler();
		text = (TextView) findViewById(R.id.text);
		Thread thread = new myThread();
		thread.start();

	}

	class myThread extends Thread {

		public void run() {
			List<BasicNameValuePair> params = new LinkedList<BasicNameValuePair>();
			params.add(new BasicNameValuePair("XX", "XXXX"));//网页的参数

			try {
				DefaultHttpClient httpclient = new DefaultHttpClient();
				String baseUrl = "XXXXXXXXXXXXXXXXXXXXXXX";//网址
				HttpPost postMethod = new HttpPost(baseUrl);
				postMethod.setEntity(new UrlEncodedFormEntity(params, "utf-8")); // 将参数填入POST
																					// Entity中

				HttpResponse response = httpclient.execute(postMethod); // 执行POST方法
				Log.v("resCode = ", String.valueOf(response.getStatusLine()
						.getStatusCode())); // 获取响应码
				// //获取响应内容
				String s = EntityUtils.toString(response.getEntity(), "utf-8");
				System.out.println(s);
                                //获取Message并发送参数
                                Message msg = handler.obtainMessage();
				msg.obj = s;
				handler.sendMessage(msg);

			} catch (UnsupportedEncodingException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (ClientProtocolException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}

		}

	}

	class myHandler extends Handler {
		@Override
		public void handleMessage(Message msg) {
			System.out.println("handler中" + Thread.currentThread().getName());
			String s = (String) msg.obj;//接受参数
			text.setText(s);//更新UI

		}

	}
<pre name="code" class="java">   }

从结果可以看出，我们从主线程中更新了UI。