android Handler(一)

這幾天一直在看Handler的用法，今天來總結下：

你或許會遇見android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

中文意思就是隻有創建這個控件的線程才能去更新該控件的內容。

爲什麼會這樣？這就要說到android保護機制了，爲了不讓UI線程（主線程）阻塞，android會要求你去建議一個新的線程去操作那些費時間的操作，而且UI的更新必需在主線程中更新，這是我們就可以用Handler來實現了。

先來個簡單的例子：

該例子的目的是聯網取得數據後在UI中顯示數據。

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"));//網頁的參數

			try {
				DefaultHttpClient httpclient = new DefaultHttpClient();
				String baseUrl = "XXXXXXXXXXXXXXXXXXXXXXX";//網址
				HttpPost postMethod = new HttpPost(baseUrl);
				postMethod.setEntity(new UrlEncodedFormEntity(params, "utf-8")); // 將參數填入POST
																					// Entity中

				HttpResponse response = httpclient.execute(postMethod); // 執行POST方法
				Log.v("resCode = ", String.valueOf(response.getStatusLine()
						.getStatusCode())); // 獲取響應碼
				// //獲取響應內容
				String s = EntityUtils.toString(response.getEntity(), "utf-8");
				System.out.println(s);
                                //獲取Message併發送參數
                                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;//接受參數
			text.setText(s);//更新UI

		}

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

從結果可以看出，我們從主線程中更新了UI。