Android中的Handler機制

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直接在UI線程中開啟子線程來更新TextView顯示的內容，運行程式我們會發現，如下錯 誤：android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.翻譯過來就是：只有建立這個控制項的線程才能去更新該控制項的內容。

    所有的UI線程要去負責View的建立並且維護它，例如更新冒個TextView的顯示，都必須在主線程中去做，我們不能直接在UI線程中去建立子線程，要利用訊息機制：handler，如下就是handler的簡單工作原理圖：

既然android給我們提供了Handler機制來解決這樣的問題，請看如下代碼：

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public class HandlerTestActivity extends Activity {    private TextView tv;    private static final int UPDATE = 0;    private Handler handler = new Handler() {         @Override        public void handleMessage(Message msg) {            // TODO 接收訊息並且去更新UI線程上的控制項內容            if (msg.what == UPDATE) {                // Bundle b = msg.getData();                // tv.setText(b.getString("num"));                tv.setText(String.valueOf(msg.obj));            }            super.handleMessage(msg);        }    };     /** Called when the activity is first created. */    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        tv = (TextView) findViewById(R.id.tv);         new Thread() {            @Override            public void run() {                // TODO 子線程中通過handler發送訊息給handler接收，由handler去更新TextView的值                try {                    for (int i = 0; i < 100; i++) {                        Thread.sleep(500);                        Message msg = new Message();                        msg.what = UPDATE;                        // Bundle b = new Bundle();                        // b.putString("num", "更新後的值：" + i);                        // msg.setData(b);                        msg.obj = "更新後的值：" + i;                        handler.sendMessage(msg);                    }                } catch (InterruptedException e) {                    e.printStackTrace();                }            }        }.start();    } }

        我們就通過Handler機制來處理了子線程去更新UI線程式控制件問題，Andrid開發中要經常用到這種機制。

Android中的Handler機制