怎样制作一个chronometer计时器

所有要改变UI对象的的action都应该在主UI中运行。

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_cronometro);

    /**********************/
    boton_iniciar = (Button) findViewById(R.id.button1);
    texto_cronometro = (TextView) findViewById(R.id.textView1);
    count= new Timer("Contador");
    boton_iniciar.setOnClickListener(new OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            count.scheduleAtFixedRate(new TimerTask() {         
                @Override
                public void run() {
                    // TODO Auto-generated method stub
                    a++;
                    ActivityCronometro.this.runOnUiThread(new Runnable() {          
                    @Override
                    public void run() {
                        texto_cronometro.setText(String.valueOf(a));

                    }
                });

                }
            }, 100, 100);
        }
    });
}