title: NSDatePicker change date method runs twice!（mac app）
body: <p>I just add one NSDatePicker to the form, set the style graphical, and set the action like this:</p>
  
  <pre><code>[datePicker setAction:@selector(datePickSelected:)];
  </code></pre>
  
  <p>in the method, it just prints out the selected date.</p>
  
  <pre><code>-(void)datePickSelected:(id)sender
  {
      NSLog(@"%@",[datePicker dateValue]);
  }
  </code></pre>
  
  <p>It works, but runs two times when you click the date in this datepicker. Why is this?</p>
  
  <pre><code>2011-05-25 15:17:09.382 xxx[6609:a0f] 2011-05-13 15:17:04 +0800
  2011-05-25 15:17:09.677 xxx[6609:a0f] 2011-05-13 15:17:04 +0800
  </code></pre>