selectedSegmentIndex属性标识UISegmentedControl的选定段.将此属性设置为任何有效的段索引，或将UISegmentedControlNoSegment(-1)设置为关闭当前 Select .
// select the first segment
segmented.selectedSegmentIndex = 0;
// turn off the current selection
segmented.selectedSegmentIndex = UISegmentedControlNoSegment;