UI组件：PickerView

1.创建PickerView类

2.实现委托UIPickerViewDataSource,UIPickerViewDelegate

func numberOfComponentsInPickerView(pickerView: UIPickerView) -> Int{

        //返回滑轮个数，比如省、市、区，返回3

        return 3

}

func pickerView(pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int{

        //返回滑轮中数据的个数

        return 0

}