UITextField默认情况下，是紧挨着左边的边框，有时候产品为了美观需要空出一定的间距
-(void)setTextFieldLeftPadding:(UITextField *)textField forWidth:(CGFloat)leftWidth
{
CGRect frame = textField.frame;
frame.size.width = leftWidth;
UIView *leftview = [[UIView alloc] initWithFrame:frame];
textField.leftViewMode = UITextFieldViewModeAlways;
textField.leftView = leftview;
}
