就是通过uitextfield的leftview来实现的，同时要设置leftviewmode。
如果设置左右边距，需要再加上rightView功能
-(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;
}