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 = left view;
}