方法不止一种，可以用category，可以直接骗用户，可以...这里说一种继承，步骤如下：
1)继承UITextfield
2)重写子类中的- (void)drawRect:(CGRect)rect 如下

• (void)drawRect:(CGRect)rect {

  CGContextRef context = UIGraphicsGetCurrentContext();
  CGContextSetFillColorWithColor(context, [UIColor blackColor].CGColor);
  CGContextFillRect(context, CGRectMake(0, CGRectGetHeight(self.frame) - 0.5, CGRectGetWidth(self.frame), 0.5));

  }

3)TestCode 如下：

import "CustomField.h"

@implementation ViewController

• (void)viewDidLoad {

  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  CustomField *textField = [CustomField new];
  textField.frame = CGRectMake(0, 200, 100, 30);
  textField.backgroundColor = [UIColor yellowColor];
  [self.view addSubview:textField];

  }