在UITextField的代理方法中添加类似如下代码

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string

{

    NSMutableString * futureString = [NSMutableString stringWithString:textField.text];

    [futureString  insertString:string atIndex:range.location];

    NSInteger flag=0;

    const NSInteger limited = 1;

    for (int i = futureString.length-1; i>=0; i--) {

        if ([futureString characterAtIndex:i] == '.') {

            if (flag > limited) {

                return NO;

            }

            break;

        }

        flag++;

    }

    return YES;

}