ios开发之--通过通知监听textfield的输入状态，判断按钮的状态

_rightBtn = [UIButton buttonWithType:UIButtonTypeCustom];
    _rightBtn.frame = CGRectMake(kWidth - 50-5, 7, kWidth - _tf.frame.size.width-10, 45);
    [_rightBtn setBackgroundImage:[UIImage imageNamed:@"send_un"] forState:UIControlStateNormal];
    [_rightBtn addTarget:self action:@selector(btnClick:) forControlEvents:UIControlEventTouchUpInside];
    [_inputView addSubview:_rightBtn];

    //监听textfield的输入状态
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(textFieldDidChangeValue:)
                                                 name:UITextFieldTextDidChangeNotification
                                               object:_tf];

//这里可以通过发送object消息获取注册时指定的UITextField对象
- (void)textFieldDidChangeValue:(NSNotification *)notification
{
    UITextField *sender = (UITextField *)[notification object];
    if(sender.text.length != 0)
    {
        [_rightBtn setBackgroundImage:[UIImage imageNamed:@"send"] forState:UIControlStateNormal];
    }else
    {
        [_rightBtn setBackgroundImage:[UIImage imageNamed:@"send_un"] forState:UIControlStateNormal];
    }
}