iOS - 准确获取UITextField/UITextView字数, 控制按钮高亮

在代理方法中, 判断UITextField/UITextView字数总是不正确

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

通过对TextField/View监听事件能准确获取字数, 在回调方法中处理按钮考虑、是否可点等动作

  [self.mobileTextField addTarget:self action:@selector(textValueDidChanged) forControlEvents:UIControlEventEditingChanged];

//Action方法
- (void)textValidChanged
{
    self.verificationCodeButton.enabled = NO;
    self.submitButton.enabled           = NO;

    self.submitButton.enabled = (self.mobileTextField.text.length >= 11) && (self.VerificationCodeTextField.text.length > 0);
    self.verificationCodeButton.enabled = self.mobileTextField.text.length >= 11;
    self.verificationCodeButton.backgroundColor = self.verificationCodeButton.enabled ? QDYellowColor : ButtonGrayColor;
}