Question

Flutter我只需要在按下按钮时隐藏我的组件，即可设置布尔值并使用if条件，但问题是它显示错误Columns子级不能包含空值。

这是我的代码

 Widget build(BuildContext context) {
    var first = true;
    var second = false;
    Size size = MediaQuery.of(context).size;
    return Background(
      child: SingleChildScrollView(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              "SIGNUP",
              style: TextStyle(fontWeight: FontWeight.bold),
            ),
            SizedBox(height: size.height * 0.03),
            SvgPicture.asset(
              "assets/icons/signup.svg",
              height: size.height * 0.35,
            ),
            first ? RoundedInputField(
              hintText: "Your Name",
              icon: Icons.person,
              onChanged: (value) {},
            ) : null ,
            first ? RoundedInputField(
              hintText: "Your Email",
              icon: Icons.mail,
              onChanged: (value) {
                print(value);
              },
            ) : null ,

            first ? RoundedPasswordField(
              onChanged: (value) {},
            ) : null ,

            second ? RoundedInputField(
              hintText: "Your Number",
              icon: Icons.phone,
              onChanged: (value) {
                print(value);
              },
            ) : null ,

            first ? RoundedButton(
              text: "NEXT",
              press: () {
                first = false;
                second = true;
              },
            ) : null ,
            second ? RoundedButton(
              text: "REGISTER",
              press: () {

              },
            ) : null ,
            SizedBox(height: size.height * 0.03),
            AlreadyHaveAnAccountCheck(
              login: false,
              press: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(
                    builder: (context) {
                      return LoginScreen();
                    },
                  ),
                );
              },
            ),

          ],
        ),
      ),
    );
  }
}

我只是在单击时隐藏了输入小部件，单击后需要显示其他输入和按钮。但是获取错误的Columns子级不能包含任何空值

Answer 1

使用SizedBox()或空的Container()代替null

Widget build(BuildContext context) {
    var first = true;
    var second = false;
    Size size = MediaQuery.of(context).size;
    return Background(
      child: SingleChildScrollView(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              "SIGNUP",
              style: TextStyle(fontWeight: FontWeight.bold),
            ),
            SizedBox(height: size.height * 0.03),
            SvgPicture.asset(
              "assets/icons/signup.svg",
              height: size.height * 0.35,
            ),
            first ? RoundedInputField(
              hintText: "Your Name",
              icon: Icons.person,
              onChanged: (value) {},
            ) : SizedBox() ,
            first ? RoundedInputField(
              hintText: "Your Email",
              icon: Icons.mail,
              onChanged: (value) {
                print(value);
              },
            ) : SizedBox() ,

            first ? RoundedPasswordField(
              onChanged: (value) {},
            ) : SizedBox() ,

            second ? RoundedInputField(
              hintText: "Your Number",
              icon: Icons.phone,
              onChanged: (value) {
                print(value);
              },
            ) : SizedBox() ,

            first ? RoundedButton(
              text: "NEXT",
              press: () {
                first = false;
                second = true;
              },
            ) : SizedBox() ,
            second ? RoundedButton(
              text: "REGISTER",
              press: () {

              },
            ) : SizedBox() ,
            SizedBox(height: size.height * 0.03),
            AlreadyHaveAnAccountCheck(
              login: false,
              press: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(
                    builder: (context) {
                      return LoginScreen();
                    },
                  ),
                );
              },
            ),

          ],
        ),
      ),
    );
  }
}

Answer 2

如果第一个和第二个变量为null，则需要返回Container（）列小部件不知道如何呈现null 您必须告诉Column小部件如果为空，则呈现空白内容。

例如：

#include <stdlib.h>
#include <stdio.h>


int main() {

char hey[5]="hello";


if(hey[5] == NULL){
    puts("Yes the last charcter is a NULL ");
}

return 0;

}

Answer 3

Flutter不接受[('a', 'b'), ('a', 'c'), ('b', 'a'), ('b', 'c'), ('c', 'a'), ('c', 'b'), ('a', 'a'), ('b', 'b'), ('c', 'c')]值作为窗口小部件。如果不想在其他窗口小部件中什么也不显示，则必须至少提供一个空窗口小部件。通常，您可以使用itertools.permutations(iterable, r=None)甚至null。

Answer 4

所有答案都是有效的，但是您可以使用Visibility显示或隐藏小部件，看起来像这样

// Get the modal
var modal = document.getElementById("myModal");

// Get the image and insert it inside the modal - use its "alt" text as a caption
var img = document.getElementById("myImg");
var modalImg = document.getElementById("img01");
img.onclick = function(){
  modal.style.display = "block";
  modalImg.src = this.src;
}

// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];

// When the user clicks on <span> (x), close the modal
span.onclick = function() { 
  modal.style.display = "none";
}

颤振在显示/隐藏小部件上显示错误

4 个答案: