Can\'t I write braces in an if statement in List?答案

【问题标题】：Can't I write braces in an if statement in List?Can\'t I write braces in an if statement in List?
【发布时间】：2022-12-26 13:46:05
【问题描述】：

The following without braces worked fine:

import 'package:flutter/material.dart';

void main() {
  runApp(const _MyApp());
}

class _MyApp extends StatelessWidget {
  const _MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    const isFlag = true;

    return MaterialApp(
      home: Scaffold(
        body: Column(
          children: const [
            Text(
              "Demo1",
            ),
            if (isFlag)
              Text(
                "Demo true",
              )
            else
              Text(
                "Demo flase",
              )
          ],
        ),
      ),
    );
  }
}

I prefer to add braces even if there is only one expression.
I did the following and it resulted in an error.

The code that causes an error:

import 'package:flutter/material.dart';

void main() {
  runApp(const _MyApp());
}

class _MyApp extends StatelessWidget {
  const _MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    const isFlag = true;

    return MaterialApp(
      home: Scaffold(
        body: Column(
          children: const [
            Text(
              "Demo1",
            ),
            if (isFlag) {
              Text(
                "Demo true",
              )
            } else {
              Text(
                "Demo flase",
              )
            }
          ],
        ),
      ),
    );
  }
}

Error:

lib/main.dart:21:25: Error: A value of type 'Set<Text>' can't be assigned to a variable of type 'Widget'.
 - 'Set' is from 'dart:core'.
 - 'Text' is from 'package:flutter/src/widgets/text.dart'
 ('../../../flutter-command/flutter/packages/flutter/lib/src/widgets/text.dart').
 - 'Widget' is from 'package:flutter/src/widgets/framework.dart'
 ('../../../flutter-command/flutter/packages/flutter/lib/src/widgets/framework.dart').
            if (isFlag) {
                        ^
lib/main.dart:25:20: Error: A value of type 'Set<Text>' can't be assigned to a variable of type 'Widget'.
 - 'Set' is from 'dart:core'.
 - 'Text' is from 'package:flutter/src/widgets/text.dart'
 ('../../../flutter-command/flutter/packages/flutter/lib/src/widgets/text.dart').
 - 'Widget' is from 'package:flutter/src/widgets/framework.dart'
 ('../../../flutter-command/flutter/packages/flutter/lib/src/widgets/framework.dart').
            } else {

Can't I write braces in an if statement in List?

Referred to the following:
How to use conditional statement within child attribute of a Flutter Widget (Center Widget)

【问题讨论】：

use ternary operator.
if you notice carefully you closed the Text() then creating new widget which will be condition statement instead you can try ternary condition or write like Text(((){conditions}). Please do check your referral link again and scroll down for more solutions
Braces containstatements, but the collection-if construct must produce anexpression(or an "element"). See What all statements can be executed inside Lists, like [ if(true), for(;;) ]. I strongly recommend reading Bob Nystrom's article explaining collection-for and collection-if..

标签： flutter dart

【解决方案1】：

Do it like this with ternary operator

isFlag?Text("Demo true"):Text("Demo flase")

【讨论】：

【解决方案2】：

Using If statements inside the UI page is Not recommended, You Can Do What you want in tow way: 1- Using Visibility Widget, to Hide One Widget and show another one.

    return MaterialApp(
  home: Scaffold(
    body: Column(
      children: const [
        Text(
          "Demo1",
        ),
        Visibility(
         visible : isFlag, 
         child: Text(
            "Demo true",
          ),
         )
         Visibility(
         visible : !(isFlag), 
         child: Text(
            "Demo false",
          ),
         )
      ],
    ),
  ),
);

2- You Can Use Ternary if statement Like this:

        return MaterialApp(
  home: Scaffold(
    body: Column(
      children: const [
        Text(
          "Demo1",
        ),
        isFlag ? Text("Demo true",) : Text("Demo false"), 
      ],
    ),
  ),
);

And as I say, using a lot of code inside UI page is note recommended. Don't be shy to ask me any thing if any.

【讨论】：