Why does the Column widget align its children to the center when a Center widget is used inside its children in Flutter?

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1

Today, while developing a screen in Flutter, I observed an unexpected behavior when using a Center widget inside a Column. The Column appeared to align all its children to the center, despite not explicitly setting mainAxisAlignment. I understand that, by default, Column aligns its children to the start along the main axis unless specified otherwise.

Could you clarify why this behavior occurs? If it is a bug, it may need to be addressed.

Column(
                children: [
                  SizedBox(
                    height: 100,
                  ),
                  Center(child: logoWidget()),
                  SizedBox(
                    height: 50,
                  ),
                  logoWidget(),
                  SizedBox(
                    height: 50,
                  ),
                  Text("Login to your Account")
                ],
              ),

The LogoWidget is a pictcure widget.

before using center

img 2
img 21920Ă—1080 298 KB

after using center

img 1
img 11920Ă—1080 297 KB

2

flutter version : 3.29.1

3

See here:

/// A widget that centers its child within itself.
///
/// This widget will be as big as possible if its dimensions are constrained and
/// [widthFactor] and [heightFactor] are null.
/// ...
class Center extends Align { ... }

So, it will be as big as possible if its dimensions are constrained.

Center does not grow vertically because the Column doesn’t constrain it vertically.

But it will grow horizontally, because the Column constrains it horizontally.

In other words, Column is in fact aligning it to the left, but Center is taking all horizontal space, which means aligning it to the left has no effect.

To learn more about Flutter layout, read my article in the Flutter docs: Understanding constraints | Flutter or here: Flutter: The Advanced Layout Rule Even Beginners Must Know | by Marcelo Glasberg | Flutter Community | Medium

4

I think the question (at least for me) is why the text widget is aligning to the center despite not being inside a center widget.

5

If you turn on the widget boundaries in devtools, I think the answer will appear obvious.

6

If that’s the question, it’s because the Scaffold passes loose constraints down to the Column. Without the Center the Column is as small as necessary to fit the logo and the text, and it aligns to the left. In other words, the Column doesn’t occupy all the horizontal space.

Adding Center forces the Column to occupy all the horizontal space. The text is then aligned to the Center because the Column aligns its children to the start only along its main axis (vertically). But horizontally the default is to center them:

const Flex({
    ...
    this.mainAxisAlignment = MainAxisAlignment.start,
    this.crossAxisAlignment = CrossAxisAlignment.center,
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7

This is hopefully clearest explanation fr beginners. As along time Flutter user I still think this.crossAxisAlignment = CrossAxisAlignment.center for Column is one of the most annoying defaults, things would typically behave more as expected if it was start too.

Also naming things horizontal and vertical in Row and Column and just using the appropriate “cross” and “main” things in Flex would have been so much easier to use for beginners and provide a much better devX, but it is what it is and you get used to it.

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