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flutter_constraints

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pixels wide, and 0 to 75 tall.”
First child: “OK, then I wish to be 290 pixels wide,
and 20 pixels tall.”
Widget: “Hmmm, since I want to put my second child below the
first one, this leaves only 55 pixels of height for
my second child.”
Widget: “Hey second child, You must be from 0 to 290 wide,
and 0 to 55 tall.”
Second child: “OK, I wish to be 140 pixels wide,
and 30 pixels tall.”
Widget: “Very well. My first child has position x: 5 and y: 5,
and my second child has x: 80 and y: 25.”
Widget: “Hey parent, I’ve decided that my size is going to be 300
pixels wide, and 60 pixels tall.”
<topic_end>
<topic_start>
Limitations
Flutter’s layout engine is designed to be a one-pass process.
This means that Flutter lays out its widgets very efficiently,
but does result in a few limitations:
A widget can decide its own size only within the
constraints given to it by its parent.
This means a widget usually
can’t have any size it wants.
A widget can’t know and doesn’t decide its own position
in the screen, since it’s the widget’s parent who decides
the position of the widget.
Since the parent’s size and position, in its turn,
also depends on its own parent, it’s impossible to
precisely define the size and position of any widget
without taking into consideration the tree as a whole.
If a child wants a different size from its parent and
the parent doesn’t have enough information to align it,
then the child’s size might be ignored.
Be specific when defining alignment.
In Flutter, widgets are rendered by their underlying
RenderBox objects. Many boxes in Flutter,
especially those that just take a single child,
pass their constraint on to their children.
Generally, there are three kinds of boxes,
in terms of how they handle their constraints:
Some widgets, for example Container,
vary from type to type based on their constructor arguments.
The Container constructor defaults
to trying to be as big as possible, but if you give it a width,
for instance, it tries to honor that and be that particular size.
Others, for example Row and Column (flex boxes)
vary based on the constraints they are given,
as described in the Flex section.
<topic_end>
<topic_start>
Examples
For an interactive experience, use the following DartPad.
Use the numbered horizontal scrolling bar to switch between
29 different examples.
<code_start>
import 'package:flutter/material.dart';
void main() => runApp(const HomePage());
const red = Colors.red;
const green = Colors.green;
const blue = Colors.blue;
const big = TextStyle(fontSize: 30);
class HomePage extends StatelessWidget {
const HomePage({super.key});
@override
Widget build(BuildContext context) {
return const FlutterLayoutArticle([
Example1(),
Example2(),
Example3(),
Example4(),
Example5(),
Example6(),
Example7(),
Example8(),
Example9(),
Example10(),
Example11(),
Example12(),
Example13(),
Example14(),
Example15(),
Example16(),
Example17(),
Example18(),
Example19(),
Example20(),
Example21(),
Example22(),
Example23(),
Example24(),
Example25(),
Example26(),
Example27(),
Example28(),