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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(), |