Building layouts
Mục Lục
Building layouts
What you’ll learn
- How Flutter’s layout mechanism works.
- How to lay out widgets vertically and horizontally.
- How to build a Flutter layout.
This is a guide to building layouts in Flutter.
You’ll build the layout for the following app:
The finished app
This guide then takes a step back to explain Flutter’s
approach to layout, and shows how to place a single widget
on the screen. After a discussion of how to lay widgets
out horizontally and vertically, some of the most common
layout widgets are covered.
If you want a “big picture” understanding of the layout mechanism,
start with Flutter’s approach to layout.
Step 0: Create the app base code
Make sure to set up your environment,
then do the following:
- Create a basic “Hello World” Flutter app.
-
Change the app bar title and the app title as follows:
layout/base/lib/{main_starter.dart → main.dart}
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@@ -12,10 +12,10 @@
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@override
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Widget build(BuildContext context) {
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return MaterialApp(
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–
title: ‘Welcome to Flutter’,
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+
title: ‘Flutter layout demo’,
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home: Scaffold(
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appBar: AppBar(
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–
title: const Text(‘Welcome to Flutter’),
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+
title: const Text(‘Flutter layout demo’),
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),
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body: const Center(
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child: Text(‘Hello World’),
Step 1: Diagram the layout
The first step is to break the layout down to its basic elements:
- Identify the rows and columns.
- Does the layout include a grid?
- Are there overlapping elements?
- Does the UI need tabs?
- Notice areas that require alignment, padding, or borders.
First, identify the larger elements. In this example,
four elements are arranged into a column: an image, two rows, and a block of text.
Column elements (circled in red)
Next, diagram each row. The first row, called the Title
section, has 3 children: a column of text, a star icon,
and a number. Its first child, the column, contains 2 lines of text.
That first column takes a lot of space, so it must be wrapped in an
Expanded widget.
The second row, called the Button section, also has
3 children: each child is a column that contains an icon and text.
Once the layout has been diagrammed, it’s easiest to
take a bottom-up approach to implementing it.
To minimize the visual confusion of deeply nested layout code,
place some of the implementation in variables and functions.
Step 2: Implement the title row
First, you’ll build the left column in the title section.
Add the following code at the top of the build()
method of the MyApp class:
lib/main.dart (titleSection)
Widget titleSection = Container(
padding: const EdgeInsets.all(32),
child: Row(
children: [
Expanded(
/*1*/
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
/*2*/
Container(
padding: const EdgeInsets.only(bottom: 8),
child: const Text(
'Oeschinen Lake Campground',
style: TextStyle(
fontWeight: FontWeight.bold,
),
),
),
Text(
'Kandersteg, Switzerland',
style: TextStyle(
color: Colors.grey[500],
),
),
],
),
),
/*3*/
Icon(
Icons.star,
color: Colors.red[500],
),
const Text('41'),
],
),
);
- Putting a
Columninside anExpandedwidget stretches
the column to use all remaining free space in the row.
Setting thecrossAxisAlignmentproperty to
CrossAxisAlignment.startpositions the column at
the start of the row. - Putting the first row of text inside a
Container
enables you to add padding. The second child in the
Column, also text, displays as grey. - The last two items in the title row are a star icon,
painted red, and the text “41”. The entire row is in
aContainerand padded along each edge by 32 pixels.
Add the title section to the app body like this:
{../base → step2}/lib/main.dart
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@@ -14,11 +48,13 @@
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return MaterialApp(
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title: ‘Flutter layout demo’,
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home: Scaffold(
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appBar: AppBar(
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title: const Text(‘Flutter layout demo’),
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),
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–
body: const Center(
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–
child: Text(‘Hello World’),
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+
body: Column(
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+
children: [
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+
titleSection,
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+
],
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),
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),
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);
Tip:
- When pasting code into your app, indentation can
become skewed. You can fix this in your Flutter editor
using the automatic reformatting support. - For a faster development experience,
try Flutter’s hot reload feature. - If you have problems, compare your code to
lib/main.dart.
Step 3: Implement the button row
The button section contains 3 columns that use the same
layout—an icon over a row of text.
The columns in this row are evenly spaced,
and the text and icons are painted with the primary color.
Since the code for building each column is almost identical,
create a private helper method named buildButtonColumn(),
which takes a color, an Icon and Text,
and returns a column with its widgets painted in the given color.
lib/main.dart (_buildButtonColumn)
class MyApp extends StatelessWidget {
const MyApp({super.key});
@override
Widget build(BuildContext context) {
// ···
}
Column _buildButtonColumn(Color color, IconData icon, String label) {
return Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(icon, color: color),
Container(
margin: const EdgeInsets.only(top: 8),
child: Text(
label,
style: TextStyle(
fontSize: 12,
fontWeight: FontWeight.w400,
color: color,
),
),
),
],
);
}
}
The function adds the icon directly to the column.
The text is inside a Container with a top-only margin,
separating the text from the icon.
Build the row containing these columns by calling the
function and passing the color, Icon, and text specific
to that column. Align the columns along the main axis
using MainAxisAlignment.spaceEvenly to arrange the
free space evenly before, between, and after each column.
Add the following code just below the
titleSection declaration inside the build() method:
lib/main.dart (buttonSection)
Color color = Theme.of(context).primaryColor;
Widget buttonSection = Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
_buildButtonColumn(color, Icons.call, 'CALL'),
_buildButtonColumn(color, Icons.near_me, 'ROUTE'),
_buildButtonColumn(color, Icons.share, 'SHARE'),
],
);
Add the button section to the body:
{step2 → step3}/lib/main.dart
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@@ -48,3 +59,3 @@
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return MaterialApp(
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title: ‘Flutter layout demo’,
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home: Scaffold(
@@ -54,8 +65,9 @@
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body: Column(
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children: [
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titleSection,
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+
buttonSection,
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],
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),
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),
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);
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}
Step 4: Implement the text section
Define the text section as a variable. Put the text
in a Container and add padding along each edge.
Add the following code just below the buttonSection
declaration:
lib/main.dart (textSection)
Widget textSection = const Padding(
padding: EdgeInsets.all(32),
child: Text(
'Lake Oeschinen lies at the foot of the Blüemlisalp in the Bernese '
'Alps. Situated 1,578 meters above sea level, it is one of the '
'larger Alpine Lakes. A gondola ride from Kandersteg, followed by a '
'half-hour walk through pastures and pine forest, leads you to the '
'lake, which warms to 20 degrees Celsius in the summer. Activities '
'enjoyed here include rowing, and riding the summer toboggan run.',
softWrap: true,
),
);
By setting softwrap to true, text lines will fill the column width before
wrapping at a word boundary.
Add the text section to the body:
{step3 → step4}/lib/main.dart
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@@ -59,3 +72,3 @@
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return MaterialApp(
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title: ‘Flutter layout demo’,
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home: Scaffold(
@@ -66,6 +79,7 @@
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children: [
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titleSection,
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buttonSection,
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+
textSection,
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],
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),
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),
Step 5: Implement the image section
Three of the four column elements are now complete,
leaving only the image. Add the image file to the example:
- Create an
imagesdirectory at the top of the project. - Add
lake.jpg.
Note that wget doesn’t work for saving this binary file.
The original image is available online under a
Creative Commons license, but it’s large and slow to fetch.
-
Update the
pubspec.yamlfile to include anassetstag.
This makes the image available to your code.{step4 → step5}/pubspec.yaml
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@@ -18,3 +18,5 @@
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flutter:
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uses-material-design: true
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assets:
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– images/lake.jpg
Tip:
- Note that
pubspec.yamlis case sensitive,
so writeassets:and the image URL
as shown above. - The pubspec file is also sensitive to white
space, so use proper indentation. - You might need to restart the running program
(either on the simulator or a connected device) for the
pubspec changes to take effect.
Now you can reference the image from your code:
{step4 → step5}/lib/main.dart
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@@ -77,6 +77,12 @@
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),
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body: Column(
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children: [
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+
Image.asset(
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‘images/lake.jpg’,
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width: 600,
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height: 240,
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fit: BoxFit.cover,
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),
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titleSection,
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buttonSection,
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textSection,
BoxFit.cover tells the framework that the image should
be as small as possible but cover its entire render box.
Step 6: Final touch
In this final step, arrange all of the elements in a
ListView, rather than a Column, because a
ListView supports app body scrolling when the app is run
on a small device.
{step5 → step6}/lib/main.dart
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@@ -72,13 +77,13 @@
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return MaterialApp(
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title: ‘Flutter layout demo’,
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home: Scaffold(
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appBar: AppBar(
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title: const Text(‘Flutter layout demo’),
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),
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–
body: Column(
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+
body: ListView(
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children: [
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Image.asset(
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‘images/lake.jpg’,
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width: 600,
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height: 240,
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fit: BoxFit.cover,
Dart code: main.dart
Image: images
Pubspec: pubspec.yaml
That’s it! When you hot reload the app,
you should see the same app layout as
the screenshot at the top of this page.
You can add interactivity to this layout by following
Adding Interactivity to Your Flutter App.
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