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<topic_start>Building adaptive apps |
<topic_end> |
<topic_start> |
Overview |
Flutter provides new opportunities to build apps that can |
run on mobile, desktop, and the web from a single codebase. |
However, with these opportunities, come new challenges. |
You want your app to feel familiar to users, |
adapting to each platform by maximizing usability and |
ensuring a comfortable and seamless experience. |
That is, you need to build apps that are not just |
multiplatform, but are fully platform adaptive.There are many considerations for developing platform-adaptive |
apps, but they fall into three major categories:This page covers all three categories in detail |
using code snippets to illustrate the concepts. |
If you’d like to see how these concepts come together, |
check out the Flokk and Folio examples that |
were built using the concepts described here.Original demo code for adaptive app development techniques from flutter-adaptive-demo.<topic_end> |
<topic_start> |
Building adaptive layouts |
One of the first things you must consider when writing |
your app for multiple platforms is how to adapt |
it to the various sizes and shapes of the screens that |
it will run on.<topic_end> |
<topic_start> |
Layout widgets |
If you’ve been building apps or websites, |
you’re probably familiar with creating responsive interfaces. |
Luckily for Flutter developers, |
there are a large set of widgets to make this easier.Some of Flutter’s most useful layout widgets include:Single childAlign—Aligns a child within itself. |
It takes a double value between -1 and 1, |
for both the vertical and horizontal alignment.AspectRatio—Attempts to size the |
child to a specific aspect ratio.ConstrainedBox—Imposes size constraints on its child, |
offering control over the minimum or maximum size.CustomSingleChildLayout—Uses a delegate function |
to position a single child. The delegate can determine |
the layout constraints and positioning for the child.Expanded and Flexible—Allows a child of a |
Row or Column to shrink or grow to fill any available space.FractionallySizedBox—Sizes its child to a fraction |
of the available space.LayoutBuilder—Builds a widget that can reflow |
itself based on its parents size.SingleChildScrollView—Adds scrolling to a single child. |
Often used with a Row or Column.MultichildColumn, Row, and Flex—Lays out children |
in a single horizontal or vertical run. |
Both Column and Row extend the Flex widget.CustomMultiChildLayout—Uses a delegate function to |
position multiple children during the layout phase.Flow—Similar to CustomMultiChildLayout, |
but more efficient because it’s performed during the |
paint phase rather than the layout phase.ListView, GridView, and |
CustomScrollView—Provides scrollable |
lists of children.Stack—Layers and positions multiple children |
relative to the edges of the Stack. |
Functions similarly to position-fixed in CSS.Table—Uses a classic table layout algorithm for |
its children, combining multiple rows and columns.Wrap—Displays its children in multiple horizontal |
or vertical runs.To see more available widgets and example code, see |
Layout widgets.<topic_end> |
<topic_start> |
Visual density |
Different input devices offer various levels of precision, |
which necessitate differently sized hit areas. |
Flutter’s VisualDensity class makes it easy to adjust the |
density of your views across the entire application, |
for example, by making a button larger |
(and therefore easier to tap) on a touch device.When you change the VisualDensity for your MaterialApp, |
MaterialComponents that support it animate their densities |
to match. By default, both horizontal and vertical densities |
are set to 0.0, but you can set the densities to any negative |
or positive value that you want. By switching between different |
densities, you can easily adjust your UI:To set a custom visual density, inject the density into |
your MaterialApp theme: |
<code_start>double densityAmt = touchMode ? 0.0 : -1.0; |
VisualDensity density = |
VisualDensity(horizontal: densityAmt, vertical: densityAmt); |
return MaterialApp( |
theme: ThemeData(visualDensity: density), |
home: MainAppScaffold(), |
debugShowCheckedModeBanner: false, |
);<code_end> |
To use VisualDensity inside your own views, |
you can look it up: |
<code_start>VisualDensity density = Theme.of(context).visualDensity;<code_end> |
Not only does the container react automatically to changes |
in density, it also animates when it changes. |
This ties together your custom components, |
along with the built-in components, |
for a smooth transition effect across the app.As shown, VisualDensity is unit-less, |
so it can mean different things to different views. |
In this example, 1 density unit equals 6 pixels, |
but this is totally up to your views to decide. |
The fact that it is unit-less makes it quite versatile, |
and it should work in most contexts.It’s worth noting that the Material Components generally |
use a value of around 4 logical pixels for each |
visual density unit. For more information about the |
supported components, see VisualDensity API. |
For more information about density principles in general, |
see the Material Design guide.<topic_end> |
<topic_start> |
Contextual layout |
If you need more than density changes and can’t find a |
widget that does what you need, you can take a more |
procedural approach to adjust parameters, calculate sizes, |
swap widgets, or completely restructure your UI to suit |
a particular form factor.<topic_end> |
<topic_start>Screen-based breakpoints |
The simplest form of procedural layouts uses |
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