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Flutter's LayoutBuilder Widget

Posted: 6/7/2021

Tagged under: dartflutter
Flutter's LayoutBuilder Widget

Flutter's LayoutBuilder Widget is a widget that allows you to adapt children widgets based off the constraints that the LayoutBuilder is contained in. This allows you to layout widgets differently in different sized containers (like a phone screen vs a tablet screen), but LayoutBuilder also allows you to change how widgets render based off the container that they're rendered within. A good example of the later is rendering a list of items as a list when the container is taller than 500 dp, but when it's smaller, rendering a select dropdown with the items as options.

Indeed, LayoutBuilder can effectively allow you to build not only responsive applications, but can help you build adaptive applications as well. Let's take a look at how to use a LayoutBuilder.

LayoutBuilder(builder: (context, constraints) {
return Container(...);
}

As we can see from the example above, the LayoutBuilder widget takes in a builder function as a named parameter. That builder function receives a BuildContext and a BoxConstraints. While the build context can be helpful, most of the code for adapting to layouts will leverage the BoxConstraints.

BoxConstraints are the immutable (unable to be changed) layout constraints for the RenderBox. These box constraints have properties to get the minimum and maximum width and height of the Renderbox.

For example, in Luna Journal, I check the BoxConstraints' maxWidth on the home screen. If it's bigger than 900 dp, I render the layout in a row to create a master detail view. If the screen is smaller than 900 dp, I render the layout in a column (top to bottom, for phone and small devices).

LayoutBuilder(builder: (context, constraints) {
if (constraints.maxWidth > 900) {
return Row(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Flexible(
flex: 1,
child: PetCard(),
),
Flexible(
flex: 2,
child: getActionGrid(context, petVm, appVm),
),
]
);
} else {
return Column(
children: [
Container(
height: 110,
child: Center(
child: PetCard(),
)
),
Flexible(
flex: 1,
fit: FlexFit.tight,
child: getActionGrid(context, petVm, appVm)
)
],
);
}

You'll notice that I render the PetCard() twice. That PetCard also uses LayoutBuilder, but instead of changing the layout of the widgets on the screen, it fundamentally changes how the PetCard is rendered. If the constraints are of a small height, then we render a select dropdown however, if the contstraints are of a large height, we render a ListView. Despite the name being _Layout_Builder, you can actually do a lot more than just change the layout!

Note A lot of small things (like TextStyles and Padding) have been removed from this example to remove some of the unrelated "noise". The intent was to focus on the use of layout builder to create a drastic shift in how a component is built. All this to say, "If you copy and paste this code, you may have issues with rendering. Probably won't, but maybe 😁"

LayoutBuilder(builder: (context, constraints) {
if (constraints.maxHeight > 400) {
// render everything as a listview
return Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
if (data != null)
Expanded(
flex: 1,
child: ListView.builder(
itemCount: data.length,
itemBuilder: (context, index) {
return GestureDetector(
onTap: () {
petVm.setActivePet(data[index].documentID);
},
child: Text(data[index].name),
);
}),
),
GestureDetector(
onTap: () {
Navigator.of(context).pushNamed("/pets");
},
child: Text("Manage Pets")
)
],
);
}
// render everything as a PopupMenuButton
return Row(
mainAxisSize: MainAxisSize.max,
mainAxisAlignment: MainAxisAlignment.center,
children: [
Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
PopupMenuButton<String>(
child: Text("${petVm.getActivePet().name} ▾",
onSelected: (String result) {
if (result == "Manage Pets") {
Navigator.of(context).pushNamed("/pets");
} else {
petVm.setActivePet(result);
}
},
itemBuilder: (BuildContext context) =>
data.map((e) => PopupMenuItem<String>(
value: e.documentID,
child: Text(e.name != null ? e.name : ""),
)).toList()
..add(PopupMenuItem<String>(
value: "Manage Pets",
child: Text("Manage Pets"),
)),
),
if (pet != null && pet.birthDate != null)
Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
Text("Birthdate:"),
Text(formatter.format(pet?.birthDate?.toDate()))
]
),
],
)
],
);

In this example you can see how we can completely change how a Widget is rendered through the constraints passed in to our LayoutBuilder. In fact, you can even have a LayoutBuilder that may import and render one specific widget when a certain constraint is met, but render other specific widgets when other constraints are met.

LayoutBuilder is ultimately a very simple widget that allows your widget tree to make some pretty powerful decisions regarding how it's children should be displayed. When building my first flutter app, I did not use LayoutBuilder at all. After some serious refactoring, almost every screen is using it in some way. This has allowed me to provide a more meaningful and expected experience on different devices and different display resolutions.

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