How to create observable variables from other observable variables in GetX

⋅ 3 min read ⋅ Flutter GetX

Table of Contents

GetX is one of the frameworks that simplify Reactive State Manager.

Using the ' .obs ' helper property, you can turn any variable into an observable type. Then, wrap a part of UI that you want to change inside Obx(() => YourWidget()).

Here is a simple user agreement screen where the next button is enabled when users accept both privacy policy and terms and conditions.

class GetApp extends StatelessWidget {
// 1
final _acceptPrivacyPolicy = false.obs;
final _acceptTermAndCondition = false.obs;

GetApp({Key? key}) : super(key: key);


Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
home: Scaffold(
appBar: AppBar(
title: const Text('User Agreement'),
),
body: SafeArea(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Text(
'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.',
style: Theme.of(context).textTheme.bodyLarge,
),
Row(
children: [
// 2
Obx(
() => Checkbox(
value: _acceptPrivacyPolicy.value,
onChanged: (checked) {
// 3
_acceptPrivacyPolicy.value = checked ?? false;
},
),
),
const Text("Accept privacy policy")
],
),
Row(
children: [
// 4
Obx(
() => Checkbox(
value: _acceptTermAndCondition.value,
onChanged: (checked) {
// 5
_acceptTermAndCondition.value = checked ?? false;
},
),
),
const Text("Accept term and condition")
],
),
// 6
Obx(
() => ElevatedButton(
onPressed: _acceptPrivacyPolicy.value &&
_acceptTermAndCondition.value
? () {}
: null,
child: const Text("Next"),
),
)
],
),
),
),
);
}
}

1 We declare observable boolean variables to keep user choice.
2 We make the checkbox adapt to change by wrap it inside Obx().
3 When checkbox value changed, we update the value, _acceptPrivacyPolicy.value = checked ?? false;;
4, 5 We do the same for other variable.
6 We use two variables value as a condition to disable/enable the button.

Here is the result.

User agreement screen.
User agreement screen.

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Problem

Even with this simple sample, our code becomes hard to read.

Obx(
() => ElevatedButton(
onPressed: _acceptPrivacyPolicy.value &&
_acceptTermAndCondition.value
? () {}
: null,
child: const Text("Next"),
),
)

We use two variables _acceptPrivacyPolicy.value && _acceptTermAndCondition.value to determine button disabled state.

I want to make a new variable out of these two, e.g., bool canGoNext. This would convey a more explicit message and hide implementation detail from the user.

Turn out this is work out of the box in GetX.

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How to create observable variables from other observable variables in GetX

To create an observable derived variable, you make a calculated property out of observable type.

bool get _canGoNext =>
_acceptPrivacyPolicy.value && _acceptTermAndCondition.value;

Then, use it instead of old condition, _acceptPrivacyPolicy.value && _acceptTermAndCondition.value.

Obx(
() => ElevatedButton(
onPressed: _canGoNext
? () {}
: null,
child: const Text("Next"),
),
)

That's all you need to do!

If your logic is more complicated than this, you can write your logic inside a function form instead of arrow syntax.

This also work.

bool get _canGoNext {
return _acceptPrivacyPolicy.value && _acceptTermAndCondition.value;
}

Read more article about Flutter, GetX, or see all available topic

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