How to define a variable based on another variable in Flutter?

Question

I am building a Flutter app with a ChangeNotifier provider. When the app is started, I make a call to the Firebase api and save the results in a Provider variable:

Map<DateTime,List> datesMap;

How can I define another variable in the same Provider, based on the first variable? for example:

List newList = datesMap[DateTime.now()]

If I try to do it I get an error:

The instance member 'params' can't be accessed in an initializer

And if I place the second variable in a Constructor, I will get an error because the first variable datesMap is null until the Firebase api is completed.

Example code:

class ShiftsProvider with ChangeNotifier {

Map<DateTime,List> datesMap;

List newList = datesMap[DateTime.now()];

Future<void> getDatesMapfromFirebase () {

some code...

datesMap = something;

notifyListeners();

return;
}

vladzaba · Accepted Answer · 2022-11-22 17:16:08Z

1

You can make getter:

List get newList {
   return datesMap[DateTime.now()];
 }

answered Nov 22, 2022 at 17:16

vladzaba

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Comments

eamirho3ein · Accepted Answer · 2022-11-22 17:13:43Z

1

You can use late like this:

Map<DateTime,List>? datesMap;

late List? newList = datesMap?[DateTime.now()];

answered Nov 22, 2022 at 17:13

eamirho3ein

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1 Comment

eamirho3ein Over a year ago

what version of flutter you are use?I think null safety is not active in your project right? @LateBoy

Gwhyyy · Accepted Answer · 2022-11-22 17:34:30Z

1

since like I see that datesMap variable is related to the specific class, you can mark it with static keyword, this will fix your problem:

class ShiftsProvider with ChangeNotifier {

static Map<DateTime,List> datesMap;

List? newList = datesMap[DateTime.now()];

Future<void> getDatesMapfromFirebase () {

some code...

datesMap = something;

notifyListeners();

return;
}
}

just note that if you want to use that static variable, you can access it like this:

ShiftsProvider.datesMap

edited Nov 22, 2022 at 17:34

answered Nov 22, 2022 at 17:16

Gwhyyy

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5 Comments

LateBoy Over a year ago

What difference the keyword "static" makes?

Gwhyyy Over a year ago

I will explain by example, when we have a class that contains some variables, when we create multiple objects of that class, each one of theme will have a those variables belongs to just that instance object.

Gwhyyy Over a year ago

with the static keyword, you make a variable belonging to the original class, and what the number of instances you made of that class, the static variable only refers to the original class and it doesn't only by an instance, but it belongs to all of them.

Gwhyyy Over a year ago

did you solve the issue ?

LateBoy Over a year ago

Thank you, I found your answer useful, but for this specific use case I found the getter method more straightforward.

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