I have a flutter application which uses a MultiProvider widget at the root of the widgets tree.
The problem is, I need to add a ChangeNotifierProvider to the MultiProvider widget, but later because I can't create it at the beginning (well I could but it makes more sense to do it later *).
How am I supposed to do?
(* for more details about why I would like to create the ChangeNotifierProvider later: The associated ChangeNotifier represents a bluetooth connection that will not be available at the start of the application.)
In these situations I create Provider with object which have some initial values and later I change initial values to actual.
Related
When creating a Flutter application,
we use a Navigator to move back and forth between screens.
In my application, I have some class instances that
I want to share with all child widgets in the hierarchy.
An example is a Data Base driver class (stateful)
which holds all functionality of reading/writing to the actual database.
Currently - the 'main' function is initializing it,
then the resulting object is passed through all
constructors of the participating widgets.
How do I make it behave like the Navigator
so it would be available in the global scope
without importing it or passing it through a chain of constructors?
I would gladly accept any other approaches,
and still, if you can also address the exact question - I would be grateful :)
I opened the code of Navigator, Stateless and Stateful widgets,
I tried to google for the source of that Navigator instance
but came up with nothing helpful.
Have you tried using InheritedWidget?
From the docs:
Base class for widgets that efficiently propagate information down the tree.
Create DataBaseDriver class as inherited Widget class, access it in its child class using context as DataBaseDriver.of(context)
I have 4 widgets named OptionCard (it's their Type) i'de like to iterate through them to check the value of their variables how can I do it ?
tried OptionCard.forEach but it doesn't work
This is not the way Flutter widgets are supposed to be used. Widgets (even StatefulWidgets) generally do not hold application state. They should only provide a view on state managed elsewhere.
There are many proven methods of application state management that are popular and well supported in Flutter.
I'm trying to build 2 screens one contains a list of products and the second has product details, I'm using Firebase to store data and so I fetch the data from Firestore in the first screen and it all works perfectly.
However, in the second screen, I pass product data correctly using Navigator but I cannot use that data in my UI.
Here is the code for my second screen:
As you can see I cannot use the name variable in my widget tree, I've seen videos on youtube and I found that some people use state management to solve this problem, but for my case I just want to pass data and display it.
The way you've passed the data is correct but the way you've accessed the data isn't.
Whenever you try to access the variables of a Class in its state, you do that using widget. So, to access the name or other variables, just do as follows:
widget.name
Additionally, if the only purpose is to pass and display the data, then, there's no need of a StatefulWidget, use a StatelessWidget instead.
A [State] object's configuration is the corresponding [StatefulWidget] instance. This property is initialized by the framework before calling [initState]. If the parent updates this location in the tree to a new widget with the same [runtimeType] and [Widget.key] as the current configuration, the framework will update this property to refer to the new widget and then call [didUpdateWidget], passing the old configuration as an argument.
For that you can use widget.key, this will work fine for you.
I get the general idea that the StatefulWidget my get rebuild very often, and even get assigned to a different State if not using key in some cases. However the State of that widget always exists in memory. I need to change some data of the State when it's not shown. Is it safe to keep the reference of that State instance for example in a singleton and change its data?
Specifically, I have a StatefulWidget(call it homapage) with PageView. Its State keeps the reference of the PageController. I want to change the controller.page when the screen is showing other tabs rather than homepage. I did this via keeping a reference of the State in a singleton. In order to do it, I need to make the State class public by deleting the _ in front of the State class name. If feels unsafe and against the Flutter design philosophy to me.
The state will be destroyed when a widget that uses that state is no longer in the widget tree. We recommend that you use GlobalKey and don't use a singleton as a reference to the State. Using InheritedWidget is also a good option.
I'm building my first Flutter app, in which I need to refresh a list of data, and every component has some modifiers.
This is the basic architecture.
A big list of data (about 5000 rows) is periodically refreshed from an API inside a RefresherWidget (which is a StatefulWidget that holds the list), and then passed along to the children.
Every RowWidget has a Switch (and Dialogs too) that modifies the data it represents.
Currently, the methods to modify the list are in the RefresherWidget, so I'm passing them as callback functions inside every children until reaching the onChanged callback of the Switch.
But I don't think it's a very clean solution, and I don't know how to implement a better one: I've tried thinking about passing these methods inside an InheritedWidget that stays between RefresherWidget and ListViewWidget, and referencing them using the of function, but I don't know about the perfomance hit I would get if the InheritedWidget gets rebuild.
Also, Streams and BLoCs seem very complicated for what I need to do.
How do you guys usually approach a problem like this?
This is definitely a situation for InheritedWidget or BuildContext in general.
I've tried [...] InheritedWidget [...] but I don't know about the perfomance hit I would get if the InheritedWidget gets rebuild.
You don't have to fear anything. InheritedWidget is built for this exact purpose.
Obtaining the InheritedWidget is very performant (O(1)). And only widgets that depends on the value gets rebuilt – which is optimal too.