I have my navigator setup and everything is working as expected, but I have a widget buried in my widget tree that needs to know about route changes and I can't figure out how to subscribe to them.
You could use the name of the route, accessible anywhere:
ModalRoute.of(context).settings.name
and listen to the changes of that value.
But generally speaking, if you want to listen to some variable changes deep in the widget tree, you should look at some state management options.
Provider, Bloc or MobX are some options.
flutter documentation on state management
Related
I been trying to understand how Riverpod works and was wondering how does it know when to destroy the state of a provider with the .autoDispose modifier when changing a route in Flutter. Is it somehow subscribed to the navigator changes?
The docs state:
A common use case is to destroy the state of a provider when it is no-longer used.
There are multiple reasons for doing so, such as:
- When using Firebase, to close the connection and avoid unnecessary cost.
- ***To reset the state when the user leaves a screen and re-enters it.****
The "autoDispose" logic doesn't depend on Flutter-specific logic.
Instead it relies purely on whether a provider is currently being listened or not.
When a route is poped, the associated widgets are destroyed. This causes the ConsumerWidgets in that route to stop listening to their "watched" providers.
Then, if a provider now has no more listener, it gets disposed.
How do pass an instance of one Bloc instance to another bloc, so that we can use BlocA state change in BlocB
Well, technically just pass it as a parameter. But you shouldn't. It is not recommended by the author of Flutter Bloc.
The recommended solution is to let blocs communicate via either the presentation layer (the UI) by using BlocListeners, OR via streams from injected repositories.
There is a segment on this in the documentation:
https://bloclibrary.dev/#/architecture?id=bloc-to-bloc-communication
I'm new to using the Bloc pattern in Flutter, and I wish to update some widgets (i.e. state) for every bloc concerned by an external change.
For example, imagine that I have a trading application, and so I have a "Trading" component that receive some information updates.
My "Trading" component receive new information, but then how does it transmit the new information to the Bloc and so, how it emits (and update) the widget.
Any idea how I can do this? What is the correct pattern to follow ?
You need to emit the state changes (your trading data here) to your state using a BLoC / Cubit which will then rebuild the UI using BlocBuilder.
I was wondering , since I started using RiverPod , if I should not use setState at all and have almost everything in StateNotifierProvider
Yes you can. You need to understand there are 2 types of state:
Ephemeral(Local): This is contained to only a single widget and not used for passing information between different components. You should use setState and internal state variables for cases like these. Like #Ruchit said in his comment above, a good example is a checkbox, switch, dropdown. Or if you want to hide/show something based on some data.
Global: This is for any information that is passed between layers, components or widgets and should be accessible and synced across different screens. For these cases you should use state management solutions like Provider, Riverpod etc. Some examples are:
Adding items to a cart.
Updating a favourites list.
Sending data to network layer to make http calls etc.
Well, I am sort of new to Flutter, My question is why we use providers in Flutter, I know it is used for state management. But I am looking to know the most common use case of providers.
You need to be able to move data between your Widgets. It's an easy way to do it.
You start your root Build method in the app with:
#override
Widget build(BuildContext context) {
return MultiProvider( // Multi means you can have more providers if you need
providers: [
ChangeNotifierProvider(builder: (context) => MyStateClass()),
],
child: MaterialApp(....
Now you can place all the data you need to share in the MyStateClass() and place underlying Widgets inside:
Consumer<MyStateClass>(builder: (context, state, child) {
// your code here - return(SomeOtherWidget());
})
or inside your Build methods:
#override
Widget build(BuildContext context) {
MyStateClass state = Provider.of<MyStateClass>(context);
// ... TODO ... return (Widget)
As you asked that why we use provider in flutter, so i will be providing only theoretical explanation which i think will surely help you to understand what
actually provider is and why it is used.
Suppose you are working on a large app with a lot of folders and files.
Now if the user have interacted with your app(say pressed a button or something like that) then the app have to build itself again to update to the changes that the user had made.But wait! this doesn't looks a good deal, to build the whole app again just to make changes in some particular section.
So, here comes the Provider to solve this problem.
A Provider is basically a container or a storage that stores and provides you with state or data.
Also, we know that widgets are arranged in an app like a tree like fashion.
so, if u assign Provider to any node in the tree then all the children of that node will have access to the data that is provided by the Provider.
Back the stage ,Provider comes with a listener and these listeners are assigned to the children of the widget to which Provider is attached.
So, If the user interferes with any widget that has a listener then the app will build only that particular (which the user interacted) widget again (not the whole app).
What is provider and how it's works ?
Provider is a simple way for state management, it works on the concept of PUB_SUB,
Which means there is one provider and multiple subscribers which is called consumers here. ex : youtube.
Whenever there is change, with NotifyChangeListener it will update all the consumers.
A wrapper around InheritedWidget to make them easier to use and more reusable.
By using provider instead of manually writing InheritedWidget, you get:
simplified allocation/disposal of resources lazy
loading a vastly reduced boilerplate over making a new class every
time devtools friendly
using Provider, the state of your application will be visible in the Flutter devtools a common way to consume these InheritedWidgets