I have tried to use bloc in flutter for a period, but the question of where to put bloc provider confuse me.
At the beginning of learning flutter, I put nearly all the providers in the main() function by using MultipleBlocProvider.
But due to some of my blocs subscribe to streams such as firebase snapshots, I think it might cost extra resources (I am not sure but at least some memory was occupied I guess) if I do not close those subscriptions by closing the bloc.
However, if the provider is in the main(), change page or pop up would not close these blocs and the subscriptions inside.
In this scenario, I try to put some bloc providers into specific pages, so the bloc can be close and recreate when I goes in and out of this page. But there are some other questions occurs.
For example, if I have a bloc called ProductDetailsBloc which is used to control the widget of product details in product details page, its events contains an event called GetProductBySku which is used to get product from firebase and set the product inside the state(ProductDetailsState).
I want to put this bloc and its provider inside product details page and put an event trigger inside product list widget (located in product list page) and its onTap() function, where users click on the product list item (this is a thumbnail product item which is from another resources and only contains very basic info such as sku, title ) inside product list page and then navigator to product details page to view the full information of this product.
But as I mentioned before, If I put the bloc provider of ProductDetailsBloc inside product details page, I can not use GetProductBySku before entering product details page
So, I personally have two ideas for this questions,
the first one is passing the product sku through arguments to product details page and then call the GetProductBySku after the bloc has been created.
the second one is the put the ProductDetailsBloc inside the main(), so I can skip this questions, and directly use the GetProductBySku in product list widget, but this turns the problems in to the very front.
I do not know which one is better, so I would be very appreciative if someone can give some suggestions.
And back to the main questions, what is the best practice of putting bloc providers and what are the concerns if I put providers inside main().
Thanks for every one that read to here because this is a bit long and this is my first time of asking on attack overflow :3
You can find more discussion regarding this in the discord server of Bloc. More info: https://discord.com/channels/649708778631200778/649708993773699083/860604230930006016
I will copy paste the message and code here. we have been using bloc
in our app for over a year and it seems we have got an issue.
Scenario: List of Posts needs to be shown in home feed. Let's say each
post is encapsulated in PostBloc (operations on post to be performed
here). I have been using moor to reactively find if anything changes
in the data (like number of likes of post which might have trigged
from post detail page where post detail was fetched again). I want to
know if it is a wise decision to create BlocFactory which will be
providing Bloc object (here different PostBloc object) based on
Post_Id.
I have written a sample BlocFactoryProvider, please let me know your
thoughts
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:provider/provider.dart';
import 'package:provider/single_child_widget.dart';
typedef CreateIfAbsent<T> = T Function();
//create bloc of <T> type
//author: https://github.com/imrhk
class BlocFactory<T extends Bloc<dynamic, dynamic>> {
Map<String, T> _cache = <String, T>{};
T createBloc({#required String id, #required CreateIfAbsent<T> createIfAbsent}) {
assert(id != null && createIfAbsent != null);
if (_cache.containsKey(id)) {
return _cache[id];
}
final value = createIfAbsent.call();
_cache[id] = value;
return value;
}
void dispose() {
_cache.values.forEach((bloc) => bloc?.close());
_cache.clear();
}
}
class BlocFactoryProvider<T extends Bloc<dynamic, dynamic>, V>
extends SingleChildStatelessWidget {
final BlocFactory<T> _blocFactory;
final Widget child;
BlocFactoryProvider({BlocFactory<T> blocFactory, this.child})
: _blocFactory = blocFactory ?? BlocFactory<T>();
#override
Widget buildWithChild(BuildContext context, Widget child) {
return InheritedProvider<BlocFactory<T>>(
create: (_) => _blocFactory,
dispose: (_, __) => _blocFactory.dispose(),
child: child,
lazy: false,
);
}
static BlocFactory<T> of<T extends Bloc<dynamic, dynamic>>(BuildContext context) {
try {
return Provider.of<BlocFactory<T>>(context, listen: false);
} on ProviderNotFoundException catch (e) {
if (e.valueType != T) rethrow;
throw FlutterError(
"""
BlocProvider.of() called with a context that does not contain a Bloc of type $T.
No ancestor could be found starting from the context that was passed to BlocProvider.of<$T>().
This can happen if the context you used comes from a widget above the BlocProvider.
The context used was: $context
""",
);
}
}
Felix's replay was
It’s hard to say without more context but I’d recommend moving the
caching to the repository layer and instead have a single PostBloc
with PostChanged event that pulls the latest details (using cache).
So he did not completely discarded the idea.
declaration: I am the author of the below code shared over discord in March'21.
Related
For a simple Email login with OTP code I have a structure as follows.
View
await _signUpCntrl.signUp(email, password);
Controller
_showOtpDialog(email);
_showOtpDialog func
return Get.dialog(
AlertDialog(
So the thing is _showOtpDialog function is inside a controller file. ie. /Controllers/controller_file.dart
I want do something like a blocListener, call the _showOtpDialog from a screen(view) file on signup success. (also relocate the _showOtpDialog to a view file)
Using GetX I have to use one of the builders either obs or getbuilder. Which is I think not a good approach to show a dialog box.
On internet it says Workers are the alternative to BlocListener. However Workers function resides on Controller file and with that the dialog is still being called on the controller file.
As OTP dialog will have its own state and a controller I wanted to put it inside a /view/viewfile.dart
How do I obtain this?
I tried using StateMixin but when I call Get.dialog() it throw an error.
visitChildElements() called during build
Unlike BLoC there's no BlocListener or BlocConsumer in GetX.
Instead GetX has RxWorkers. You can store your response object in a Rx variable:
class SomeController extends GetxController{
final response= Rxn<SomeResponse>();
Future<void> someMethod()async{
response.value = await someApiCall();
}
}
And then right before the return of your widget's build method:
class SomeWidget extends StatelessWidget{
final controller = Get.put(SomeController());
#override
Widget build(BuildContext context){
ever(controller.response, (SomeResponse res){
if(res.success){
return Get.dialog(SuccessDialog()); //Or snackbar, or navigate to another page
}
....
});
return UI();
}
First thing, you will need to enhance the quality of your question by making things more clearly. Add the code block and the number list, highlight those and making emphasize texts are bold. Use the code block instead of quote.
Seconds things, Depends on the state management you are using, we will have different approaches:
Bloc (As you already added to the question tag). By using this state management, you controller ( business logic handler) will act like the view model in the MVVM architecture. In terms of that, You will need to emit a state (e.g: Sent success event). Afterward, the UI will listen to the changes and update it value according to the event you have emitted. See this Bloc example
GetX (As your code and question pointed out): GetX will acts a little bit different. you have multiple ways to implement this:
Using callbacks (passed at the start when calling the send otp function)
Declare a general dialog for your application ( this is the most used when it comes to realization) and calling show Dialog from Bloc
Using Rx. You will define a Reactive Variable for e.g final success = RxBool(true). Then the view will listen and update whenever the success changes.
controller.dart
class MyController extends GetxController {
final success = RxBool(false);
void sendOtp() async {
final result = await repository.sendOTP();
success.update((val) => {true});
}
}
view.dart
class MyUI extends GetView<MyController> {
#override
Widget build(BuildContext context) {
ever(controller.success, (bool success) {
// This will update things whenever success is updated
if (success) {
Get.dialog(AlertDialog());
}
});
return Container();
}
}
I'm new in flutter and I read lot of documentations on multiple subjets. One of them is sharing data between screens. I found a lot of solutions and as I'm creating a base project with lot of features, I want to know which one is the best
Solution 1 : In constructor
Basically, when we navigate, we send the data through the next constructor widget.
Solution 2 : In session
Creates a singleton which can be access from everywhere in the application and each time you need to send data, you add it in the session and the next widget can retrieve it.
Solution 3 : In blocs
I read this solution which looks good:
I made a BLoC container class where I've instantiated the BLoCs of the two screen. Then here I set the reference from the BLoC A to a stream of the BLoC B where I want to send the data. The BLoCs are still decoupled cause they don't know anything about each other, the BLoC A doesn't get passed through the constructor on the BLoC B and vice versa, but it just knows that it will receive some data on one of its streams.
UPDATED:
Solution 4 : Inherited widget
With a static function like :
static InheritedDataProvider of(BuildContext context) => context.inheritFromWidgetOfExactType(InheritedDataProvider);
}
So you can access to the data initialized in the parent with something like :
final data = InheritedDataProvider.of(context).data;
Maybe there are others solutions and I'll be glad to know them.
Thanks
The best way is passing a parameter to the constructor
Navigator.push(
context,
PageTransition(
type: PageTransitionType.fade,
child: LoginPage(
userModel: model,
)));
then :
class LoginPage extends StatefulWidget {
LoginPage({this.userModel});
User userModel;
#override
_LoginPageState createState() => _LoginPageState(userModel: userModel);
}
class _LoginPageState extends State with TickerProviderStateMixin {
User userModel;
_LoginPageState({this.userModel});
}
}
What I use:
Call next screen with Navigator.pushNamed(ctx, '/foo', arguments: bar). Only include arguments if needed (e.g. for "update detail" screen).
Centralize all possible routes in the widget that contains MaterialApp.
Pass a clone of the passed argument to the next screen (immutable data).
In code:
...
MaterialApp(
onGenerateRoute: (s) => MaterialPageRoute(builder: (ctx) => _route(s), settings: s),
)
...
Widget _route(RouteSettings route) {
switch (route.name) {
case '/signup':
return SignupRoute();
case '/vendor':
return VendorRoute((route.arguments as Vendor)?.data?.clone());
default:
throw ('No match for ${route.name}');
}
}
i started using Provider package for state management, and using it in a basic way.
As the app gets more complex i want to extend the usage.
Now i have this model structure in mind: List<Client> having a List<Product> (and deeper having a List<Component>).
I have a MultiProvider using a ChangeNotifierProvider for the Clients, means the List<Client> is managed by the provider, so far so good.
Now i want to directly use the List<Product> in a provider, or later the List<Component> inside the List<Product>. I do not want to go the way through the List<Client>...down to the Component.
Here i have an image map of the structure to visualize.
Here is some simplified code:
// Just an example idea of..
Class Product with ChangeNotifier {
final String title;
}
Class Client with ChangeNotifier {
final String name;
final String List<Product>;
}
Class Clients with ChangeNotifier {
final List<Client> _items;
}
void main() {
// start the app
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MultiProvider(
providers: [
ChangeNotifierProvider(create: (ctx) => Clients()),
// How to provide a List<Product> that actually in the model
// belongs to a Client in the List<Client>
],
child: MaterialApp(
body: ...
)
);
}
}
So the main question is how to provide a List<Product> that actually in the model
belongs to a Client in the List<Client>?
Thanks for your help in the comments.
There seem to be two possible solutions to it.
Do not use nested structure. This is possible by keeping the id of the parent inside the before nested list and throw it out the of the parent list. Then give it an own ChangeNotifierProvider and filter the items by the parent id when needed.
So in my example List and List are on the same level and both have an
ChangeNotifierProvider. The product model holds the id of it's client and the list has an method getbyClientId(clientId) that results in a filtered productslist to the specified client. That's it.
Use ChangeNotifierProxyProvider to delegate the request into the nested list.
I have chosen the first way as in my case it seemed the right way. And after working some time now with it i can not say it was a wrong decision, so i have my freedom with it.
Probably for other cases the second approach might be better. I haven't invested too much time in ChangeNotifierProxyProvider, so please check other sources for more details on that.
I'm new to dart/flutter and having a little trouble getting my head around communication patterns.
One reoccurring problem is that I keep looking to expose a public method on a widget so it can be called by other widgets.
The problem is with stateful widgets. In these cases, I need to call down to the widgets state to do the actual work.
The problem is that the widget doesn't have a copy of the state.
I have been saving a copy of the state in the widget but of course this throws a warning as it makes the widget mutable.
Let me give a specific example:
I have a specialised menu which can have a set of menu items.
Each are stateful.
When the menu is closing it needs to iterate over the list of menu items that it owns and tell each one to hide (the menu items are not visually contained within the menu so hiding the menu doesn't work).
So the menu has the following code:
class Menu{
closeMenu() {
for (var menuItem in menuItems) {
menuItem.close();
}
}
So that works fine, but of course in the MenuItem class I need to:
class MenuItem {
MenuItemState state;
close()
{
state.close();
}
But of course having the state object stored In the MenuItem is a problem given that MenuItem is meant to be immutable. (It is only a warning so the code works, but its clearly not the intended design pattern).
I could do with seeing more of your code to get a better idea of how to solve your specific issue but it appears that the Flutter documentation will help you in some regard, specifically the section on Lifting state up:
In Flutter, it makes sense to keep the state above the widgets that use it.
Why? In declarative frameworks like Flutter, if you want to change the UI, you have to rebuild it.
…it’s hard to imperatively change a widget from outside, by calling a method on it. And even if you could make this work, you would be fighting the framework instead of letting it help you.
It appears you're trying to fight the framework in your example and that you were correct to be apprehensive about adding public methods to your Widgets. What you need to do is something closer to what's detailed in the documentation (which details all of the new classes etc you'll see below). I've put a quick example together based on this and the use of Provider which makes this approach to state management easy. Here's a Google I/O talk from this year encouraging its use.
void main() {
runApp(
ChangeNotifierProvider(
builder: (context) => MenuModel(),
child: MyApp(),
),
);
}
class MyApp extends StatelessWidget {
…
// call this when the menu is closed
void onMyMenuClosed(BuildContext context) {
var menuModel = getMyMenuModel(context);
menuModel.hideMenuItems();
}
}
class MenuModel extends ChangeNotifier {
bool _displayItems = false;
void hideMenuItems() {
_displayItems = false;
notifyListeners();
}
void showMenuItems() {
_displayItems = true;
notifyListeners();
}
}
Calling hideMenuItems() makes a call to notifyListeners() that'll do just that; notify any listeners of a change which in turn prompts a rebuild of the Widget/s you wrap in a Consumer<MenuModel> Now, when the Widget that displays the menu is rebuilt, it just grabs the appropriate detail from the MenuModel class - the one source of truth for the state. This reduces the number of code paths you'd otherwise have to deal with to one and makes it far easier to see what's happening when you make further changes.
#override
Widget build(BuildContext context) {
return Consumer<MenuModel>(
builder: (context, menuModel, child) {
return menuModel._displayItems() ? MenuItemsWidget() : Container();
},
);
}
I recommend you read the entire page on state management.
I am new to Dart/Flutter and after "attending" a Udemy course,
everything has been going well.
Until now ;-)
As in the sample application in the Udemy course i am using the BLOC pattern.
Like this:
class App extends StatelessWidget {
Widget build(context) {
return AppBlocProvider(
child: MaterialApp(
(See "AppBlocProvider" which I later on use to get the "AppBloc")
The App as well as all the screens are StatelessWidget's.
The AppBlocProvider extends the InheritedWidget.
Like this:
class AppBlocProvider extends InheritedWidget {
final AppBloc bloc;
AppBlocProvider({Key key, Widget child})
: bloc = AppBloc(),
super(key: key, child: child);
bool updateShouldNotify(_) => true;
static AppBloc of(BuildContext context) {
return (context.inheritFromWidgetOfExactType(AppBlocProvider) as AppBlocProvider).bloc;
}
}
The AppBlocProvider provides an "AppBloc" containing two further bloc's to separate the different data a bit.
Like this:
class AppBloc {
//Variables holding the continuous state
Locale appLocale;
final UserBloc userBloc;
final GroupsBloc groupsBlock;
In my application I have a "GroupSearchScreen" with just one entry field, where you can enter a fragment of a group name. When clicking a button, a REST API call is done and list of group names is returned.
As in the sample application, I put the data in a stream too.
In the sample application the data fetching and putting it in the stream is done in the bloc itself.
On the next line, the screen that uses the data, is created.
Like this:
//Collecting data and putting it in the stream
storiesBloc.fetchTopIds();
//Creating a screen ths shows a list
return NewsList();
In my case however, there are two major differences:
After collecting the data in the GroupSearchScreen, I call/create the GroupsListScreen, where the list of groups shall be shown, using regular routing.
Like this:
//Add data to stream ("changeGroupList" privides the add function of the stream!)
appBloc.groupsBlock.changeGroupList(groups);
//Call/create screen to show list of groups
Navigator.pushNamed(context, '/groups_list');
In the GroupsListScreen, that is created, I fetch the bloc.
Like this:
Widget build(context) {
final AppBloc appBloc = AppBlocProvider.of(context);
These are the routes:
Route routes(RouteSettings settings) {
switch (settings.name) {
case '/':
return createLoginScreen();
case '/search_group':
return createSearchGroupScreen();
case '/groups_list':
return createGroupsListScreen();
default:
return null;
}
}//routes
And "/groups_list" points to this function:
Route createSearchGroupScreen() {
return MaterialPageRoute(
builder: (context) {
//Do we need a DashboardScreen BLOC?
//final storiesBloc = StoriesProvider.of(context);
//storiesBloc.fetchTopIds();
return GroupSearchScreen();
}
);
}
As you can see, the "AppBlocProvider" is only used once.
(I ran into that problem too ;-)
Now to the problem:
When the GroupsListScreen starts rendering the list, the stream/snapshot has no data!
(See "if (!snapshot.hasData)" )
Widget buildList(AppBloc appBloc) {
return StreamBuilder(
stream: appBloc.groupsBlock.groups,
builder: (context, AsyncSnapshot<List<Map<String, dynamic>>>snapshot) {
if (!snapshot.hasData) {
In order to test if all data in the bloc gets lost, I tried not to put the data in the stream directly, but in a member variable (in the bloc!).
In GroupSearchScreen I put the json data in a member variable in the bloc.
Now, just before the GroupsListScreen starts rendering the list, I take the data (json) out of the bloc, put it in the stream, which still resides in the bloc, and everything works fine!
The snapshot has data...
Like this (in the GroupsListScreen):
//Add data to Stream
appBloc.groupsBlock.changeGroupList(appBloc.groupsBlock.groupSearchResult);
Why on earth is the stream "losing" its data on the way from "GroupSearchScreen" to "GroupsListScreen" when the ordinary member variable is not? Both reside in the same bloc!
At the start of the build method of the GroupsListScreen, I have a print statement.
Hence I can see that GroupsListScreen is built twice.
That should be normal, but could that be the reason for not finding data in the stream?
Is the Stream listened on twice?
Widget buildList(AppBloc appBloc) {
return StreamBuilder(
stream: appBloc.groupsBlock.groups,
I tried to explain my problem this way, not providing tons of code.
But I don't know if it's enough to give a hint where I can continue to search...
Update 16.04.2019 - SOLUTION:
I built up my first app using another app seen in a Udemy course...
The most important difference between "his" app and mine is that he creates the Widget that listens to the stream and then adds data to the stream.
I add data to the stream and then navigate to the Widget that shows the data.
Unfortunately I used an RX-Dart "PublishSubject." If you listen to that one you will get all the data put in the stream starting at that time you started listening!
An RX-Dart "BehaviorSubject" however, will also give you the last data, just before you started listening.
And that's the behavior I needed here:
Put data on stream
Create Widget and start listening
I can encourage all Flutter newbies to read both of these very good tutorials:
https://medium.com/flutter-community/reactive-programming-streams-bloc-6f0d2bd2d248
https://www.didierboelens.com/2018/12/reactive-programming---streams---bloc---practical-use-cases/
In the first one, both of the streams mentioned, are explained very well.