Suresh Sharma's Blog

Suresh Sharma's Blog

State Management in Flutter using Provider: Part 2

State Management in Flutter using Provider: Part 2

In the previous article, we have discussed the problem of using setState to update the app's state in the moderate or complex app, This part will cover the solution to those problems. Let's start.

What we are going to learn in this article

  1. What is Provider, Why we use it?
  2. What are the types of Provider?
  3. How to use Provider in our code?
  4. What is Consumer & Selector?

1. What is Provider, Why we use it

Provider is one of the most popular state management solution, created by Remi Rousselet, used for flutter app development.

On the beginner stage, we use setState to update the app's state, because it is easy, and switching from setState to other complex techniques like flutter_bloc or Mobx is not that easy to follow. 😓

Provider uses InheritedWidget internally, to manage all complex computations and provide a simple feature to manage our app's state

Why provider? We have some other techniques that can be used for managing state. Passing data via constructors is not a good idea. In doing so, all those widgets will rebuild just for passing the data. This may cause the performance issue and our code get too much complicated.

So, what we should do now? We will use the provider

flutter_bloc & Mobx uses provider internally 🥳 And now the flutter team also recommends using the provider. So, let's learn about the types of provider

2. What are the types of provider

Provider can handle different type of objects

Complete list of all objects

NameDescription
ProviderThe most basic form of provider. It takes a value and exposes it, whatever the value is.
ListenableProviderA specific provider for Listenable object. ListenableProvider will listen to the object and ask widgets which depend on it to rebuild whenever the listener is called.
ChangeNotifierProviderA specification of ListenableProvider for ChangeNotifier. It will automatically call ChangeNotifier.dispose when needed.
ValueListenableProviderListen to a ValueListenable and only expose ValueListenable.value.
StreamProviderListen to a Stream and expose the latest value emitted.
FutureProviderTakes a Future and updates dependents when the future completes.

So, let's discover more about the categories

Provider

This is a very basic form of Provider, manages the lifecycle of the value it provides by delegating to a pair of Create and Dispose. This can provide any type of value.

class Counter {
    int count = 0;
  void increment() {
      count += 1;
      notifyListeners();
  }
}
class Stateless extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Provider<Counter>(
      create: (context) =>  Counter(),
      dispose: (context, value) => value.dispose(),
      child: ...,
    );
  }
}

ListenableProvider

ListenableProvider is a bit different from the other providers in the list, as this provider provides state source as the value.

Other providers generally provide the single value from the object<T>, but ListenableProvider provides the whole model as the value to its dependents. When the state changes in Listenable, it notifies the provider about the state change. Now provider, via it's InheritedWidget marks it's listening dependent to rebuild.

In the above example, the whole Counter() object is being returned from create, as this value is provided to the dependents, dependents use this value as Counter() and updates it's state and rebuilds again to show the updated value on the screen.

ChangeNotifierProvider

In the change notifier provider, everything is the same as ListenableProvider, one additional specification of ListenableProvider for ChangeNotifier is, it automatically calls ChangeNotifier.dispose to dispose the provider when needed.

ValueListenableProvider

ValueListenableProvider gets the value change notification from ValueListenable, and provides the value to its dependent as state. ValueListenableProvider<T> receives values of type T from a ValueListenable<T>.

StreamProvider

StreamProvider gets the events from a subscribed stream and provides these events(values) to its dependent widgets. we can use StreamProvider<T> to receive events of type T from a Stream<T>.

StreamProvider<int>(
  initialData: 0,
  builder: (context) {
    return Stream<int>
      .periodic(Duration(milliseconds: 100), (count) => count + 1)
      .take(100);
  },
  child: ...
)

In the above example, StreamProvider emits count value ranges from 0 to 100, on the interval of 100 milliseconds, dependents can use these events for some purpose.

FutureProvider

FutureProvider gets the value of a future and provides to its dependents. We can use FutureProvider<T> to get the Future<T> value and this value represents the state.

FutureProvider<int>(
  initialData: 0,
  builder: (context) {
    return Future.delayed(Duration(seconds: 4), () => 1);
  },
  child: ...
),

This, FutureProvider<int> gives the binary value 0 and 1. In the initial state, it provides the 0, and as soon as future resolves (after 4 seconds)this provides the 1 as a value

3. How to use a provider in our code

Provider has some methods and we will look at 3 major one

  • context.watch<T>(), which makes the widget listen to changes on T
  • context.read<T>(), which returns T without listening to it
  • context.select<T, R>(R cb(T value)), which allows a widget to listen to only a small part of T.

Here <T> is a type of value, like String, int, double or any other complex type We can also use the static method Provider.of<T>(context) or Provider.of<T>(context, listen : false) which will behave similarly to watch/read respectively

How these methods work 🤔

These methods will look up in the widget tree starting from the widget associated with the BuildContext, and will return the nearest variable of type T found (or throw if nothing is found).

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Text(
      context.watch<String>(),
    );
  }
}

we must pass the type<> here in watch(), searches the same type of data in the widget tree. this watch() will listen to a state change, and rebuilds with the new value, whenever the state changes.

Similarly, read<T>() will look for the variable of type T in the widget tree from it's BuildContext and retrieves the data.

In the case of select((T value)), it only listens to those parts of T, if state change happens in this part, only then it's dependents will update there value. It prevents widget from unnecessarily rebuilding if something other changes.

Now, one static method which works the same as the above method Provider.of<T>(context) this method performs the same as watch(), listen to changes in state T

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
      String provider = Provider.of<String>(context);
    return Text(provider
    );
  }
}

Provider.of<T>(context) method has listen, which is set to true by default, which means if we are using this method it will listen to state change.

If we set listen to false then, this will not listen to state change

Alternatively, the same thing we can also obtain by using Consumer and Selector

Introduction to Consumer and Selector

The consumer gets the value from ancestors using Provider<T> and passes this value to it's builder

Consumer widget gets the value from the Provider<T> when there is no BuildContext Selector works same as select((T value)), it rebuilds only when, if there is a change in selected value, for which this is listing, no rebuilds happen when others value changes

Selector determines, if builder needs to be called again by comparing the previous and new result of selector using DeepCollectionEquality from the package collection

Takeaways

  • We learned how provider helps to overcome the limitations of setState in a complex project
  • Discussed 5 different types of provider
  • We got to know that flutter_bloc & Mobx also uses provider internally
  • Got introduction about Consumer & Selector Widget

So, that's all for this article. I hope this helped you to understand the provider in Flutter. If I'm left with something crucial, please let me know in the comment section.

Feel free to connect with me on twitter

 
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