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This package brings back context.read and context.watch for riverpod

This package brings back the context extensions for riverpod that were discontinued in version 1.0.0.

  • To read any provider, do context.read(myProvider)
  • To watch any provider, do context.watch(myProvider)

This package is meant to be used alongside riverpod and offers an alternative to the official ConsumerWidget and ConsumerStatefulWidget.

Getting Started #

This assumes you already have flutter_riverpod (or hooks_riverpod) set up.

First, add riverpod_context as a dependency.

flutter pub add riverpod_context

Next, add InheritedConsumer underneath the root ProviderScope:

// Before
ProviderScope(
  child: MyApp(),
)
// After
ProviderScope(
  child: InheritedConsumer(
    child: MyApp(),
  ),
)

That's all.

Context Extensions #

riverpod_context provides four convenient context extensions to interact with your providers.

context.read #

context.read can be used anywhere without any special consideration. It naturally supports any providers, provider families as well as the new .select() syntax.

Widget build(BuildContext context) {
  // this won't rebuild based on 'myValue'
  String myValue = context.read(myProvider);
  return Text(myValue);
}

context.watch #

context.watch watches the provider and triggers a rebuild of the given context when the providers state changes. It again supports any providers, provider families as well as the new .select() syntax.

Widget build(BuildContext context) {
  // this will rebuild each time 'myValue' changes
  String myValue = context.watch(myProvider);
  return Text(myValue);
}

🚨 There are a few important considerations to make when using context.watch. With those, you can also safely use any .autoDispose providers.

1. Only use inside build()

context.watch can only be used inside the build() method of a widget. Especially interaction callbacks (like onPressed) and StatefulWidgets initState, didChangeDependencies and other lifecycle handlers are not allowed.

2. Be cautious when conditionally watching providers

It is possible to conditionally watch providers. This is the case when context.watch may not be called on every rebuild.

Widget build(BuildContext context) {
  if (myCondition) {
    return Text(context.watch(myProvider));
  } else {
    return Container();
  }
}

In this example, when myCondition is false, context.watch is not called. This leads to an issue where the dependencies of the provider are not clearly defined.

It is important to make sure that this does not happen, since it can lead to leaking memory and wrong behavior!

Preventing this is however pretty simple.

If there exists another context.watch on the same context, this issue is resolved. Generally speaking, it requires at least one context.watch call on every build to be safe.

If in the example above, the myCondition actually comes from another context.watch call, you are safe.

Widget build(BuildContext context) {
  if (context.watch(myConditionProvider)) {
    // don't worry about this being called conditionally, 
    // we already have called context.watch once before
    return Text(context.watch(myProvider));
  } else {
    return Container();
  }
}

If not, use context.prime().

3. Or use context.prime() when conditionally watching providers

If context.watch is - under certain conditions - not called on every rebuild, you have to "prime" the context for the missing provider. This can be done using a simple context.prime() call.

In the previous example, this can be placed either in the else, or unconditionally at the top. It also has no effect to do it multiple times.

Widget build(BuildContext context) {
  context.prime(); // option 1: always prime
  if (myCondition) {
    return Text(context.watch(myProvider));
  } else {
    context.prime(); // option 2: prime to account for missing context.watch call
    return Container();
  }
}

As a rule just remember this:

Wherever you use context.watch conditionally, make sure to either have another unconditional context.watch or use context.prime on the same context.

Or in other words:

You are safe if on each rebuild there always is at least one call to either context.watch or context.prime.

context.refresh #

context.refresh refreshes any provider.

Widget build(BuildContext context) {
  return TextButton(
    onPressed: () {
      context.refresh(myProvider);
    },
    child: const Text('Refresh'),
  );
}

context.listen #

context.listen Listens to a provider without triggering a rebuild. This can be used inside the build() method or in the didChangeDependencies() of a stateful widget.

Widget build(BuildContext context) {
  
  context.listen(myProvider, (previous, value) {
    // do something
  });
  
  return SomeWidget();
}
Idempotent listeners

There will only ever be a single active listener for a specific context, meaning that calling context.listen multiple times for the same provider will only have the last listener active. Only because of this it is safe to use context.listen inside the build() method across rebuilds.

You can set fireImmediate: true to immediately fire the listener once. This will be ignored when re-listening to a provider, i.e. after a rebuild.

Widget build(BuildContext context) {

  // across multiple rebuilds, there will only exist a single listener on this provider
  // only on the first build, the listener will fire immediately
  context.listen(myProvider, (previous, value) {
    // do something
  }, fireImmediately: true);
  
  return SomeWidget();
}
Closing listeners

All listeners will only be closed when the context is disposed. Therefore it has no effect to call context.listen conditionally, especially with .autoDispose providers.

There are two ways to control the closing of a listener:

  • By using context.unlisten you can close the active listener on a provider.
  • When wanting more control over a listener, use context.subscribe.
Widget build(BuildContext context) {

  if (myCondition) {
    context.listen(myProvider, (previous, value) {
      // do something
    });
  } else {
    // this will remove the active listener on this provider
    // and properly dispose an .autoDispose provider
    context.unlisten(myProvider);
  }
  
  return SomeWidget();
}

context.subscribe #

context.subscribe listens to a provider and returns the ProviderSubscription. Use this when you need to manually manage the subscription of a provider.

  • This can be used wherever you have a BuildContext, even in the initState() method.
  • Make sure to call subscription.close() when the listener is no longer needed.
class MyWidgetState extends State<MyWidget> {
  
  late ProviderSubscription subscription;
  
  @override
  void initState() {
    // store the returned subscription in a variable
    subscription = context.subscribe(myProvider, (previous, value) {
      // do something
    });
  }
  
  // ...
  
  @override dispose() {
    // make sure to properly close the subscription
    subscription.close();
  }
}

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Publisher

verified publisher iconschultek.de

This package brings back context.read and context.watch for riverpod

Repository (GitHub)
View/report issues

Documentation

API reference

License

MIT (LICENSE)

Dependencies

flutter, flutter_riverpod

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