Provider: ProxyProvider* does not wait for non-null value to available.

Created on 26 Aug 2019 · 15Comments · Source: rrousselGit/provider

I have this setup

 FutureProvider<RemoteWeatherDataSource>.value(
    value: OpenWeatherMapRestApiClient.newInstance(),
  ),
  FutureProvider<LocalWeatherDataSource>.value(
    value: SQLiteWeatherDataSource.newInstance(),
  ),
  ProxyProvider2<LocalWeatherDataSource, RemoteWeatherDataSource, GetWeather>(
    builder: (context, localWeather, remoteWeather, _) => GetWeather(
      localWeather,
      remoteWeather,
    ),
  ),

the builder is being call with null values because the futureProvider provide initial null values.

but the getweather class is excepting non-null values.

the expected behavior is that as soon as both two values are non-null then the builder is called, because otherwise what's the purpose of receiving null value ?

documentation

Source

bitsydarel

All 15 comments

The builder will be called again when the future's value is available

But it is necessary to handle the "null" scenario. It is critical that your widget tree should be able to render at any time.

rrousselGit on 26 Aug 2019

so this work as excepted ?

Just not what i was expecting from a DI point of view.

The documentation of all of those provider and even the consumer docs does not also specify that the value might be null.

I had to check and see that it's use FutureBuilder internally that the reason.

bitsydarel on 26 Aug 2019

would not be correct to just provide two final event ? success or failure ?

bitsydarel on 26 Aug 2019

It's expected. But fair point, the documentation on this matter could be improved

rrousselGit on 26 Aug 2019

👍1

I have handled the null case and on the UI everything seems ok but it's seems that if a proxyProvider return null it's print some long debug messages, is that really expected ?

does the library really allow null values ?

bitsydarel on 26 Aug 2019

This seems to be the tree summary from where the null value has been returned...

bitsydarel on 26 Aug 2019

I can confirm that it works. Here's a widget test showcasing it:

  testWidgets('a', (tester) async {
    await tester.pumpWidget(MultiProvider(
      providers: [
        FutureProvider(builder: (_) => Future.value(42)),
        ProxyProvider<int, String>(builder: (_, a, __) => a.toString()),
      ],
      child: Consumer<String>(
        builder: (_, str, __) => Text(str, textDirection: TextDirection.ltr),
      ),
    ));
    expect(find.text('null'), findsOneWidget);

    await tester.pump();

    expect(find.text('42'), findsOneWidget);
  });

rrousselGit on 26 Aug 2019

👍1

  providers: [
    FutureProvider(builder: (_) => Future.value(42)),
    ProxyProvider<int, String>(builder: (_, a, __) => a.toString()),

Thanks for this! Have just spend a while wondering if I need to create a new class just to calculate a single value from 2 providers.. maybe there could be examples of this in the docs too?

castaway on 5 Sep 2019

What examples do you have in mind, or what do you feel that is lacking in the current one?

rrousselGit on 5 Sep 2019

Caveat: I'm fairly new to dart/flutter in general, and have only so far used ChangeNotifier+Provider in probably basic ways - the example in the README for ProxyProvider creates a value using a class Translations.. what I'm after is a single value thats generated from 2 providers

Is there a better place to chat about this? someone elses issue probably isnt the sanest one ;)

castaway on 5 Sep 2019

what I'm after is a single value thats generated from 2 providers

So ProxyProvider2?

ProxyProvider2<A, B, String>(
  builder: (_, a, b, __) => '$a $b',
  child: ...,
)

rrousselGit on 5 Sep 2019

Yess.. but.. see #211 ;)

castaway on 5 Sep 2019

should we close this issue or keep it open for other devs ?

bitsydarel on 25 Sep 2019

@rrousselGit

MultiProvider(
      providers: [
        FutureProvider(create: (_) => Future.value(42)),
        ProxyProvider<int, String>(update: (_, a, __) => a.toString()),
      ]

This works fine. It's the equivalent of:

var a = await Future.value(42);
var b = a.toString();

But what if you have two FutureProviders? This does not work. The value of num never reaches String.

MultiProvider(
      providers: [
        FutureProvider<int>(create: (_) => Future.value(42)),
        FutureProvider<num>(create: (context) => Future.value(context.watch<int>() + 0.2)),
        ProxyProvider<num, String>(update: (_, b, __) => b.toString()),
      ]

which is equivalent to:

var a = await Future.value(42);
var b = await Future.value(a + 0.2);
var c = b.toString();

(plus we'd have to handle nulls in Provider<num>)

How would you set up providers in this case?

It seems the num Provider would have to be both a FutureProvider (for async) AND a ProxyProvider (for update, and eventually dispose)

frank06 on 19 Mar 2020

👍1

I'll close this as this is not actionable with the current architecture.
I have some plans to improve such thing (that I talked about here: https://www.youtube.com/watch?v=zjBulEwjzeI), but this is a experimental project.

rrousselGit on 5 Jun 2020

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