Simplify the integration of custom widgets with MultiProvider

[rrousselGit]

Problem:

MultiProvider is a very important feature for a provider. But it is currently tedious and error-prone to make a widget compatible with MultiProvider:

• we have to implement SingleChildCloneableWidget, that is very error-prone (easy to forget a parameter)
• it usually requires an extra private constructor, just for the clone implementation

All in all, it's too complex, which makes peoples not want to make custom providers and stick to the existing ones.

Solution

I made a POC of an alternate implementation of MultiProvider, which doesn't relly on a "clone" method.

Instead, it uses a new kind of widget (neither Stateless nor Stateful): SingleChildStatelessWidget / SingleChildStatefulWidget.

The difference is that the build method takes an extra Widget child parameter:

class Example extends SingleChildStatelessWidget {
  Example({Key key, Widget child}): super(key: key, child: child);

  @override
  Widget build(BuildContext context, {Widget child}) {
    // TODO: do something with `child`
  }
}

Which is then instantly compatible with MultiProvider:

MultiProvider(
  providers: [
    Example(),
  ],
)

Consequences:

The SingleChildCloneableWidget interface will be removed, and replaced by a SingleChildWidget interface and two implementations:

• SingleChildStatelessWidget
• SingleChildStatefulWidget

MultiProvider will then accept a SingleChildWidget instead of SingleChildCloneableWidget

Existing implementations of SingleChildCloneableWidget should be migrated to use one of the two SingleChildWidget implementation instead.

Before:

class Example extends StatelessWidget implements SingleChildCloneableWidget {
  Example({Key key, int value, this.child}): this(key: key, foo: Foo(value), child: child);

  Example._({Key key, this.foo, this.child): super(key: key);

  final Foo foo;
  final Widget child;

  @override
  Widget build(BuildContext context) {
    return Whatever(
      foo: foo,
      child: child,
    );
  }

  @override
  Example cloneWithChild(Widget child) {
    return Example._(
      key: key,
      foo: foo,
      child: child,
    );
  }
}

After:

class Example extends SingleChildStatelessWidget {
  Example({Key key, int value, Widget child}): foo = Foo(value), super(key: key, child: child);

  final Foo foo;

  @override
  Widget build(BuildContext context, {Widget child}) {
    return Whatever(
      foo: foo,
      child: child,
    );
  }
}

[lookfirst]

I had to specially import this:

import 'package:provider/single_child_widget.dart';

[awhitford]

Is there a reason why single_child_widget is not included when importing package:provider/provider.dart?

[rrousselGit]

It pollutes the auto complete for things not directly related to provider.