ChangeNotifierProxyProvider2<T, T2, R extends ChangeNotifier> class

A ChangeNotifierProvider that builds and synchronizes a ChangeNotifier from values obtained from other providers.

To understand better this variation of ChangeNotifierProvider, we can look into the following code using the original provider:

ChangeNotifierProvider(
  builder: (context) {
    return MyChangeNotifier(
      foo: Provider.of<Foo>(context, listen: false),
    );
  },
  child: ...
)

In example, we built a MyChangeNotifier from a value coming from another provider: Foo.

This works as long as Foo never changes. But if it somehow updates, then our ChangeNotifier will never update accordingly.

To solve this issue, we could instead use this class, like so:

ChangeNotifierProxyProvider<Foo, MyChangeNotifier>(
  initialBuilder: (_) => MyChangeNotifier(),
  builder: (_, foo, myNotifier) => myNotifier
    ..foo = foo,
  child: ...
);

In that situation, if Foo were to update, then MyChangeNotifier will be able to update accordingly.

Notice how MyChangeNotifier doesn't receive Foo in its constructor anymore. It is now passed through a custom setter instead.

A typical implementation of such MyChangeNotifier would be:

class MyChangeNotifier with ChangeNotifier {
  Foo _foo;
  set foo(Foo value) {
    if (_foo != value) {
      _foo = value;
      // do some extra work, that may call `notifyListeners()`
    }
  }
}
  • DON'T create the ChangeNotifier inside builder directly.

    This will cause your state to be lost when one of the values used updates. It will also cause uncesserary overhead because it will dispose the previous notifier, then subscribes to the new one.

Instead use properties with custom setters like shown previously, or methods.

ChangeNotifierProxyProvider<Foo, MyChangeNotifier>(
  // may cause the state to be destroyed unvoluntarily
  builder: (_, foo, myNotifier) => MyChangeNotifier(foo: foo),
  child: ...
);
  • PREFER using ProxyProvider when possible.

    If the created object is only a combination of other objects, without http calls or similar side-effects, then it is likely that an immutable object built using ProxyProvider will work.

Inheritance

Constructors

ChangeNotifierProxyProvider2({Key key, @required ValueBuilder<R> initialBuilder, @required ProxyProviderBuilder2<T, T2, R> builder, Widget child })
Initializes key for subclasses.

Properties

builder ProxyProviderBuilder2<T, T2, R>
Builds the value passed to InheritedProvider by combining InheritedWidget. [...]
read-only, inherited
child Widget
The widget that is below the current Provider widget in the tree.
final, inherited
dispose Disposer<R>
Optionally allows to clean-up resources when the widget is removed from the tree.
final, inherited
hashCode → int
The hash code for this object.
read-only, inherited
initialBuilder ValueBuilder<R>
Builds the initial value passed as previous to didChangeDependencies.
final, inherited
key Key
Controls how one widget replaces another widget in the tree. [...]
final, inherited
runtimeType → Type
A representation of the runtime type of the object.
read-only, inherited

Methods

build(BuildContext context, R value) Widget
An equivalent of StatelessWidget.build.
inherited
cloneWithChild(Widget child) → _NumericProxyProvider<T, T2, Void, Void, Void, Void, R>
Clones the current provider with a new child.
inherited
createElement() ProxyProviderElement
Creates a StatefulElement to manage this widget's location in the tree. [...]
inherited
createState() → _ProxyProviderState<R>
Creates the mutable state for this widget at a given location in the tree.
inherited
debugDescribeChildren() → List<DiagnosticsNode>
Returns a list of DiagnosticsNode objects describing this node's children. [...]
@protected, inherited
debugFillProperties(DiagnosticPropertiesBuilder properties) → void
Add additional properties associated with the node. [...]
inherited
didChangeDependencies(BuildContext context, R previous) → R
Builds the value passed to build by combining InheritedWidget.
inherited
noSuchMethod(Invocation invocation) → dynamic
Invoked when a non-existent method or property is accessed.
inherited
toDiagnosticsNode({String name, DiagnosticsTreeStyle style }) DiagnosticsNode
Returns a debug representation of the object that is used by debugging tools and by DiagnosticsNode.toStringDeep. [...]
inherited
toString({DiagnosticLevel minLevel: DiagnosticLevel.debug }) → String
Returns a string representation of this object.
inherited
toStringDeep({String prefixLineOne: '', String prefixOtherLines, DiagnosticLevel minLevel: DiagnosticLevel.debug }) → String
Returns a string representation of this node and its descendants. [...]
inherited
toStringShallow({String joiner: ', ', DiagnosticLevel minLevel: DiagnosticLevel.debug }) → String
Returns a one-line detailed description of the object. [...]
inherited
toStringShort() → String
A short, textual description of this widget.
inherited

Operators

operator ==(dynamic other) → bool
The equality operator.
inherited