ChangeNotifierProxyProvider2<T, T2, R extends ChangeNotifier> class

A ChangeNotifierProvider that builds and synchronizes a ChangeNotifier with external values.

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

ChangeNotifierProvider(
  create: (context) {
    return MyChangeNotifier(
      myModel: Provider.of<MyModel>(context, listen: false),
    );
  },
  child: ...
)

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

This works as long as MyModel 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<MyModel, MyChangeNotifier>(
  create: (_) => MyChangeNotifier(),
  update: (_, myModel, myNotifier) => myNotifier
    ..update(myModel),
  child: ...
);

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

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

A typical implementation of such MyChangeNotifier could be:

class MyChangeNotifier with ChangeNotifier {
  void update(MyModel myModel) {
    // Do some custom work based on myModel that may call `notifyListeners`
  }
}
  • DON'T create the ChangeNotifier inside update directly.

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

Instead reuse the previous instance, and update some properties or call some methods.

ChangeNotifierProxyProvider<MyModel, MyChangeNotifier>(
  // may cause the state to be destroyed involuntarily
  update: (_, myModel, myNotifier) => MyChangeNotifier(myModel: myModel),
  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 Create<R> create, @required ProxyProviderBuilder2<T, T2, R> update, bool lazy, TransitionBuilder builder, Widget child})
Initializes key for subclasses.

Properties

builder TransitionBuilder
Syntax sugar for obtaining a BuildContext that can read the provider created. [...]
final, inherited
hashCode int
The hash code for this object. [...]
@nonVirtual, read-only, 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) Widget
Describes the part of the user interface represented by this widget. [...]
inherited
buildWithChild(BuildContext context, Widget child) Widget
A build method that receives an extra child parameter. [...]
inherited
createElement() → _InheritedProviderElement<R>
Creates a StatelessElement to manage this widget's 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
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.info}) 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 ==(Object other) bool
The equality operator. [...]
@nonVirtual, inherited