SubHandler<Model, Msg> class

Provides dispatch and model access within a subscription's setup function.

handler(msg) / handler.dispatch(msg) — send a message. handler.model — read the current model (always fresh, never stale).

Avoiding stale model captures

The subscriptions(Model model) function is called with the current model on every model change, but a subscription's setup function is only invoked once when its ID first appears. Any model value captured via closure from the outer call will be frozen at that instant and will not update.

Wrong — model is captured and goes stale:

Subs<MyModel, MyMsg> subscriptions(MyModel model) => [
  (['my-sub'], (handler) {
    return stream.listen((_) => handler(DoThing(model.count))).cancel;
    //                                            ^^^^^^^^^^^
    //                                            stale after first registration
  }),
];

Correct — read handler.model at event time:

Subs<MyModel, MyMsg> subscriptions(MyModel model) => [
  (['my-sub'], (handler) {
    return stream.listen((_) => handler(DoThing(handler.model.count))).cancel;
  }),
];

Preferred (Elm-style) — emit a plain message and let update access the model, which always receives the freshest state:

Subs<MyModel, MyMsg> subscriptions(MyModel model) => [
  (['my-sub'], (handler) {
    return stream.listen((_) => handler(EventOccurred())).cancel;
  }),
];
// Then in update:
(MyModel, Cmd<MyMsg>) update(MyMsg msg, MyModel model) => switch (msg) {
  EventOccurred() => (model.copyWith(count: model.count + 1), Cmd.none()),
  // ...
};

Constructors

SubHandler(Dispatch<Msg> _dispatch, Model _getModel())

const

Properties

hashCode → int

The hash code for this object.

no setter, inherited

model → Model

The current model — always reflects the latest state.

no setter

runtimeType → Type

A representation of the runtime type of the object.

no setter, inherited

Methods

call(Msg msg) → void

Dispatch a message. Can also be invoked directly as handler(msg).

dispatch(Msg msg) → void

Dispatch a message.

noSuchMethod(Invocation invocation) → dynamic

Invoked when a nonexistent method or property is accessed.

inherited

toString() → String

A string representation of this object.

inherited

Operators

operator ==(Object other) → bool

The equality operator.

inherited