Skip to main content
Version: 0.3.1

SignalComponent

The typed event bus. A SignalComponent is a dcc.Store under the hood, wrapped in a WeaverletComponent so producers and consumers reference it via self.<signal> instead of a string ID.

from weaverlet import SignalComponent

Signature​

class SignalComponent(WeaverletComponent):

    signal_id = Identifier()
    signal_group_id = Identifier()
    signal_attr: str = 'data'
    signal_default_retval: dict = {}

    def __init__(self, name: str = "unnamed") -> None: ...

    def get_layout(self) -> "dash.dcc.Store": ...
    def register_callbacks(self, app) -> None: ...   # no-op

Public attributes​

AttributeTypeDescription
signal_idIdentifierThe underlying dcc.Store's element ID.
signal_attrstr ('data')The property on the store callbacks read/write.
signal_group_idIdentifierGroup ID for use with dash_extensions.enrich's group= multi-output multiplexing.
signal_default_retvaldict ({})Default value used when callbacks need to "no-op emit". E.g. StoreComponent's CLEAN op.

What get_layout returns​

dcc.Store(id=self.signal_id, data={})

A dcc.Store with data={} initial value. You must render the signal exactly once per layout. Calling self.sig() inside the layout of whichever component owns it.

register_callbacks is a no-op; SignalComponent has no callbacks of its own. It's pure plumbing.

Usage​

from weaverlet import (
    WeaverletComponent, WeaverletApp, Identifier,
    SignalComponent, SignalOutput, SignalInput,
)
from dash_extensions.enrich import Input, Output
from dash import html


class Demo(WeaverletComponent):
    btn_id = Identifier()
    out_id = Identifier()

    def __init__(self):
        super().__init__()
        self.sig = SignalComponent()         # owned here

    def get_layout(self):
        return html.Div([
            self.sig(),                       # rendered exactly once
            html.Button("Emit", id=self.btn_id),
            html.P(id=self.out_id),
        ])

    def register_callbacks(self, app):
        @app.callback(SignalOutput(self.sig), Input(self.btn_id, "n_clicks"))
        def emit(n):
            return {"clicks": n}

        @app.callback(Output(self.out_id, "children"), SignalInput(self.sig))
        def show(payload):
            return f"Payload: {payload}"


WeaverletApp(root_component=Demo()).app.run()

Wiring callbacks to signals​

Don't write Input(sig.signal_id, sig.signal_attr) yourself. Use the signal adapters:

To …UseReturns
Emit payloadSignalOutput(sig)Output
Receive payloadSignalInput(sig)Input
React to edge onlySignalTrigger(sig)Trigger
Read as State (no trigger)SignalState(sig)State
Use as a group IDSignalGroup(sig)str
Serverside-cached outputServersideSignalOutput(sig)Output (same as SignalOutput in 0.3+; wrap return value with Serverside(...))

Multiple writers to one signal​

Multiple producer callbacks can target the same signal because WeaverletApp.app is a DashProxy with group= multi-output multiplexing. No setup needed:

@app.callback(SignalOutput(self.sig), Input(self.btn_a, "n_clicks"))
def from_a(n): return {"source": "A", "n": n}

@app.callback(SignalOutput(self.sig), Input(self.btn_b, "n_clicks"))
def from_b(n): return {"source": "B", "n": n}

Multiple readers​

Just as straightforward:

@app.callback(Output(self.first_out, "children"), SignalInput(self.sig))
def show_one(payload): ...

@app.callback(Output(self.second_out, "children"), SignalInput(self.sig))
def show_two(payload): ...

Both fire on every emission.

Pitfalls​

Render self.sig() exactly once

A SignalComponent is a dcc.Store underneath. Rendering it twice in the same layout triggers DuplicateIdError. The owning component is the natural place to render it. If multiple components need to share a signal, the lowest common ancestor owns it and passes references to its descendants.

Producer callbacks must return a dict

Even if you have no meaningful payload, return {}. Don't return None, a string, or a non-dict. Downstream consumers expect a dict and may break.

The consumer fires on every emission, including the initial empty value

At app startup, dcc.Store.data is {} and SignalInput-based callbacks may fire once with that empty value. Use prevent_initial_call=True to skip startup firing.

See also​