Signal-based decoupling

Table of contents

• Introduction
• Defining and sending a signal
• Listening to a signal
• Signals or template hooks
• See also

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Introduction

Features announce business events through blinker signals. The feature that sends one does not know who listens, so you add or remove a listener without touching it.

# splent_feature_reports/signals.py
from splent_framework.signals.signal_utils import connect_signal

@connect_signal("note-created", "splent_feature_reports")
def on_note_created(sender, note, **kwargs):
    ...  # count the note in today's report

When somebody writes a note, reports counts it, and notes never imports reports.

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Defining and sending a signal

# splent_feature_notes/signals.py
from splent_framework.signals.signal_utils import define_signal

note_created = define_signal("note-created", "splent_feature_notes")

# splent_feature_notes/services.py
note_created.send(current_app._get_current_object(), note=note)

Signal names use hyphens, and feature:contract lists them under provides.signals.

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Listening to a signal

Connect in your feature’s own signals.py, by name, as the introduction shows. A handler whose sender has not loaded yet is connected when its define_signal runs, so load order does not matter. If the sender is not installed, the handler never fires.

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Signals or template hooks

Mechanism	Layer	Use it to
Signals	Business logic	React to domain events, such as an order placed
Template hooks	Presentation	Put HTML into another feature’s templates

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See also

• Signals, the API reference