Display
No reading yet
Shows the latest reading
OBSERVER / THE IDEA IN A MINUTE
One change.
Several interested listeners.
Observer lets objects subscribe to events from another object, so each can react when something changes.
The publisher keeps a list of interested observers. It sends each one a notification through the same contract. The caller controls who joins and who leaves.
- What changes?
- The listeners interested in an event.
- What stays stable?
- The publisher and the notification contract.
- Who manages the connection?
- The caller subscribes listeners and cleans up when they leave.
WATCH THE CONNECTIONS
One reading. Only interested listeners.
Choose who subscribes, then publish a reading. Watch it reach only the connected panels.
THE PUBLISHER
Weather station
18 °C
2 of 3 panels subscribed
History
No reading yet
Remembers the last five
Frost alert
No reading yet
Warns at 0 °C or below
Display and History are subscribed. Publish a reading to see them react.
Useful when the audience changes.
Several independent objects need to react to the same event, and listeners join or leave over time.
Connections need care.
Subscriptions add lifecycle work and make the call flow less obvious. A direct call may be simpler for one fixed receiver. Observer does not automatically make delivery asynchronous or reliable.
Choose your next step.
LEARNING IT FOR THE FIRST TIME
Learn through an example
Start with one display, feel the cost of adding listeners, then connect panels through subscriptions.
Live weather station · optional practiceBACK FOR A REFRESHERQuick reference
Review the roles, subscription lifecycle, TypeScript/Kotlin code, and mistakes to avoid.
Bookmarkable · no walkthrough required