Exercise: Observer
This exercise puts the Observer pattern into code.
Scenario
A Publisher manages a registry of Subscriber objects. Each subscriber holds a name and
an update(msg) method that returns "<name> got <msg>". When the publisher receives a
message, it calls update on each subscriber in subscription order and collects the results.
Duplicate subscriptions are silently ignored; the original position is preserved.
Commands
| Command | Behavior | Output |
|---|---|---|
["subscribe", name] | Register the subscriber; ignore if already subscribed | (none) |
["unsubscribe", name] | Remove the subscriber if present; do nothing otherwise | (none) |
["publish", msg] | Notify every current subscriber in subscription order | one line per subscriber: "<name> got <msg>"; if no subscribers: "No subscribers" |
A single publish command can emit multiple output lines — one per active subscriber.
Example
Input
6 subscribe Alice subscribe Bob publish hello unsubscribe Bob subscribe Alice publish bye
Output
Alice got hello Bob got hello Alice got bye
Explanation
Alice and Bob subscribe, so the first `publish` notifies both in subscription order, producing two lines. Bob then unsubscribes. The second `subscribe Alice` is a duplicate and is silently ignored — Alice keeps her original position. The second `publish` notifies only Alice. Note that `subscribe` and `unsubscribe` produce no output; only `publish` prints lines.
Your task
Implement the skeleton in the editor below so the commands produce the output described above.