Contract events: listener

Focus: one listener contract that subscribes to a publisher event and stores the last payload.

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const PRICE_EVENT: u64 = 100
const LAST_PRICE_KEY: string = "last_price"
 
fn on_price(args: any[]) -> u64 {
    let storage: Storage = Storage::new()
    storage.store(LAST_PRICE_KEY, args)
    return 0
}
 
entry listen_to(oracle_hash: Hash) {
    let oracle: Contract = Contract::new(oracle_hash).expect("oracle contract not found")
    require(oracle.listen_event(PRICE_EVENT, on_price, 100000), "listen failed")
    return 0
}
 
entry last_price() -> any[] {
    let storage: Storage = Storage::new()
    return storage.load(LAST_PRICE_KEY).unwrap_or([])
}

How it works

• PRICE_EVENT must match the publisher’s event id.
• on_price(args: any[]) is the callback signature for contract event payloads.
• The callback stores the raw payload under LAST_PRICE_KEY.
• listen_to(oracle_hash) loads the publisher contract and registers the callback with a gas limit.
• last_price() is a small read entry that stores the last observed payload as a typed exit value, or an empty array when no price was observed.

Run listen_to once with the publisher hash, then invoke publish_price on the publisher.