Class PollingSourceTransfer<T>

Output transfer with internal polling of a data source.

Capabilities: isOutput, isPollingSource, isPullable, isSubscribable, isTriggerable, isGate

Mechanics:

1. The constructor accepts config.fetcher: DataFetcher — a function to retrieve data
2. An internal Ticker (RAFTicker by default) calls trigger() at the specified interval
3. trigger() — calls fetcher(), writes the result, notifies subscribers
4. pull() — calls fetcher() directly (without writing to state)
5. subscribe(handler) — subscribes to periodic updates
6. activate()/deactivate()/toggle() — control polling
7. destroy() — stops the ticker, unsubscribes subscribers

Error handling:

• If fetcher() throws an exception in trigger() or pull(), onError is called.
• With onError provided, the exception is suppressed (polling continues).
• Without onError, the exception is rethrown.

Configuration (PollingSourceTransferConfig):

• fetcher: DataFetcher — data retrieval function
• interval: number — polling interval (ms)
• activated: boolean — initial polling state
• tickerFactory?: TickerFactory — custom ticker factory (default: RAFTicker.factory)
• onError?: ErrorHandler — fetcher error handler

Use cases:

• Periodic polling of an API or external source
• Timer emitting the current time
• Animation at a fixed FPS