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# AsyncSRC20EventWatcher

Polling-based SRC20 event watcher that runs as an `asyncio.Task`.

Typically created via [`async_watch_src20_events()`](/clients/python/src20/event-watching/watch-src20-events.md) or [`async_watch_src20_events_with_key()`](/clients/python/src20/intelligence-providers/watch-src20-events-with-key.md) rather than instantiated directly.

## Constructor

```python
class AsyncSRC20EventWatcher:
    def __init__(
        self,
        w3: AsyncWeb3,
        aes_key: Bytes32,
        *,
        token_address: ChecksumAddress | None = None,
        on_transfer: AsyncTransferCallback | TransferCallback | None = None,
        on_approval: AsyncApprovalCallback | ApprovalCallback | None = None,
        on_error: AsyncErrorCallback | ErrorCallback | None = None,
        poll_interval: float = 2.0,
        from_block: int | str = "latest",
    ) -> None
```

## Parameters

| Parameter       | Type                                                        | Default    | Description                                    |
| --------------- | ----------------------------------------------------------- | ---------- | ---------------------------------------------- |
| `w3`            | `AsyncWeb3`                                                 |            | Async Web3 instance                            |
| `aes_key`       | [`Bytes32`](/clients/python/api-reference/types/bytes32.md) |            | 32-byte AES-256 viewing key                    |
| `token_address` | `ChecksumAddress \| None`                                   | `None`     | SRC20 contract to filter (`None` = all tokens) |
| `on_transfer`   | Callback                                                    | `None`     | Async or sync callback for Transfer events     |
| `on_approval`   | Callback                                                    | `None`     | Async or sync callback for Approval events     |
| `on_error`      | Callback                                                    | `None`     | Async or sync callback for errors              |
| `poll_interval` | `float`                                                     | `2.0`      | Seconds between polls                          |
| `from_block`    | `int \| "latest"`                                           | `"latest"` | Starting block                                 |

## Methods

| Method          | Description                          |
| --------------- | ------------------------------------ |
| `await start()` | Start the async polling task         |
| `await stop()`  | Cancel the task and wait for cleanup |
| `is_running`    | Property — `True` if task is active  |

## Async Context Manager

```python
async with AsyncSRC20EventWatcher(w3, key, on_transfer=cb) as watcher:
    await asyncio.sleep(60)
# Automatically stopped
```

## Notes

* Callbacks can be sync or async — the watcher detects and awaits coroutines automatically
* `CancelledError` is suppressed during shutdown
* If `on_error` is not provided, errors are logged at DEBUG level

## See Also

* [SRC20EventWatcher](/clients/python/src20/event-watching/src20-event-watcher.md) — Sync variant
* [watch\_src20\_events](/clients/python/src20/event-watching/watch-src20-events.md) — Factory with auto key fetch
* [watch\_src20\_events\_with\_key](/clients/python/src20/intelligence-providers/watch-src20-events-with-key.md) — Factory with explicit key
* [DecryptedTransferLog](/clients/python/src20/types/decrypted-transfer-log.md) — Transfer callback data
* [DecryptedApprovalLog](/clients/python/src20/types/decrypted-approval-log.md) — Approval callback data


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