# onOpened

Sets up a listener for the event when a mail is opened by the receiver.

### Usage

```ts
const receiver = '0x...';

mail.onOpened(receiver, null, (index, openedAt) => {
  console.log(index);
  console.log(openedAt);
});
```

#### Stop watching

```ts
const stopWatching = mail.onOpened(receiver, null, (index, openedAt) => {
  console.log(index);
  console.log(openedAt);
});

// ... Later, to stop watching for the events
stopWatching();
```

### Returns

`UnwatchFn`

A function that can be invoked to stop watching for new event logs.

### Parameters

#### receiver

* **Type** `Address | null`

The mail receiver's address. If set to null, it listens for any receiver.

#### index

* **Type** `bigint | null`

The index of the mail. If set to null, it listens for the opening of any mail.

#### callback

* **Type** `(index: bigint, openedAt: bigint) => void`

The callback function to be executed when a mail-opened event is detected. It receives the index of the opened mail and its opened timestamp as its arguments.