# set_interval

## Function

Registers a fixed-delay repeating callback and returns a `Timer` object immediately. The next trigger time is calculated only after each round of callback execution is completed, so the same interval will not re-enter, and events will not accumulate due to slow callbacks.

## Syntax

```bt
timer = set_interval(fn, delay_ms)
```

## Parameters

| Parameters | Type | Required | Default value | Description |
| ------ | ------ | ------ | ------ | ------ |
| fn | Fn | is | None | A BT function that is executed repeatedly. |
| delay_ms | Int | No | 1 | Interval in milliseconds; if it is less than 1, it will be treated as 1. |

## Return value

| Type | Description |
| ------ | ------ |
| Timer | Timer handle, you can call `cancel()` to stop subsequent triggering. |

## Example

```bt
count = 0

set_interval(fn(self) {
    count++

    if count >= 3 {
        self.cancel()
    }

    // Output: current count
    println(count)
}, 10)
```

## Notes

- The callback can declare that the first parameter receives the current `Timer` object to facilitate calling `self.cancel()` inside the callback.
- When the callback does not declare parameters, the extra `self` parameters will be ignored.
- If an error is thrown in the callback, the error will be recorded, and subsequent intervals will continue to execute by default; if you need to stop, please capture the error in the callback and cancel.
- Timers cannot be created in the context of a web request.