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# `useStateValidator`
Each time given state changes - validator function is invoked.
## Usage
```ts
import * as React from 'react';
import { useCallback } from 'react';
import { useStateValidator } from 'react-use';
const DemoStateValidator = s => [s === '' ? null : (s * 1) % 2 === 0];
const Demo = () => {
const [state, setState] = React.useState<string | number>(0);
const [[isValid]] = useStateValidator(state, DemoStateValidator);
return (
<div>
<div>Below field is valid only if number is even</div>
<input
type="number"
min="0"
max="10"
value={state}
onChange={(ev: React.ChangeEvent<HTMLInputElement>) => {
setState(ev.target.value);
}}
/>
{isValid !== null && <span>{isValid ? 'Valid!' : 'Invalid'}</span>}
</div>
);
};
```
## Reference
```ts
const [validity, revalidate] = useStateValidator(
state: any,
validator: (state, setValidity?)=>[boolean|null, ...any[]],
initialValidity: any
);
```
- **`validity`**_`: [boolean|null, ...any[]]`_ result of validity check. First element is strictly nullable boolean, but others can contain arbitrary data;
- **`revalidate`**_`: ()=>void`_ runs validator once again
- **`validator`**_`: (state, setValidity?)=>[boolean|null, ...any[]]`_ should return an array suitable for validity state described above;
- `state` - current state;
- `setValidity` - if defined hook will not trigger validity change automatically. Useful for async validators;
- `initialValidity` - validity value which set when validity is nt calculated yet;
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