feat: (#174) Introduce onCheck event to register a callback

README.md

@@ -73,6 +73,7 @@ Props
 ----
 
 - `onChange`: callback for whenever the element changes from being within the window viewport or not. Function is called with 1 argument `(isVisible: boolean)`
+- `onCheck`: callback for whenever the element being checked if it is within the window viewport or not. Function is called with 1 argument `(isVisible: boolean)` after the check is done. - Same as `onChange` but called even if the visibility hasn't changed.
 - `active`: (default `true`) boolean flag for enabling / disabling the sensor.  When `active !== true` the sensor will not fire the `onChange` callback.
 - `partialVisibility`: (default `false`) consider element visible if only part of it is visible. Also possible values are - 'top', 'right', 'bottom', 'left' - in case it's needed to detect when one of these become visible explicitly.
 - `offset`: (default `{}`) with offset you can define amount of px from one side when the visibility should already change. So in example setting `offset={{top:10}}` means that the visibility changes hidden when there is less than 10px to top of the viewport. Offset works along with `partialVisibility`

visibility-sensor.js

@@ -38,6 +38,7 @@ export default class VisibilitySensor extends React.Component {
 
   static propTypes = {
     onChange: PropTypes.func,
+    onCheck: PropTypes.func,
     active: PropTypes.bool,
     partialVisibility: PropTypes.oneOfType([
       PropTypes.bool,
@@ -321,6 +322,7 @@ export default class VisibilitySensor extends React.Component {
       if (this.props.onChange) this.props.onChange(isVisible);
     }
 
+    if (this.props.onCheck) this.props.onCheck(isVisible);
     return state;
   };