# Widget testing passed in function


In this post, we going to go through how, at least in my opinion, one would go about testing that a function is called within a widget when that function is one of the Widgets arguments.

Take the following overly simplified example…

```
class SampleWidget extends StatelessWidget {
  final Function(String fileId) onDelete;
  final String fileId;

  const SampleWidget({
    @required this.onDelete,
    @required this.fileId,
    foundation.Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    throw FlatButton(
      onPressed: () => onDelete(fileId),
      child: const Text('Delete'),
    );
  }
}
```


As you can see we have a simple “reusable” widget that takes a `fileId` and a `onDelete` function.

Testing this in the implementing widget is quite simple as you would simply `verify` the resulting function is called.

However, if you wished to test this widget in isolation, there are a few more steps required.

In our current project, we added a very simple helper class:

```
class TestCallbackFunctions {
  void onFileDelete(String fileId) => null;
}
```


This class will then hold simply “mock” functions that we can pass into reusable widgets to test it completely.

I hope you found this interesting, and if you have any questions, comments, or improvements, feel free to drop a comment. Enjoy your Flutter development journey :D

If you liked it, a like would be awesome, and if you really liked it, a [cup of coffee](https://www.buymeacoffee.com/remelehane) would be great.

Thanks for reading.

If you wish to carry on with the subject of testing, why not take a look at:

[**Bringing localization into your Widget testing**
*The more accurate you make your test, the higher the quality of the test itself.*](https://remelehane.hashnode.dev/bringing-localization-into-your-widget-testing-121ef7eec7c)

[**Widget testing when your app needs access to directories.**
*Many times in an application you would have a need to store files, temporarily or even permanently. For this, you are…*](https://remelehane.hashnode.dev/widget-testing-when-your-app-needs-access-to-directories-2c9c7aa73b8a)