Enable custom validation of Settings through `ConfigurationChangeEvent`

[robole]

Currently, it is not possible to do custom validation of Settings programmatically. There are some options in the Extension Manifest (package.json) e.g use a regex to validate a string. However, if you have a group of options (like a checkbox group), and you want to have at least one to be selected, this is not possible to validate. I have this scenario for my extension Marky Stats, I always want at least one stat to be selected. I can do this validation with the QuickPick, but I cannot do the equivalent with the configuration.

Currently, you can use the workspace.onDidConfigurationChange event to respond to config options being changed, but you can only tell if something has changed or not. This is done through ConfigurationChangeEvent.affectsConfiguration(). It would be great if you could update the API of ConfigurationChangeEvent to enable a change to be validated.

[roblourens]

How do you want to display validations? Currently you could show a notification or something

[robole]

I think reporting the validation error through window.showErrorMessage() would be sufficient. You would need the concept of a checkbox group to give inline feedback.

[roblourens]

So what's keeping you from being able to do that?

[robole]

So what's keeping you from being able to do that?

As I stated at the outset, you can only tell if something has changed or not. You cannot tell what values have changed. You cannot reject a change, which is typically what validation does.

[roblourens]

You can read the configuration at any time to see whether the values are valid, using getConfiguration. But we won't enable custom validation to reject changes like this. You can either show a warning in some other way, or you can change the way your settings work to be able to validate it with JSON schema