I have had some time to look more into this and these are my findings.

1. The "Save and preview" works by first saving the content, and then "ui" does the go-to-preview bit.
2. It is not directly possible by default, to interupt the event, since umbraco does not produce such an event.

Solution: Hijack the preview url using a new controller class, and do the custom handling of data, before redirecting the user further.

Hope this can help other people in the future, but please Umbraco, add this to the event system - please :D

I usually don't like to give links, but doesn't this help you? https://docs.umbraco.com/umbraco-cms/reference/notifications/contentservice-notifications ?

I just copy-pasted this example and it works ok. All the events you need are covered here.

You can only rename DontShout to something more edible :)

Regards

I might be a little late to the party, but check this page out: https://docs.umbraco.com/umbraco-cms/reference/routing/custom-controllers

public class CustomControllerRouteController : RenderController
{
    public CustomControllerRouteController(ILogger<RenderController> logger, ICompositeViewEngine compositeViewEngine, IUmbracoContextAccessor umbracoContextAccessor) : base(logger, compositeViewEngine, umbracoContextAccessor)
    {

    }
    public override IActionResult Index()
    {
        // you can access the CurrentPage directly in the controller, but just so you know you can use it:
        var currentPage = CurrentPage;
        if (UmbracoContext.InPreviewMode)
        {
            // Do something
        }
        // maybe do an else here, depending on what you want to do above
        return CurrentTemplate(CurrentPage);
    }
}

Then in your program.cs below the .Build() of your builder.CreateUmbracoBuilder add:

builder.Services.Configure<UmbracoRenderingDefaultsOptions>(c =>
{
    c.DefaultControllerType = typeof(CustomControllerRouteController);
});

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