Setting the value of a field when loading a page for editing

[BillH]

It feels like this should be obvious or it should be easy to find a solution in the forums, but I've got stuck!

I'm trying to set the value of a field (based on other field values) when an existing page loads for editing.

Here's a reduced test version of the code that doesn't work, from ready.php:

wire()->addHookAfter('Page::loaded', function($event) {

    $page = $event->object;
    
    if ($page->template == 'article') {

        // Get field values and do calculations here
		// but for testing:
        $calculatedValue = "Testing 123";
    
        $page->of(false);
        $page->set('test_field', $calculatedValue);

    }

});

Note that if I start the code like this, it works fine when the page is saved:

wire()->addHookAfter('Pages::saveReady', function($event) {

    $page = $event->arguments[0];

	if ($page->template...

So I presume there's something wrong with the hook or getting the page.

But what?

[wbmnfktr]

Maybe it's enough to hook into it before the page is ready to be saved.

wire()->addHookBefore('Pages::saveReady', function($event) {