Flag link field is not click sortable in views

Such kind of bugs are testable

+++ b/src/FlaggingViewsData.php
@@ -47,6 +47,7 @@ class FlaggingViewsData extends EntityViewsData {
+      'real field' => 'uid',

Huh?

Why would click-sorting this sort by user ID? I don't see how this makes sense.

This field is not data; it's function. Not being able to sort on it is correct.

> I have the following using case, where there is a need to sort Checked\Unchecked content. Also, this field is used to change content state.

Don't we have a flag status field which could be sorted on?

I can see that in your case you've made the flag link texts look like descriptions rather than actions, but for most people's setup, it won't make sense to sort on 'Flag this item'.

Very probably it fails because of related issue

Hm. I'm not sure it is. I had the same error on #2955701: Add method to get flag in ConfirmFormBase but a retest made it vanish...

Yes, it looks like some random fail https://www.drupal.org/pift-ci-job/929080
Very probably kinda "wait-for-ajax" used cos failed only Ajax tests

Created #2957888: Ajax Tests randomly fail.

I need this as well; my flags are "just" checkboxes (or, rather: FontAwesome square/check-squares) in a table, and I want to be able to visually sort by checked or unchecked. Screenshot attached.

This patch is working great for me.

I'm gonna set this to RTBC, given:

1. There's tests.
2. The "Flag link" clickSort matches the prior-art of the "Flagged" clickSort.
3. There's currently nothing in Flag core that PREVENTS a site builder from setting a "Flagging: Flag link" field, in a View, as "sortable" in a View's Table format. If a site builder does check "sortable", it then causes a PHP error / white screen for end-users who click that table header. This patch prevents that error. If this is uncommitable then we, at least, need a different patch that PREVENTS "sortable" from being checked by a site builder.
4. Given two use cases in this issue, and the long standing #353534: Add Checkboxes as Display Type, there's clearly enough demand for this sort of functionality for the flag links.

Agree that the uid sort seems a bit weird, but indeed similar to what the other definition does. That said, I'd also be fine with the initial fix, it's consistent then. But no deal breaker.

What is one, unfortunately, is that this needs some updates to be D9 compatible, can't commit a patch that isn't anymore. Hopefully not too much work, new base class, default theme, D9 compatibility in the test module...

Just wanted to add a comment, I had the same use case as @Oleksiy, where I needed the functional flag/unflag link in the view, but wanted to sort by whether they were flagged or unflagged. I was able to solve it by adding both 'Flagging' and 'Flagged' to my view, rewriting 'Flagged' as 'Flagging', and making 'Flagged' sortable in the table. (And excluding 'Flagging' from display)

Flagging/unflagging links work, and the view sorts correctly.

Resolved all PHPCS Issues

Rebased branch and updated the test. Changing status to needs review as I did not manually test it, but tests are passing.

Needs check why it failed https://git.drupalcode.org/issue/flag-2943153/-/jobs/7076733#L55

Failed because the older version of phpstan does not have that method it seems. Replaced the method with the waitForElementRemoved as there is no actual element to check on ajax complete and that method also seems to not be available. As I did not want to complicate things added that specific error message to the phpstan.neon to be ignored and that also fails (as the issue is just with previous version). Open to suggestions as locally can't really reproduce. My POV, it's not a big deal, as it's only test and a previous version, so would leave as is.