#2668 - Add null-checks in on_outro callback for IfBlock

[IOuser]

#2668

[Rich-Harris]

Thanks, this is an excellent test case and a compact fix. It feels like there might be a deeper bug here — normally, I don't think it's possible to have 'nested' groups of outros. At least, if I modify transitions.js like so...

+let grouping = false;
+
export function group_outros() {
+	if (grouping) {
+		throw new Error('already grouping');
+	}
+
+	grouping = true;
+
	outros = {
		remaining: 0,
		callbacks: []
	};
}

export function check_outros() {
+	grouping = false;
+
	if (!outros.remaining) {
		run_all(outros.callbacks);
	}
}

...then the only test that fails is the one in this PR. So I wonder if there's an alternative fix that renders the null check unnecessary? (Unfortunately my brain is only capable of asking the question right now, not delivering the answer. It's very possible that I'm barking up the wrong tree.)

[IOuser]

@Rich-Harris, thank you for reply.
This snippet brought me to another fix without null checks. I tried to add counter for group_outros and check_outros. This allows to avoid override outros for 'nested' groups of outros. And also this allows to run callbacks only when 'outer' check_outros being called.

+let n = 0;
+
export function group_outros() {
+	if (n++ > 1) {
+		return; // already grouping
+	}
+
	outros = {
		remaining: 0,
		callbacks: []
	};
}

export function check_outros() {
+	if (n-- > 1) {
+		return; // skip inner calls
+	}
+
	if (!outros.remaining) {
		run_all(outros.callbacks);
	}
}

In that case tests passed without null checks in outro callbacks.

But unfortunately in that case 'out' animations are broken because nested blocks are still not detached when create_animation calls and it returns noop =)

I will continue investigating and share example with animations in separate repository a little bit later.

[IOuser]

I made a repository with an list animations example.
I think best solution for this issue - reset outros object after run_all callbacks.
I'm also added test case for multiple branches IfBlock.

[nikku]

@IOuser is this ready to be looked into? @Rich-Harris Could you give this PR another look?

[Rich-Harris]

The underlying issue is closed, having (apparently) been fixed by #3172, so I'll close this PR. Thanks