[docs] Update 03-template-syntax.md - on:click on:click emits twice to parents - explanation

[Tommertom]

Added explanation on double event handlers that propagate to the parent. To make sure the developer understands the true power of adding multiple handlers. (and I have been in a situation forgetting about this, which took me a while to figure out).

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[Conduitry]

When is it useful to emit two instances of the event to the parent component?

[Tommertom]

When is it useful to emit two instances of the event to the parent component?

While I can't say never, it is likely not the best way to achieve something close to this at the parent level.

It did happen to me by accident keeping me busy debugging for a while until I found I added on:click twice (I had many event handlers added to a component). It was a bit of a rabbithole, so I felt I needed to share to help other avoiding this issue.

So maybe my PR suggests to document it as a feature or even a desirable pattern. That is not my intention. It's more like a caution.

How best to cover this in the documentation, or somewhere else? Happy to make adjustments.

[dummdidumm]

To me this feels expected - if I add an event listener multiple times, I'll be notified multiple times. This is the same with regular DOM event listeners. So I don't think we should mention it since it serves no use case other than "be careful" - to me this doesn't add enough value to the documentation vs the added lines. We have to leave out some things - if we add everything, the documentation will be too long.

[Tommertom]

Hi all

I agree adding to the doc wont make it easier to digest as a whole.

I do believe it is a pitfall for beginning devs and I bet this will occur in the future.

I see a reference being made to another issue where a different solution is suggested

To me, we can close this. Enough said and possibly Google indexing will help out too.