Learning to Filter

[flyerbear]

This is such an awesome tool, but I feel like I'm barely scratching the surface of how to really use it. Are there good tutorials anyone can recommend for learning CSS, JSONPath, JQ, and XPath filters so I can only trigger on exactly the content I want? I know what I want to do on each page, but I'm struggling to turn that into the filters necessary to make it happen.

Also, is there a way to view the content that the browser sees before the text is extracted into the Preview / Diff page so that we can see what tags, etc. to filter on? I've been using "View Page Source" from my browser for some pages, but for dynamically generated javascript pages, this method doesn't work. If I could see what changedetection's browser sees, I could tailor my filters accordingly.

Finally, I know there is a built-in price tool, but is there a way to trigger when certain data exceed / go below thresholds? Is this something that can be written into to the filter/trigger fields?

Thank you!

[flyerbear]

Oh my goodness I just figured out how to directly copy an element's xpath using the Web Inspector from a browser, without any real knowledge of how xpath works. Complete game changer.

Now to figure out how to trigger on values within the remaining text. If anyone can offer help, I would be very grateful!

[flyerbear]

I was able to do this in a rough way with regex, but I'm not sure that's the best way to implement what is ultimately a comparison of values. Is there a better way to do a value comparison in one of the filter/trigger fields?

[ta7tUHIJHx4RwR21]

Can you share your watch? That way people can give specific help rather than try to answer your question with generalities.

[flyerbear]

Sure, here's the watch: https://changedetection.io/share/U68BOuTDKrYa

This watches mempool.space, a bitcoin blockchain visualization and data analysis site. The idea is to monitor for when transaction fees fall below a set threshold. I currently have it looking at the "No Priority" fee rate and triggering when this falls to 5 sat/vB or less. I have it jury-rigged with a regex trigger, but this will get messier if I ever want to trigger on higher values with more than one digit. It would be more straightforward to do a simple <= comparison.

If there's a way to do that, my next goal is to be able to look at the same site, but set different triggers and notifications for different people. That way, if I were supporting a group of people with interest in the same page, but wanted to be notified at different thresholds, the watch would do that. I thought I could implement this (inefficiently) with multiple watches all going to the same URL, but it looks like changedetection.io won't let me add more than one watch to the same address.

This particular example is a little esoteric in that it is bitcoin-specific, but I'm interested in the same functionality for product price watches.

[flyerbear]

I just noticed the "Create a copy" button at the bottom of all the edit tabs...this will definitely achieve the goal of looking at the same site with different triggers and notifications.

[Sudlon]

@flyerbear Did you ever figure out how to set a trigger for a certain threshold?

[flyerbear]

I didn't I'm afraid. I was able to get what I needed in this specific case using regex but it's def something I'd like to figure out in the future. I think it might be possible with js but I don't know it well enough. If I figure it out, I'll post!

[Sudlon]

Ok so I got it to work with XPATH! I didn't think it was exactly intuitive and also it seems that changedetection.io only supports a subset (or maybe I found some obscure functions), so I'll just share what I ended up using as a more elaborate example so that people can adjust it to their needs.

This is what I used for checking whether the price, which is represented in an element with id "product_price", is below a certain point; in this case 60:
//*[@id='product_price' and number(translate(text(), ",", ".")) < 60]

What happens here is I grab the inner value, using "text()". Then I replace the comma by a period so I can then change the string value to a number value. This is needed so I can then compare it with the threshold of 60.

For some reason I didn't need to do the conversion to number on all websites. For example (somewhat similar to your use case) when setting an alert on the ethereum gas price at https://etherscan.io/gastracker, this also works directly:
//span[@id='spanLowPrice' and text()<20]

[flyerbear]

This is great! Thanks for the follow up. I'll have to play with it a bit.

[CanYouJustWorkPlease]

@Sudlon Thanks a lot! This helped.