special handling for _reload-hash in dev tools

[chriddyp]

when my app crashes, the reload request fails. this causes repeated error message in dash dev tools:

It would be nice if we could treat this message with more care:

• Only display the error once or twice
• Update the error message to be something like "The app has likely crashed."

[alexcjohnson]

Good call - also if you forget and leave a dash app open in a tab after halting its server... next day the whole browser starts dragging as those thousands of calls clog up the page.

[raghavendrajain]

Hi. In my case, even when there is NO error in the code, page displays this error whenever a change is made. I think, it is a problem with hot-reloading feature, as it appears irrespective of whether code is correct or wrong.

[alexcjohnson]

@raghavendrajain that's not an issue I've seen, can you say more? Does it happen with any app or just a specific one? What versions of Dash packages do you have? (pip list | grep dash if installed by pip)

[kenhui521]

@alexcjohnson
I got the same error as well. It's not to an app specific. My dash version is dash==1.2.0.
I have noticed that this error appeared when there is a long running method executed by dash button.

[alexcjohnson]

Thanks @kenhui521 - I've tried creating extremely slow callbacks, clogging the server, and still haven't been able to generate that error. If you're able to find an app simple enough to share that I can use to replicate this error, I'd love to see it!

[alexcjohnson]

Perhaps we should convert this situation from an error to a simple status indicator - when the most recent _reload-hash request failed, we add a little red light somewhere that has a tooltip saying "the back end is not responding" - and when a request succeeds we remove that indicator.

Also perhaps we should skip these requests entirely - or at least make them much less frequent - when the window is inactive (document.visibilityState !== 'visible' is what I used in dash-notes) - just means it may have the old state for a couple seconds when you come back to it.