Bust the assets cache on modification.

[T4rk1n]

Append time modified as query string to assets urls to trick the cache to refresh the file.

cc @plotly/dash

Resolves #303

[rmarren1]

I like this, annoying to always have dev tools open and the cache probably trips up beginners.

[chriddyp]

While this will work when developing locally, assets will still be cached by the browser when the user deploys the app. The use case I'm imagining is:

1. Developer deploys their app
2. Person A visits their app
3. Developer changes CSS, deploys again
4. Person A visits again - the CSS is cached from when they visited the app before the previous deploy.

The developer won't see an issue because they got the new version when they were developing, so they might not even know that their visitors will have an issue.

So, I think we should append a time modified query string or some other solution

[T4rk1n]

@chriddyp I changed this PR to use time modified query string.

[chriddyp]

Awesome, thanks for updating this! Once this is released, could you update https://github.com/plotly/dash-docs/blob/master/tutorial/external_css_and_js.py#L160-L163?

[T4rk1n]

@chriddyp Ok, I'll release this along with #305 tomorrow.

[chriddyp]

@T4rk1n - Could we get this one merged and released?

[T4rk1n]

@chriddyp
Sorry I thought @bpostlethwaite wanted to chime in, we discussed that in local, we shouldn't add the modified query string for the assets to be mapped to the right local file and instead use the no-cache while on local and use the query string only in a prod settings.

[chriddyp]

we discussed that in local, we shouldn't add the modified query string for the assets to be mapped to the right local file and instead use the no-cache while on local and use the query string only in a prod settings

OK. Why? It seems to complicate things to have two different ways that caches are invalidated in local vs deployed. Also, we don't have the config abstraction around local vs production yet.

[T4rk1n]

That is also my feeling, I'll merge & release.