Support BFCache using the pageshow event about history

[mjackson]

@taurose Would you like to take a stab at this?

[agundermann]

I would, but I'm not sure how to best implement this. When going back to a different page, do we run transition hooks as if the page was never left, or immediately update the location? Should we let the user know that the transition came from pageshow instead of popstate, and should we let him know in case the URL hasn't changed and we don't need to transition?

[mjackson]

When going back to a different page, do we run transition hooks as if the page was never left, or immediately update the location?

From what I understand of the original bug, it seems like pageshow should be treated just like our initial POP event which bypasses transition hooks and immediately updates the location, but I'm not 100% sure.

In the steps to reproduce that bug, when is pageshow actually fired? When you hit Enter or when you hit the back button?

[agundermann]

It fires when you come back to the app, i.e. when hitting the back button. Instead of re-executing the app, we get the pageshow event with everything else being in the state that the app was left.

The example I posted there may be a bit confusing since the app isn't really left, but pressing enter on the URL bar has the same effect of creating a new document, reloading HTML&JS etc. This example should be easier to follow.

I'm leaning towards treating it like the initial event, too, since it would be confusing to go back to a page and be presented with a dialog about leaving the page I left a long time ago. On the other hand, the initial DOM will reflect the old page, so maybe we should make it distinguishable somehow.

[mjackson]

On the other hand, the initial DOM will reflect the old page, so maybe we should make it distinguishable somehow.

What would be the use case here? I guess someone might want to preserve that DOM for some reason. I'm trying to imagine maybe a half-filled out form or something, where you'd want to keep data the user had already entered. But then you'd have to pull all of your application state from the DOM... maybe I'm thinking too much about the React paradigm?

In any case, I think for now we can just treat it like the initial POP and wait for someone to say "hey, I need to do something special on pageshow!" and we can continue this discussion at that point.

[agundermann]

I guess someone might want to preserve that DOM for some reason. I'm trying to imagine maybe a half-filled out form or something, where you'd want to keep data the user had already entered. But then you'd have to pull all of your application state from the DOM... maybe I'm thinking too much about the React paradigm?

You wouldn't have to do that since javascript state is cached, too. From the app's perspective, the site was never left. That's where pageshow comes in.

In any case, I think for now we can just treat it like the initial POP and wait for someone to say "hey, I need to do something special on pageshow!" and we can continue this discussion at that point.

Sounds good.

When trying to implement this, I stumbled upon another problem. Firefox doesn't seem to update window.history.state to the current entry. Instead, it still has the state of the cached page that the user has navigated away from. So when calling updateLocation(getCurrentLocation()) in response to a persisted pageshow event, location.key will be wrong. Pressing back then forward or F5 will fix it.

I can't think of a way to work around this and fix location.key. Seems like a browser issue to me with window.location and window.history.state being out of sync.

[mjackson]

What would be your recommendation here, @taurose? I'm sorry I haven't had more time to dig into this myself.

[agundermann]

I don't think it's worth pursuing this:

• I still couldn't solve the problem mentioned above, so I don't think we can make it work without losing state for the first page the user sees when coming back to our app
• the cache seems mostly useful for multi page apps/websites to speed up transitions between pages. Transitions from other websites back to one's app via back button aren't as important I'd say
• the cache is disabled in many cases (https, beforeunload, ...)
• no one else ran into this issue or asked for this functionality as far as I can tell; perhaps due to the reasons above

[ZebraFlesh]

I think I have a use case for this feature: our site is currently part single page apps and part legacy server side HTML. We step our users through what's essentially a multipage form (essentially, https://www.example.com/try-us-for-free/{guid1}/step1, /step2, etc). When they complete the process, the temporary {guid1} resource is converted into a permanent resource (something like, https://www.example.com/success/{guid2}). This permanent resource page is server side HTML. If the user presses the back button at this point, they go back into the single page app. The fetch for this page includes a server side redirect (HTTP 302 response) and they're forced back to the permanent resource. (Indeed, any request for a step in the multipage form with the consumed temporary resource results in a server side redirect.)

This all works great until you get to a BFCache supporting browser such as Safari. Safari ignores the no-cache header and returns the user to the single page application (https://www.example.com/try-us-for-free/{guid1}/stepN), loading it from the cache and not performing any server requests.

Right now, the only solution to this problem is to manually hooking into the pageshow event, outside of history. It feels and looks terribly hacky.

[mjackson]

Closing due to inactivity.