Proposal: pass isPreview value into the event detail in render events

[Matt-Yorkley]

Passes the renderer's isPreview attribute into the event detail on 'turbo:before-render' and 'turbo:render' events.

isPreview signifies whether the content currently being rendered is from the turbo cache or not, and this bit of information is very useful for any listeners that might want to act on those two events differently for cached or non-cached content.

Eg, the following listeners:

document.addEventListener("turbo:before-render", (event) => {
  console.log("turbo:before-render")
  console.log(event.detail)
})

document.addEventListener("turbo:render", (event) => {
  console.log("turbo:render")
  console.log(event.detail)
})

now produce these event details (when navigating to a page that has a cache entry, both events fire twice):

So it's now possible to do things like...

document.addEventListener("turbo:before-render", (event) => {
  if (event.detail.isPreview) {
    resetTheThings()
    modifyTurboCachedContent(event.detail.newBody)
  } else {
    modifyFinalContent(event.detail.body)
  }
})

[afcapel]

Looks good, thanks @Matt-Yorkley 👍

[domchristie]

Could this be inferred from html[data-turbo-preview] attribute?:

document.addEventListener("turbo:before-render", () => {
  console.log("turbo:before-render")
  console.log(document.documentElement.hasAttribute('data-turbo-preview'))
})

document.addEventListener("turbo:render", () => {
  console.log("turbo:render")
  console.log(document.documentElement.hasAttribute('data-turbo-preview'))
})

[afcapel]

@Matt-Yorkley @domchristie is making a good point. Can you use data-turbo-preview instead?

[Matt-Yorkley]

@Matt-Yorkley @domchristie is making a good point. Can you use data-turbo-preview instead?

Ah, I must have missed data-turbo-preview in the documentation. Yeah, @domchristie's comment about preferring to store state in the DOM sounds good #1007 (comment)