Turbo Streams: Preserve permanent elements

Closes #623

Refactor the `Snapshot` implementation to make the permanent element
finding code re-usable outside that module.

Then, introduce the `StreamMessageRenderer` class, and re-use that code.

The `StreamMessageRenderer` class also implements `BardoDelegate`, and
relies on `Bardo.preservingPermanentElements` to manage elements across
their `<turbo-stream>` rendering lifespan.

src/core/session.ts

@@ -11,6 +11,7 @@ import { Navigator, NavigatorDelegate } from "./drive/navigator"
 import { PageObserver, PageObserverDelegate } from "../observers/page_observer"
 import { ScrollObserver } from "../observers/scroll_observer"
 import { StreamMessage } from "./streams/stream_message"
+import { StreamMessageRenderer } from "./streams/stream_message_renderer"
 import { StreamObserver } from "../observers/stream_observer"
 import { Action, Position, StreamSource, isAction } from "./types"
 import { clearBusyState, dispatch, markAsBusy } from "../util"
@@ -62,6 +63,7 @@ export class Session
   readonly streamObserver = new StreamObserver(this)
   readonly formLinkClickObserver = new FormLinkClickObserver(this, document.documentElement)
   readonly frameRedirector = new FrameRedirector(this, document.documentElement)
+  readonly streamMessageRenderer = new StreamMessageRenderer()
 
   drive = true
   enabled = true
@@ -129,7 +131,7 @@ export class Session
   }
 
   renderStreamMessage(message: StreamMessage | string) {
-    document.documentElement.appendChild(StreamMessage.wrap(message).fragment)
+    this.streamMessageRenderer.render(StreamMessage.wrap(message))
   }
 
   clearCache() {

src/core/snapshot.ts

@@ -37,11 +37,11 @@ export class Snapshot<E extends Element = Element> {
   }
 
   get permanentElements() {
-    return [...this.element.querySelectorAll("[id][data-turbo-permanent]")]
+    return queryPermanentElementsAll(this.element)
   }
 
   getPermanentElementById(id: string) {
-    return this.element.querySelector(`#${id}[data-turbo-permanent]`)
+    return getPermanentElementById(this.element, id)
   }
 
   getPermanentElementMapForSnapshot(snapshot: Snapshot) {
@@ -59,4 +59,12 @@ export class Snapshot<E extends Element = Element> {
   }
 }
 
+export function getPermanentElementById(node: ParentNode, id: string) {
+  return node.querySelector(`#${id}[data-turbo-permanent]`)
+}
+
+export function queryPermanentElementsAll(node: ParentNode) {
+  return node.querySelectorAll("[id][data-turbo-permanent]")
+}
+
 export type PermanentElementMap = Record<string, [Element, Element]>

src/core/streams/stream_message_renderer.ts

@@ -0,0 +1,36 @@
+import { StreamMessage } from "./stream_message"
+import { StreamElement } from "../../elements/stream_element"
+import { Bardo, BardoDelegate } from "../bardo"
+import { PermanentElementMap, getPermanentElementById, queryPermanentElementsAll } from "../snapshot"
+
+export class StreamMessageRenderer implements BardoDelegate {
+  render({ fragment }: StreamMessage) {
+    Bardo.preservingPermanentElements(this, getPermanentElementMapForFragment(fragment), () =>
+      document.documentElement.appendChild(fragment)
+    )
+  }
+
+  enteringBardo(currentPermanentElement: Element, newPermanentElement: Element) {
+    newPermanentElement.replaceWith(currentPermanentElement.cloneNode(true))
+  }
+
+  leavingBardo() {}
+}
+
+function getPermanentElementMapForFragment(fragment: DocumentFragment): PermanentElementMap {
+  const permanentElementsInDocument = queryPermanentElementsAll(document.documentElement)
+  const permanentElementMap: PermanentElementMap = {}
+  for (const permanentElementInDocument of permanentElementsInDocument) {
+    const { id } = permanentElementInDocument
+
+    for (const streamElement of fragment.querySelectorAll<StreamElement>("turbo-stream")) {
+      const elementInStream = getPermanentElementById(streamElement.templateElement.content, id)
+
+      if (elementInStream) {
+        permanentElementMap[id] = [permanentElementInDocument, elementInStream]
+      }
+    }
+  }
+
+  return permanentElementMap
+}

src/tests/functional/rendering_tests.ts

@@ -395,6 +395,36 @@ test("test preserves permanent element video playback", async ({ page }) => {
   assert.equal(timeAfterRender, timeBeforeRender, "element state is preserved")
 })
 
+test("test preserves permanent element through Turbo Stream update", async ({ page }) => {
+  await page.evaluate(() => {
+    window.Turbo.renderStreamMessage(`
+      <turbo-stream action="update" target="frame">
+        <template>
+          <div id="permanent-in-frame" data-turbo-permanent>Ignored</div>
+        </template>
+      </turbo-stream>
+    `)
+  })
+  await nextBeat()
+
+  assert.equal(await page.textContent("#permanent-in-frame"), "Rendering")
+})
+
+test("test preserves permanent element through Turbo Stream append", async ({ page }) => {
+  await page.evaluate(() => {
+    window.Turbo.renderStreamMessage(`
+      <turbo-stream action="append" target="frame">
+        <template>
+          <div id="permanent-in-frame" data-turbo-permanent>Ignored</div>
+        </template>
+      </turbo-stream>
+    `)
+  })
+  await nextBeat()
+
+  assert.equal(await page.textContent("#permanent-in-frame"), "Rendering")
+})
+
 test("test preserves input values", async ({ page }) => {
   await page.fill("#text-input", "test")
   await page.click("#checkbox-input")