Extract SnapshotSubstitution from FrameController

To refactor the `proposeVisitIfNavigatedWithAction()` implementation,
this commit extracts the `SnapshotSubstitution` to manage replacing
`Snapshot` references to `<turbo-frame>` elements whose visits are
promoted to page-wide state. The class partially implement
the `VisitDelegate` interface so that it can be passed directly as a
`delegate:` option to the `session.visit()` call.

src/core/frames/frame_controller.ts

@@ -4,7 +4,7 @@ import { FetchResponse } from "../../http/fetch_response"
 import { AppearanceObserver, AppearanceObserverDelegate } from "../../observers/appearance_observer"
 import { clearBusyState, getAttribute, parseHTMLDocument, markAsBusy } from "../../util"
 import { FormSubmission, FormSubmissionDelegate } from "../drive/form_submission"
-import { Visit } from "../drive/visit"
+import { Visit, VisitDelegate } from "../drive/visit"
 import { Snapshot } from "../snapshot"
 import { ViewDelegate } from "../view"
 import { getAction, expandURL, urlsAreEqual, locationIsVisitable, Locatable } from "../url"
@@ -261,23 +261,12 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
     const action = getAttribute("data-turbo-action", submitter, element, frame)
 
     if (isAction(action)) {
-      const clone = frame.cloneNode(true)
+      const delegate = new SnapshotSubstitution(frame)
       const proposeVisit = () => {
-        const { ownerDocument, id, src } = frame
-        if (src) {
-          const snapshotHTML = ownerDocument.documentElement.outerHTML
-          let snapshot: Snapshot
-
-          const delegate = {
-            visitStarted(visit: Visit) {
-              snapshot = visit.view.snapshot
-            },
-            visitCachedSnapshot() {
-              snapshot.element.querySelector("#" + id)?.replaceWith(clone)
-            }
-          }
-
-          session.visit(src, { willRender: false, action, snapshotHTML, delegate })
+        if (frame.src) {
+          const snapshotHTML = frame.ownerDocument.documentElement.outerHTML
+
+          session.visit(frame.src, { willRender: false, action, snapshotHTML, delegate })
         }
       }
 
@@ -403,6 +392,29 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
   }
 }
 
+class SnapshotSubstitution implements Partial<VisitDelegate> {
+  private readonly clone: Node
+  private readonly id: string
+  private snapshot?: Snapshot
+
+  constructor(element: FrameElement) {
+    this.clone = element.cloneNode(true)
+    this.id = element.id
+  }
+
+  visitStarted(visit: Visit) {
+    this.snapshot = visit.view.snapshot
+  }
+
+  visitCachedSnapshot() {
+    const { snapshot, id, clone } = this
+
+    if (snapshot) {
+      snapshot.element.querySelector("#" + id)?.replaceWith(clone)
+    }
+  }
+}
+
 function getFrameElementById(id: string | null) {
   if (id != null) {
     const element = document.getElementById(id)