Extract DriveDelegate interface

The `FrameController` depends on the application's singleton `Session`
instance in several ways:

* it drives the `History`
* it determines whether or not an `Element` is "enabled" to navigate a
  page or frame
* it preloads links
* it dispatches `turbo:frame-render` and `turbo:frame-load` events
* it can initiate a `Visit`

This commit codifies those dependent behaviors by creating the
`DriveDelegate` interface and declaring the `Session` class as an
extension of that interface.

With the codification of the `DriveDelegate` responsibilities
established, managing `turbo:frame-render` and `turbo:frame-load` events
feels out of scope. To resolve that, this commit migrates the
`Session.frameRendered` and `Session.frameLoaded` methods to the
`FrameController`, and omits them from the `DriveDelegate` interface.

The `FrameController` still directly imports the singleton instance of
the `Session` from the `src/core/index` module, but assigns it to a
`delegate: DriveDelegate` property for use throughout the class.

The next step would be to find a way to forward the `session` instance
into the [FrameElement.delegateConstructor][] assignment.

[FrameElement.delegateConstructor]: https://github.com/hotwired/turbo/blob/3a18111412cb4190b818b96c9c6aad7264cc4441/src/elements/index.ts#L6

src/core/frames/frame_controller.js

@@ -34,8 +34,9 @@ export class FrameController {
   #ignoredAttributes = new Set()
   action = null
 
-  constructor(element) {
+  constructor(element, delegate = session) {
     this.element = element
+    this.delegate = delegate
     this.view = new FrameView(this, this.element)
     this.appearanceObserver = new AppearanceObserver(this, this.element)
     this.formLinkClickObserver = new FormLinkClickObserver(this, this.element)
@@ -279,7 +280,7 @@ export class FrameController {
   viewRenderedSnapshot(_snapshot, _isPreview) {}
 
   preloadOnLoadLinksForView(element) {
-    session.preloadOnLoadLinksForView(element)
+    this.delegate.preloadOnLoadLinksForView(element)
   }
 
   viewInvalidated() {}
@@ -313,14 +314,27 @@ export class FrameController {
 
       await this.view.render(renderer)
       this.complete = true
-      session.frameRendered(fetchResponse, this.element)
-      session.frameLoaded(this.element)
+      this.frameRendered(this.element, fetchResponse)
+      this.frameLoaded(this.element)
+
       await this.fetchResponseLoaded(fetchResponse)
     } else if (this.#willHandleFrameMissingFromResponse(fetchResponse)) {
       this.#handleFrameMissingFromResponse(fetchResponse)
     }
   }
 
+  frameLoaded(frame) {
+    return dispatch("turbo:frame-load", { target: frame })
+  }
+
+  frameRendered(frame, fetchResponse) {
+    return dispatch("turbo:frame-render", {
+      detail: { fetchResponse },
+      target: frame,
+      cancelable: true
+    })
+  }
+
   async #visit(url) {
     const request = new FetchRequest(this, FetchMethod.get, url, new URLSearchParams(), this.element)
 
@@ -370,7 +384,7 @@ export class FrameController {
 
           if (this.action) options.action = this.action
 
-          session.visit(frame.src, options)
+          this.delegate.visit(frame.src, options)
         }
       }
     }
@@ -379,7 +393,7 @@ export class FrameController {
   changeHistory() {
     if (this.action) {
       const method = getHistoryMethodForAction(this.action)
-      session.history.update(method, expandURL(this.element.src || ""), this.restorationIdentifier)
+      this.delegate.history.update(method, expandURL(this.element.src || ""), this.restorationIdentifier)
     }
   }
 
@@ -427,7 +441,7 @@ export class FrameController {
     const responseHTML = await wrapped.responseHTML
     const { location, redirected, statusCode } = wrapped
 
-    return session.visit(location, { response: { redirected, statusCode, responseHTML } })
+    return this.delegate.visit(location, { response: { redirected, statusCode, responseHTML } })
   }
 
   #findFrameElement(element, submitter) {
@@ -482,11 +496,11 @@ export class FrameController {
       }
     }
 
-    if (!session.elementIsNavigatable(element)) {
+    if (!this.delegate.elementIsNavigatable(element)) {
       return false
     }
 
-    if (submitter && !session.elementIsNavigatable(submitter)) {
+    if (submitter && !this.delegate.elementIsNavigatable(submitter)) {
       return false
     }
 

src/core/frames/frame_redirector.js

@@ -4,8 +4,8 @@ import { LinkInterceptor } from "./link_interceptor"
 import { expandURL, getAction, locationIsVisitable } from "../url"
 
 export class FrameRedirector {
-  constructor(session, element) {
-    this.session = session
+  constructor(delegate, element) {
+    this.delegate = delegate
     this.element = element
     this.linkInterceptor = new LinkInterceptor(this, element)
     this.formSubmitObserver = new FormSubmitObserver(this, element)
@@ -62,8 +62,8 @@ export class FrameRedirector {
   #shouldRedirect(element, submitter) {
     const isNavigatable =
       element instanceof HTMLFormElement
-        ? this.session.submissionIsNavigatable(element, submitter)
-        : this.session.elementIsNavigatable(element)
+        ? this.delegate.submissionIsNavigatable(element, submitter)
+        : this.delegate.elementIsNavigatable(element)
 
     if (isNavigatable) {
       const frame = this.#findFrameElement(element, submitter)

src/core/session.js

@@ -276,16 +276,6 @@ export class Session {
     this.adapter.pageInvalidated(reason)
   }
 
-  // Frame element
-
-  frameLoaded(frame) {
-    this.notifyApplicationAfterFrameLoad(frame)
-  }
-
-  frameRendered(fetchResponse, frame) {
-    this.notifyApplicationAfterFrameRender(fetchResponse, frame)
-  }
-
   // Application events
 
   applicationAllowsFollowingLinkToLocation(link, location, ev) {
@@ -347,19 +337,7 @@ export class Session {
     )
   }
 
-  notifyApplicationAfterFrameLoad(frame) {
-    return dispatch("turbo:frame-load", { target: frame })
-  }
-
-  notifyApplicationAfterFrameRender(fetchResponse, frame) {
-    return dispatch("turbo:frame-render", {
-      detail: { fetchResponse },
-      target: frame,
-      cancelable: true
-    })
-  }
-
-  // Helpers
+  // Drive delegate
 
   submissionIsNavigatable(form, submitter) {
     if (this.formMode == "off") {