hotwired · dhh · Jun 18, 2022 · Mar 17, 2022 · Mar 17, 2022 · Mar 17, 2022
diff --git a/src/core/drive/preloader.ts b/src/core/drive/preloader.ts
@@ -0,0 +1,54 @@
+import { Navigator } from "./navigator"
+import { PageSnapshot } from "./page_snapshot"
+import { SnapshotCache } from "./snapshot_cache"
+
+export interface PreloaderDelegate {
+  readonly navigator: Navigator
+}
+
+export class Preloader {
+  readonly delegate: PreloaderDelegate
+  readonly selector: string = 'a[data-turbo-preload]'
+
+  constructor(delegate: PreloaderDelegate) {
+    this.delegate = delegate
+  }
+
+  get snapshotCache(): SnapshotCache {
+    return this.delegate.navigator.view.snapshotCache
+  }
+
+  start() {
+    if (document.readyState === 'loading') {
+      return document.addEventListener('DOMContentLoaded', () => {
+        this.preloadOnLoadLinksForView(document.body)
+      });
+    } else {
+      this.preloadOnLoadLinksForView(document.body)
+    }
+  }
+
+  preloadOnLoadLinksForView(element: Element) {
+    for (const link of element.querySelectorAll<HTMLAnchorElement>(this.selector)) {
+      this.preloadURL(link)
+    }
+  }
+
+  async preloadURL(link: HTMLAnchorElement) {
+    const location = new URL(link.href)
+
+    if (this.snapshotCache.has(location)) {
+      return
+    }
+
+    try {
+      const response = await fetch(location.toString(), { headers: { 'VND.PREFETCH': 'true', 'Accept': 'text/html' } })
+      const responseText = await response.text()
+      const snapshot = PageSnapshot.fromHTMLString(responseText)
+
+      this.snapshotCache.put(location, snapshot)
+    } catch(_) {
+      // If we cannot preload that is ok!
+    }
+  }
+}
diff --git a/src/core/frames/frame_controller.ts b/src/core/frames/frame_controller.ts
@@ -243,6 +243,10 @@ export class FrameController
 
   viewRenderedSnapshot(_snapshot: Snapshot, _isPreview: boolean) {}
 
+  preloadOnLoadLinksForView(element: Element) {
+    session.preloadOnLoadLinksForView(element)
+  }
+
   viewInvalidated() {}
 
   // Private

diff --git a/src/core/session.ts b/src/core/session.ts
@@ -18,6 +18,7 @@ import { Visit, VisitOptions } from "./drive/visit"
 import { PageSnapshot } from "./drive/page_snapshot"
 import { FrameElement } from "../elements/frame_element"
 import { FetchResponse } from "../http/fetch_response"
+import { Preloader, PreloaderDelegate } from "./drive/preloader"
 
 export type TimingData = unknown
 
@@ -28,10 +29,12 @@ export class Session
     LinkClickObserverDelegate,
     NavigatorDelegate,
     PageObserverDelegate,
-    PageViewDelegate
+    PageViewDelegate,
+    PreloaderDelegate
 {
   readonly navigator = new Navigator(this)
   readonly history = new History(this)
+  readonly preloader = new Preloader(this)
   readonly view = new PageView(this, document.documentElement)
   adapter: Adapter = new BrowserAdapter(this)
 
@@ -59,6 +62,7 @@ export class Session
       this.streamObserver.start()
       this.frameRedirector.start()
       this.history.start()
+      this.preloader.start()
       this.started = true
       this.enabled = true
     }
@@ -267,6 +271,10 @@ export class Session
     this.notifyApplicationAfterRender()
   }
 
+  preloadOnLoadLinksForView(element: Element) {
+    this.preloader.preloadOnLoadLinksForView(element)
+  }
+
   viewInvalidated(reason: ReloadReason) {
     this.adapter.pageInvalidated(reason)
   }

diff --git a/src/core/view.ts b/src/core/view.ts
@@ -6,6 +6,7 @@ import { getAnchor } from "./url"
 
 export interface ViewDelegate<S extends Snapshot> {
   allowsImmediateRender(snapshot: S, resume: (value: any) => void): boolean
+  preloadOnLoadLinksForView(element: Element): void
   viewRenderedSnapshot(snapshot: S, isPreview: boolean): void
   viewInvalidated(reason: ReloadReason): void
 }
@@ -89,6 +90,7 @@ export abstract class View<
 
         await this.renderSnapshot(renderer)
         this.delegate.viewRenderedSnapshot(snapshot, isPreview)
+        this.delegate.preloadOnLoadLinksForView(this.element)
         this.finishRenderingSnapshot(renderer)
       } finally {
         delete this.renderer

diff --git a/src/tests/fixtures/frame_preloading.html b/src/tests/fixtures/frame_preloading.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Page With Preloading Frame</title>
+  <script src="/dist/turbo.es2017-umd.js" data-turbo-track="reload"></script>
+</head>
+
+<body>
+  <turbo-frame id="menu" src="/src/tests/fixtures/frames/preloading.html"></turbo-frame>
+</body>
+
+</html>
diff --git a/src/tests/fixtures/frames/preloading.html b/src/tests/fixtures/frames/preloading.html
@@ -0,0 +1,4 @@
+<turbo-frame id="menu">
+  <a href="/src/tests/fixtures/preloaded.html" id="frame_preload_anchor" data-turbo-preload="true">Visit preloaded
+    page</a>
+</turbo-frame>
diff --git a/src/tests/fixtures/hot_preloading.html b/src/tests/fixtures/hot_preloading.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Page That Links to Preloading Page</title>
+  <script src="/dist/turbo.es2017-umd.js" data-turbo-track="reload"></script>
+</head>
+
+<body>
+  <a href="/src/tests/fixtures/preloading.html" id="hot_preload_anchor">Next page has preloading</a>
+</body>
+
+</html>
diff --git a/src/tests/fixtures/preloaded.html b/src/tests/fixtures/preloaded.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Preloaded Page</title>
+  <script src="/dist/turbo.es2017-umd.js" data-turbo-track="reload"></script>
+</head>
+
+<body>
+  <div>
+    This page was hopefully preloaded
+  </div>
+</body>
+
+</html>
diff --git a/src/tests/fixtures/preloading.html b/src/tests/fixtures/preloading.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<html>
+
+<head>
+  <meta charset="utf-8">
+  <title>Preloading Page</title>
+  <script src="/dist/turbo.es2017-umd.js" data-turbo-track="reload"></script>
+</head>
+
+<body>
+  <a href="/src/tests/fixtures/preloaded.html" id="preload_anchor" data-turbo-preload="true">
+    Visit preloaded page
+  </a>
+</body>
+
+</html>
diff --git a/src/tests/functional/index.ts b/src/tests/functional/index.ts
@@ -11,6 +11,7 @@ export * from "./loading_tests"
 export * from "./navigation_tests"
 export * from "./pausable_rendering_tests"
 export * from "./pausable_requests_tests"
+export * from "./preloader_tests"
 export * from "./rendering_tests"
 export * from "./scroll_restoration_tests"
 export * from "./stream_tests"

diff --git a/src/tests/functional/preloader_tests.ts b/src/tests/functional/preloader_tests.ts
@@ -0,0 +1,49 @@
+import { TurboDriveTestCase } from "../helpers/turbo_drive_test_case"
+
+export class PreloaderTests extends TurboDriveTestCase {
+  async "test preloads snapshot on initial load"() {
+    // contains `a[rel="preload"][href="http://localhost:9000/src/tests/fixtures/preloaded.html"]`
+    await this.goToLocation("/src/tests/fixtures/preloading.html")
+    await this.nextBeat
+
+    this.assert.ok(await this.remote.execute(() => {
+      const preloadedUrl = "http://localhost:9000/src/tests/fixtures/preloaded.html"
+      const cache = window.Turbo.session.preloader.snapshotCache.snapshots
+
+      return preloadedUrl in cache
+    }))
+  }
+
+  async "test preloads snapshot on page visit"() {
+    // contains `a[rel="preload"][href="http://localhost:9000/src/tests/fixtures/preloading.html"]`
+    await this.goToLocation("/src/tests/fixtures/hot_preloading.html")
+
+    // contains `a[rel="preload"][href="http://localhost:9000/src/tests/fixtures/preloaded.html"]`
+    await this.clickSelector("#hot_preload_anchor")
+    await this.waitUntilSelector("#preload_anchor")
+    await this.nextBeat
+
+    this.assert.ok(await this.remote.execute(() => {
+      const preloadedUrl = "http://localhost:9000/src/tests/fixtures/preloaded.html"
+      const cache = window.Turbo.session.preloader.snapshotCache.snapshots
+
+      return preloadedUrl in cache
+    }))
+  }
+
+  async "test navigates to preloaded snapshot from frame"() {
+    // contains `a[rel="preload"][href="http://localhost:9000/src/tests/fixtures/preloaded.html"]`
+    await this.goToLocation("/src/tests/fixtures/frame_preloading.html")
+    await this.waitUntilSelector("#frame_preload_anchor")
+    await this.nextBeat
+
+    this.assert.ok(await this.remote.execute(() => {
+      const preloadedUrl = "http://localhost:9000/src/tests/fixtures/preloaded.html"
+      const cache = window.Turbo.session.preloader.snapshotCache.snapshots
+
+      return preloadedUrl in cache
+    }))
+  }
+}
+
+PreloaderTests.registerSuite()