perf(mobile): warm WebKit's processes on iOS before the first page opens (#39851)

* perf(mobile): warm WebKit's processes on iOS before the first page opens

Builds a blank, non-persistent WKWebView a second after Home has a paired
host, so the first page open starts against WebContent, GPU and Network
processes already running. A release before the delayed start now makes
the start a no-op.

Based on #32163 by Jinwoo-H.

Co-authored-by: Jinwoo-H <73622457+Jinwoo-H@users.noreply.github.com>

* fix(mobile): guard warmUp on older binaries and release the warm view on page failure

Co-authored-by: Jinwoo-H <73622457+Jinwoo-H@users.noreply.github.com>

---------

Co-authored-by: m4air <m4air@m4airs-MacBook-Air.local>
Co-authored-by: Jinwoo-H <73622457+Jinwoo-H@users.noreply.github.com>

mobile/modules/orca-mobile-web-shell/ios/MobileWebShellView.swift

@@ -603,6 +603,7 @@ final class OrcaMobileWebShellView: ExpoView, WKNavigationDelegate, WKUIDelegate
 
   /// history.replaceState changes the path before finish; finished() uses commit state instead of URL equality.
   func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
+    MobileWebShellWarmUp.release()
     emit(loadState.finished())
   }
 
@@ -611,15 +612,18 @@ final class OrcaMobileWebShellView: ExpoView, WKNavigationDelegate, WKUIDelegate
     didFailProvisionalNavigation navigation: WKNavigation!,
     withError error: Error
   ) {
+    MobileWebShellWarmUp.release()
     reportNavigationFailure(error)
   }
 
   func webView(_ webView: WKWebView, didFail navigation: WKNavigation!, withError error: Error) {
+    MobileWebShellWarmUp.release()
     reportNavigationFailure(error)
   }
 
   /// Report renderer death; memory pressure/provider updates look alike, so the caller owns retry policy.
   func webViewWebContentProcessDidTerminate(_ webView: WKWebView) {
+    MobileWebShellWarmUp.release()
     clearBridgeTarget()
     // Jetsam takes WebContent while Orca is backgrounded; the report can arrive as the app resumes, before it is active.
     let background = window == nil || UIApplication.shared.applicationState != .active

mobile/modules/orca-mobile-web-shell/ios/MobileWebShellWarmUp.swift

@@ -0,0 +1,41 @@
+import UIKit
+import WebKit
+
+/// The first page view in a process waits for WebKit to launch its helper processes. A blank view
+/// loaded while Home is idle pays that instead. It is held until the first page has loaded, so that
+/// page starts against processes already up, or until the app backgrounds, memory runs low or a
+/// minute passes. It has its own non-persistent store and no bridge or scripts, so it shares no
+/// state with a page.
+enum MobileWebShellWarmUp {
+  private static var view: WKWebView?
+  private static var started = false
+  private static var observers: [NSObjectProtocol] = []
+
+  static func start() {
+    dispatchPrecondition(condition: .onQueue(.main))
+    // A launch into the background may never show a page, so it gets no web process.
+    guard !started, UIApplication.shared.applicationState == .active else { return }
+    started = true
+    let configuration = WKWebViewConfiguration()
+    configuration.websiteDataStore = .nonPersistent()
+    let view = WKWebView(frame: .zero, configuration: configuration)
+    view.loadHTMLString("", baseURL: nil)
+    self.view = view
+    DispatchQueue.main.asyncAfter(deadline: .now() + 60) { release() }
+    observers = [
+      UIApplication.didReceiveMemoryWarningNotification,
+      UIApplication.didEnterBackgroundNotification
+    ].map { name in
+      NotificationCenter.default.addObserver(forName: name, object: nil, queue: .main) { _ in release() }
+    }
+  }
+
+  static func release() {
+    dispatchPrecondition(condition: .onQueue(.main))
+    // A page that finished before the delayed start already has its processes, so start must no-op.
+    started = true
+    view = nil
+    observers.forEach { NotificationCenter.default.removeObserver($0) }
+    observers = []
+  }
+}

mobile/modules/orca-mobile-web-shell/ios/OrcaMobileWebShellModule.swift

@@ -7,6 +7,14 @@ public class OrcaMobileWebShellModule: Module {
   public func definition() -> ModuleDefinition {
     Name("OrcaMobileWebShell")
 
+    // Home calls this once it has a paired host, so a user with nothing to open pays nothing.
+    Function("warmUp") {
+      // A second's delay keeps the warm-up off the launch's first frames.
+      DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
+        MobileWebShellWarmUp.start()
+      }
+    }
+
     AsyncFunction("presentPdf") { (url: URL) in
       guard let presenter = self.appContext?.utilities?.currentViewController() else {
         throw PdfPresenterUnavailableException()

mobile/modules/orca-mobile-web-shell/src/index.ts

@@ -1,6 +1,10 @@
-import { requireNativeModule, requireNativeViewManager } from 'expo-modules-core'
+import {
+  requireNativeModule,
+  requireOptionalNativeModule,
+  requireNativeViewManager
+} from 'expo-modules-core'
 import type { ComponentType, RefAttributes } from 'react'
-import type { NativeSyntheticEvent, ViewProps } from 'react-native'
+import { Platform, type NativeSyntheticEvent, type ViewProps } from 'react-native'
 import type { MobileWebShellLoadStatePayload } from './load-state'
 
 /** One raw JSON envelope, exactly as the page posted it. Parsing is the caller's. */
@@ -66,6 +70,18 @@ export function saveImageToPhotos(uri: string, mime: string): Promise<void> {
   return requireNativeModule<ImageModule>('OrcaMobileWebShell').saveImageToPhotos(uri, mime)
 }
 
+// iOS only: starts WebKit's helper processes ahead of the first page. Later calls are no-ops.
+export function warmUpWebShell(): void {
+  if (Platform.OS !== 'ios') {
+    return
+  }
+  // A binary built before warmUp existed has the module without the function.
+  const shell = requireOptionalNativeModule<{ warmUp?: () => void }>('OrcaMobileWebShell')
+  if (typeof shell?.warmUp === 'function') {
+    shell.warmUp()
+  }
+}
+
 export {
   MOBILE_WEB_SHELL_FAILURE_REASONS,
   parseMobileWebShellLoadState,

mobile/src/home/use-mobile-home-data.ts

@@ -1,6 +1,7 @@
 import AsyncStorage from '@react-native-async-storage/async-storage'
 import { useRouter, useFocusEffect } from 'expo-router'
 import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
+import { warmUpWebShell } from '../../modules/orca-mobile-web-shell/src'
 import { loadHomeSnapshot, saveHomeSnapshot } from '../cache/home-snapshot-cache'
 import type { AccountsSnapshot } from '../components/AccountUsage'
 import { getCachedWorktrees, setCachedWorktrees } from '../cache/worktree-cache'
@@ -93,6 +94,13 @@ export function useMobileHomeData() {
     }
   }, [worktreeInfo])
 
+  const hasPairedHost = hosts.length > 0
+  useEffect(() => {
+    if (hasPairedHost) {
+      warmUpWebShell()
+    }
+  }, [hasPairedHost])
+
   useEffect(() => {
     allClientsRef.current = connections.allClients
   }, [connections.allClients])