feat: mobile web bridge (#1597)

packages/mobile/src/AppStateObserverService.ts

@@ -0,0 +1,62 @@
+import { AbstractService, InternalEventBus, ReactNativeToWebEvent } from '@standardnotes/snjs'
+import { AppState, AppStateStatus, NativeEventSubscription } from 'react-native'
+
+export class AppStateObserverService extends AbstractService<ReactNativeToWebEvent> {
+  private mostRecentState?: ReactNativeToWebEvent
+  private removeListener: NativeEventSubscription
+  private ignoringStateChanges = false
+
+  constructor() {
+    const bus = new InternalEventBus()
+    super(bus)
+
+    this.removeListener = AppState.addEventListener('change', async (nextAppState: AppStateStatus) => {
+      if (this.ignoringStateChanges) {
+        return
+      }
+
+      // if the most recent state is not 'background' ('inactive'), then we're going
+      // from inactive to active, which doesn't really happen unless you, say, swipe
+      // notification center in iOS down then back up. We don't want to lock on this state change.
+      const isResuming = nextAppState === 'active'
+      const isResumingFromBackground = isResuming && this.mostRecentState === ReactNativeToWebEvent.EnteringBackground
+      const isEnteringBackground = nextAppState === 'background'
+      const isLosingFocus = nextAppState === 'inactive'
+
+      if (isEnteringBackground) {
+        this.notifyStateChange(ReactNativeToWebEvent.EnteringBackground)
+      }
+
+      if (isResumingFromBackground || isResuming) {
+        if (isResumingFromBackground) {
+          this.notifyStateChange(ReactNativeToWebEvent.ResumingFromBackground)
+        }
+
+        // Notify of GainingFocus even if resuming from background
+        this.notifyStateChange(ReactNativeToWebEvent.GainingFocus)
+        return
+      }
+
+      if (isLosingFocus) {
+        this.notifyStateChange(ReactNativeToWebEvent.LosingFocus)
+      }
+    })
+  }
+
+  beginIgnoringStateChanges() {
+    this.ignoringStateChanges = true
+  }
+
+  stopIgnoringStateChanges() {
+    this.ignoringStateChanges = false
+  }
+
+  deinit() {
+    this.removeListener.remove()
+  }
+
+  private notifyStateChange(state: ReactNativeToWebEvent): void {
+    this.mostRecentState = state
+    void this.notifyEvent(state)
+  }
+}