Treat camera unavailability in a more friendly way.

Also creates a `components-lib` directory for major webcomponent support code.

src/components-lib/cached-value-controller.ts

@@ -0,0 +1,99 @@
+import { ReactiveController, ReactiveControllerHost } from 'lit';
+import { Timer } from '../utils/timer';
+
+export class CachedValueController<T> implements ReactiveController {
+  protected _value?: T;
+  protected _host: ReactiveControllerHost;
+  protected _timerSeconds: number;
+  protected _callback: () => T;
+  protected _timerStartCallback?: () => void;
+  protected _timerStopCallback?: () => void;
+  protected _timer = new Timer();
+
+  constructor(
+    host: ReactiveControllerHost,
+    timerSeconds: number,
+    callback: () => T,
+    timerStartCallback?: () => void,
+    timerStopCallback?: () => void,
+  ) {
+    this._timerSeconds = timerSeconds;
+    this._callback = callback;
+    this._timerStartCallback = timerStartCallback;
+    this._timerStopCallback = timerStopCallback;
+    (this._host = host).addController(this);
+  }
+
+  /**
+   * Remove the controller for the host.
+   */
+  public removeController(): void {
+    this.stopTimer();
+    this._host.removeController(this);
+  }
+
+  /**
+   * Get the value.
+   */
+  get value(): T | undefined {
+    return this._value;
+  }
+
+  /**
+   * Update the cached value.
+   */
+  public updateValue(): void {
+    this._value = this._callback();
+  }
+
+  /**
+   * Clear the cached value.
+   */
+  public clearValue(): void {
+    this._value = undefined;
+  }
+
+  /**
+   * Disable the timer.
+   */
+  public stopTimer(): void {
+    if (this._timer.isRunning()) {
+      this._timer.stop();
+      this._timerStopCallback?.();
+    }
+  }
+
+  /**
+   * Enable the timer. Repeated calls will have no effect.
+   */
+  public startTimer(): void {
+    this.stopTimer();
+
+    this._timerStartCallback?.();
+    this._timer.startRepeated(this._timerSeconds, () => {
+      this.updateValue();
+      this._host.requestUpdate();
+    });
+  }
+
+  public hasTimer(): boolean {
+    return this._timer.isRunning();
+  }
+
+  /**
+   * Host has connected to the cache.
+   */
+  hostConnected(): void {
+    this.updateValue();
+    this.startTimer();
+    this._host.requestUpdate();
+  }
+
+  /**
+   * Host has disconnected from the cache.
+   */
+  hostDisconnected(): void {
+    this.clearValue();
+    this.stopTimer();
+  }
+}