Optimize live queries for useLiveQuery with dexie 3 (#1728)

src/classes/observable/observable.ts

@@ -11,6 +11,9 @@ const symbolObservable: typeof Symbol.observable =
 
 export class Observable<T> implements IObservable<T> {
   private _subscribe: (observer: Observer<T>) => Subscription;
+  hasValue?: ()=>boolean;
+  getValue?: ()=>T;
+
   constructor(subscribe: (observer: Observer<T>) => Subscription) {
     this._subscribe = subscribe;
   }

src/live-query/live-query.ts

@@ -17,7 +17,9 @@ import { extendObservabilitySet } from "./extend-observability-set";
 import { rangesOverlap } from "../helpers/rangeset";
 
 export function liveQuery<T>(querier: () => T | Promise<T>): IObservable<T> {
-  return new Observable<T>((observer) => {
+  let hasValue = false;
+  let currentValue: T = undefined as any;
+  const observable = new Observable<T>((observer) => {
     const scopeFuncIsAsync = isAsyncFunction(querier);
     function execute(subscr: ObservabilitySet) {
       if (scopeFuncIsAsync) {
@@ -83,6 +85,8 @@ export function liveQuery<T>(querier: () => T | Promise<T>): IObservable<T> {
       querying = true;
       Promise.resolve(ret).then(
         (result) => {
+          hasValue = true;
+          currentValue = result;
           querying = false;
           if (closed) return;
           if (shouldNotify()) {
@@ -97,6 +101,7 @@ export function liveQuery<T>(querier: () => T | Promise<T>): IObservable<T> {
         },
         (err) => {
           querying = false;
+          hasValue = false;
           observer.error && observer.error(err);
           subscription.unsubscribe();
         }
@@ -106,4 +111,7 @@ export function liveQuery<T>(querier: () => T | Promise<T>): IObservable<T> {
     doQuery();
     return subscription;
   });
+  observable.hasValue = () => hasValue;
+  observable.getValue = () => currentValue;
+  return observable;
 }

src/public/types/observable.d.ts

@@ -5,14 +5,22 @@ declare global {
     readonly observable: symbol;
   }
 }
+
+interface Subscribable<T> {
+  subscribe(observer: Partial<Observer<T>>): Unsubscribable;
+}
+
+interface Unsubscribable {
+  unsubscribe(): void;
+}
+
 export interface Observable<T = any> {
-  subscribe(
-    onNext?: ((value: T) => void) | null,
-    onError?: ((error: any) => void) | null,
-    onComplete?: (() => void) | null
-  ): Subscription;
-  subscribe(observer?: Observer<T> | null): Subscription;
-  [Symbol.observable]: () => Observable<T>;
+  subscribe(observerOrNext?: Observer<T> | ((value: T) => void)): Subscription;
+  subscribe(next?: ((value: T) => void) | null, error?: ((error: any) => void) | null, complete?: (() => void) | null): Subscription;
+  getValue?(): T;
+  hasValue?(): boolean;
+
+	[Symbol.observable]: () => Subscribable<T>;
 }
 
 export interface Subscription {