j9tjQcec

1import { Injectable } from "@angular/core";
2import { Subject, Observable } from "rxjs";
3import { of } from "rxjs";
4import { tap } from "rxjs/operators";
5
6@Injectable({
7  providedIn: "root"
8})
9export class CacheService<T> {
10  private cache: Map<string, CacheContent<T>> = new Map<string, CacheContent<T>>();
11  private inFlightObservables: Map<string, Subject<T>> = new Map<string, Subject<T>>();
12  readonly DEFAULT_MAX_AGE: number = 300000;
13
14  constructor() { }
15
16  /**
17   * Gets the value from cache if the key is provided.
18   * If no value exists in cache, then check if the same call exists
19   * in flight, if so return the subject. If not create a new
20   * Subject inFlightObservable and return the source observable.
21   */
22  get(key: string, fallback?: Observable<T>, maxAge?: number): Observable<T> | Subject<T> {
23
24    if (this.hasValidCachedValue(key)) {
25      console.log(`%cGetting from cache ${key}`, "color: green");
26      return of(this.cache.get(key).value);
27    }
28
29    if (!maxAge) {
30      maxAge = this.DEFAULT_MAX_AGE;
31    }
32
33    if (this.inFlightObservables.has(key)) {
34      return this.inFlightObservables.get(key);
35    } else if (fallback && fallback instanceof Observable) {
36      this.inFlightObservables.set(key, new Subject());
37      console.log(`%c Calling api for ${key}`, "color: purple");
38      return fallback.pipe(
39        tap((value) => { this.set(key, value, maxAge); })
40      );
41    }
42  }
43
44  set(key: string, value: T, maxAge: number = this.DEFAULT_MAX_AGE): void {
45    this.cache.set(key, { value: value, expiry: Date.now() + maxAge });
46    this.notifyInFlightObservers(key, value);
47  }
48
49  has(key: string): boolean {
50    return this.cache.has(key);
51  }
52
53  /**
54   * Publishes the value to all observers of the given
55   * in progress observables if observers exist.
56   */
57  private notifyInFlightObservers(key: string, value: T): void {
58    if (this.inFlightObservables.has(key)) {
59      const inFlight = this.inFlightObservables.get(key);
60      const observersCount = inFlight.observers.length;
61      if (observersCount) {
62        console.log(`%cNotifying ${inFlight.observers.length} flight subscribers for ${key}`, "color: blue");
63        inFlight.next(value);
64      }
65      inFlight.complete();
66      this.inFlightObservables.delete(key);
67    }
68  }
69
70  private hasValidCachedValue(key: string): boolean {
71    if (this.cache.has(key)) {
72    // if (this.cache.get(key).expiry < Date.now()) {
73    //     this.cache.delete(key);
74    // }
75      return true;
76    } else {
77      return false;
78    }
79  }
80}
81
82interface CacheContent<T> {
83  expiry: number;
84  value: T;
85}
86