52YSHHYz

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