Singleton class not working with component navigation

Question

I'm trying to create a singleton class so I could avoid opening the same database again.

I've read about creating a class provider with a static variable so it could stay the same, but I've seen that each time I navigate to another component, the static variable content is lost.

Here's my code so far couchbase.service.ts

import { Injectable } from "@angular/core";
import { Couchbase } from "nativescript-couchbase";
import * as connection from "tns-core-modules/connectivity";
import { isAndroid } from "platform";

@Injectable()
export class CouchbaseService {
    private static database: any;

    constructor() {
        console.log("enter constructor")
    }

    public static init(){
        if(!CouchbaseService.database) {
            console.log("enter init")
            CouchbaseService.database = new Couchbase("data");
        }
        return CouchbaseService.database;
    }

    public getDatabase() {
        return CouchbaseService.database;
    }
    ...

}

app.component.ts: I've read that calling init from this parent class would make the app keep it for the children (note that I also tried passing the CouchbaseService as a constructor parameter, changing the method init to non-static)

import { Component } from "@angular/core";
import * as Platform from "platform";
import { CouchbaseService } from './services/couchbase.service';

@Component({
    selector: "ns-app",
    templateUrl: "app.component.html",
})

export class AppComponent { 
    constructor() {
        CouchbaseService.init();
    }
}

In my app.module.ts file, I added CouchbaseService to the providers list.

import { NgModule, ErrorHandler, NO_ERRORS_SCHEMA } from "@angular/core";
import { NativeScriptModule } from "nativescript-angular/nativescript.module";
import { NativeScriptUIListViewModule } from "nativescript-ui-listview/angular";
import { NativeScriptUISideDrawerModule } from "nativescript-ui-sidedrawer/angular";
import { NativeScriptHttpModule } from "nativescript-angular/http";
import { Http } from "@angular/http";
import { NativeScriptUIDataFormModule } from "nativescript-ui-dataform/angular";
import { NativeScriptFormsModule } from "nativescript-angular/forms";
import { registerElement } from 'nativescript-angular/element-registry';
import { CardView } from 'nativescript-cardview';
import { AppRoutingModule } from "./app.routing";
import { AppComponent } from "./app.component";

import { CouchbaseService } from "./services/couchbase.service";


import { HomeComponent } from "./components/home/home.component";
registerElement('CardView', () => CardView);
registerElement("MapboxView", () => require("nativescript-mapbox").MapboxView);
registerElement("Fab", () => require("nativescript-floatingactionbutton").Fab);

@NgModule({
    bootstrap: [
        AppComponent
    ],
    imports: [
        NativeScriptModule,
        NativeScriptUIListViewModule,
        NativeScriptUISideDrawerModule,
        AppRoutingModule,
        NativeScriptUIDataFormModule,
        NativeScriptFormsModule,
        NativeScriptHttpModule
    ],
    declarations: [
        AppComponent,
        HomeComponent
    ],
    providers: [
        CouchbaseService,
    ],
    schemas: [
        NO_ERRORS_SCHEMA
    ],
    entryComponents: [
    ],
})
/*
Pass your application module to the bootstrapModule function located in main.ts to start your app
*/
export class AppModule { }

When I look at the logs in the terminal, seems like each time I navigate to another component the service is restarted, so it looses the value of database.

I'm using NativeScript 4 with Angular 5 and TypeScript 2.7.2.

Answer 1

This is not how should make use of a singleton service. Let's do this way:

Change your service like this:

import { Injectable } from "@angular/core";
import { Couchbase } from "nativescript-couchbase";
import * as connection from "tns-core-modules/connectivity";
import { isAndroid } from "platform";

@Injectable()
export class CouchbaseService {

    private database: any;

    constructor() {
        console.log("enter constructor")
        this.init();
    }

    private init(){
        if(!this.database) {
            console.log("enter init")
            this.database = new Couchbase("data");
        }
    }

    public getDatabase() {
        return this.database;
    }
    ...

}

In your AppModule [or module where you have your service] specify the service in @NgModule's provider array like this:

@NgModule({
  declarations: [
    AppComponent,
    YourComponent
  ],
  imports: [
    BrowserModule
  ],
  providers: [CouchbaseService],
  bootstrap: [AppComponent]
})
export class AppModule { }

Now in your component use constructor dependency injection like this:

import { Component } from "@angular/core";
import * as Platform from "platform";
import { CouchbaseService } from './services/couchbase.service';

@Component({
    selector: "ns-app",
    templateUrl: "app.component.html",
})

export class AppComponent { 
    constructor(private service: CouchbaseService) {
        console.log(this.service.getDatabase());
    }
}