Observable Merge Scan (Reduce)

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### Angular Starter Plunker - Typescript

System.config({
  //use typescript for compilation
  transpiler: 'typescript',
  //typescript compiler options
  typescriptOptions: {
    emitDecoratorMetadata: true
  },
  paths: {
    'npm:': 'https://unpkg.com/'
  },
  //map tells the System loader where to look for things
  map: {
    
    'app': './src',
    
    '@angular/core': 'npm:@angular/core/bundles/core.umd.js',
    '@angular/common': 'npm:@angular/common/bundles/common.umd.js',
    '@angular/compiler': 'npm:@angular/compiler/bundles/compiler.umd.js',
    '@angular/platform-browser': 'npm:@angular/platform-browser/bundles/platform-browser.umd.js',
    '@angular/platform-browser-dynamic': 'npm:@angular/platform-browser-dynamic/bundles/platform-browser-dynamic.umd.js',
    '@angular/http': 'npm:@angular/http/bundles/http.umd.js',
    '@angular/router': 'npm:@angular/router/bundles/router.umd.js',
    '@angular/forms': 'npm:@angular/forms/bundles/forms.umd.js',
    
    '@angular/core/testing': 'npm:@angular/core/bundles/core-testing.umd.js',
    '@angular/common/testing': 'npm:@angular/common/bundles/common-testing.umd.js',
    '@angular/compiler/testing': 'npm:@angular/compiler/bundles/compiler-testing.umd.js',
    '@angular/platform-browser/testing': 'npm:@angular/platform-browser/bundles/platform-browser-testing.umd.js',
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    '@angular/http/testing': 'npm:@angular/http/bundles/http-testing.umd.js',
    '@angular/router/testing': 'npm:@angular/router/bundles/router-testing.umd.js',
    
    'rxjs': 'npm:rxjs',
    'typescript': 'npm:typescript@2.0.2/lib/typescript.js'
  },
  //packages defines our app package
  packages: {
    app: {
      main: './main.ts',
      defaultExtension: 'ts'
    },
    rxjs: {
      defaultExtension: 'js'
    }
  }
});

//main entry point
import {AppModule} from './app';
import {platformBrowserDynamic} from '@angular/platform-browser-dynamic';

platformBrowserDynamic().bootstrapModule(AppModule)

//our root app component
import {Component, NgModule} from '@angular/core'
import {BrowserModule} from '@angular/platform-browser'
import { DemoComponent } from './demo.component'

@Component({
  selector: 'my-app',
  template: `
    <div>
      <h2>{{name}}</h2>
    </div>
    <br><br>
     <demo></demo>
     <br><br>
  `,
})
export class App {
  name:string;
  constructor() {
    this.name = 'Angular2 Demos'
  }
}

@NgModule({
  imports: [ BrowserModule ],
  declarations: [ App,
  DemoComponent],
  bootstrap: [ App ]
})
export class AppModule {}

import { Component, OnInit, Input, ViewChild, ElementRef } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/fromEvent';
import 'rxjs/add/observable/merge';
import 'rxjs/add/operator/scan';
import 'rxjs/add/operator/map';

@Component({
    selector: 'demo',
    template: `
    <div>{{demoInfo}}</div>
    <button #incrementButton id="incrementButton" class="btn btn-outline-success" value="1">Increment</button>
    <span #totalSpan id="totalSpan" style="margin-left:10px;">0</span>
    <button #decrementButton id="decrementButton" class="btn btn-outline-success" style="margin-left:10px;" value="-1">Decrement</button>
    <button id="addListenerButton" class="btn btn-secondary" (click)="addListeners()" [disabled]=listenersActive>Add Listeners</button>
    <button id="removeListenerButton" class="btn btn-secondary" (click)="removeListeners()" [disabled]=!listenersActive>Remove Listeners</button>
    <button class="btn btn-primary" (click)="reset()">Reset</button>
    <div *ngFor="let output of outputArray">
    {{output}}
    </div>
    `,
})

export class DemoComponent implements OnInit{
    demoInfo = 'Observable Merge & Scan (Reduce)';
    outputArray = new Array(); 

    @ViewChild('incrementButton') incrementButton: ElementRef;
    @ViewChild('decrementButton') decrementButton: ElementRef;

    @ViewChild('totalSpan') totalSpan: ElementRef;
    listenersActive: boolean = true;

    getValue = ev => parseInt(ev.target.value, 10);

    increment$: Observable<{}>;
    decrement$: Observable<{}>;
    changes$: Observable<{}>;
    total$: Observable<{}>;

    changeListener;
    totalListener;


    ngOnInit(){
        //establish streams of click events
        this.increment$ = Observable.fromEvent(this.incrementButton.nativeElement, 'click');
        this.decrement$ = Observable.fromEvent(this.decrementButton.nativeElement, 'click');

        //merge them into a single stream of changes
        this.changes$ = Observable.merge(this.increment$, this.decrement$).map(this.getValue);

        //reduce the stream of changes to a running total
        this.total$ = this.changes$.scan((total, value) => (<number>total) + (<number>value), 0);
        
        this.addListeners();
    }

    addListeners(){
            //remove any previously subscribed listeners
            this.removeListeners();

            this.outputArray.push("Adding Change Listener");
            this.changeListener = this.changes$.subscribe(val => console.log(val));

            this.outputArray.push("Adding Total Listener");
            this.totalListener = this.total$.subscribe(count => (<HTMLSpanElement> this.totalSpan.nativeElement).innerText = count.toString());
            this.listenersActive = true;            
    }

    removeListeners(){
        if (this.changeListener != null){
            this.outputArray.push("Unsubscribing Change Listener");
            this.changeListener.unsubscribe();
        }
        
        if (this.totalListener != null){
            this.outputArray.push("Unsubscribing Total Listener");
            this.totalListener.unsubscribe();
        }

        (<HTMLSpanElement> this.totalSpan.nativeElement).innerText = "0";
        this.listenersActive = false;
    }

    reset(){
      this.addListeners();
      this.outputArray.length = 0;
    }

}