demo of switchMap and stringing dependent observables.

<!DOCTYPE html>
<html>

  <head>
    <base href="." />
    <title>angular2 playground</title>
    <link rel="stylesheet" href="style.css" />
    <script src="https://unpkg.com/zone.js/dist/zone.js"></script>
    <script src="https://unpkg.com/zone.js/dist/long-stack-trace-zone.js"></script>
    <script src="https://unpkg.com/reflect-metadata@0.1.3/Reflect.js"></script>
    <script src="https://unpkg.com/systemjs@0.19.31/dist/system.js"></script>
    <script src="config.js"></script>
    <script>
    System.import('app')
      .catch(console.error.bind(console));
  </script>
  </head>

  <body>
    <my-app>
    loading...
  </my-app>
  </body>

</html>

/* Styles go here */

demo of switchMap and stringing dependent observables.

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',
    '@angular/platform-browser-dynamic/testing': 'npm:@angular/platform-browser-dynamic/bundles/platform-browser-dynamic-testing.umd.js',
    '@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 {platformBrowserDynamic} from '@angular/platform-browser-dynamic';
import {AppModule} from './app';

platformBrowserDynamic().bootstrapModule(AppModule)

//our root app component
import {Component, NgModule, OnInit} from '@angular/core'
import {BrowserModule} from '@angular/platform-browser'
import {SessionService} from './session.service'
import {DataService} from './data.service'

@Component({
  selector: 'my-app',
  template: `
    <div>
    <button (click)="getstuff()">get data</button>
      <h2>session {{sessionToken}}</h2>
      <p> data {{data.data}}</p>
    </div>
  `,
})
export class App implements OnInit{
  name:string;
  private data={data:"test"};
  private sesisonToken;
  constructor(private sessionService:SessionService, private dataService:DataService) {
    this.name = 'Angular2'
  }
  ngOnInit(){
      this.sessionToken = 'empty';
      this.data = {data:"null"};
  }
  getstuff(){
    this.sessionService.getSession().subscribe(
      t => {
        this.sessionToken = t;
      });


    this.dataService.getData().subscribe(
      d => {
        this.data = d;
      });
      
  }
}

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

import { Injectable, Inject, EventEmitter } from '@angular/core';
import { Observable } from 'rxjs/Rx';

@Injectable()
export class SessionService {
  private sessionObservable: Observable<any>;
  private sessionToken: string;
  // http response will look similar to:
  private sessonTokenResponse = {headers: {"X-Session-Token": "ORECUHDLRUECHOELURCH54581", responseCode: 201}, body: null};
  
  constructor(){}
  public getSession(): Observable<any>{
    if (this.sessionToken)
      return Observable.of(this.sessionToken);
    else if(this.sessionObservable)
      return this.sessionObservable;
    else {
      // simulate http call 
      this.sessionObservable = Observable.of(this.sessonTokenResponse)
      .map(res => {
        this.sessionObservable = null;
        return res.headers["X-Session-Token"];
      })
      .delay(500)
      .share();
      return this.sessionObservable;
    }
  }
}

import { Injectable, Inject } from '@angular/core';
import { Observable } from 'rxjs/Rx';
import { SessionService } from './session.service';

@Injectable()
export class DataService {
  private dataObservable: Observable<any>;
  private dataResponse = { headers: 
    {"X-Session-Token": "the same one as earlier",
      responseCode: 200
    }, body: {data:'somJSON data'}};
    
    private dataResponse_noSession = { 
      headers: 
      {
        responseCode: 400
      },
      body: "error"
    };
    
  constructor(private sessionService: SessionService){}
  
  public getData() {
    if (this.dataObservable)
      return this.dataObservable;
    else {
      this.dataObservable = this.sessionService.getSession()
        .switchMap((sessionToken:string, index:number) =>{
          //simulate data http call
            return Observable.of(this.dataResponse)
            .map(res => {
              this.dataObservable = null;
              return res.body;
            })
            .delay(1200)
          })
          .map ( data => {
            return data;
          })
          .catch(err => {
            console.log("err in data service", err);
           // return err;
          })
          .share();
      return this.dataObservable;
    }
  }
  public getData_noSession(){
    return Observable.of(this.dataResponse_noSession)
          .map(res => {res.body})
          .catch(err => {console.log(err);})
          .delay(10);
  }
}