Untitled

<!DOCTYPE html>
<html>
<head>

  <!-- Load common libraries -->
  <script src="https://cdnjs.cloudflare.com/ajax/libs/typescript/2.1.1/typescript.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/core-js/2.4.1/core.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/zone.js/0.8.5/zone.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/systemjs/0.19.38/system.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/hammer.js/2.0.8/hammer.min.js"></script>

  <!-- Configure SystemJS -->
  <script src="systemjs.config.js"></script>

  <script>
    System
      .import('main.ts')
      .catch(console.error.bind(console));
  </script>

  <style>body { font-family: Roboto, Arial, sans-serif; }</style>
</head>

<body>
<app>Loading ...</app>
</body>

</html>

import {platformBrowserDynamic} from '@angular/platform-browser-dynamic';
import {BrowserModule} from '@angular/platform-browser';
import {BrowserAnimationsModule} from '@angular/platform-browser/animations';
import {NgModule} from '@angular/core';
import {AppComponent} from './app.component';
import {TurnGreenWhenInViewComponent} from './turn-green-when-in-view.component.ts';
import {HttpModule} from '@angular/http';

@NgModule({
  exports: [
  ]
})
export class PlunkerMaterialModule {}

@NgModule({

  imports: [
    BrowserModule,
    BrowserAnimationsModule
  ],

  declarations: [AppComponent, TurnGreenWhenInViewComponent],
  bootstrap: [AppComponent],
  providers: []
})
export class PlunkerAppModule {}
platformBrowserDynamic().bootstrapModule(PlunkerAppModule);

import {Component} from '@angular/core';


@Component({
  selector: 'app',
  template: `
    <div style="height: 1500px; background: #ccc;">scroll down slowly</div>
    
    <turn-green-when-in-view></turn-green-when-in-view>
    
    <div style="height: 1500px; background: #ccc;"></div>
  `
})
export class AppComponent {}

/** Add Transpiler for Typescript */
System.config({
  transpiler: 'typescript',
  typescriptOptions: {
    emitDecoratorMetadata: true
  },
  packages: {
    '.': {
      defaultExtension: 'ts'
    },
    'vendor': {
      defaultExtension: 'js'
    }
  }
});

System.config({
  map: {
    'main': 'main.js',

    // Angular specific mappings.
    '@angular/core': 'https://unpkg.com/@angular/core/bundles/core.umd.js',
    '@angular/animations': 'https://unpkg.com/@angular/animations/bundles/animations.umd.js',
    '@angular/common': 'https://unpkg.com/@angular/common/bundles/common.umd.js',
    '@angular/compiler': 'https://unpkg.com/@angular/compiler/bundles/compiler.umd.js',
    '@angular/http': 'https://unpkg.com/@angular/http/bundles/http.umd.js',
    '@angular/forms': 'https://unpkg.com/@angular/forms/bundles/forms.umd.js',
    '@angular/router': 'https://unpkg.com/@angular/router/bundles/router.umd.js',
    '@angular/platform-browser': 'https://unpkg.com/@angular/platform-browser/bundles/platform-browser.umd.js',
    '@angular/platform-browser-dynamic': 'https://unpkg.com/@angular/platform-browser-dynamic/bundles/platform-browser-dynamic.umd.js',
    '@angular/animations/browser': 'https://unpkg.com/@angular/animations/bundles/animations-browser.umd.js',
    '@angular/platform-browser/animations': 'https://unpkg.com/@angular/platform-browser/bundles/platform-browser-animations.umd.js',
    '@angular/cdk': 'https://unpkg.com/@angular/cdk/bundles/cdk.umd.js',

    // Rxjs mapping
    'rxjs': 'https://unpkg.com/rxjs',
  },
  packages: {
    // Thirdparty barrels.
    'rxjs': { main: 'index' },
  }
});

import {Component} from '@angular/core';
import {AppearOnce} from './appear-once.ts';

@Component({
  selector: 'turn-green-when-in-view',
  template: `
    <div [ngStyle]="{backgroundColor: appearedOnce ? 'green' : 'red'}" style="height: 500px; line-height: 500px; color: #fff;">
      <span [hidden]="appearedOnce">I haven't appeared once yet.</span>
      <span [hidden]="!appearedOnce">I have appeared (at least once).</span>
    </div>
  `,
  styles: [`
    :host {
      display: block;
    }
  `]
})
export class TurnGreenWhenInViewComponent extends AppearOnce {
}

import {
  ElementRef,
  OnInit, OnDestroy, ChangeDetectorRef, Injectable
} from '@angular/core';
import {Observable} from 'rxjs/Observable';
import {Subscription} from 'rxjs/Subscription';
import 'rxjs/add/observable/fromEvent';
import 'rxjs/add/operator/startWith';

@Injectable()
export class AppearOnce implements OnInit, OnDestroy {
  appearedOnce: boolean;

  elementPos: number;
  elementHeight: number;

  scrollPos: number;
  windowHeight: number;

  subscriptionScroll: Subscription;
  subscriptionResize: Subscription;

  constructor(private element: ElementRef, private cdRef: ChangeDetectorRef){}
  
  onResize() {
    this.elementPos = this.getOffsetTop(this.element.nativeElement);
    this.elementHeight = this.element.nativeElement.clientHeight;
    this.checkVisibility();
  }
  onScroll() {
    this.scrollPos = window.scrollY;
    this.windowHeight = window.innerHeight;
    this.checkVisibility();
  }
  getOffsetTop(element: any){
    let offsetTop = element.offsetTop || 0;
    if(element.offsetParent){
      offsetTop += this.getOffsetTop(element.offsetParent);
    }
    return offsetTop;
  }

  checkVisibility(){
    if(!this.appearedOnce){
      if(this.scrollPos >= this.elementPos || (this.scrollPos + this.windowHeight) >= (this.elementPos + this.elementHeight)){
        this.appearedOnce = true;
        this.unsubscribe();
        this.cdRef.detectChanges();
      }
    }
  }

  subscribe(){
    this.subscriptionScroll = Observable.fromEvent(window, 'scroll').startWith(null)
      .subscribe(() => this.onScroll());
    this.subscriptionResize = Observable.fromEvent(window, 'resize').startWith(null)
      .subscribe(() => this.onResize());
  }
  unsubscribe(){
    if(this.subscriptionScroll){
      this.subscriptionScroll.unsubscribe();
    }
    if(this.subscriptionResize){
      this.subscriptionResize.unsubscribe();
    }
  }

  ngOnInit(){
    this.subscribe();
  }
  ngOnDestroy(){
    this.unsubscribe();
  }
}