feat(image-loader): Set image asynchronously and placeholder immediately

src/app/app.component.html

@@ -23,6 +23,22 @@ <h3>Bottom image</h3>
   #imgEl>
 </sn-image-loader>
 <h3>Bottom image imageLoaded event count: <span class="image-loaded-count">{{ imageLoadedEventCount }}</span></h3>
+
+<h1>Scroll down for async image</h1>
+<h3>Async image placeholder loaded: <span class="placeholder-loaded">{{ placeholderAsyncImageLoaded }}</span></h3>
+<div class="spacer"></div>
+
+<h3>Async image data after 3 seconds (placeholder immediately)</h3><button (click)="startAsyncLoading()">Start timer</button>
+<sn-image-loader
+  [image]="asyncImage"
+  [sizes]="sizes"
+  imgClass="img img--bottom"
+  alt="lorem ipsum"
+  (placeholderLoaded)="onAsyncImagePlaceholderLoad($event)"
+  (imageLoaded)="onAsyncImageLoad($event)"
+  class="sn-image-loader sn-image-loader--bottom">
+</sn-image-loader>
+<h3>Async image imageLoaded event count: <span class="image-loaded-count">{{ asyncImageLoadedEventCount }}</span></h3>
 <div class="spacer"></div>
 
 <sn-video-loader

src/app/app.component.ts

@@ -1,4 +1,4 @@
-import { Component } from '@angular/core';
+import { Component, OnInit } from '@angular/core';
 
 import { Breakpoint } from './image-loader/shared/breakpoint.model';
 import { ResponsiveImage, Size } from './image-loader/shared/image.model';
@@ -11,10 +11,11 @@ import { video, image, sizes } from './app-data';
   templateUrl: './app.component.html',
   styleUrls: ['./app.component.scss'],
 })
-export class AppComponent {
+export class AppComponent implements OnInit {
   sizes: Breakpoint[] = sizes;
   image: ResponsiveImage = image;
   video: ResponsiveVideo = video;
+  asyncImage: ResponsiveImage;
 
   /**
    * Set to true on placeholder loaded event.
@@ -24,6 +25,14 @@ export class AppComponent {
    */
   placeholderLoaded = false;
 
+  /**
+   * Set to true on placeholder loaded event.
+   *
+   * @type {boolean}
+   * @memberof AppComponent
+   */
+  placeholderAsyncImageLoaded = false;
+
   /**
    * Incremented on each image load event.
    *
@@ -32,6 +41,14 @@ export class AppComponent {
    */
   imageLoadedEventCount = 0;
 
+  /**
+   * Incremented on each async image load event.
+   *
+   * @type {number}
+   * @memberof AppComponent
+   */
+  asyncImageLoadedEventCount = 0;
+
   /**
    * Increments event count on each image loaded event.
    * Counter displayed in component template.
@@ -42,6 +59,16 @@ export class AppComponent {
     this.placeholderLoaded = true;
   }
 
+  /**
+   * Increments event count on each image loaded event.
+   * Counter displayed in component template.
+   *
+   * @memberof AppComponent
+   */
+  public onAsyncImagePlaceholderLoad(imageLoadedEvent: ImageLoadedEvent) {
+    this.placeholderAsyncImageLoaded = true;
+  }
+
   /**
    * Increments event count on each image loaded event.
    * Counter displayed in component template.
@@ -51,4 +78,48 @@ export class AppComponent {
   public onImageLoad(imageLoadedEvent: ImageLoadedEvent) {
     this.imageLoadedEventCount++;
   }
+
+  /**
+   * Increments event count on each image loaded event.
+   * Counter displayed in component template.
+   *
+   * @memberof AppComponent
+   */
+  public onAsyncImageLoad(imageLoadedEvent: ImageLoadedEvent) {
+    this.asyncImageLoadedEventCount++;
+  }
+
+  /**
+   * Resets async image data and simulates loading
+   *
+   * @memberof AppComponent
+   */
+  public startAsyncLoading() {
+    this.asyncImageLoadedEventCount = 0;
+    this.placeholderAsyncImageLoaded = false;
+    this.loadAsyncImageData();
+  }
+
+  ngOnInit() {
+    this.asyncImage = {
+      placeholder: image.placeholder,
+      images: [],
+      fallback: image.fallback,
+    };
+  }
+
+  /**
+   * Sets a placeholder for the image and simulates loading of image data (typically from some API endpoint)
+   * After 3 seconds the image data is set
+   */
+  private loadAsyncImageData() {
+    this.asyncImage = {
+      placeholder: image.placeholder,
+      images: [],
+      fallback: image.fallback,
+    };
+    setTimeout(() => {
+      this.asyncImage = image;
+    }, 3000);
+  }
 }

src/app/image-loader/image-loader/image-loader.component.spec.ts

@@ -219,4 +219,45 @@ describe('ImageLoaderComponent', () => {
     component.ngOnChanges();
     expect(spy).toHaveBeenCalled();
   });
+
+  describe('async image', () => {
+    let spyPreloadImage, spySetPlaceholder;
+
+    beforeEach(() => {
+      spyPreloadImage = spyOn(component, 'preloadImage').and.callThrough();
+      spySetPlaceholder = spyOn(component, 'setPlaceholder').and.callThrough();
+
+      component.image = {
+        placeholder: 'someplaceholder',
+        images: null,
+        fallback: null,
+      };
+      component.inViewport = true;
+      component.loaded = false;
+    });
+
+    it('should not preload image if no images are set and is in viewport', () => {
+      component.ngOnChanges();
+      expect(spyPreloadImage).toHaveBeenCalled();
+      expect(spySetPlaceholder).toHaveBeenCalled();
+      expect(component.preloadSrc).toEqual('');
+      expect(component.preloadSrcset).toEqual('');
+    });
+
+    it('should preload image after already scrolled in and set later on', () => {
+      component.ngOnChanges();
+      expect(component.preloadSrc).toEqual('');
+      expect(component.preloadSrcset).toEqual('');
+      expect(component.src).toEqual('someplaceholder');
+
+      component.image = image;
+      component.ngOnChanges();
+      expect(component.preloadSrc).toEqual(
+        'http://via.placeholder.com/150x350?text=xs+1x',
+      );
+      expect(component.preloadSrcset).toEqual(
+        'http://via.placeholder.com/150x350?text=xs+1x 1x, http://via.placeholder.com/300x700?text=xs+2x 2x',
+      );
+    });
+  });
 });

src/app/image-loader/image-loader/image-loader.component.ts

@@ -250,16 +250,18 @@ export class ImageLoaderComponent
    */
   public preloadImage(): void {
     if (this.inViewport && this.notLoaded) {
-      const retinaImg = this.image.images.find(
-        retinaImage => retinaImage.size === this.size,
-      );
-      const imageNormal = retinaImg.x1;
-      const imageRetina = retinaImg.x2;
-      if ('srcset' in this.img.nativeElement) {
-        this.supportsSrcSet = true;
+      const retinaImg =
+        this.image.images &&
+        this.image.images.find(retinaImage => retinaImage.size === this.size);
+      if (retinaImg) {
+        const imageNormal = retinaImg.x1;
+        const imageRetina = retinaImg.x2;
+        if ('srcset' in this.img.nativeElement) {
+          this.supportsSrcSet = true;
+        }
+        this.preloadSrcset = `${imageNormal} 1x, ${imageRetina} 2x`;
+        this.preloadSrc = imageNormal;
       }
-      this.preloadSrcset = `${imageNormal} 1x, ${imageRetina} 2x`;
-      this.preloadSrc = imageNormal;
     }
   }
   /**