angular · yharaskrik · Sep 4, 2022 · Oct 10, 2022
diff --git a/goldens/public-api/common/errors.md b/goldens/public-api/common/errors.md
@@ -25,6 +25,8 @@ export const enum RuntimeErrorCode {
     // (undocumented)
     REQUIRED_INPUT_MISSING = 2954,
     // (undocumented)
+    TOO_MANY_PRELOADED_IMAGES = 2961,
+    // (undocumented)
     UNEXPECTED_DEV_MODE_CHECK_IN_PROD_MODE = 2958,
     // (undocumented)
     UNEXPECTED_INPUT_CHANGE = 2953,

diff --git a/packages/common/src/directives/ng_optimized_image/ng_optimized_image.ts b/packages/common/src/directives/ng_optimized_image/ng_optimized_image.ts
@@ -6,14 +6,16 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Directive, ElementRef, inject, InjectionToken, Injector, Input, NgZone, OnChanges, OnDestroy, OnInit, Renderer2, SimpleChanges, ɵformatRuntimeError as formatRuntimeError, ɵRuntimeError as RuntimeError} from '@angular/core';
+import {Directive, ElementRef, inject, InjectionToken, Injector, Input, NgZone, OnChanges, OnDestroy, OnInit, PLATFORM_ID, Renderer2, SimpleChanges, ɵformatRuntimeError as formatRuntimeError, ɵRuntimeError as RuntimeError} from '@angular/core';
 
 import {RuntimeErrorCode} from '../../errors';
+import {isPlatformServer} from '../../platform_id';
 
 import {imgDirectiveDetails} from './error_helper';
 import {IMAGE_LOADER} from './image_loaders/image_loader';
 import {LCPImageObserver} from './lcp_image_observer';
 import {PreconnectLinkChecker} from './preconnect_link_checker';
+import {PreloadLinkCreator} from './preload-link-creator';
 
 /**
  * When a Base64-encoded image is passed as an input to the `NgOptimizedImage` directive,
@@ -207,6 +209,8 @@ export class NgOptimizedImage implements OnInit, OnChanges, OnDestroy {
   private renderer = inject(Renderer2);
   private imgElement: HTMLImageElement = inject(ElementRef).nativeElement;
   private injector = inject(Injector);
+  private readonly isServer = isPlatformServer(inject(PLATFORM_ID));
+  private readonly preloadLinkChecker = inject(PreloadLinkCreator);
 
   // a LCP image observer - should be injected only in the dev mode
   private lcpObserver = ngDevMode ? this.injector.get(LCPImageObserver) : null;
@@ -386,14 +390,28 @@ export class NgOptimizedImage implements OnInit, OnChanges, OnDestroy {
     this.setHostAttribute('fetchpriority', this.getFetchPriority());
     // The `src` and `srcset` attributes should be set last since other attributes
     // could affect the image's loading behavior.
-    this.setHostAttribute('src', this.getRewrittenSrc());
+    const rewrittenSrc = this.getRewrittenSrc();
+    this.setHostAttribute('src', rewrittenSrc);
+
+    let rewrittenSrcset: string|undefined = undefined;
+
     if (this.sizes) {
       this.setHostAttribute('sizes', this.sizes);
     }
+
     if (this.ngSrcset) {
-      this.setHostAttribute('srcset', this.getRewrittenSrcset());
+      rewrittenSrcset = this.getRewrittenSrcset();
     } else if (!this._disableOptimizedSrcset && !this.srcset) {
-      this.setHostAttribute('srcset', this.getAutomaticSrcset());
+      rewrittenSrcset = this.getAutomaticSrcset();
+    }
+
+    if (rewrittenSrcset) {
+      this.setHostAttribute('srcset', rewrittenSrcset);
+    }
+
+    if (this.isServer && this.priority) {
+      this.preloadLinkChecker.createPreloadLinkTag(
+          this.renderer, rewrittenSrc, rewrittenSrcset, this.sizes);
     }
   }
 

diff --git a/packages/common/src/directives/ng_optimized_image/preload-link-creator.ts b/packages/common/src/directives/ng_optimized_image/preload-link-creator.ts
@@ -0,0 +1,79 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {inject, Injectable, Renderer2, ɵRuntimeError as RuntimeError} from '@angular/core';
+
+import {DOCUMENT} from '../../dom_tokens';
+import {RuntimeErrorCode} from '../../errors';
+
+import {DEFAULT_PRELOADED_IMAGES_LIMIT, PRELOADED_IMAGES} from './tokens';
+
+/**
+ * @description Contains the logic needed to track and add preload link tags to the `<head>` tag. It
+ * will also track what images have already had preload link tags added so as to not duplicate link
+ * tags.
+ *
+ * In dev mode this service will validate that the number of preloaded images does not exceed the
+ * configured default preloaded images limit: {@link DEFAULT_PRELOADED_IMAGES_LIMIT}.
+ */
+@Injectable({providedIn: 'root'})
+export class PreloadLinkCreator {
+  private readonly preloadedImages = inject(PRELOADED_IMAGES);
+  private readonly document = inject(DOCUMENT);
+
+  /**
+   * @description Add a preload `<link>` to the `<head>` of the `index.html` that is served from the
+   * server while using Angular Universal and SSR to kick off image loads for high priority images.
+   *
+   * The `sizes` (passed in from the user) and `srcset` (parsed and formatted from `ngSrcset`)
+   * properties used to set the corresponding attributes, `imagesizes` and `imagesrcset`
+   * respectively, on the preload `<link>` tag so that the correctly sized image is preloaded from
+   * the CDN.
+   *
+   * {@link https://web.dev/preload-responsive-images/#imagesrcset-and-imagesizes}
+   *
+   * @param renderer The `Renderer2` passed in from the directive
+   * @param src The original src of the image that is set on the `ngSrc` input.
+   * @param srcset The parsed and formatted srcset created from the `ngSrcset` input
+   * @param sizes The value of the `sizes` attribute passed in to the `<img>` tag
+   */
+  createPreloadLinkTag(renderer: Renderer2, src: string, srcset?: string, sizes?: string): void {
+    if (ngDevMode) {
+      if (this.preloadedImages.size >= DEFAULT_PRELOADED_IMAGES_LIMIT) {
+        throw new RuntimeError(
+            RuntimeErrorCode.TOO_MANY_PRELOADED_IMAGES,
+            ngDevMode &&
+                `The \`NgOptimizedImage\` directive has detected that more than ` +
+                    `${DEFAULT_PRELOADED_IMAGES_LIMIT} images were marked as priority. ` +
+                    `This might negatively affect an overall performance of the page. ` +
+                    `To fix this, remove the "priority" attribute from images with less priority.`);
+      }
+    }
+
+    if (this.preloadedImages.has(src)) {
+      return;
+    }
+
+    this.preloadedImages.add(src);
+
+    const preload = renderer.createElement('link');
+    renderer.setAttribute(preload, 'as', 'image');
+    renderer.setAttribute(preload, 'href', src);
+    renderer.setAttribute(preload, 'rel', 'preload');
+
+    if (sizes) {
+      renderer.setAttribute(preload, 'imageSizes', sizes);
+    }
+
+    if (srcset) {
+      renderer.setAttribute(preload, 'imageSrcset', srcset);
+    }
+
+    renderer.appendChild(this.document.head, preload);
+  }
+}
diff --git a/packages/common/src/directives/ng_optimized_image/tokens.ts b/packages/common/src/directives/ng_optimized_image/tokens.ts
@@ -0,0 +1,27 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {InjectionToken} from '@angular/core';
+
+/**
+ * In SSR scenarios, a preload `<link>` element is generated for priority images.
+ * Having a large number of preload tags may negatively affect the performance,
+ * so we warn developers (by throwing an error) if the number of preloaded images
+ * is above a certain threshold. This const specifies this threshold.
+ */
+export const DEFAULT_PRELOADED_IMAGES_LIMIT = 5;
+
+/**
+ * Helps to keep track of priority images that already have a corresponding
+ * preload tag (to avoid generating multiple preload tags with the same URL).
+ *
+ * This Set tracks the original src passed into the `ngSrc` input not the src after it has been
+ * run through the specified `IMAGE_LOADER`.
+ */
+export const PRELOADED_IMAGES = new InjectionToken<Set<string>>(
+    'NG_OPTIMIZED_PRELOADED_IMAGES', {providedIn: 'root', factory: () => new Set<string>()});
diff --git a/packages/common/src/errors.ts b/packages/common/src/errors.ts
@@ -31,4 +31,5 @@ export const enum RuntimeErrorCode {
   UNEXPECTED_DEV_MODE_CHECK_IN_PROD_MODE = 2958,
   INVALID_LOADER_ARGUMENTS = 2959,
   OVERSIZED_IMAGE = 2960,
+  TOO_MANY_PRELOADED_IMAGES = 2961,
 }