Add support for prefetching images for the Feed.

Bug: 1128169
Change-Id: I423085d3d3f6ac7b9434b72cb794cd7f19ace64e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2531365
Commit-Queue: Jonathan Freed <freedjm@chromium.org>
Reviewed-by: Dan H <harringtond@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827127}

chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/FeedProcessScopeDependencyProvider.java

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.chrome.browser.feed.v2;
+
+import android.content.Context;
+
+import org.chromium.base.BundleUtils;
+import org.chromium.base.ContextUtils;
+import org.chromium.base.Log;
+import org.chromium.base.ThreadUtils;
+import org.chromium.base.task.PostTask;
+import org.chromium.base.task.TaskTraits;
+import org.chromium.chrome.browser.profiles.Profile;
+import org.chromium.chrome.browser.signin.IdentityServicesProvider;
+import org.chromium.chrome.browser.xsurface.ImageFetchClient;
+import org.chromium.chrome.browser.xsurface.ProcessScopeDependencyProvider;
+import org.chromium.components.signin.base.CoreAccountInfo;
+import org.chromium.components.signin.identitymanager.ConsentLevel;
+import org.chromium.content_public.browser.UiThreadTaskTraits;
+
+/**
+ * Provides logging and context for all surfaces.
+ */
+public class FeedProcessScopeDependencyProvider implements ProcessScopeDependencyProvider {
+    private static final String FEED_SPLIT_NAME = "feedv2";
+
+    private Context mContext;
+    private ImageFetchClient mImageFetchClient;
+    private LibraryResolver mLibraryResolver;
+
+    FeedProcessScopeDependencyProvider() {
+        mContext = createFeedContext(ContextUtils.getApplicationContext());
+        mImageFetchClient = new FeedImageFetchClient();
+        if (BundleUtils.isIsolatedSplitInstalled(mContext, FEED_SPLIT_NAME)) {
+            mLibraryResolver = (libName) -> {
+                return BundleUtils.getNativeLibraryPath(libName);
+            };
+        }
+    }
+
+    @Override
+    public Context getContext() {
+        return mContext;
+    }
+
+    @Deprecated
+    @Override
+    public String getAccountName() {
+        assert ThreadUtils.runningOnUiThread();
+        CoreAccountInfo primaryAccount =
+                IdentityServicesProvider.get()
+                        .getIdentityManager(Profile.getLastUsedRegularProfile())
+                        .getPrimaryAccountInfo(ConsentLevel.NOT_REQUIRED);
+        return (primaryAccount == null) ? "" : primaryAccount.getEmail();
+    }
+
+    @Deprecated
+    @Override
+    public int[] getExperimentIds() {
+        // Note: this is thread-safe.
+        return FeedStreamSurfaceJni.get().getExperimentIds();
+    }
+
+    @Deprecated
+    @Override
+    public String getClientInstanceId() {
+        assert ThreadUtils.runningOnUiThread();
+        return FeedServiceBridge.getClientInstanceId();
+    }
+
+    @Override
+    public ImageFetchClient getImageFetchClient() {
+        return mImageFetchClient;
+    }
+
+    @Override
+    public void logError(String tag, String format, Object... args) {
+        Log.e(tag, format, args);
+    }
+
+    @Override
+    public void logWarning(String tag, String format, Object... args) {
+        Log.w(tag, format, args);
+    }
+
+    @Override
+    public void postTask(int taskType, Runnable task, long delayMs) {
+        TaskTraits traits;
+        switch (taskType) {
+            case ProcessScopeDependencyProvider.TASK_TYPE_UI_THREAD:
+                traits = UiThreadTaskTraits.DEFAULT;
+                break;
+            case ProcessScopeDependencyProvider.TASK_TYPE_BACKGROUND_MAY_BLOCK:
+                traits = TaskTraits.BEST_EFFORT_MAY_BLOCK;
+                break;
+            default:
+                assert false : "Invalid task type";
+                return;
+        }
+        PostTask.postDelayedTask(traits, task, delayMs);
+    }
+
+    @Override
+    public LibraryResolver getLibraryResolver() {
+        return mLibraryResolver;
+    }
+
+    public static Context createFeedContext(Context context) {
+        if (!BundleUtils.isIsolatedSplitInstalled(context, FEED_SPLIT_NAME)) {
+            return context;
+        }
+        return BundleUtils.createIsolatedSplitContext(context, FEED_SPLIT_NAME);
+    }
+}

chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/FeedServiceBridge.java

@@ -10,13 +10,24 @@ import org.chromium.base.ContextUtils;
 import org.chromium.base.annotations.CalledByNative;
 import org.chromium.base.annotations.JNINamespace;
 import org.chromium.base.annotations.NativeMethods;
+import org.chromium.chrome.browser.AppHooks;
 import org.chromium.chrome.browser.feed.library.common.locale.LocaleUtils;
-
+import org.chromium.chrome.browser.xsurface.ProcessScope;
 /**
  * Bridge for FeedService-related calls.
  */
 @JNINamespace("feed")
 public final class FeedServiceBridge {
+    private static ProcessScope sXSurfaceProcessScope;
+
+    public static ProcessScope xSurfaceProcessScope() {
+        if (sXSurfaceProcessScope == null) {
+            sXSurfaceProcessScope = AppHooks.get().getExternalSurfaceProcessScope(
+                    new FeedProcessScopeDependencyProvider());
+        }
+        return sXSurfaceProcessScope;
+    }
+
     public static boolean isEnabled() {
         return FeedServiceBridgeJni.get().isEnabled();
     }
@@ -39,6 +50,11 @@ public final class FeedServiceBridge {
         FeedStreamSurface.clearAll();
     }
 
+    @CalledByNative
+    public static void prefetchImage(String url) {
+        xSurfaceProcessScope().provideImagePrefetcher().prefetchImage(url);
+    }
+
     /** Called at startup to trigger creation of |FeedService|. */
     public static void startup() {
         FeedServiceBridgeJni.get().startup();

chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/FeedStreamSurface.java

@@ -19,9 +19,7 @@ import androidx.recyclerview.widget.LinearLayoutManager;
 import androidx.recyclerview.widget.RecyclerView;
 import androidx.recyclerview.widget.RecyclerView.ItemAnimator.ItemAnimatorFinishedListener;
 
-import org.chromium.base.BundleUtils;
 import org.chromium.base.Callback;
-import org.chromium.base.ContextUtils;
 import org.chromium.base.Log;
 import org.chromium.base.ObserverList;
 import org.chromium.base.ThreadUtils;
@@ -30,7 +28,6 @@ import org.chromium.base.annotations.JNINamespace;
 import org.chromium.base.annotations.NativeMethods;
 import org.chromium.base.supplier.Supplier;
 import org.chromium.base.task.PostTask;
-import org.chromium.base.task.TaskTraits;
 import org.chromium.chrome.R;
 import org.chromium.chrome.browser.AppHooks;
 import org.chromium.chrome.browser.base.SplitCompatUtils;
@@ -52,9 +49,7 @@ import org.chromium.chrome.browser.ui.messages.snackbar.Snackbar;
 import org.chromium.chrome.browser.ui.messages.snackbar.SnackbarManager;
 import org.chromium.chrome.browser.xsurface.FeedActionsHandler;
 import org.chromium.chrome.browser.xsurface.HybridListRenderer;
-import org.chromium.chrome.browser.xsurface.ImageFetchClient;
 import org.chromium.chrome.browser.xsurface.ProcessScope;
-import org.chromium.chrome.browser.xsurface.ProcessScopeDependencyProvider;
 import org.chromium.chrome.browser.xsurface.SurfaceActionsHandler;
 import org.chromium.chrome.browser.xsurface.SurfaceScope;
 import org.chromium.chrome.browser.xsurface.SurfaceScopeDependencyProvider;
@@ -240,95 +235,6 @@ public class FeedStreamSurface implements SurfaceActionsHandler, FeedActionsHand
         }
     }
 
-    /**
-     * Provides logging and context for all surfaces.
-     *
-     * TODO(rogerm): Find a more global home for this.
-     */
-    private static class FeedProcessScopeDependencyProvider
-            implements ProcessScopeDependencyProvider {
-        private Context mContext;
-        private ImageFetchClient mImageFetchClient;
-        private LibraryResolver mLibraryResolver;
-
-        FeedProcessScopeDependencyProvider() {
-            mContext = createFeedContext(ContextUtils.getApplicationContext());
-            mImageFetchClient = new FeedImageFetchClient();
-            if (BundleUtils.isIsolatedSplitInstalled(mContext, FEED_SPLIT_NAME)) {
-                mLibraryResolver = (libName) -> {
-                    return BundleUtils.getNativeLibraryPath(libName);
-                };
-            }
-        }
-
-        @Override
-        public Context getContext() {
-            return mContext;
-        }
-
-        @Deprecated
-        @Override
-        public String getAccountName() {
-            assert ThreadUtils.runningOnUiThread();
-            CoreAccountInfo primaryAccount =
-                    IdentityServicesProvider.get()
-                            .getIdentityManager(Profile.getLastUsedRegularProfile())
-                            .getPrimaryAccountInfo(ConsentLevel.NOT_REQUIRED);
-            return (primaryAccount == null) ? "" : primaryAccount.getEmail();
-        }
-
-        @Deprecated
-        @Override
-        public int[] getExperimentIds() {
-            // Note: this is thread-safe.
-            return FeedStreamSurfaceJni.get().getExperimentIds();
-        }
-
-        @Deprecated
-        @Override
-        public String getClientInstanceId() {
-            assert ThreadUtils.runningOnUiThread();
-            return FeedServiceBridge.getClientInstanceId();
-        }
-
-        @Override
-        public ImageFetchClient getImageFetchClient() {
-            return mImageFetchClient;
-        }
-
-        @Override
-        public void logError(String tag, String format, Object... args) {
-            Log.e(tag, format, args);
-        }
-
-        @Override
-        public void logWarning(String tag, String format, Object... args) {
-            Log.w(tag, format, args);
-        }
-
-        @Override
-        public void postTask(int taskType, Runnable task, long delayMs) {
-            TaskTraits traits;
-            switch (taskType) {
-                case ProcessScopeDependencyProvider.TASK_TYPE_UI_THREAD:
-                    traits = UiThreadTaskTraits.DEFAULT;
-                    break;
-                case ProcessScopeDependencyProvider.TASK_TYPE_BACKGROUND_MAY_BLOCK:
-                    traits = TaskTraits.BEST_EFFORT_MAY_BLOCK;
-                    break;
-                default:
-                    assert false : "Invalid task type";
-                    return;
-            }
-            PostTask.postDelayedTask(traits, task, delayMs);
-        }
-
-        @Override
-        public LibraryResolver getLibraryResolver() {
-            return mLibraryResolver;
-        }
-    }
-
     /**
      * Provides activity and darkmode context for a single surface.
      */
@@ -337,7 +243,8 @@ public class FeedStreamSurface implements SurfaceActionsHandler, FeedActionsHand
         final boolean mDarkMode;
 
         FeedSurfaceScopeDependencyProvider(Context activityContext, boolean darkMode) {
-            mActivityContext = createFeedContext(activityContext);
+            mActivityContext =
+                    FeedProcessScopeDependencyProvider.createFeedContext(activityContext);
             mDarkMode = darkMode;
         }
 

chrome/android/feed/feed_java_sources.gni

@@ -47,6 +47,7 @@ feed_java_sources = [
   "//chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/CardMenuBottomSheetContent.java",
   "//chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/FeedImageFetchClient.java",
   "//chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/FeedListContentManager.java",
+  "//chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/FeedProcessScopeDependencyProvider.java",
   "//chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/FeedServiceBridge.java",
   "//chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/FeedSliceViewTracker.java",
   "//chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/FeedStream.java",

chrome/browser/android/feed/v2/feed_service_bridge.cc

@@ -92,4 +92,10 @@ bool FeedServiceBridge::IsEnabled() {
   return FeedService::IsEnabled(*profile->GetPrefs());
 }
 
+void FeedServiceBridge::PrefetchImage(const GURL& url) {
+  JNIEnv* env = base::android::AttachCurrentThread();
+  Java_FeedServiceBridge_prefetchImage(
+      env, base::android::ConvertUTF8ToJavaString(env, url.spec()));
+}
+
 }  // namespace feed

chrome/browser/android/feed/v2/feed_service_bridge.h

@@ -18,6 +18,7 @@ class FeedServiceBridge {
   static DisplayMetrics GetDisplayMetrics();
   static void ClearAll();
   static bool IsEnabled();
+  static void PrefetchImage(const GURL& url);
 };
 
 }  // namespace feed

chrome/browser/android/feed/v2/feed_service_factory.cc

@@ -46,6 +46,9 @@ class FeedServiceDelegateImpl : public FeedService::Delegate {
     return FeedServiceBridge::GetDisplayMetrics();
   }
   void ClearAll() override { FeedServiceBridge::ClearAll(); }
+  void PrefetchImage(const GURL& url) override {
+    FeedServiceBridge::PrefetchImage(url);
+  }
 };
 
 }  // namespace

chrome/browser/xsurface/BUILD.gn

@@ -9,6 +9,7 @@ android_library("java") {
     "android/java/src/org/chromium/chrome/browser/xsurface/FeedActionsHandler.java",
     "android/java/src/org/chromium/chrome/browser/xsurface/HybridListRenderer.java",
     "android/java/src/org/chromium/chrome/browser/xsurface/ImageFetchClient.java",
+    "android/java/src/org/chromium/chrome/browser/xsurface/ImagePrefetcher.java",
     "android/java/src/org/chromium/chrome/browser/xsurface/ListContentManager.java",
     "android/java/src/org/chromium/chrome/browser/xsurface/ListContentManagerObserver.java",
     "android/java/src/org/chromium/chrome/browser/xsurface/ProcessScope.java",

chrome/browser/xsurface/android/java/src/org/chromium/chrome/browser/xsurface/ImagePrefetcher.java

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.chrome.browser.xsurface;
+
+/**
+ * Interface to prefetch an image and cache it on disk. This
+ * allows native code to call to the image loader across the
+ * xsurface.
+ */
+public interface ImagePrefetcher {
+    default void prefetchImage(String url) {}
+}

chrome/browser/xsurface/android/java/src/org/chromium/chrome/browser/xsurface/ProcessScope.java

@@ -34,4 +34,9 @@ public interface ProcessScope {
     default SurfaceScope obtainSurfaceScope(Context activityContext) {
         return null;
     }
+
+    @Nullable
+    default ImagePrefetcher provideImagePrefetcher() {
+        return null;
+    }
 }

components/feed/core/v2/BUILD.gn

@@ -65,6 +65,8 @@ source_set("feed_core_v2") {
     "tasks/load_stream_from_store_task.h",
     "tasks/load_stream_task.cc",
     "tasks/load_stream_task.h",
+    "tasks/prefetch_images_task.cc",
+    "tasks/prefetch_images_task.h",
     "tasks/upload_actions_task.cc",
     "tasks/upload_actions_task.h",
     "tasks/wait_for_store_initialize_task.cc",

components/feed/core/v2/config.cc

@@ -86,6 +86,11 @@ void OverrideWithFinch(Config* config) {
           kInterestFeedV2, "session_id_max_age_days",
           config->session_id_max_age.InDays()));
 
+  config->max_prefetch_image_requests_per_refresh =
+      base::GetFieldTrialParamByFeatureAsInt(
+          kInterestFeedV2, "max_prefetch_image_requests_per_refresh",
+          config->max_prefetch_image_requests_per_refresh);
+
   // Erase any capabilities with "enable_CAPABILITY = false" set.
   base::EraseIf(config->experimental_capabilities, CapabilityDisabled);
 }

components/feed/core/v2/config.h

@@ -42,6 +42,8 @@ struct Config {
   bool send_signed_out_session_logs = true;
   // The max age of a signed-out session token.
   base::TimeDelta session_id_max_age = base::TimeDelta::FromDays(30);
+  // Maximum number of images prefetched per refresh.
+  int max_prefetch_image_requests_per_refresh = 50;
   // Set of optional capabilities included in requests. See
   // CreateFeedQueryRequest() for required capabilities.
   base::flat_set<feedwire::Capability> experimental_capabilities = {

components/feed/core/v2/feed_stream.cc

@@ -34,6 +34,7 @@
 #include "components/feed/core/v2/tasks/clear_all_task.h"
 #include "components/feed/core/v2/tasks/get_prefetch_suggestions_task.h"
 #include "components/feed/core/v2/tasks/load_stream_task.h"
+#include "components/feed/core/v2/tasks/prefetch_images_task.h"
 #include "components/feed/core/v2/tasks/upload_actions_task.h"
 #include "components/feed/core/v2/tasks/wait_for_store_initialize_task.h"
 #include "components/feed/feed_feature_list.h"
@@ -278,6 +279,10 @@ std::string FeedStream::GetSessionId() const {
   return GetMetadata()->GetSessionIdToken();
 }
 
+void FeedStream::PrefetchImage(const GURL& url) {
+  delegate_->PrefetchImage(url);
+}
+
 void FeedStream::AttachSurface(SurfaceInterface* surface) {
   metrics_reporter_->SurfaceOpened(surface->GetSurfaceId());
 
@@ -703,6 +708,10 @@ void FeedStream::BackgroundRefreshComplete(LoadStreamTask::Result result) {
   if (result.loaded_new_content_from_network && prefetch_service_)
     prefetch_service_->NewSuggestionsAvailable();
 
+  // Add prefetch images to task queue without waiting to finish
+  // since we treat them as best-effort.
+  task_queue_.AddTask(std::make_unique<PrefetchImagesTask>(this));
+
   refresh_task_scheduler_->RefreshTaskComplete();
 }
 

components/feed/core/v2/feed_stream.h

@@ -70,6 +70,7 @@ class FeedStream : public FeedStreamApi,
     virtual std::string GetLanguageTag() = 0;
     virtual void ClearAll() = 0;
     virtual bool IsSignedIn() = 0;
+    virtual void PrefetchImage(const GURL& url) = 0;
   };
 
   // Forwards to |feed::TranslateWireResponse()| by default. Can be overridden
@@ -225,6 +226,8 @@ class FeedStream : public FeedStreamApi,
   const Metadata* GetMetadata() const { return &metadata_; }
   MetricsReporter* GetMetricsReporter() const { return metrics_reporter_; }
 
+  void PrefetchImage(const GURL& url);
+
   // Returns the time of the last content fetch.
   base::Time GetLastFetchTime();
 

components/feed/core/v2/feed_stream_unittest.cc

@@ -602,6 +602,10 @@ class FeedStreamTest : public testing::Test, public FeedStream::Delegate {
   std::string GetLanguageTag() override { return "en-US"; }
   void ClearAll() override {}
   bool IsSignedIn() override { return is_signed_in_; }
+  void PrefetchImage(const GURL& url) override {
+    prefetched_images_.push_back(url);
+    prefetch_image_call_count_++;
+  }
 
   // For tests.
 
@@ -692,6 +696,8 @@ class FeedStreamTest : public testing::Test, public FeedStream::Delegate {
   bool is_offline_ = false;
   bool is_signed_in_ = true;
   base::test::ScopedFeatureList scoped_feature_list_;
+  int prefetch_image_call_count_ = 0;
+  std::vector<GURL> prefetched_images_;
 };
 
 class FeedStreamConditionalActionsUploadTest : public FeedStreamTest {
@@ -744,6 +750,21 @@ TEST_F(FeedStreamTest, BackgroundRefreshSuccess) {
   EXPECT_EQ(1, prefetch_service_.NewSuggestionsAvailableCallCount());
 }
 
+TEST_F(FeedStreamTest, BackgroundRefreshPrefetchesImages) {
+  // Trigger a background refresh.
+  response_translator_.InjectResponse(MakeTypicalInitialModelState());
+  stream_->ExecuteRefreshTask();
+  EXPECT_EQ(0, prefetch_image_call_count_);
+  WaitForIdleTaskQueue();
+
+  std::vector<GURL> expected_fetches(
+      {GURL("http://image0/"), GURL("http://favicon0/"), GURL("http://image1/"),
+       GURL("http://favicon1/")});
+  // Verify that images were prefetched.
+  EXPECT_EQ(4, prefetch_image_call_count_);
+  EXPECT_EQ(expected_fetches, prefetched_images_);
+}
+
 TEST_F(FeedStreamTest, BackgroundRefreshNotAttemptedWhenModelIsLoading) {
   response_translator_.InjectResponse(MakeTypicalInitialModelState());
   TestSurface surface(stream_.get());

components/feed/core/v2/public/feed_service.cc

@@ -121,6 +121,9 @@ class FeedService::StreamDelegateImpl : public FeedStream::Delegate {
     return service_delegate_->GetLanguageTag();
   }
   void ClearAll() override { service_delegate_->ClearAll(); }
+  void PrefetchImage(const GURL& url) override {
+    service_delegate_->PrefetchImage(url);
+  }
   bool IsSignedIn() override { return identity_manager_->HasPrimaryAccount(); }
 
  private:

components/feed/core/v2/public/feed_service.h

@@ -67,6 +67,8 @@ class FeedService : public KeyedService {
     virtual DisplayMetrics GetDisplayMetrics() = 0;
     // Clear all stored data.
     virtual void ClearAll() = 0;
+    // Fetch the image and store it in the disk cache.
+    virtual void PrefetchImage(const GURL& url) = 0;
   };
 
   // Construct a FeedService given an already constructed FeedStream.

components/feed/core/v2/tasks/prefetch_images_task.cc

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/feed/core/v2/tasks/prefetch_images_task.h"
+
+#include <utility>
+
+#include "base/callback.h"
+#include "base/logging.h"
+#include "components/feed/core/proto/v2/wire/stream_structure.pb.h"
+#include "components/feed/core/v2/config.h"
+#include "components/feed/core/v2/feed_store.h"
+#include "components/feed/core/v2/feed_stream.h"
+#include "components/feed/core/v2/stream_model.h"
+#include "components/feed/core/v2/tasks/load_stream_from_store_task.h"
+
+namespace feed {
+namespace {
+
+// Converts a URL string into a GURL. If the string is not a valid URL, returns
+// an empty GURL. Since GURL::spec() asserts on invalid URLs, this is necessary
+// to scrub the incoming data from the wire.
+GURL SpecToGURL(const std::string& url_string) {
+  GURL url(url_string);
+  if (!url.is_valid())
+    url = GURL();
+  return url;
+}
+
+}  // namespace
+
+PrefetchImagesTask::PrefetchImagesTask(FeedStream* stream) : stream_(stream) {
+  max_images_per_refresh_ =
+      GetFeedConfig().max_prefetch_image_requests_per_refresh;
+}
+
+PrefetchImagesTask::~PrefetchImagesTask() = default;
+
+void PrefetchImagesTask::Run() {
+  if (stream_->GetModel()) {
+    PrefetchImagesFromModel(*stream_->GetModel());
+    return;
+  }
+
+  load_from_store_task_ = std::make_unique<LoadStreamFromStoreTask>(
+      LoadStreamFromStoreTask::LoadType::kFullLoad, stream_->GetStore(),
+      stream_->GetClock(),
+      base::BindOnce(&PrefetchImagesTask::LoadStreamComplete,
+                     base::Unretained(this)));
+
+  load_from_store_task_->Execute(base::DoNothing());
+}
+
+void PrefetchImagesTask::LoadStreamComplete(
+    LoadStreamFromStoreTask::Result result) {
+  if (!result.update_request) {
+    TaskComplete();
+    return;
+  }
+
+  // It is a bit dangerous to retain the model loaded here. The normal
+  // LoadStreamTask flow has various considerations for metrics and signalling
+  // surfaces to update. For this reason, we're not going to retain the loaded
+  // model for use outside of this task.
+  StreamModel model;
+  model.Update(std::move(result.update_request));
+  PrefetchImagesFromModel(model);
+}
+
+void PrefetchImagesTask::PrefetchImagesFromModel(const StreamModel& model) {
+  for (ContentRevision rev : model.GetContentList()) {
+    const feedstore::Content* content = model.FindContent(rev);
+    if (!content)
+      continue;
+    for (const feedwire::PrefetchMetadata& metadata :
+         content->prefetch_metadata()) {
+      MaybePrefetchImage(SpecToGURL(metadata.image_url()));
+      MaybePrefetchImage(SpecToGURL(metadata.favicon_url()));
+      for (const std::string& url : metadata.additional_image_urls()) {
+        MaybePrefetchImage(SpecToGURL(url));
+      }
+    }
+  }
+
+  TaskComplete();
+}
+
+void PrefetchImagesTask::MaybePrefetchImage(const GURL& gurl) {
+  // If we've already fetched this url, or we've hit the max number of fetches,
+  // then don't send a fetch request.
+  if ((previously_fetched_.find(gurl.spec()) != previously_fetched_.end()) ||
+      previously_fetched_.size() >=
+          static_cast<size_t>(max_images_per_refresh_))
+    return;
+  previously_fetched_.insert(gurl.spec());
+  stream_->PrefetchImage(gurl);
+}
+
+}  // namespace feed

components/feed/core/v2/tasks/prefetch_images_task.h

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_FEED_CORE_V2_TASKS_PREFETCH_IMAGES_TASK_H_
+#define COMPONENTS_FEED_CORE_V2_TASKS_PREFETCH_IMAGES_TASK_H_
+
+#include <memory>
+#include <vector>
+
+#include "base/memory/weak_ptr.h"
+#include "components/feed/core/v2/tasks/load_stream_from_store_task.h"
+#include "components/offline_pages/task/task.h"
+
+namespace feed {
+class FeedStream;
+class StreamModel;
+
+// Prefetch the images in the model.
+class PrefetchImagesTask : public offline_pages::Task {
+ public:
+  explicit PrefetchImagesTask(FeedStream* stream);
+  ~PrefetchImagesTask() override;
+  PrefetchImagesTask(const PrefetchImagesTask&) = delete;
+  PrefetchImagesTask& operator=(const PrefetchImagesTask&) = delete;
+
+ private:
+  base::WeakPtr<PrefetchImagesTask> GetWeakPtr() {
+    return weak_ptr_factory_.GetWeakPtr();
+  }
+
+  // offline_pages::Task.
+  void Run() override;
+
+  void LoadStreamComplete(LoadStreamFromStoreTask::Result result);
+
+  void PrefetchImagesFromModel(const StreamModel& model);
+
+  void MaybePrefetchImage(const GURL& gurl);
+
+  FeedStream* stream_;
+  std::unordered_set<std::string> previously_fetched_;
+  unsigned long max_images_per_refresh_;
+
+  std::unique_ptr<LoadStreamFromStoreTask> load_from_store_task_;
+
+  base::WeakPtrFactory<PrefetchImagesTask> weak_ptr_factory_{this};
+};
+}  // namespace feed
+
+#endif  // COMPONENTS_FEED_CORE_V2_TASKS_PREFETCH_IMAGES_TASK_H_