[iOS] Maintain scroll position for Discover feed

Adds enough height to the CS collection view to allow for scroll
position to be maintained in Discover feed. Added height is calculated
from the cached NTP offset.

Bug: 1085419, 1108403
Change-Id: Ia9c2ed7dd7d5d3d8aa3413266404f956ae524cbf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317811
Commit-Queue: Adam Trudeau-Arcaro <adamta@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796864}

ios/chrome/browser/ui/content_suggestions/content_suggestions_coordinator.mm

@@ -66,7 +66,10 @@
 #include "ios/chrome/grit/ios_strings.h"
 #import "ios/public/provider/chrome/browser/chrome_browser_provider.h"
 #import "ios/public/provider/chrome/browser/discover_feed/discover_feed_provider.h"
-#include "ui/base/l10n/l10n_util_mac.h"
+#import "ios/web/public/navigation/navigation_item.h"
+#import "ios/web/public/navigation/navigation_manager.h"
+#import "ios/web/public/web_state.h"
+#import "ui/base/l10n/l10n_util_mac.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
@@ -211,8 +214,20 @@
   self.metricsRecorder = [[ContentSuggestionsMetricsRecorder alloc] init];
   self.metricsRecorder.delegate = self.contentSuggestionsMediator;
 
+  // Offset to maintain Discover feed scroll position.
+  CGFloat offset = 0;
+  if (IsDiscoverFeedEnabled() && contentSuggestionsEnabled) {
+    web::NavigationManager* navigationManager =
+        self.webState->GetNavigationManager();
+    web::NavigationItem* item = navigationManager->GetVisibleItem();
+    if (item) {
+      offset = item->GetPageDisplayState().scroll_state().content_offset().y;
+    }
+  }
+
   self.suggestionsViewController = [[ContentSuggestionsViewController alloc]
-      initWithStyle:CollectionViewControllerStyleDefault];
+      initWithStyle:CollectionViewControllerStyleDefault
+             offset:offset];
   [self.suggestionsViewController
       setDataSource:self.contentSuggestionsMediator];
   self.suggestionsViewController.suggestionCommandHandler = self.NTPMediator;

ios/chrome/browser/ui/content_suggestions/content_suggestions_layout.h

@@ -13,6 +13,14 @@
 // is pinned to the top of the collection.
 @interface ContentSuggestionsLayout : MDCCollectionViewFlowLayout
 
+// The cached scroll position of the NTP.
+@property(nonatomic, assign) CGFloat offset;
+
+// Creates layout with |offset| as additional height. Allows the view's height
+// to be increased enough to maintain the scroll position. Only needed if
+// Discover feed is enabled.
+- (instancetype)initWithOffset:(CGFloat)offset;
+
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_CONTENT_SUGGESTIONS_CONTENT_SUGGESTIONS_LAYOUT_H_

ios/chrome/browser/ui/content_suggestions/content_suggestions_layout.mm

@@ -16,6 +16,13 @@
 
 @implementation ContentSuggestionsLayout
 
+- (instancetype)initWithOffset:(CGFloat)offset {
+  if (self = [super init]) {
+    _offset = offset;
+  }
+  return self;
+}
+
 - (CGSize)collectionViewContentSize {
   CGFloat collectionViewHeight = self.collectionView.bounds.size.height;
   CGFloat headerHeight = [self firstHeaderHeight];
@@ -40,6 +47,11 @@
   CGSize contentSize = [super collectionViewContentSize];
   if (contentSize.height < minimumHeight) {
     contentSize.height = minimumHeight;
+    // Increases the minimum height to allow the page to scroll to the cached
+    // position.
+    if (self.offset > 0) {
+      contentSize.height += self.offset;
+    }
   }
   return contentSize;
 }

ios/chrome/browser/ui/content_suggestions/content_suggestions_view_controller.h

@@ -34,8 +34,10 @@ extern NSString* const
     : CollectionViewController <ContentSuggestionsCollectionControlling,
                                 ContentSuggestionsConsumer>
 
+// Inits view controller with |offset| to maintain scroll position if needed.
+// Offset is only required if Discover feed is enabled.
 - (instancetype)initWithStyle:(CollectionViewControllerStyle)style
-    NS_DESIGNATED_INITIALIZER;
+                       offset:(CGFloat)offset NS_DESIGNATED_INITIALIZER;
 
 - (instancetype)initWithLayout:(UICollectionViewLayout*)layout
                          style:(CollectionViewControllerStyle)style

ios/chrome/browser/ui/content_suggestions/content_suggestions_view_controller.mm

@@ -74,6 +74,10 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
 // The FeedView CollectionView contained by discoverFeedVC.
 @property(nonatomic, strong) UICollectionView* feedView;
 
+// Navigation offset applied to the layout height to maintain the scroll
+// position, since the feed height is dynamic.
+@property(nonatomic) CGFloat offset;
+
 @end
 
 @implementation ContentSuggestionsViewController
@@ -90,8 +94,11 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
 
 #pragma mark - Lifecycle
 
-- (instancetype)initWithStyle:(CollectionViewControllerStyle)style {
-  UICollectionViewLayout* layout = [[ContentSuggestionsLayout alloc] init];
+- (instancetype)initWithStyle:(CollectionViewControllerStyle)style
+                       offset:(CGFloat)offset {
+  _offset = offset;
+  UICollectionViewLayout* layout =
+      [[ContentSuggestionsLayout alloc] initWithOffset:offset];
   self = [super initWithLayout:layout style:style];
   if (self) {
     _collectionUpdater = [[ContentSuggestionsCollectionUpdater alloc] init];
@@ -290,6 +297,13 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
   // Resize the collection as it might have been rotated while not being
   // presented (e.g. rotation on stack view).
   [self updateConstraints];
+  // Remove forced height if it was already applied, since the scroll position
+  // was already maintained.
+  if (self.offset > 0) {
+    ContentSuggestionsLayout* layout = static_cast<ContentSuggestionsLayout*>(
+        self.collectionView.collectionViewLayout);
+    layout.offset = 0;
+  }
 }
 
 - (void)viewDidLayoutSubviews {

ios/showcase/content_suggestions/sc_content_suggestions_coordinator.mm

@@ -39,7 +39,8 @@
   _dataSource = [[SCContentSuggestionsDataSource alloc] init];
 
   _suggestionViewController = [[ContentSuggestionsViewController alloc]
-      initWithStyle:CollectionViewControllerStyleDefault];
+      initWithStyle:CollectionViewControllerStyleDefault
+             offset:0];
   [_suggestionViewController setDataSource:_dataSource];
 
   _suggestionViewController.suggestionCommandHandler =