[ios] Remove use of default snapshot image

The default snapshot image is a simply a 2x2 white pixel image. This image is
unnecessarily passed around and unnecessarily complicates code.
This CL removes use of a default snapshot image, and updates the API
contract to use nil as the sentinel value for failed retrieval or
generation.

Bug: 918032
Change-Id: Ie7d8312cad6d05912474c6205c1bbb4ec9d242e0
Reviewed-on: https://chromium-review.googlesource.com/c/1392264Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619565}

ios/chrome/app/main_controller.mm

@@ -1423,21 +1423,18 @@ enum class EnterTabSwitcherSnapshotResult {
     BOOL loading = currentWebState->IsLoading();
     SnapshotTabHelper::FromWebState(currentWebState)
         ->UpdateSnapshotWithCallback(^(UIImage* snapshot) {
-          BOOL failed =
-              !snapshot ||
-              snapshot == SnapshotTabHelper::GetDefaultSnapshotImage();
           EnterTabSwitcherSnapshotResult snapshotResult;
-          if (loading && failed) {
+          if (loading && !snapshot) {
             snapshotResult =
                 EnterTabSwitcherSnapshotResult::kPageLoadingAndSnapshotFailed;
-          } else if (loading && !failed) {
+          } else if (loading && snapshot) {
             snapshotResult = EnterTabSwitcherSnapshotResult::
                 kPageLoadingAndSnapshotSucceeded;
-          } else if (!loading && failed) {
+          } else if (!loading && !snapshot) {
             snapshotResult = EnterTabSwitcherSnapshotResult::
                 kPageNotLoadingAndSnapshotFailed;
           } else {
-            DCHECK(!loading && !failed);
+            DCHECK(!loading && snapshot);
             snapshotResult = EnterTabSwitcherSnapshotResult::
                 kPageNotLoadingAndSnapshotSucceeded;
           }

ios/chrome/browser/snapshots/snapshot_generator.h

@@ -31,8 +31,7 @@ class WebState;
 - (CGSize)snapshotSize;
 
 // Gets a color snapshot for the current page, calling |callback| once it has
-// been retrieved or regenerated. If the snapshot cannot be generated, the
-// |callback| will be called with nil.
+// been retrieved. Invokes |callback| with nil if a snapshot does not exist.
 - (void)retrieveSnapshot:(void (^)(UIImage*))callback;
 
 // Gets a grey snapshot for the current page, calling |callback| once it has
@@ -59,9 +58,6 @@ class WebState;
 // Requests deletion of the current page snapshot from disk and memory.
 - (void)removeSnapshot;
 
-// Returns an image to use as replacement of a missing snapshot.
-+ (UIImage*)defaultSnapshotImage;
-
 // The SnapshotGenerator delegate.
 @property(nonatomic, weak) id<SnapshotGeneratorDelegate> delegate;
 

ios/chrome/browser/snapshots/snapshot_generator.mm

@@ -87,20 +87,11 @@ BOOL ViewHierarchyContainsWKWebView(UIView* view) {
 
 - (void)retrieveSnapshot:(void (^)(UIImage*))callback {
   DCHECK(callback);
-
-  void (^wrappedCallback)(UIImage*) = ^(UIImage* image) {
-    if (!image) {
-      image = [SnapshotGenerator defaultSnapshotImage];
-    }
-    callback(image);
-  };
-
-  SnapshotCache* snapshotCache = self.snapshotCache;
-  if (snapshotCache) {
-    [snapshotCache retrieveImageForSessionID:_snapshotSessionId
-                                    callback:wrappedCallback];
+  if (self.snapshotCache) {
+    [self.snapshotCache retrieveImageForSessionID:_snapshotSessionId
+                                         callback:callback];
   } else {
-    wrappedCallback(nil);
+    callback(nil);
   }
 }
 
@@ -128,12 +119,9 @@ BOOL ViewHierarchyContainsWKWebView(UIView* view) {
 
 - (UIImage*)updateSnapshot {
   UIImage* snapshot = [self generateSnapshotWithOverlays:YES];
-  // Return default snapshot without caching it if the generation failed.
-  if (!snapshot) {
-    return [[self class] defaultSnapshotImage];
+  if (snapshot) {
+    [self.snapshotCache setImage:snapshot withSessionID:_snapshotSessionId];
   }
-
-  [self.snapshotCache setImage:snapshot withSessionID:_snapshotSessionId];
   return snapshot;
 }
 
@@ -218,23 +206,6 @@ BOOL ViewHierarchyContainsWKWebView(UIView* view) {
   [self.snapshotCache removeImageWithSessionID:_snapshotSessionId];
 }
 
-+ (UIImage*)defaultSnapshotImage {
-  static UIImage* defaultSnapshotImage = nil;
-  if (!defaultSnapshotImage) {
-    CGRect frame = CGRectMake(0, 0, 2, 2);
-    UIGraphicsBeginImageContext(frame.size);
-    [[UIColor whiteColor] setFill];
-    CGContextFillRect(UIGraphicsGetCurrentContext(), frame);
-
-    UIImage* result = UIGraphicsGetImageFromCurrentImageContext();
-    UIGraphicsEndImageContext();
-
-    defaultSnapshotImage =
-        [result stretchableImageWithLeftCapWidth:1 topCapHeight:1];
-  }
-  return defaultSnapshotImage;
-}
-
 #pragma mark - Private methods
 
 // Returns the frame of the snapshot. Will return an empty rectangle if the

ios/chrome/browser/snapshots/snapshot_tab_helper.h

@@ -44,27 +44,31 @@ class SnapshotTabHelper : public infobars::InfoBarManager::Observer,
   // Retrieves a color snapshot for the current page, invoking |callback|
   // with the image. The callback may be called synchronously is there is
   // a cached snapshot available in memory, otherwise it will be invoked
-  // asynchronously after retrieved from disk or re-generated.
+  // asynchronously after retrieved from disk. Invokes |callback| with nil if a
+  // snapshot does not exist.
   void RetrieveColorSnapshot(void (^callback)(UIImage*));
 
   // Retrieves a grey snapshot for the current page, invoking |callback|
   // with the image. The callback may be called synchronously is there is
   // a cached snapshot available in memory, otherwise it will be invoked
-  // asynchronously after retrieved from disk or re-generated.
+  // asynchronously after retrieved from disk or re-generated. Invokes
+  // |callback| with nil if a snapshot does not exist.
   void RetrieveGreySnapshot(void (^callback)(UIImage*));
 
   // Asynchronously generates a new snapshot, updates the snapshot cache, and
-  // runs |callback| with the new snapshot image.
+  // invokes |callback| with the new snapshot image. Invokes |callback| with nil
+  // if snapshot generation fails.
   void UpdateSnapshotWithCallback(void (^callback)(UIImage*));
 
   // DEPRECATED(crbug.com/917929): Use the asynchronous function
   // |UpdateSnapshotWithCallback()| for all new callsites.
   // Generates a new snapshot, updates the snapshot cache, and returns the new
-  // snapshot image.
+  // snapshot image. Returns nil if snapshot generation fails.
   UIImage* UpdateSnapshot();
 
   // Generates a new snapshot without any overlays, and returns the new snapshot
-  // image. This does not update the snapshot cache.
+  // image. This does not update the snapshot cache. Returns nil if snapshot
+  // generation fails.
   UIImage* GenerateSnapshotWithoutOverlays();
 
   // Requests deletion of the current page snapshot from disk and memory.
@@ -73,9 +77,6 @@ class SnapshotTabHelper : public infobars::InfoBarManager::Observer,
   // Instructs the helper not to snapshot content for the next page load event.
   void IgnoreNextLoad();
 
-  // Returns an image to use as replacement of a missing snapshot.
-  static UIImage* GetDefaultSnapshotImage();
-
  private:
   SnapshotTabHelper(web::WebState* web_state, NSString* session_id);
 

ios/chrome/browser/snapshots/snapshot_tab_helper.mm

@@ -99,11 +99,6 @@ void SnapshotTabHelper::IgnoreNextLoad() {
   ignore_next_load_ = true;
 }
 
-// static
-UIImage* SnapshotTabHelper::GetDefaultSnapshotImage() {
-  return [SnapshotGenerator defaultSnapshotImage];
-}
-
 SnapshotTabHelper::SnapshotTabHelper(web::WebState* web_state,
                                      NSString* session_id)
     : web_state_(web_state),
@@ -158,8 +153,7 @@ void SnapshotTabHelper::PageLoaded(
                   return;
                 PageLoadedSnapshotResult snapshotResult =
                     PageLoadedSnapshotResult::kSnapshotSucceeded;
-                if (!snapshot ||
-                    snapshot == SnapshotTabHelper::GetDefaultSnapshotImage()) {
+                if (!snapshot) {
                   snapshotResult =
                       PageLoadedSnapshotResult::kSnapshotAttemptedAndFailed;
                 }

ios/chrome/browser/snapshots/snapshot_tab_helper_unittest.mm

@@ -164,8 +164,8 @@ TEST_F(SnapshotTabHelperTest, RetrieveColorSnapshotCachedSnapshot) {
   EXPECT_EQ(delegate_.snapshotTakenCount, 0u);
 }
 
-// Tests that RetrieveColorSnapshot returns the default snapshot image when
-// there is no cached snapshot and the WebState web usage is disabled.
+// Tests that RetrieveColorSnapshot returns nil when there is no cached snapshot
+// and the WebState web usage is disabled.
 TEST_F(SnapshotTabHelperTest, RetrieveColorSnapshotWebUsageDisabled) {
   web_state_.SetWebUsageEnabled(false);
 
@@ -181,15 +181,12 @@ TEST_F(SnapshotTabHelperTest, RetrieveColorSnapshotWebUsageDisabled) {
 
   run_loop.Run();
 
-  ASSERT_TRUE(snapshot);
-  EXPECT_TRUE(
-      UIImagesAreEqual(snapshot, SnapshotTabHelper::GetDefaultSnapshotImage()));
+  EXPECT_FALSE(snapshot);
   EXPECT_EQ(delegate_.snapshotTakenCount, 0u);
 }
 
-// Tests that RetrieveColorSnapshot returns the default snapshot image when
-// there is no cached snapshot and the delegate says it is not possible to
-// take a snapshot.
+// Tests that RetrieveColorSnapshot returns nil when there is no cached snapshot
+// and the delegate says it is not possible to take a snapshot.
 TEST_F(SnapshotTabHelperTest, RetrieveColorSnapshotCannotTakeSnapshot) {
   delegate_.canTakeSnapshot = YES;
 
@@ -205,9 +202,7 @@ TEST_F(SnapshotTabHelperTest, RetrieveColorSnapshotCannotTakeSnapshot) {
 
   run_loop.Run();
 
-  ASSERT_TRUE(snapshot);
-  EXPECT_TRUE(
-      UIImagesAreEqual(snapshot, SnapshotTabHelper::GetDefaultSnapshotImage()));
+  EXPECT_FALSE(snapshot);
   EXPECT_EQ(delegate_.snapshotTakenCount, 0u);
 }
 
@@ -234,8 +229,8 @@ TEST_F(SnapshotTabHelperTest, RetrieveGreySnapshotCachedSnapshot) {
   EXPECT_EQ(delegate_.snapshotTakenCount, 0u);
 }
 
-// Tests that RetrieveGreySnapshot returns the default snapshot image when
-// there is no cached snapshot and the WebState web usage is disabled.
+// Tests that RetrieveGreySnapshot returns nil when there is no cached snapshot
+// and the WebState web usage is disabled.
 TEST_F(SnapshotTabHelperTest, RetrieveGreySnapshotWebUsageDisabled) {
   web_state_.SetWebUsageEnabled(false);
 
@@ -251,14 +246,12 @@ TEST_F(SnapshotTabHelperTest, RetrieveGreySnapshotWebUsageDisabled) {
 
   run_loop.Run();
 
-  ASSERT_TRUE(snapshot);
-  EXPECT_TRUE(UIImagesAreEqual(
-      snapshot, GreyImage(SnapshotTabHelper::GetDefaultSnapshotImage())));
+  EXPECT_FALSE(snapshot);
   EXPECT_EQ(delegate_.snapshotTakenCount, 0u);
 }
 
-// Tests that RetrieveGreySnapshot returns the default snapshot image when
-// there is no cached snapshot and the WebState web usage is disabled.
+// Tests that RetrieveGreySnapshot returns nil when there is no cached snapshot
+// and the WebState web usage is disabled.
 TEST_F(SnapshotTabHelperTest, RetrieveGreySnapshotCannotTakeSnapshot) {
   delegate_.canTakeSnapshot = YES;
   base::RunLoop run_loop;
@@ -273,9 +266,7 @@ TEST_F(SnapshotTabHelperTest, RetrieveGreySnapshotCannotTakeSnapshot) {
 
   run_loop.Run();
 
-  ASSERT_TRUE(snapshot);
-  EXPECT_TRUE(UIImagesAreEqual(
-      snapshot, GreyImage(SnapshotTabHelper::GetDefaultSnapshotImage())));
+  EXPECT_FALSE(snapshot);
   EXPECT_EQ(delegate_.snapshotTakenCount, 0u);
 }
 

ios/chrome/browser/ui/browser_view_controller.mm

@@ -4195,7 +4195,7 @@ NSString* const kBrowserViewControllerSnackbarCategory =
         ->RetrieveColorSnapshot(^(UIImage* image) {
           if (PagePlaceholderTabHelper::FromWebState(webStateBeingActivated)
                   ->will_add_placeholder_for_next_navigation()) {
-            [swipeView setImage:SnapshotTabHelper::GetDefaultSnapshotImage()];
+            [swipeView setImage:nil];
           } else {
             [swipeView setImage:image];
           }

ios/chrome/browser/ui/side_swipe/card_side_swipe_view.mm

@@ -191,7 +191,7 @@ const CGFloat kResizeFactor = 4;
         if (PagePlaceholderTabHelper::FromWebState(tab.webState)
                 ->will_add_placeholder_for_next_navigation() &&
             !ios::device_util::IsSingleCoreDevice()) {
-          [card setImage:SnapshotTabHelper::GetDefaultSnapshotImage()];
+          [card setImage:nil];
           dispatch_async(priorityQueue, ^{
             UIImage* greyImage = [self smallGreyImage:image];
             dispatch_async(dispatch_get_main_queue(), ^{

ios/chrome/browser/ui/side_swipe/swipe_view.mm

@@ -40,6 +40,7 @@
     _imageView = [[UIImageView alloc] initWithFrame:CGRectZero];
     [_imageView setClipsToBounds:YES];
     [_imageView setContentMode:UIViewContentModeScaleAspectFill];
+    [_imageView setBackgroundColor:[UIColor whiteColor]];
     [self addSubview:_imageView];
 
     _topToolbarSnapshot = [[UIImageView alloc] initWithFrame:CGRectZero];