PlatformView Blur for Backdrop Filter

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm

@@ -33,6 +33,9 @@ - (BOOL)flt_hasFirstResponderInViewHierarchySubtree {
 @end
 
 namespace flutter {
+// Becomes NO if Apple's API changes and blurred backdrop filters cannot be applied.
+BOOL canApplyBlurBackdrop = YES;
+// int changingBlurRadius = 0; // TODO EMILY part of visual tests. delete before landing PR
 
 std::shared_ptr<FlutterPlatformViewLayer> FlutterPlatformViewLayerPool::GetLayer(
     GrDirectContext* gr_context,
@@ -414,12 +417,35 @@ - (BOOL)flt_hasFirstResponderInViewHierarchySubtree {
                                CGRectGetWidth(flutter_view.bounds),
                                CGRectGetHeight(flutter_view.bounds))] autorelease];
 
+  NSMutableArray* blurRadii = [[[NSMutableArray alloc] init] autorelease];
+
+  // TODO EMILY: this line is for visual simulator tests, delete before landing PR
+  //                        int numFilters = 0;
+
   auto iter = mutators_stack.Begin();
   while (iter != mutators_stack.End()) {
     switch ((*iter)->GetType()) {
       case kTransform: {
         CATransform3D transform = GetCATransform3DFromSkMatrix((*iter)->GetMatrix());
         finalTransform = CATransform3DConcat(transform, finalTransform);
+
+        //             TODO EMILY: these lines are for visual simulator tests, delete before landing
+        //             PR
+        //                                  if(numFilters < 1) {
+        //                                    flutter::DlBlurImageFilter filter =
+        //                                        flutter::DlBlurImageFilter(changingBlurRadius,
+        //                                        changingBlurRadius, flutter::DlTileMode::kDecal);
+        //
+        //                                    NSNumber* blurRadius =
+        //                                    @(filter.asBlur()->sigma_x());
+        //                                    [blurRadii addObject:blurRadius];
+        //
+        //                                    numFilters++;
+        //                                    FML_DLOG(ERROR) << "****checkpoint***" <<
+        //                                    changingBlurRadius;
+        //
+        //                                    changingBlurRadius++;
+        //                                  }
         break;
       }
       case kClipRect:
@@ -434,11 +460,25 @@ - (BOOL)flt_hasFirstResponderInViewHierarchySubtree {
       case kOpacity:
         embedded_view.alpha = (*iter)->GetAlphaFloat() * embedded_view.alpha;
         break;
-      case kBackdropFilter:
+      case kBackdropFilter: {
+        // We only support DlBlurImageFilter for BackdropFilter.
+        if ((*iter)->GetFilter().asBlur() && canApplyBlurBackdrop) {
+          // sigma_x is arbitrarily chosen as the radius value because Quartz sets
+          // sigma_x and sigma_y equal to each other. DlBlurImageFilter's Tile Mode
+          // is not supported in Quartz's gaussianBlur CAFilter, so it is not used
+          // to blur the PlatformView.
+          [blurRadii addObject:@((*iter)->GetFilter().asBlur()->sigma_x())];
+        }
         break;
+      }
     }
     ++iter;
   }
+
+  if (canApplyBlurBackdrop) {
+    canApplyBlurBackdrop = [clipView applyBlurBackdropFilters:blurRadii];
+  }
+
   // Reverse the offset of the clipView.
   // The clipView's frame includes the final translate of the final transform matrix.
   // So we need to revese this translate so the platform view can layout at the correct offset.