Reader: fix unfollowed sites not being removed from list of followed sites

WordPress/Classes/Services/ReaderTopicService.h

@@ -241,6 +241,7 @@ extern NSString * const ReaderTopicFreshlyPressedPathCommponent;
 @end
 
 @interface ReaderTopicService (Tests)
+- (void)mergeFollowedSites:(NSArray *)sites withSuccess:(void (^)(void))success;
 - (void)mergeMenuTopics:(NSArray *)topics withSuccess:(void (^)(void))success;
 - (NSString *)formatTitle:(NSString *)str;
 @end

WordPress/Classes/Services/ReaderTopicService.m

@@ -51,14 +51,7 @@ - (void)fetchFollowedSitesWithSuccess:(void(^)(void))success failure:(void(^)(NS
 {
     ReaderTopicServiceRemote *service = [[ReaderTopicServiceRemote alloc] initWithWordPressComRestApi:[self apiForRequest]];
     [service fetchFollowedSitesWithSuccess:^(NSArray *sites) {
-        for (RemoteReaderSiteInfo *siteInfo in sites) {
-            [self siteTopicForRemoteSiteInfo:siteInfo];
-        }
-        [[ContextManager sharedInstance] saveContext:self.managedObjectContext withCompletionBlock:^{
-            if (success) {
-                success();
-            }
-        }];
+        [self mergeFollowedSites:sites withSuccess:success];
     } failure:^(NSError *error) {
         if (failure) {
             failure(error);
@@ -925,6 +918,39 @@ - (NSString *)formatTitle:(NSString *)str
     return [title capitalizedStringWithLocale:[NSLocale currentLocale]];
 }
 
+/**
+Saves the specified `ReaderSiteTopics`. Any `ReaderSiteTopics` not included in the passed
+array are marked as being unfollowed in Core Data.
+
+@param topics An array of `ReaderSiteTopics` to save.
+*/
+- (void)mergeFollowedSites:(NSArray *)sites withSuccess:(void (^)(void))success
+{
+     [self.managedObjectContext performBlock:^{
+         NSArray *currentSiteTopics = [self allSiteTopics];
+         NSMutableArray *remoteFeedIds = [NSMutableArray array];
+
+         for (RemoteReaderSiteInfo *siteInfo in sites) {
+             [remoteFeedIds addObject:siteInfo.feedID];
+             [self siteTopicForRemoteSiteInfo:siteInfo];
+         }
+
+         for (ReaderSiteTopic *siteTopic in currentSiteTopics) {
+             // If a site fetched from Core Data isn't included in the list of sites
+             // fetched from remote, that means it's no longer being followed.
+             if (![remoteFeedIds containsObject:siteTopic.feedID]) {
+                 siteTopic.following = NO;
+             }
+         }
+
+         [[ContextManager sharedInstance] saveContext:self.managedObjectContext withCompletionBlock:^{
+             if (success) {
+                 success();
+             }
+         }];
+     }];
+}
+
 /**
  Saves the specified `ReaderAbstractTopics`. Any `ReaderAbstractTopics` not included in the passed
  array are removed from Core Data.

WordPress/WordPressTest/ReaderTopicServiceTest.swift

@@ -95,6 +95,25 @@ final class ReaderTopicSwiftTest: XCTestCase {
         }
     }
 
+    func remoteSiteInfoForTests() -> [RemoteReaderSiteInfo] {
+        let foo = RemoteReaderSiteInfo()
+        foo.feedID = 1
+        foo.isFollowing = true
+        foo.postsEndpoint = "/sites/foo"
+
+        let bar = RemoteReaderSiteInfo()
+        bar.feedID = 2
+        bar.isFollowing = true
+        bar.postsEndpoint = "/sites/bar"
+
+        let baz = RemoteReaderSiteInfo()
+        baz.feedID = 3
+        baz.isFollowing = true
+        baz.postsEndpoint = "/sites/baz"
+
+        return [foo, bar, baz]
+    }
+
     func remoteTopicsForTests() -> [RemoteReaderTopic] {
         let foo = RemoteReaderTopic()
         foo.topicID = 1
@@ -138,6 +157,48 @@ final class ReaderTopicSwiftTest: XCTestCase {
 
     // MARK: Tests
 
+    /**
+    Ensure that followed sites a user unfollows from are set to unfollowed in core data when merging
+    results from the REST API.
+    */
+    func testUnfollowedSiteIsUnfollowedDuringSync() {
+        // Arrange: Setup
+        let remoteSites = remoteSiteInfoForTests()
+        let service = ReaderTopicService(managedObjectContext: context!)
+        let foo = remoteSites.first as RemoteReaderSiteInfo?
+
+        // Act: Save sites
+        var expect = expectation(description: "sites saved expectation")
+        service.mergeFollowedSites(remoteSites, withSuccess: { () -> Void in
+            expect.fulfill()
+        })
+        waitForExpectations(timeout: expectationTimeout, handler: nil)
+
+        // Assert: Sites exist in the context
+        let request = NSFetchRequest<NSFetchRequestResult>(entityName: ReaderSiteTopic.classNameWithoutNamespaces())
+        request.predicate = NSPredicate(format: "following = YES")
+        var count = try! context!.count(for: request)
+        XCTAssertEqual(count, remoteSites.count, "Number of sites in context did not match expectations")
+
+        // Act: Merge new set of sites
+        expect = expectation(description: "sites saved expectation")
+        service.mergeFollowedSites([foo!], withSuccess: { () -> Void in
+            expect.fulfill()
+        })
+        waitForExpectations(timeout: expectationTimeout, handler: nil)
+
+        // Assert: Unfollowed sites were unfollowed when merged
+        count = try! context!.count(for: request)
+        XCTAssertEqual(count, 1, "Number of sites in context did not match expectations")
+        do {
+            let results = try context!.fetch(request)
+            let site = results.first as! ReaderSiteTopic
+            XCTAssertEqual(site.feedID, foo?.feedID, "The site returned was not the one expected.")
+        } catch let error as NSError {
+            XCTAssertNil(error, "Error executing fetch request.")
+        }
+    }
+
     /**
     Ensure that topics a user unsubscribes from are removed from core data when merging
     results from the REST API.