Use Transformations.map and PagingDataTransforms.filter to remove read posts. It is an ugly design but I cannot think of a better way.

app/src/main/java/ml/docilealligator/infinityforreddit/adapters/PostRecyclerViewAdapter.java

@@ -2636,6 +2636,7 @@ public class PostRecyclerViewAdapter extends PagingDataAdapter<Post, RecyclerVie
                 if (mActivity != null && mActivity instanceof MarkPostAsReadInterface) {
                     ((MarkPostAsReadInterface) mActivity).markPostAsRead(post);
                 }
+                mFragment.addCurrentlyReadPostId(post.getId());
             }
         }
     }
@@ -3620,6 +3621,7 @@ public class PostRecyclerViewAdapter extends PagingDataAdapter<Post, RecyclerVie
                 if (mActivity != null && mActivity instanceof MarkPostAsReadInterface) {
                     ((MarkPostAsReadInterface) mActivity).markPostAsRead(post);
                 }
+                mFragment.addCurrentlyReadPostId(post.getId());
             }
         }
     }
@@ -3854,6 +3856,7 @@ public class PostRecyclerViewAdapter extends PagingDataAdapter<Post, RecyclerVie
                 if (mActivity != null && mActivity instanceof MarkPostAsReadInterface) {
                     ((MarkPostAsReadInterface) mActivity).markPostAsRead(post);
                 }
+                mFragment.addCurrentlyReadPostId(post.getId());
             }
         }
     }

app/src/main/java/ml/docilealligator/infinityforreddit/fragments/PostFragment.java

@@ -55,9 +55,11 @@ import org.greenrobot.eventbus.Subscribe;
 
 import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Random;
+import java.util.Set;
 import java.util.concurrent.Executor;
 
 import javax.inject.Inject;
@@ -250,6 +252,7 @@ public class PostFragment extends Fragment implements FragmentCommunicator {
     private float swipeActionThreshold;
     private ItemTouchHelper touchHelper;
     private ArrayList<ReadPost> readPosts;
+    private Set<String> currentlyReadPostIds = new HashSet<>();
     private Unbinder unbinder;
     private Map<String, String> subredditOrUserIcons = new HashMap<>();
 
@@ -1317,6 +1320,10 @@ public class PostFragment extends Fragment implements FragmentCommunicator {
         }
     }
 
+    public void addCurrentlyReadPostId(String id) {
+        currentlyReadPostIds.add(id);
+    }
+
     @Override
     public void changeNSFW(boolean nsfw) {
         postFilter.allowNSFW = !mSharedPreferences.getBoolean(SharedPreferencesUtils.DISABLE_NSFW_FOREVER, false) && nsfw;
@@ -1473,10 +1480,11 @@ public class PostFragment extends Fragment implements FragmentCommunicator {
 
     @Override
     public void hideReadPosts() {
-        if (mAdapter != null) {
+        /*if (mAdapter != null) {
             mAdapter.prepareToHideReadPosts();
             refreshAdapter();
-        }
+        }*/
+        mPostViewModel.setCurrentlyReadPostIds(currentlyReadPostIds);
     }
 
     @Override

app/src/main/java/ml/docilealligator/infinityforreddit/post/PostViewModel.java

@@ -14,9 +14,12 @@ import androidx.lifecycle.ViewModelProvider;
 import androidx.paging.Pager;
 import androidx.paging.PagingConfig;
 import androidx.paging.PagingData;
+import androidx.paging.PagingDataTransforms;
 import androidx.paging.PagingLiveData;
 
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 import java.util.concurrent.Executor;
 
 import ml.docilealligator.infinityforreddit.SortType;
@@ -39,8 +42,10 @@ public class PostViewModel extends ViewModel {
     private PostFilter postFilter;
     private String userWhere;
     private List<ReadPost> readPostList;
+    private MutableLiveData<Set<String>> currentlyReadPostIdsLiveData = new MutableLiveData<>();
 
     private LiveData<PagingData<Post>> posts;
+    private LiveData<PagingData<Post>> postsWithReadPostsHidden;
 
     private MutableLiveData<SortType> sortTypeLiveData;
     private MutableLiveData<PostFilter> postFilterLiveData;
@@ -74,6 +79,15 @@ public class PostViewModel extends ViewModel {
                     sortTypeLiveData.getValue(), postFilterLiveData.getValue());
             return PagingLiveData.cachedIn(PagingLiveData.getLiveData(pager), ViewModelKt.getViewModelScope(this));
         });
+
+        postsWithReadPostsHidden = PagingLiveData.cachedIn(Transformations.switchMap(currentlyReadPostIdsLiveData,
+                currentlyReadPostIds -> Transformations.map(
+                        posts,
+                        postPagingData -> PagingDataTransforms.filter(
+                                postPagingData, executor,
+                                post -> !currentlyReadPostIds.contains(post.getId())))), ViewModelKt.getViewModelScope(this));
+
+        currentlyReadPostIdsLiveData.setValue(new HashSet<>());
     }
 
     public PostViewModel(Executor executor, Retrofit retrofit, String accessToken, String accountName,
@@ -106,6 +120,15 @@ public class PostViewModel extends ViewModel {
                     sortTypeLiveData.getValue(), postFilterLiveData.getValue());
             return PagingLiveData.cachedIn(PagingLiveData.getLiveData(pager), ViewModelKt.getViewModelScope(this));
         });
+
+        postsWithReadPostsHidden = PagingLiveData.cachedIn(Transformations.switchMap(currentlyReadPostIdsLiveData,
+                currentlyReadPostIds -> Transformations.map(
+                        posts,
+                        postPagingData -> PagingDataTransforms.filter(
+                                postPagingData, executor,
+                                post -> !currentlyReadPostIds.contains(post.getId())))), ViewModelKt.getViewModelScope(this));
+
+        currentlyReadPostIdsLiveData.setValue(new HashSet<>());
     }
 
     public PostViewModel(Executor executor, Retrofit retrofit, String accessToken, String accountName,
@@ -140,6 +163,15 @@ public class PostViewModel extends ViewModel {
                     sortTypeLiveData.getValue(), postFilterLiveData.getValue());
             return PagingLiveData.cachedIn(PagingLiveData.getLiveData(pager), ViewModelKt.getViewModelScope(this));
         });
+
+        postsWithReadPostsHidden = PagingLiveData.cachedIn(Transformations.switchMap(currentlyReadPostIdsLiveData,
+                currentlyReadPostIds -> Transformations.map(
+                        posts,
+                        postPagingData -> PagingDataTransforms.filter(
+                                postPagingData, executor,
+                                post -> !currentlyReadPostIds.contains(post.getId())))), ViewModelKt.getViewModelScope(this));
+
+        currentlyReadPostIdsLiveData.setValue(new HashSet<>());
     }
 
     public PostViewModel(Executor executor, Retrofit retrofit, String accessToken, String accountName,
@@ -174,10 +206,23 @@ public class PostViewModel extends ViewModel {
                     sortTypeLiveData.getValue(), postFilterLiveData.getValue());
             return PagingLiveData.cachedIn(PagingLiveData.getLiveData(pager), ViewModelKt.getViewModelScope(this));
         });
+
+        postsWithReadPostsHidden = PagingLiveData.cachedIn(Transformations.switchMap(currentlyReadPostIdsLiveData,
+                currentlyReadPostIds -> Transformations.map(
+                        posts,
+                        postPagingData -> PagingDataTransforms.filter(
+                                postPagingData, executor,
+                                post -> !currentlyReadPostIds.contains(post.getId())))), ViewModelKt.getViewModelScope(this));
+
+        currentlyReadPostIdsLiveData.setValue(new HashSet<>());
     }
 
     public LiveData<PagingData<Post>> getPosts() {
-        return posts;
+        return postsWithReadPostsHidden;
+    }
+
+    public void setCurrentlyReadPostIds(Set<String> currentlyReadPostIds) {
+        currentlyReadPostIdsLiveData.setValue(currentlyReadPostIds);
     }
 
     public PostPagingSource returnPagingSoruce() {