Delete messages by swiping and add snackbar for undoing deletion.

app/src/main/java/com/github/gotify/messages/ListMessageAdapter.java

@@ -48,11 +48,11 @@ public class ListMessageAdapter extends RecyclerView.Adapter<ListMessageAdapter.
                         .build();
     }
 
-    public MessageWithImage getItem(int position) {
-        return items.get(position);
+    public List<MessageWithImage> getItems() {
+        return items;
     }
 
-    void items(List<MessageWithImage> items) {
+    public void setItems(List<MessageWithImage> items) {
         this.items = items;
     }
 

app/src/main/java/com/github/gotify/messages/MessagesActivity.java

@@ -58,8 +58,11 @@
 import com.github.gotify.messages.provider.MessageFacade;
 import com.github.gotify.messages.provider.MessageState;
 import com.github.gotify.messages.provider.MessageWithImage;
+import com.github.gotify.messages.provider.PositionPair;
 import com.github.gotify.service.WebSocketService;
 import com.google.android.material.navigation.NavigationView;
+import com.google.android.material.snackbar.BaseTransientBottomBar;
+import com.google.android.material.snackbar.Snackbar;
 import com.squareup.picasso.OkHttp3Downloader;
 import com.squareup.picasso.Picasso;
 import com.squareup.picasso.Target;
@@ -212,8 +215,8 @@ public void openDocumentation() {
         startActivity(browserIntent);
     }
 
-    public void delete(Message message) {
-        new DeleteMessage().execute(message);
+    public void commitDelete() {
+        new CommitDeleteMessage().execute();
     }
 
     protected void onUpdateApps(List<Application> applications) {
@@ -360,6 +363,54 @@ protected void onDestroy() {
         picasso.shutdown();
     }
 
+    private void scheduleDeletion(int position, Message message) {
+        ListMessageAdapter adapter = (ListMessageAdapter) messagesView.getAdapter();
+
+        messages.deleteLocal(message);
+        adapter.setItems(messages.get(appId));
+        adapter.notifyItemRemoved(position);
+
+        showDeletionSnackbar();
+    }
+
+    private void undoDelete() {
+        PositionPair positionPair = messages.undoDeleteLocal();
+
+        if (positionPair != null) {
+            ListMessageAdapter adapter = (ListMessageAdapter) messagesView.getAdapter();
+            adapter.setItems(messages.get(appId));
+            int insertPosition =
+                    appId == MessageState.ALL_MESSAGES
+                            ? positionPair.getAllPosition()
+                            : positionPair.getAppPosition();
+            adapter.notifyItemInserted(insertPosition);
+            messagesView.smoothScrollToPosition(insertPosition);
+        }
+    }
+
+    private void showDeletionSnackbar() {
+        View view = swipeRefreshLayout;
+        Snackbar snackbar = Snackbar.make(view, R.string.snackbar_deleted, Snackbar.LENGTH_LONG);
+        snackbar.setAction(R.string.snackbar_undo, v -> undoDelete());
+        snackbar.addCallback(new SnackbarCallback());
+        snackbar.show();
+    }
+
+    private class SnackbarCallback extends BaseTransientBottomBar.BaseCallback<Snackbar> {
+        @Override
+        public void onDismissed(Snackbar transientBottomBar, int event) {
+            super.onDismissed(transientBottomBar, event);
+            if (event != DISMISS_EVENT_ACTION && event != DISMISS_EVENT_CONSECUTIVE) {
+                // Execute deletion when the snackbar disappeared without pressing the undo button
+                // DISMISS_EVENT_CONSECUTIVE should be excluded as well, because it would cause the
+                // deletion to be sent to the server twice, since the deletion is sent to the server
+                // in MessageFacade if a message is deleted while another message was already
+                // waiting for deletion.
+                MessagesActivity.this.commitDelete();
+            }
+        }
+    }
+
     private class SwipeToDeleteCallback extends ItemTouchHelper.SimpleCallback {
         private ListMessageAdapter adapter;
         private Drawable icon;
@@ -394,8 +445,8 @@ public boolean onMove(
         @Override
         public void onSwiped(@NonNull RecyclerView.ViewHolder viewHolder, int direction) {
             int position = viewHolder.getAdapterPosition();
-            Message message = adapter.getItem(position).message;
-            MessagesActivity.this.delete(message);
+            MessageWithImage message = adapter.getItems().get(position);
+            scheduleDeletion(position, message.message);
         }
 
         @Override
@@ -561,11 +612,11 @@ protected void onPostExecute(Void data) {
         }
     }
 
-    private class DeleteMessage extends AsyncTask<Message, Void, Void> {
+    private class CommitDeleteMessage extends AsyncTask<Void, Void, Void> {
 
         @Override
-        protected Void doInBackground(Message... messages) {
-            MessagesActivity.this.messages.delete(first(messages));
+        protected Void doInBackground(Void... messages) {
+            MessagesActivity.this.messages.commitDelete();
             return null;
         }
 
@@ -648,7 +699,7 @@ private void updateMessagesAndStopLoading(List<MessageWithImage> messageWithImag
         }
 
         ListMessageAdapter adapter = (ListMessageAdapter) messagesView.getAdapter();
-        adapter.items(messageWithImages);
+        adapter.setItems(messageWithImages);
         adapter.notifyDataSetChanged();
     }
 

app/src/main/java/com/github/gotify/messages/provider/MessageFacade.java

@@ -10,6 +10,7 @@ public class MessageFacade {
     private final MessageRequester requester;
     private final MessageStateHolder state;
     private final MessageImageCombiner combiner;
+    private Message messagePendingDeletion;
 
     public MessageFacade(MessageApi api, ApplicationHolder applicationHolder) {
         this.applicationHolder = applicationHolder;
@@ -33,6 +34,13 @@ public List<MessageWithImage> loadMore(Integer appId) {
         if (state.hasNext || !state.loaded) {
             PagedMessages pagedMessages = requester.loadMore(state);
             this.state.newMessages(appId, pagedMessages);
+
+            // If there is a message with pending removal, it should not reappear in the list when
+            // reloading. Thus, it needs to be removed from the local list again after loading new
+            // messages.
+            if (messagePendingDeletion != null) {
+                this.state.removeMessage(messagePendingDeletion);
+            }
         }
         return get(appId);
     }
@@ -52,9 +60,24 @@ public int getLastReceivedMessage() {
         return state.getLastReceivedMessage();
     }
 
-    public void delete(Message message) {
-        this.requester.asyncRemoveMessage(message);
+    public void deleteLocal(Message message) {
         this.state.removeMessage(message);
+        // If there is already a deletion pending, that one should be executed before scheduling the
+        // next deletion.
+        if (messagePendingDeletion != null) {
+            commitDelete();
+        }
+        messagePendingDeletion = message;
+    }
+
+    public void commitDelete() {
+        this.requester.asyncRemoveMessage(messagePendingDeletion);
+        messagePendingDeletion = null;
+    }
+
+    public PositionPair undoDeleteLocal() {
+        messagePendingDeletion = null;
+        return this.state.undoLastRemoveMessage();
     }
 
     public boolean deleteAll(Integer appId) {

app/src/main/java/com/github/gotify/messages/provider/MessageStateHolder.java

@@ -9,6 +9,10 @@ class MessageStateHolder {
     private int lastReceivedMessage = -1;
     private Map<Integer, MessageState> states = new HashMap<>();
 
+    private Message lastRemovedMessage;
+    private int lastRemovedAllPosition;
+    private int lastRemovedAppPosition;
+
     synchronized void clear() {
         states = new HashMap<>();
     }
@@ -30,17 +34,7 @@ synchronized void newMessages(Integer appId, PagedMessages pagedMessages) {
     }
 
     synchronized void newMessage(Message message) {
-        MessageState allMessages = state(MessageState.ALL_MESSAGES);
-        MessageState appMessages = state(message.getAppid());
-
-        if (allMessages.loaded) {
-            allMessages.messages.add(0, message);
-        }
-
-        if (appMessages.loaded) {
-            appMessages.messages.add(0, message);
-        }
-
+        addMessage(message, 0, 0);
         lastReceivedMessage = message.getId();
     }
 
@@ -77,11 +71,45 @@ void removeMessage(Message message) {
         MessageState appMessages = state(message.getAppid());
 
         if (allMessages.loaded) {
-            allMessages.messages.remove(message);
+            int allPosition = allMessages.messages.indexOf(message);
+            allMessages.messages.remove(allPosition);
+            lastRemovedAllPosition = allPosition;
         }
 
         if (appMessages.loaded) {
-            appMessages.messages.remove(message);
+            int appPosition = appMessages.messages.indexOf(message);
+            appMessages.messages.remove(appPosition);
+            lastRemovedAppPosition = appPosition;
+        }
+
+        lastRemovedMessage = message;
+    }
+
+    PositionPair undoLastRemoveMessage() {
+        PositionPair result = null;
+
+        if (lastRemovedMessage != null) {
+            addMessage(lastRemovedMessage, lastRemovedAllPosition, lastRemovedAppPosition);
+            result = new PositionPair(lastRemovedAllPosition, lastRemovedAppPosition);
+
+            lastRemovedMessage = null;
+            lastRemovedAllPosition = -1;
+            lastRemovedAppPosition = -1;
+        }
+
+        return result;
+    }
+
+    private void addMessage(Message message, int allPosition, int appPosition) {
+        MessageState allMessages = state(MessageState.ALL_MESSAGES);
+        MessageState appMessages = state(message.getAppid());
+
+        if (allMessages.loaded && allPosition != -1) {
+            allMessages.messages.add(allPosition, message);
+        }
+
+        if (appMessages.loaded && appPosition != -1) {
+            appMessages.messages.add(appPosition, message);
         }
     }
 }

app/src/main/java/com/github/gotify/messages/provider/PositionPair.java

@@ -0,0 +1,19 @@
+package com.github.gotify.messages.provider;
+
+public final class PositionPair {
+    private final int allPosition;
+    private final int appPosition;
+
+    public PositionPair(int allPosition, int appPosition) {
+        this.allPosition = allPosition;
+        this.appPosition = appPosition;
+    }
+
+    public int getAllPosition() {
+        return allPosition;
+    }
+
+    public int getAppPosition() {
+        return appPosition;
+    }
+}

app/src/main/res/values/strings.xml

@@ -25,6 +25,8 @@
     <string name="grouped_message">Received %d messages while being disconnected</string>
     <string name="delete_all">Delete all</string>
     <string name="delete_logs">Delete logs</string>
+    <string name="snackbar_deleted">Message deleted</string>
+    <string name="snackbar_undo">Undo</string>
     <string name="all_messages">All Messages</string>
     <string name="logs">Logs</string>
     <string name="message_image_desc">The image of a message</string>