[android] fix concurrency issues when attachments are received delayed

Do not observe attachment live data multiple times
and don't miss received attachments in ImageActivity resp. ImageViewModel.

briar-android/src/main/java/org/briarproject/briar/android/conversation/ConversationActivity.java

@@ -1109,6 +1109,10 @@ public class ConversationActivity extends BriarActivity
 				attachmentRetriever.getAttachmentItems(h);
 		List<AttachmentItem> items = new ArrayList<>(liveDataList.size());
 		for (LiveData<AttachmentItem> liveData : liveDataList) {
+			// first remove all our observers to avoid having more than one
+			// in case we reload the conversation, e.g. after deleting messages
+			liveData.removeObservers(this);
+			// add a new observer
 			liveData.observe(this, new AttachmentObserver(h.getId(), liveData));
 			items.add(requireNonNull(liveData.getValue()));
 		}

briar-android/src/main/java/org/briarproject/briar/android/conversation/ImageActivity.java

@@ -103,9 +103,20 @@ public class ImageActivity extends BriarActivity
 			setSceneTransitionAnimation(transition, null, transition);
 		}
 
+		// Intent Extras
+		Intent i = getIntent();
+		attachments =
+				requireNonNull(i.getParcelableArrayListExtra(ATTACHMENTS));
+		int position = i.getIntExtra(ATTACHMENT_POSITION, -1);
+		if (position == -1) throw new IllegalStateException();
+		String name = i.getStringExtra(NAME);
+		long time = i.getLongExtra(DATE, 0);
+		byte[] messageIdBytes = requireNonNull(i.getByteArrayExtra(ITEM_ID));
+
 		// get View Model
 		viewModel = ViewModelProviders.of(this, viewModelFactory)
 				.get(ImageViewModel.class);
+		viewModel.expectAttachments(attachments);
 		viewModel.getSaveState().observeEvent(this,
 				this::onImageSaveStateChanged);
 
@@ -129,17 +140,11 @@ public class ImageActivity extends BriarActivity
 		TextView contactName = toolbar.findViewById(R.id.contactName);
 		TextView dateView = toolbar.findViewById(R.id.dateView);
 
-		// Intent Extras
-		Intent i = getIntent();
-		attachments = i.getParcelableArrayListExtra(ATTACHMENTS);
-		int position = i.getIntExtra(ATTACHMENT_POSITION, -1);
-		if (position == -1) throw new IllegalStateException();
-		String name = i.getStringExtra(NAME);
-		long time = i.getLongExtra(DATE, 0);
+		// Set contact name and message time
 		String date = formatDateAbsolute(this, time);
 		contactName.setText(name);
 		dateView.setText(date);
-		conversationMessageId = new MessageId(i.getByteArrayExtra(ITEM_ID));
+		conversationMessageId = new MessageId(messageIdBytes);
 
 		// Set up image ViewPager
 		viewPager = findViewById(R.id.viewPager);

briar-android/src/main/java/org/briarproject/briar/android/conversation/ImageFragment.java

@@ -95,8 +95,6 @@ public class ImageFragment extends Fragment
 
 		viewModel = ViewModelProviders.of(requireNonNull(getActivity()),
 				viewModelFactory).get(ImageViewModel.class);
-		viewModel.getOnAttachmentLoaded()
-				.observeEvent(this, this::onAttachmentLoaded);
 
 		photoView = v.findViewById(R.id.photoView);
 		photoView.setScaleLevels(1, 2, 4);
@@ -111,6 +109,9 @@ public class ImageFragment extends Fragment
 		} else {
 			photoView.setImageResource(R.drawable.ic_image_missing);
 			startPostponedTransition();
+			// state is not final, so observe state changes
+			viewModel.getOnAttachmentReceived(attachment.getMessageId())
+					.observeEvent(this, this::onAttachmentReceived);
 		}
 
 		return v;
@@ -127,8 +128,8 @@ public class ImageFragment extends Fragment
 				.into(photoView);
 	}
 
-	private void onAttachmentLoaded(MessageId messageId) {
-		if (attachment.getMessageId().equals(messageId)) loadImage();
+	private void onAttachmentReceived(Boolean received) {
+		if (received) loadImage();
 	}
 
 	@Override

briar-android/src/main/java/org/briarproject/briar/android/conversation/ImageViewModel.java

@@ -27,7 +27,9 @@ import java.io.InputStream;
 import java.io.OutputStream;
 import java.text.SimpleDateFormat;
 import java.util.Date;
+import java.util.List;
 import java.util.Locale;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.Executor;
 import java.util.logging.Logger;
 
@@ -40,6 +42,7 @@ import androidx.lifecycle.AndroidViewModel;
 import static android.media.MediaScannerConnection.scanFile;
 import static android.os.Environment.DIRECTORY_PICTURES;
 import static android.os.Environment.getExternalStoragePublicDirectory;
+import static java.util.Objects.requireNonNull;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.util.IoUtils.copyAndClose;
@@ -57,8 +60,10 @@ public class ImageViewModel extends AndroidViewModel implements EventListener {
 	@IoExecutor
 	private final Executor ioExecutor;
 
-	private final MutableLiveEvent<MessageId> attachmentLoaded =
-			new MutableLiveEvent<>();
+	private volatile boolean receivedAttachmentsInitialized = false;
+	private ConcurrentHashMap<MessageId, MutableLiveEvent<Boolean>>
+			receivedAttachments = new ConcurrentHashMap<>();
+
 	/**
 	 * true means there was an error saving the image, false if image was saved.
 	 */
@@ -91,13 +96,36 @@ public class ImageViewModel extends AndroidViewModel implements EventListener {
 	@Override
 	public void eventOccurred(Event e) {
 		if (e instanceof AttachmentReceivedEvent) {
-			attachmentLoaded
-					.postEvent(((AttachmentReceivedEvent) e).getMessageId());
+			MessageId id = ((AttachmentReceivedEvent) e).getMessageId();
+			MutableLiveEvent<Boolean> oldEvent;
+			if (receivedAttachmentsInitialized) {
+				oldEvent = receivedAttachments.get(id);
+			} else {
+				oldEvent = receivedAttachments
+						.putIfAbsent(id, new MutableLiveEvent<>(true));
+			}
+			if (oldEvent != null) oldEvent.postEvent(true);
 		}
 	}
 
-	LiveEvent<MessageId> getOnAttachmentLoaded() {
-		return attachmentLoaded;
+	@UiThread
+	public void expectAttachments(List<AttachmentItem> attachments) {
+		for (AttachmentItem item : attachments) {
+			// no need to track items that are in a final state already
+			if (item.getState().isFinal()) continue;
+			// add new live events, if not added concurrently by Event
+			MessageId id = item.getMessageId();
+			receivedAttachments.putIfAbsent(id, new MutableLiveEvent<>());
+		}
+		receivedAttachmentsInitialized = true;
+	}
+
+	@UiThread
+	LiveEvent<Boolean> getOnAttachmentReceived(MessageId messageId) {
+		if (receivedAttachments.size() == 0) {
+			throw new IllegalStateException("expectAttachments() not called");
+		}
+		return requireNonNull(receivedAttachments.get(messageId));
 	}
 
 	void clickImage() {

briar-android/src/main/java/org/briarproject/briar/android/viewmodel/LiveEvent.java

@@ -12,6 +12,22 @@ import androidx.lifecycle.Observer;
 @NotNullByDefault
 public class LiveEvent<T> extends LiveData<LiveEvent.ConsumableEvent<T>> {
 
+	/**
+	 * Creates a LiveEvent initialized with the given {@code value}.
+	 *
+	 * @param value initial value
+	 */
+	public LiveEvent(T value) {
+		super(new ConsumableEvent<>(value));
+	}
+
+	/**
+	 * Creates a LiveEvent with no value assigned to it.
+	 */
+	public LiveEvent() {
+		super();
+	}
+
 	public void observeEvent(LifecycleOwner owner,
 			LiveEventHandler<T> handler) {
 		LiveEventObserver<T> observer = new LiveEventObserver<>(handler);

briar-android/src/main/java/org/briarproject/briar/android/viewmodel/MutableLiveEvent.java

@@ -5,6 +5,22 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 @NotNullByDefault
 public class MutableLiveEvent<T> extends LiveEvent<T> {
 
+	/**
+	 * Creates a MutableLiveEvent initialized with the given {@code value}.
+	 *
+	 * @param value initial value
+	 */
+	public MutableLiveEvent(T value) {
+		super(value);
+	}
+
+	/**
+	 * Creates a MutableLiveEvent with no value assigned to it.
+	 */
+	public MutableLiveEvent() {
+		super();
+	}
+
 	public void postEvent(T value) {
 		super.postValue(new ConsumableEvent<>(value));
 	}