[android] Introduce a (Mutable)LiveEvent for single-use LiveData

briar-android/src/main/java/org/briarproject/briar/android/contact/add/remote/AddContactActivity.java

@@ -49,8 +49,8 @@ public class AddContactActivity extends BriarActivity implements
 
 		viewModel = ViewModelProviders.of(this, viewModelFactory)
 				.get(AddContactViewModel.class);
-		viewModel.getRemoteLinkEntered().observe(this, entered -> {
-			if (entered != null && entered) {
+		viewModel.getRemoteLinkEntered().observeEvent(this, entered -> {
+			if (entered) {
 				NicknameFragment f = new NicknameFragment();
 				showNextFragment(f);
 			}

briar-android/src/main/java/org/briarproject/briar/android/contact/add/remote/AddContactViewModel.java

@@ -12,6 +12,8 @@ import org.briarproject.bramble.api.contact.ContactManager;
 import org.briarproject.bramble.api.db.DatabaseExecutor;
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.android.viewmodel.LiveEvent;
+import org.briarproject.briar.android.viewmodel.MutableLiveEvent;
 
 import java.util.concurrent.Executor;
 import java.util.logging.Logger;
@@ -35,8 +37,8 @@ public class AddContactViewModel extends AndroidViewModel {
 
 	private final MutableLiveData<String> handshakeLink =
 			new MutableLiveData<>();
-	private final MutableLiveData<Boolean> remoteLinkEntered =
-			new MutableLiveData<>();
+	private final MutableLiveEvent<Boolean> remoteLinkEntered =
+			new MutableLiveEvent<>();
 	private final MutableLiveData<Boolean> addContactResult =
 			new MutableLiveData<>();
 	@Nullable
@@ -81,14 +83,13 @@ public class AddContactViewModel extends AndroidViewModel {
 		return link != null && LINK_REGEX.matcher(link).find();
 	}
 
-	LiveData<Boolean> getRemoteLinkEntered() {
+	LiveEvent<Boolean> getRemoteLinkEntered() {
 		return remoteLinkEntered;
 	}
 
 	void onRemoteLinkEntered() {
 		if (remoteHandshakeLink == null) throw new IllegalStateException();
-		remoteLinkEntered.setValue(true);
-		remoteLinkEntered.postValue(false);  // reset, so we can navigate back
+		remoteLinkEntered.setEvent(true);
 	}
 
 	void addContact(String nickname) {

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

@@ -0,0 +1,61 @@
+package org.briarproject.briar.android.viewmodel;
+
+import android.arch.lifecycle.LifecycleOwner;
+import android.arch.lifecycle.LiveData;
+import android.arch.lifecycle.Observer;
+import android.support.annotation.Nullable;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@NotNullByDefault
+public class LiveEvent<T> extends LiveData<LiveEvent.ConsumableEvent<T>> {
+
+	public void observeEvent(LifecycleOwner owner,
+			LiveEventHandler<T> handler) {
+		LiveEventObserver<T> observer = new LiveEventObserver<>(handler);
+		super.observe(owner, observer);
+	}
+
+	public static class ConsumableEvent<T> {
+		private final T content;
+		private boolean consumed = false;
+
+		public ConsumableEvent(T content) {
+			this.content = content;
+		}
+
+		@Nullable
+		public T getContentIfNotConsumed() {
+			if (consumed) return null;
+			else {
+				consumed = true;
+				return content;
+			}
+		}
+	}
+
+	@Immutable
+	public static class LiveEventObserver<T>
+			implements Observer<ConsumableEvent<T>> {
+		private final LiveEventHandler<T> handler;
+
+		public LiveEventObserver(LiveEventHandler<T> handler) {
+			this.handler = handler;
+		}
+
+		@Override
+		public void onChanged(@Nullable ConsumableEvent<T> consumableEvent) {
+			if (consumableEvent != null) {
+				T content = consumableEvent.getContentIfNotConsumed();
+				if (content != null) handler.onEventUnconsumedContent(content);
+			}
+		}
+
+	}
+
+	public interface LiveEventHandler<T> {
+		void onEventUnconsumedContent(T t);
+	}
+}

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

@@ -0,0 +1,15 @@
+package org.briarproject.briar.android.viewmodel;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public class MutableLiveEvent<T> extends LiveEvent<T> {
+
+	public void postEvent(T value) {
+		super.postValue(new ConsumableEvent<>(value));
+	}
+
+	public void setEvent(T value) {
+		super.setValue(new ConsumableEvent<>(value));
+	}
+}