Allow observers to be removed.

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ConnectivityChecker.java

@@ -22,10 +22,21 @@ interface ConnectivityChecker {
 	 * the check succeeds. If a check is already running then the observer is
 	 * called when the check succeeds. If a connectivity check has recently
 	 * succeeded then the observer is called immediately.
+	 * <p>
+	 * Observers are removed after being called, or when the checker is
+	 * {@link #destroy() destroyed}.
 	 */
 	void checkConnectivity(MailboxProperties properties,
 			ConnectivityObserver o);
 
+	/**
+	 * Removes an observer that was added via
+	 * {@link #checkConnectivity(MailboxProperties, ConnectivityObserver)}. If
+	 * there are no remaining observers and a connectivity check is running
+	 * then the check will be cancelled.
+	 */
+	void removeObserver(ConnectivityObserver o);
+
 	interface ConnectivityObserver {
 		void onConnectivityCheckSucceeded();
 	}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ConnectivityCheckerImpl.java

@@ -80,8 +80,7 @@ abstract class ConnectivityCheckerImpl implements ConnectivityChecker {
 						> CONNECTIVITY_CHECK_FRESHNESS_MS) {
 					// The last connectivity check is stale, start a new one
 					connectivityObservers.add(o);
-					ApiCall task =
-							createConnectivityCheckTask(properties);
+					ApiCall task = createConnectivityCheckTask(properties);
 					connectivityCheck = mailboxApiCaller.retryWithBackoff(task);
 				} else {
 					// The last connectivity check is fresh
@@ -108,4 +107,16 @@ abstract class ConnectivityCheckerImpl implements ConnectivityChecker {
 			o.onConnectivityCheckSucceeded();
 		}
 	}
+
+	@Override
+	public void removeObserver(ConnectivityObserver o) {
+		synchronized (lock) {
+			if (destroyed) return;
+			connectivityObservers.remove(o);
+			if (connectivityObservers.isEmpty() && connectivityCheck != null) {
+				connectivityCheck.cancel();
+				connectivityCheck = null;
+			}
+		}
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitor.java

@@ -38,6 +38,12 @@ interface TorReachabilityMonitor {
 	 */
 	void addOneShotObserver(TorReachabilityObserver o);
 
+	/**
+	 * Removes an observer that was added via
+	 * {@link #addOneShotObserver(TorReachabilityObserver)}.
+	 */
+	void removeObserver(TorReachabilityObserver o);
+
 	interface TorReachabilityObserver {
 
 		void onTorReachable();

bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitorImpl.java

@@ -87,6 +87,14 @@ class TorReachabilityMonitorImpl
 		if (callNow) o.onTorReachable();
 	}
 
+	@Override
+	public void removeObserver(TorReachabilityObserver o) {
+		synchronized (lock) {
+			if (destroyed) return;
+			observers.remove(o);
+		}
+	}
+
 	@Override
 	public void eventOccurred(Event e) {
 		if (e instanceof TransportActiveEvent) {