diff --git a/core/src/main/java/org/mercury_im/messenger/core/centers/ConnectionCenter.java b/core/src/main/java/org/mercury_im/messenger/core/centers/ConnectionCenter.java
index 3e19f83..2f3e6e8 100644
--- a/core/src/main/java/org/mercury_im/messenger/core/centers/ConnectionCenter.java
+++ b/core/src/main/java/org/mercury_im/messenger/core/centers/ConnectionCenter.java
@@ -30,6 +30,7 @@ import javax.inject.Inject;
 import javax.inject.Singleton;
 
 import io.reactivex.disposables.CompositeDisposable;
+import io.reactivex.disposables.Disposable;
 import io.reactivex.functions.Consumer;
 import io.reactivex.schedulers.Schedulers;
 
@@ -82,50 +83,54 @@ public class ConnectionCenter {
         }
 
         // otherwise subscribe to accounts and create connections.
-        disposable.add(
-                accountRepository.getAllAccounts()
-                        .observeOn(Schedulers.io())
-                        .subscribeOn(Schedulers.computation())
-                        .subscribe((Consumer<List<? extends AccountModel>>) accounts -> {
-                            LOGGER.log(Level.INFO, "Accounts changed.");
-                            Set<Long> accountIds = new HashSet<>();
-                            // Add missing connections to the map
-                            for (AccountModel account : accounts) {
-                                accountIds.add(account.getId());
-                                if (connectionMap.get(account.getId()) != null) {
-                                    continue;
-                                }
+        Disposable allAccounts = accountRepository.getAllAccounts()
+                .observeOn(Schedulers.io())
+                .subscribeOn(Schedulers.computation())
+                .subscribe((Consumer<List<? extends AccountModel>>) accounts -> {
+                    LOGGER.log(Level.INFO, "Accounts changed.");
+                    Set<Long> accountIds = new HashSet<>();
 
-                                LOGGER.log(Level.INFO, "Add new connection " + account.getJid().toString() + " to ConnectionCenter list.");
-                                MercuryConnection connection = createConnection(account);
-                                connectionMap.put(account.getId(), connection);
+                    // Add missing connections to the map
+                    for (AccountModel account : accounts) {
+                        accountIds.add(account.getId());
+                        if (connectionMap.get(account.getId()) != null) {
+                            continue;
+                        }
 
-                                RosterStore rosterStore = new RosterStore(rosterRepository);
-                                rosterStore.setAccountId(account.getId());
-                                rosterStore.subscribe();
-                                connection.getRoster().setRosterStore(rosterStore);
+                        LOGGER.log(Level.INFO, "Add new connection " + account.getJid().toString() + " to ConnectionCenter list.");
+                        MercuryConnection connection = createConnection(account);
+                        connectionMap.put(account.getId(), connection);
 
-                                messageStore.registerForMercuryConnection(connection);
+                        // initialize new connection
+                        RosterStore rosterStore = new RosterStore(rosterRepository);
+                        rosterStore.setAccountId(account.getId());
+                        rosterStore.subscribe();
+                        connection.getRoster().setRosterStore(rosterStore);
 
-                                if (account.getEnabled()) {
-                                    LOGGER.log(Level.INFO, "Connecting...");
-                                    connection.connect();
-                                    LOGGER.log(Level.INFO, "Connected!");
-                                } else {
-                                    LOGGER.log(Level.INFO, "Account not enabled. Skip.");
-                                }
-                            }
+                        messageStore.registerForMercuryConnection(connection);
+
+                        if (account.getEnabled()) {
+                            LOGGER.log(Level.INFO, "Connecting...");
+                            connection.connect();
+                            LOGGER.log(Level.INFO, "Connected!");
+                        } else {
+                            LOGGER.log(Level.INFO, "Account not enabled. Skip.");
+                        }
+                    }
+
+                    // Remove unwanted connections from the map
+                    for (long connectionId : connectionMap.keySet()) {
+                        if (!accountIds.contains(connectionId)) {
+                            AbstractXMPPConnection con =
+                                    (AbstractXMPPConnection) connectionMap.get(connectionId).getConnection();
+                            con.disconnect();
+                            connectionMap.remove(connectionId);
+                        }
+                    }
+                });
+
+        disposable.add(allAccounts);
 
-                            // Remove unwanted connections from the map
-                            for (long connectionId : connectionMap.keySet()) {
-                                if (!accountIds.contains(connectionId)) {
-                                    AbstractXMPPConnection con =
-                                            (AbstractXMPPConnection) connectionMap.get(connectionId).getConnection();
-                                    con.disconnect();
-                                    connectionMap.remove(connectionId);
-                                }
-                            }
-                        }));
     }
 
     public MercuryConnection getConnection(AccountModel account) {
@@ -140,6 +145,15 @@ public class ConnectionCenter {
         connectionMap.put(connection.getAccountId(), connection);
     }
 
+    /**
+     * Create a new {@link MercuryConnection} with an underlying {@link XMPPTCPConnection}
+     * from the credentials of an {@link AccountModel}.
+     * The new connection will not be connected.
+     *
+     * @param accountModel accountModel
+     *
+     * @return disconnected mercury connection
+     */
     public MercuryConnection createConnection(AccountModel accountModel) {
         LOGGER.log(Level.INFO, "Create Connection for " + accountModel.getJid().toString());
         XMPPTCPConnectionConfiguration configuration = XMPPTCPConnectionConfiguration.builder()
@@ -154,6 +168,11 @@ public class ConnectionCenter {
         return new MercuryConnection(tcpConnection, accountModel.getId());
     }
 
+    /**
+     * Set Client State Indication status to active.
+     *
+     * @see <a href="https://xmpp.org/extensions/xep-0352.html">XEP-0352: Client State Indication</a>
+     */
     public void clientStateActive() {
         LOGGER.log(Level.INFO, "CSI: App is going to foreground -> active");
         for (MercuryConnection mercuryConnection : connectionMap.values()) {
@@ -161,15 +180,18 @@ public class ConnectionCenter {
             if (connection.isConnected() && ClientStateIndicationManager.isSupported(connection)) {
                 try {
                     ClientStateIndicationManager.active(mercuryConnection.getConnection());
-                } catch (SmackException.NotConnectedException e) {
-                    e.printStackTrace();
-                } catch (InterruptedException e) {
+                } catch (SmackException.NotConnectedException | InterruptedException e) {
                     e.printStackTrace();
                 }
             }
         }
     }
 
+    /**
+     * Set Client State Indication status to inactive.
+     *
+     * @see <a href="https://xmpp.org/extensions/xep-0352.html">XEP-0352: Client State Indication</a>
+     */
     public void clientStateInactive() {
         LOGGER.log(Level.INFO, "CSI: App is going to background -> inactive");
         for (MercuryConnection mercuryConnection : connectionMap.values()) {
@@ -177,9 +199,7 @@ public class ConnectionCenter {
             if (connection.isConnected() && ClientStateIndicationManager.isSupported(connection)) {
                 try {
                     ClientStateIndicationManager.inactive(connection);
-                } catch (SmackException.NotConnectedException e) {
-                    e.printStackTrace();
-                } catch (InterruptedException e) {
+                } catch (SmackException.NotConnectedException | InterruptedException e) {
                     e.printStackTrace();
                 }
             }