Developing for Windows Phone and Xbox Live : Multiplayer Games (part 6) - Searching for an Available Network Session

Joining an Available Network Session

Finally, implement the JoinSession method, which joins one of the available network sessions, registesr for session events, and sets the next GameState. Add the following method to your game:

// Creates a session using a AvailableNetworkSession index
private void JoinSession(int sessionID)
{
    // Join an existing NetworkSession
    try
    {
        networkSession = NetworkSession.Join(availableSessions[sessionID]);
    }
    catch (NetworkSessionJoinException ex)
    {
        gameMessages.Add(new DisplayMessage("Faild to connect to session: " +
                                            ex.JoinError.ToString(),
                                            TimeSpan.FromSeconds(2)));

        // Check for sessons again
        FindSession();
    }
    // Register for NetworkSession events
    networkSession.GameStarted +=
             new EventHandler<GameStartedEventArgs>(networkSession_GameStarted);
    networkSession.GameEnded +=
             new EventHandler<GameEndedEventArgs>(networkSession_GameEnded);
    networkSession.GamerJoined +=
             new EventHandler<GamerJoinedEventArgs>(networkSession_GamerJoined);
    networkSession.GamerLeft +=
             new EventHandler<GamerLeftEventArgs>(networkSession_GamerLeft);
    networkSession.SessionEnded +=
             new EventHandler<NetworkSessionEndedEventArgs>(networkSessionSessionEnded);

    // Set the correct GameState. The NetworkSession may have already started a game.
    if (networkSession.SessionState == NetworkSessionState.Playing)
        gameState = GameState.PlayingGame;
    else
        gameState = GameState.GameLobby;
}

					  

The JoinSession method that you added to your game takes the index value into the availableSessions collection to use when joining the session. The NetworkSession.Join method takes a single argument, which is the AvailableNetworkSession to join. Join can throw a NetworkSessionJoinException if for some reason the session is not joinable anymore—for example, if the session becomes full between searching for available sessions and trying to join the session. If you receive the exception, you search again and print an error message.

After joining the session, you have a valid NetworkSession similar to when you created the session, so register for all of the same events. Finally, set the current gameState depending on whether the session you are joining is already playing the game or is in the lobby.

Running the sample now should enable you to create a session on one machine and join that session from another. The lobby should look similar to Figure 10.

Figure 10. Game lobby with multiple players

Both players can set their ready state, which should update on both machines. If the host presses the Start button, the game begins. In the game playing state, both players can move their name around the screen (see Figure 11).

Figure 11. Two players moving their names around the screen

If the host presses the Back button, the game ends and all players move back into the lobby. If the host presses the Back button again, the session ends and all players return to the main menu (see Figure 12).

Figure 12. Host exiting the session sends all players to the main menu

Sending Player Invites

Xbox LIVE enables you to connect with players from around the world. While playing a game, you might want to invite your friends to join you. You might also receive invitations from others to join their game. Adding support for game invites is tremendously easy with XNA Game Studio.

In the GameLobbyUpdate method, add the following code:

// Invite other players
if (ButtonPressed(Buttons.X) && !Guide.IsVisible)
    Guide.ShowGameInvite(PlayerIndex.One, null);

Guide.ShowGameInvite shows a list of the player’s friends to invite to the current game.

Now, you need to register for the NetworkSession.InviteAccepted event, which fires after a player accepts the invite.

// Register for the invite event
NetworkSession.InviteAccepted +=
                new EventHandler<InviteAcceptedEventArgs>(NetworkSessionInviteAccepted);

					  

Finally, implement the NetworkSession_InviteAccepted method.

void NetworkSession_InviteAccepted(object sender, InviteAcceptedEventArgs e)
{
    if (networkSession != null && !networkSession.IsDisposed)
        networkSession.Dispose();

    if (!e.IsCurrentSession)
        networkSession = NetworkSession.JoinInvited(1);
}

					  

First, NetworkSession_InviteAccepted disposes of any NetworkSession that might be active. Then, check to make sure you are not already in the session before you call the NetworkSession.JoinInvited method. This method takes the number of local gamers that are joining. Now, your game is ready for game invites.

Party Invites

A party allows for up to eight players to group together no matter which game they are playing. The players in the party can chat with each other even while playing different games. This is great if you want to chat with a friend while you play separate games.

While you are playing a game, you can view available network sessions of your party members that you can join. To display the list of sessions you can join, add the following to the MainMenuUpdate method:

// Show a party sessions the player can join
if (ButtonPressed(Buttons.Y) && !Guide.IsVisible)
    Guide.ShowPartySessions(PlayerIndex.One);

You can also invite the players in your party to your game session. To show the party screen from your game, add the following to the GameLobbyUpdate method:

// Show the party screen
if (ButtonPressed(Buttons.Y) && !Guide.IsVisible)
    Guide.ShowParty(PlayerIndex.One);

Simulating Real World Network Conditions

As expected, the conditions and speed that the network traffic performs in your game is slower than what you are currently seeing. Your connection from your PC to your Xbox 360 over your local area network is close to perfect. The physical distance is low and the amount of other traffic your packets have to deal with is relatively low. Out in the wild, Internet conditions can vary greatly. Your connection to another machine has a higher latency and you can experience packet loss.

Developers have a difficult time handling these conditions unless they could simulate them within their development environment. NetworkSession provides two properties that enable the developer to simulate real-world network conditions.

The NetworkSession.SimulatedLatency property is a TimeSpan, which is a simulated latency between the machines in the session. Games running on Xbox LIVE games are expected to handle latencies of 200 milliseconds while still enabling the game to be playable.

The NetworkSession.SimulatedPacketLoss property is a float value in the rage of 0 for no packet loss to 1 for 100 percent packet loss. Xbox LIVE games are expected to handle packet loss around 10 percent or a value of 0.1f.