1
Session 16.2
Network Games and
State
Chapter 16.2: Network Games and State
2
Session Overview
 We now know the fundamentals of network
communication
 Now we are going to see how XNA uses the
underlying network to provide network game play
 We are going to discover the ways in which you can
use XNA to create connected games and how to
create a state machine to manage a network game
 We are also going to find out how C# programs can
bind to events caused by the network
Chapter 16.2: Network Games and State
3
Xbox Live
 “Xbox Live” is the name for the multiplayer gaming
interface which provides network game play
 On the Windows PC it is know as “Games for
Windows – Live”
 Paid up members have a unique gamertag that
gives them an identity (and an avatar) on the
network
 Xbox Live contains Xbox Live Arcade where
members can buy and download games, including
some created using XNA
Chapter 16.2: Network Games and State
4
XNA and Xbox Live
 XNA games can use Xbox Live in exactly the same
way as other Xbox games
 XNA games that you write can be uploaded as
“Indie Games” for purchase and download from the
Xbox Live Arcade
 This is a great way to get started as a game
developer
 You have to pay $99 to become a “Premium”
member of the XNA Creators Club to do this
Chapter 16.2: Network Games and State
XNA and Zune Networked Game Play
 The Zune music player will run XNA games
 It also has WIFI networking built in
 XNA games can use this to create network game
play between Zune devices
 The network connections that are set up and
managed during the game are exactly the same
way for a Zune as any other XNA device
 This makes it very easy to create portable,
networked game play
5
Chapter 16.2: Network Games and State
6
Connected XNA Games
 XNA provides a set of classes and types that are
used to set up network games
 Our network games will use these classes to initiate
network connections, set up games, and then pass
game information between the systems in the
game
 The type of game we are going to use lets Xbox and
Windows PC gamers play against each other
 However, Zunes can only link to other Zunes
Chapter 16.2: Network Games and State
7
“System Link” XNA Games
 While it is possible to create fully featured Xbox Live
games using XNA we are going to start with a
simpler version
 A “System Link” game lets machines on the same
local area network connect for game play
 These network connections are simple to set up and
do not require Xbox Live membership to work
 System Link is the only form of network game play
that is supported by the Zune device
Chapter 16.2: Network Games and State
8
Gamer Profiles and Gamer Tags
 Xbox users can create user profiles on their
consoles to store their game play achievements
 These profiles can be linked to the Xbox Live Gamer
Tags
 On the Windows PC you can create Gamer Profiles
that work in the same way
 A “System Link” game uses these profiles to locate
players connected to the same local area network
 Fully-featured Xbox Live games use Xbox Live
servers to locate players
Chapter 16.2: Network Games and State
9
Setting Up an XNA Networked Game
 Before you can create a networked game you need
to be signed in on your system
 You use a particular Gamer Profile to sign in
 This sign-in provides you with an identity on the
network that others can find and invite to take part
in games
 It is the profile that is addressed, not the computer
or console itself
Chapter 16.2: Network Games and State
10
Managing Game Profiles in XNA
 The Xbox 360 provides a “Guide” interface that
players can use to create and manage their gamer
profile
 You press the X in the middle of the gamepad to open
up the Guide and interact with it
 Windows PC users have the same facility in their
XNA games
 They can start the guide by pressing the Home key on
the keyboard
Chapter 16.2: Network Games and State
11
Loading the GamerServices Component
public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = "Content";
this.Components.Add(new GamerServicesComponent(this));
}
 To get the Guide to work we have to load the
GamerServicesComponent to a game program
 We can do this in the constructor for the game
 Once this component is present, the Home key can
be used to start the Guide on a Windows PC
Chapter 16.2: Network Games and State
12
1. Guide Demo
 If you add the GamerServicesComponent
to the game you can use
the Home key to open up
Guide on a Windows PC
the
Chapter 16.2: Network Games and State
13
Ensuring a Gamer Is Signed In
if (Gamer.SignedInGamers.Count == 0)
{
if (Guide.IsVisible == false)
{
Guide.ShowSignIn(1, false);
}
}
 XNA provides a type called Gamer that is used to
manage network gaming
 This provides a property called SignedInGamers
that contains a collection of all the gamers signed in
 If this is empty there is nobody signed in
Chapter 16.2: Network Games and State
14
Ensuring a Gamer Is Signed In
if (Gamer.SignedInGamers.Count == 0)
{
if (Guide.IsVisible == false)
{
Guide.ShowSignIn(1, false);
}
}
 The Guide class lets a program control the guide
 An XNA game can request that the Guide be
displayed during a game
 It has a property IsVisible that is tested to make
sure that the guide is not displayed twice
Chapter 16.2: Network Games and State
15
Ensuring a Gamer Is Signed In
if (Gamer.SignedInGamers.Count == 0)
{
if (Guide.IsVisible == false)
{
Guide.ShowSignIn(1, false);
}
}
 The ShowSignIn method displays the sign in
screen and has two parameters:
 The number of players to sign in on the device (which
must be one for the Windows PC)
 Whether to restrict the sign in to only allow online
players (those with Xbox Live accounts)
Chapter 16.2: Network Games and State
16
Gamer Sign In
 By using the Gamer.SignedInGamers type and
the Guide, an XNA game can ensure that a gamer is
signed in
 The gamer can then take part in network games
 Xbox and Windows PC players can see each other
and take part in games together
 The Zune doesn’t need a gamer to sign in, a gamer
on a Zune has a profile with the name of the Zune
device
Chapter 16.2: Network Games and State
17
Setting Up a Network Game Lobby
 A lobby is managed by one system on the network
 The system “proposes” a particular game and waits
for players to join the lobby and start game play
 XNA provides features that manage a game lobby
Chapter 16.2: Network Games and State
Bread and Cheese Pong
 We are going to create a
simple two-player game
 “Bread and Cheese Pong”
 The game will need a lobby
where players can gather
 The lobby will be very small,
but can be expanded to
handle larger numbers if
required
18
Chapter 16.2: Network Games and State
19
Network Games and State
public enum GameState
{
titleScreen,
NotSignedIn,
SelectingRole,
WaitingAsHost,
WaitingAsPlayer,
PlayingAsPlayer,
PlayingAsHost
}
 A network game is a state machine with additional
states which include the lobby state
 These are all the states our game will occupy
Chapter 16.2: Network Games and State
Pong States
 This is a state
diagram
 It shows how
a game moves
between
states
 It also shows
the screens
and the events
that cause the
state changes
20
Chapter 16.2: Network Games and State
titleScreen State Update Action
case GameState.titleScreen:
if (gamePad1.Buttons.A == ButtonState.Pressed)
{
state = GameState.NotSignedIn;
}
break;
 We can use a switch construction to select the
actions performed during Update
 If the game is in the titleScreen state it must
test for button A on the gamepad
 If this is pressed the game state will change to
NotSignedIn
21
Chapter 16.2: Network Games and State
Special Behavior for the Zune
case GameState.NotSignedIn:
#if ZUNE
state = GameState.SelectingRole;
#else
// code to test players and display guide
#endif
 The Zune version of XNA does not include Guide
support – attempts to use the Guide stop the
program from compiling
 We can use conditional compilation to skip code
that won’t work on the Zune
22
Chapter 16.2: Network Games and State
Hosting a Game
if (gamePad1.DPad.Left == ButtonState.Pressed)
{
// Selected Host role
// Create the session
session = NetworkSession.Create(
NetworkSessionType.SystemLink,
1, // only 1 local gamer
2 // no more than 2 players
);
 A player can decide to host a game
 They will have to wait for players to join the game
before the game can start
 A host starts by making a network session
23
Chapter 16.2: Network Games and State
24
The NetworkSession Class
// Game World
NetworkSession session = null;
 The NetworkSession class provides methods and
properties which manage a network game
 The host creates an instance of this class and uses
it to set up the game, manage the players in the
game, and send messages to them
 The class also generates events that we can use to
trigger actions in our game
Chapter 16.2: Network Games and State
25
Network Session Type
session = NetworkSession.Create(
NetworkSessionType.SystemLink,
1, // only 1 local gamer
2 // no more than 2 players
);
 There are a number of different types of network
games
 Some of them involve the use of Xbox Live
 The SystemLink type of game works on a local area
network
 It is the only kind of game supported by the Zune
Chapter 16.2: Network Games and State
Number of Local Players
session = NetworkSession.Create(
NetworkSessionType.SystemLink,
1, // only 1 local gamer
2 // no more than 2 players
);
 An Xbox can support up to 4 local gamers for split
screen multi-player game play
 The Windows PC can only support one player on a
system
 This value must be set to 1 to indicate this
26
Chapter 16.2: Network Games and State
Number of Players
session = NetworkSession.Create(
NetworkSessionType.SystemLink,
1, // only 1 local gamer
2 // no more than 2 players
);
 This is where you set the maximum number of
players the session will accept
 We only need two players for Pong
 If you wanted to allow more players you would
increase this number
27
Chapter 16.2: Network Games and State
Lobbies and Events
 The XNA program creating the session does not
want to have to wait around for people to join the
game
 XNA wants to call Update and Draw 60 times a
second to keep the game running
 We therefore need some way that the
NetworkSession value can tell the game when
someone turns up to play the game
 This is done using C# events
28
Chapter 16.2: Network Games and State
Events in Programs
 We use events all the time:
 You don’t wait in the garage for your car to be
mended, you give the garage your number and say
“Call me when it is fixed”
 Events let programs do the same thing
 They let us give a NetworkSession object a
reference to a method and say “Call this method
when someone joins the game we are hosting”
 Events are used in Windows to implement screen
button presses, mouse clicks, and timers
29
Chapter 16.2: Network Games and State
30
Detecting Gamer Joined Events
session.GamerJoined +=
new EventHandler<GamerJoinedEventArgs>(hostSession_GamerJoined);
 A session provides a GamerJoined event
 We can add new EventHandler delegates to this
event by using the += operator
 A delegate is a reference to a method in an object
 In the code above, a new EventHandler is
created referring to the method
hostSession_GamerJoined
 When the event occurs the method is called
Chapter 16.2: Network Games and State
31
Creating a Delegate Instance
session.GamerJoined +=
new EventHandler<GamerJoinedEventArgs>(hostSession_GamerJoined);
 The type EventHandler is a delegate type
 It creates an object that contains a reference to a
method in an object
 The constructor of the EventHandler is given the
name of the method the delegate must refer to
 The EventHandler that is created is added to the
list of delegates to be called when the event occurs
Chapter 16.2: Network Games and State
The Bottom Line with Delegates
 We want a way that the NetworkSession value
can alert our game to a player entering the lobby
and joining the game
 A delegate does exactly this for us
 We connect the delegate (a reference to our
method) to the GamerJoined event that the
NetworkSession object provides
 When a gamer joins the game, the method
hostSession_GamerJoined will be called
32
Chapter 16.2: Network Games and State
33
The hostSession_GamerJoined Method
void hostSession_GamerJoined(object sender, GamerJoinedEventArgs e)
{
if (session.RemoteGamers.Count == 1)
{
StartGame ();
state = GameState.PlayingAsHost;
}
}
 This method in our game is called when a gamer
joins the game
 It checks to see if we have 1 remote gamer
 If we do it calls StartGame to start the game and
changes the game state to PlayingAsHost
Chapter 16.2: Network Games and State
34
Hosting the Lobby
 We now know what happens when a system hosts a
game of “Bread and Cheese Pong”:
 It makes a new NetworkSession object, referred to
by a game world variable called session
 It uses a delegate to connect a method to the
GamerJoined event provided by the session
variable
 When a gamer joins the game the method is called
 It starts the game running
 There is also an event, GamerLeft, that is fired if
a gamer leaves the lobby
Chapter 16.2: Network Games and State
35
Waiting as a Player
if (gamePad1.DPad.Right == ButtonState.Pressed)
{
// Selected Player role
state = GameState.WaitingAsPlayer;
}
 If a player presses Right they will be a player in the
game, and must look for hosted games to join
 The state of the game changes to reflect this
 Their game must now try to find any hosted games
and join them to start the game running
Chapter 16.2: Network Games and State
36
Finding Games
AvailableNetworkSessionCollection sessions =
NetworkSession.Find(NetworkSessionType.SystemLink, 1, null);
if (sessions.Count > 0)
{
AvailableNetworkSession mySession = sessions[0];
session = NetworkSession.Join(mySession);
session.GamerLeft +=
new EventHandler<GamerLeftEventArgs>(playerSession _GamerLeft);
StartGame();
state = GameState.PlayingAsPlayer;
}
 This code looks for available network sessions and
joins the first one that it finds
 You could make it more discerning
Chapter 16.2: Network Games and State
37
Using the Find Method
AvailableNetworkSessionCollection sessions =
NetworkSession.Find(NetworkSessionType.SystemLink, 1, null);
if (sessions.Count > 0)
{
AvailableNetworkSession mySession = sessions[0];
session = NetworkSession.Join(mySession);
session.GamerLeft +=
new EventHandler<GamerLeftEventArgs>(playerSession _GamerLeft);
StartGame();
state = GameState.PlayingAsPlayer;
}
 The Find method delivers a list of games that
match the criteria specified
 SystemLink games with 1 local player
Chapter 16.2: Network Games and State
38
Joining a Session
AvailableNetworkSessionCollection sessions =
NetworkSession.Find(NetworkSessionType.SystemLink, 1, null);
if (sessions.Count > 0)
{
AvailableNetworkSession mySession = sessions[0];
session = NetworkSession.Join(mySession);
session.GamerLeft +=
new EventHandler<GamerLeftEventArgs>(playerSession _GamerLeft);
StartGame();
state = GameState.PlayingAsPlayer;
}
 If the list of sessions returned by Find is not empty
we join the first one on the list
 This is the one with subscript 0 on the list
Chapter 16.2: Network Games and State
39
Creating a Network Session
AvailableNetworkSessionCollection sessions =
NetworkSession.Find(NetworkSessionType.SystemLink, 1, null);
if (sessions.Count > 0)
{
AvailableNetworkSession mySession = sessions[0];
session = NetworkSession.Join(mySession);
session.GamerLeft +=
new EventHandler<GamerLeftEventArgs>(playerSession _GamerLeft);
StartGame();
state = GameState.PlayingAsPlayer;
}
 The Join method returns a NetworkSession
that represents the game that has been joined
 This is the session that will be used for this game
Chapter 16.2: Network Games and State
40
Detecting When the Game Ends
AvailableNetworkSessionCollection sessions =
NetworkSession.Find(NetworkSessionType.SystemLink, 1, null);
if (sessions.Count > 0)
{
AvailableNetworkSession mySession = sessions[0];
session = NetworkSession.Join(mySession);
session.GamerLeft +=
new EventHandler<GamerLeftEventArgs>(playerSession _GamerLeft);
StartGame();
state = GameState.PlayingAsPlayer;
}
 This event is fired if the host leaves the game
 We need to get control then so that we can abandon
the game
Chapter 16.2: Network Games and State
41
Starting the Player Game
AvailableNetworkSessionCollection sessions =
NetworkSession.Find(NetworkSessionType.SystemLink, 1, null);
if (sessions.Count > 0)
{
AvailableNetworkSession mySession = sessions[0];
session = NetworkSession.Join(mySession);
session.GamerLeft +=
new EventHandler<GamerLeftEventArgs>(playerSession _GamerLeft);
StartGame();
state = GameState.PlayingAsPlayer;
}
 This starts the game and sets the state to
PlayingAsPlayer
 The game play can now begin
Chapter 16.2: Network Games and State
42
Summary
 XNA provides a set of objects that are used to
create and manage network game play
 A networked game has an expanded state machine
that contains states required to set up the network
 XNA games have a lobby which is hosted by one
player and which other players can find and join
 A C# delegate is a reference to a method in an
object
 Delegates are used to connect program events to
the code that will deal with them