class is where all of the magic in your game takes place. Almost
everything in your game is driven in some part by this class, and it is
the root of your project. Let’s dive in and see what it is made of.
The Game object
you’re class derives from has many knobs and buttons (figuratively) to
help control your game and its flow. It has several virtual methods you
can override to help control different features, some of which we’ve
you would expect, is called once just after the object is created. It
is where you would do most of your initialization (aside from loading
content) for your game. This is an ideal place to add new game
components for example (which are discussed later in this chapter). LoadContent and UnloadContent
are two other areas where you should load (or unload) your content.
Content is loosely defined and can be any external data your game
needs, whether it;s graphics, audio, levels, XML files, and so on.
method is where you handle updating anything your game requires, and in
many games, you can do things such as handle user input. Because most
games are essentially a simulation with external forces, you need a
central spot where you can perform the operations to advance that
simulation. Common things you’d do include moving objects around the
world, physics calculations, and other simulation updates.
Draw probably needs
no further explanation. All drawing code for each scene occurs here.
There are two other drawing methods you can override: BeginDraw and EndDraw. These are called directly before and after Draw is called, respectively. If you override the EndDraw call, you need to ensure you call the base.EndDraw or manually call GraphicsDevice.Present; otherwise, you never see your scenes drawn on screen.
Present is the last call made at the end of drawing and tells the graphics device, “Ok, I’m done drawing now; show it all on screen.”
Much like the pre- and post-drawing methods, there are also BeginRun and EndRun
methods you can override that are called before the game begins and
just after the game run ends. In most cases, you do not override these,
unless you are doing something such as running multiple simulations as
individual game objects.
You can override the method ShowMissingRequirementMessage.
Most people probably don’t even realize it is there. By default, this
does nothing on non-Windows platforms, and on Windows, it shows a
message box giving you the exception detail. This enables customization
if the platform you run on doesn’t meet the requirements of your game,
which is normally only an issue on a platform such as Windows where you
can’t guarantee which features it supports.
The last three methods you can override are mirrors of events you can hook. OnActivated is called at the same time the Activated
event is fired, and it happens when your game becomes activated. Your
game is activated once at startup, and then anytime it regains focus
after it has lost focus. To mirror that, you use the OnDeactivated method, which is called when the Deactivated
event is fired, and that happens when your game becomes deactivated,
such as it exits or it has lost focus. On Windows and Windows Phone 7,
your game can lose focus for any number of reasons (switching to a new
app, for example), whereas on Xbox 360, you see this only if the guide
Finally, the OnExiting method is called along with the Exiting event. As you can probably guess, this happens just before the game exits.
Most of the methods on the
Game class are virtual so there aren’t many here to discuss, and
they’re almost all named well, so you can guess what they do. The Exit, which causes the game to start shutting down. The ResetElapsedTime method resets the current elapsed time, and you’ll learn more about it later in this chapter. The Run
method is what starts the game running, and this method does not return
until the game exits. On Xbox 360 and Windows, this method is called by
the autogenerated main method in program.cs at startup, and on Windows
Phone 7, this method throws an exception. Due to platform rules, you
can’t have a blocking method happen during startup on Windows Phone 7.
A timer starts and periodically calls RunOneFrame,
which does the work of a single frame. You can use this method on Xbox
360 and Windows, but you shouldn’t have to use it since the game object
is doing that for you.
The SuppressDraw method stops calling Draw until the next time Update is called. Finally, the Tick method advances one frame; namely, it calls Update and Draw.
After covering all of the methods, properties are naturally the next item on the list. You’ve already seen the Services property, which enables you to add new services to the game and query for existing ones. You’ve also already seen the Content and GraphicsDevice properties, which store the default content manager and graphics device.
A property called InactiveSleepTime
gives you some control of how your game handles itself when it is not
the foreground window. This value tells the system how long to “sleep”
when it is not the active process before checking to see if it is
active again. The default value of this is 20 milliseconds. This is
important on Windows were you can have many processes run at once. You
don’t want your game to run at full speed when it isn’t even active.
Speaking of being active, the IsActive property tells you the current state of the game. This maps to the Activated and Deactivated events, too, as it turns true during Activated and false during Deactivated.
property is used for Windows Phone 7 to get information about
parameters required for launching, but this can be used for any
platformand translates the command-line parameters on Windows into this
object. It is a dictionary of key value pairs. On Windows, if your
command line is as follows, the object would have a dictionary with
game.exe /p /x:abc "/w:hello world"
The first member would have a
key of “p” with a value of an empty string. The second member would
have a key of “x” with a value of “abc.” The third member has a key of
“w” with a value of “hello world.”
On Windows Phone 7,
applications are launched with a URI that includes these parameters;
for example, if your launch command is as follows, the object would
have a dictionary with two members:
The first member would have
a key of “myparam1” and a value of “one.” The second member would have
a key of “myparam2” and a value of “two.”
The last two properties are IsFixedTimeStep and TargetElapsedTime.
Timing is so important to game development there is a whole section on
that! Because anticipation is probably overwhelming, that section is
You may not realize it, but a
lot of things in a game depend on time. If you create a race game and
your cars are going 60 miles per hour, you need to know how much to
move them based on a given time. The framework tries to do a lot of the
work of handling time for you.
There are two major ways to
run a game, and in the framework, they are referred to as “fixed time
step,” and “variable time step.” The two properties mentioned in the
previous section—IsFixedTimeStep and TargetElapsedTime—control how time is handled. IsFixedTimeStep
being true naturally puts the game into fixed time step mode, whereas
false puts the game into variable time step mode. If you are in fixed
time step mode, TargetElapsedTime is the target time for each frame. The defaults for projects are true for IsFixedTimeStep and 60 frames per second for TargetElapsedTime (which is measured in time, so approximately 16.6667 milliseconds).
What do these time steps
actually mean? Variable time step means that the amount of time between
frames is not constant. Your game gets one Update, then one Draw, and then it repeats until the game exits. If you noticed, the parameter to the Update method is the GameTime.
The GameTime object has three properties that you can use. First, it has the ElapsedGameTime, which is the amount of time that has passed since the last call to Update. It also includes TotalGameTime, which is the amount of time that has passed since the game has started. Finally, it includes IsRunningSlowly, which is only important in fixed time step mode.
During variable time step mode, the amount of time recorded in ElapsedGameTime passed to update can change depending on how long the frame actually takes (hence, the name “variable” time step).
Fixed time step is different. Every call to Update has the same elapsed time (hence, it is “fixed”). It is also different from variable time step in the potential order of Update and Draw calls. While in variable time step, you get one update for every draw call; in fixed time step, you potentially get numerous Update calls in between each Draw.
The logic used for fixed time step is as follows (assuming you’ve asked for a TargetElapsedTime of 60 frames per second).
Update is called as many times as necessary to catch up to the current time. For example, if your TargetElapsedTime is 16.667 milliseconds, and it has been 33 milliseconds since your last Update call, Update is called, and then immediately it is called a second time. Draw is then called. At the end of any Draw, if it is not time for an Update to occur, the framework waits until it is time for the next Update before continuing.
If at any time, the
runtime detects things are going too slow (for example, you need to
call Update multiple times to catch up), it sets the IsRunningSlowly
property to true. This gives the game the opportunity to do things to
run faster (such as rendering less or doing fewer calculations).
If the game gets extremely far behind, though, as would happen if you paused the debugger inside the Update call if your computer just isn’t fast enough, or if your Update method takes longer than the TargetElapsedTime,
the runtime eventually decides it cannot catch up. When this happens,
it assumes it cannot catch up, resets the elapsed time, and starts
executing as normal again. If you paused in the debugger, things should
just start working normally. If your computer isn’t good enough to run
your game well, you should notice things running slowly instead.
You can also reset the
elapsed time yourself if you know you are going to run a long
operation, such as loading a level or what have you. At the end of any
long operation such as this, you can call ResetElapsedTime on the game object to signify that this operation takes a while, don’t try to catch up, and just start updating from now.
Notice that in Windows Phone 7 projects, the new project instead sets the TargetElapsedTime
to 30 frames per second, rather than the 60 used on Windows and Xbox
360. This is done to save battery power, among other reasons. Running
at half the speed can be a significant savings of battery life.
Which time step mode you
actually use is a matter of personal preference. We personally choose
fixed time step mode, but either can work for any type of game. During
performance measurement, though, you should use variable time step