At the heart of an XNA project is a class that inherits from Game. This class handles initializing the graphics device, manages components, and most importantly, implements the game loop.

The MonoGame Game Loop

As you saw in the Game Loop Chapter of Game Programming Patterns:

A game loop runs continuously during gameplay. Each turn of the loop, it processes user input without blocking, updates the game state, and renders the game. It tracks the passage of time to control the rate of gameplay.

This is precisely what the Game class implements for us - a loop that 1) processes user input, 2) updates the game state, and 3) renders the game.

As a MonoGame developer, you create a new class that inherits from Game (if you use one of the MonoGame templates, this class will probably be named Game1, but feel free to rename it). Then, you can write code to execute during steps 2 and 3 of the loop by overriding the virtual methods: Update(GameTime gameTime) and Draw(GameTime gameTime). These methods are invoked by Game each time it executes the game loop. In software engineering parlance, we call this kind of method a “hook,” as we can use it to pull new functionality into the existing class.

Time and the Game Loop

Time in the MonoGame framework is typically measured using System.TimeSpan struct. While this struct has many general uses, for games we almost totally rely on the TimeSpan.TotalSeconds property, which is a double representing the full length of time the TimeSpan represents as a double measured in seconds.

You probably noticed that both methods have a GameTime object as a parameter. This is a class used to store measurements of time in the game. It is basically a data object with three properties:

• GameTime.ElapsedGameTime is a TimeSpan measuring the time that elapsed between this and the previous call to Update(GameTime). In other words, it is the time between passes in the game loop.
• GameTime.TotalGameTime is a TimeSpan measuring the total time since the game was started.
• IsRunningSlowly is a Boolean indicating that the game is lagging (more on this shortly)

As you saw in the Game Programming Patterns, the game loop can be clamped (fixed), or run as fast as possible. MonoGame allows you to choose either strategy. You can specify the strategy you want by changing the Game.IsFixedTimeStep boolean property. When using a fixed time step, you can specify the desired time step (the time between game loop passes) by setting the Game.TargetElapsedTime property to a TimeSpan of the desired duration.

By default, MonoGame adopts the fixed update time step, variable rendering strategy from Game Programming Patterns. If a pass through the game loop takes too long, it skips rendering (the Game.Draw() is not invoked), and the TimeSpan provided to the Game.Update() method has its IsRunningSlowly property set to true. The game will continue to drop rendering frames until the Game.MaximumElapsedTime value is reached, at which point it will invoke Game.Draw(). ¹

Setting the Game.IsFixedTimeStep property to false instead runs the game loop as fast as possible.