Flappy Bird in Excel VBA - The Game Timing Loop

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Introduction

This part of the tutorial introduces some extra Windows API functions and explains how to set up the sequence of individual procedures that will form our main game loop.

The SetTimer and KillTimer Functions

The function that will control our main game loop is called SetTimer. Copy the following declaration into your modPublicDeclarations module.

If you're using a 64-bit edition of Office 2010 or later you'll need this version of the declaration:

This function starts a timer which calls a specific procedure at a specified interval. The two parameters which interest us are uElapse, which is the number of milliseconds between each call to the procedure, and lpTimerFunction, which is a reference to the procedure that we want to call.

As we'll need to stop the timer when the game ends we should declare the KillTimer function as well. Copy this into the same module:

The 64-bit version of this declaration is shown below:

If you're using the version-independent declarations for your Windows API functions your entire modPublicDeclarations module should now look like this:

Adding the Basic Game Procedures

When writing complex systems it pays to be organised as early as possible in the process. We're going to create a separate module to hold our game code, as well as separate modules for the main objects involved in the game. Ordinarily I'd use class modules to do this, but we'll stick to normal modules for now and look at how to switch to class modules later in the tutorial.

Start by inserting a new module called modGameCode. When you've done that, write or copy the following code into it:

The comments in each procedure describe reasonably neatly what each procedure is going to do. The InitialiseGame procedure will be called when we start the game, eventually this will be done by clicking a button on the worksheet. This procedure also sets the game timer running. The game timer will call the UpdateAndDrawGame procedure continuously until something in our game logic tells it to stop. At that point we'll call the TerminateGame procedure which will stop the timer and reset the game.

Creating the Timer Procedures

The next job is to create two timer procedures, one to start the game timer and one to stop it. Insert a new module called modTimerCode and add the following code to it:

The comments should describe what's happening here reasonably well. The GameTimerID variable is used to remember which timer was started by the game. It's declared as private because it's only used in this module.

We set the GameTimerInterval variable to 50, meaning that the game will update every 50 milliseconds, or 20 times per second. We could just as easily have used an expression here, for example:

In most real games the game updates at either 30 times per second or 60 times per second. For our rudimentary Excel game this isn't quite so important so you'll be able to play with this value later to speed up or slow down the game. Leave it set to 50 for now.

The most important line is the one which calls the SetTimer function. We pass in the value of our timer interval and tell it to call our UpdateAndDrawGame procedure. The AddressOf operator is used to pass a pointer to that procedure into the SetTimer function. Windows API functions don't call procedures in quite the same way as normal VBA procedures do so the AddressOf operator is important here.

Starting and Stopping the Timer

Now we can go back to the modGameCode module by double-clicking on its name in the Project Explorer. To start the game timer we simply need to add this line to the end of the InitialiseGame subroutine:

We should also add this line to the end of the TerminateGame procedure:

The next thing we need to do is make the game actually do something when it runs!

Creating a Module for the Bird Code

Just as we've created separate modules to store code for the game and timer, we'll have a separate module for the bird called modBirdCode. Create it and then add the following code to it:

Each of these subroutines will need to be called in the appropriate place from the modGameCode module, so head back there next. Add a line to the InitialiseGame procedure so that it looks like this:

Now add two lines to the UpdateAndDrawGame procedure so that it looks like this:

What's Next

That's it for our basic game logic. When we finally start the game it will continue to run through the UpdateAndDrawGame procedure until the timer is stopped. Now all we have to do is provide a way for the player to start and stop the game! The next part of the tutorial describes how to do that.