The game runs in a browser window and it communicates with the game pad through a Named Pipe or FIFO. There are multiple processes running which can read from the game pad and write the required commands to the FIFO for controlling the game. The JavaScript written in the game code simply reads from the FIFO for the input control data.
Fig. 2: Block Diagram Of Raspberry Pi Gamepad
The parent process here creates four child processes which are dedicated for each of the keys in the gamepad. Whenever a key is pressed or released, they will generate a signal and send it to the parent process with a value. Using this method the parent process can identify which key has been pressed or released.
As soon as the parent process detects a key press from the value it received along with the signal, it simply writes a particular command value to the NAMED PIPE or FIFO. The HTML game will be reading continuously from the FIFO every 100ms and change the direction of the snake according to the command read.
The following part of the JavaScript reads from the file every 100ms and changes the direction of movement of the snake in the game.
//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++//
function loadFile()
{
reader = new XMLHttpRequest() || new ActiveXObject(‘MSXML2.XMLHTTP’);
reader.open(‘get’, ‘xyz’, true);
reader.onreadystatechange = displayContents;
reader.send(null);
}
function displayContents()
{
ch = reader.responseText[0];
if(ch == ‘l’)
heading = (heading == EAST) ? EAST : WEST;
else if(ch == ‘r’)
heading = (heading == WEST) ? WEST : EAST;
else if(ch == ‘u’)
heading = (heading == SOUTH) ? SOUTH : NORTH;
else if(ch == ‘d’)
heading = (heading == NORTH) ? NORTH : SOUTH;
}
//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++//
The functions used in the coding of the Gamepad code for writing operations on the FIFO are explained in the following section.
The FIFO needs to be created one using the following command in the same directory where the executable for the gamepad and the HTML game files exist.
mkfifo xyz
The FIFO can also be created using the code itself by adding the function mkfifo(), open() as explained in a previous documentation.
The user can write data into the temporary file from the terminal itself using the ‘echo’ command as shown in the following example.
echo abcdef >> /tmp/my_fifo
The FIFO writing parent process is written in such a way that it will write commands to the FIFO in respond to the signals received from the child process. The commands that will be written to the FIFO for controlling the direction of the snake are listed below;
u -> Snake Upwards
d -> Snake Downwards
l -> Snake Left
r -> Snake RIght
These commands can be written into the FIFO with the help of the system() function.
system ( “echo r >> xyz );
The above function call will turn the Snake towards right.
