Tetris on Niecza

Niecza, the Other Perl 6 Implementation on Mono and .NET, recently gained the ability to call almost any Common Language Runtime library. In Niecza’s examples directory, a simple 30 line script called gtk1.pl shows how to use gtk-sharp, and thus Gtk and Gdk, the graphical basis of Gnome. Here is gtk1’s central working part:

my $btn = Button.new("Hello World");
$btn.add_Clicked: sub ($obj, $args) { #OK
    # runs when the button is clicked.
    say "Hello World";
    Application.Quit;
};

The add_Clicked method defines a callback routine, essential to process user input. Running gtk1.pl makes the following resizeable button in a window, and it closes when clicked:

From gtk1 to Tetris is not far, see the source also in niecza/examples. Two extra ingredients make it possible: a timer tick callback routine to animate the falling pieces, and non blocking keyboard input to give the user the illusion of control. Add some simple physics and Cairo graphics and you have a playable game (modulo scoring and similar low hanging fruit) in under 170 lines of Perl 6.

Animation by timer tick works by causing the whole window to be redrawn by an ExposeEvent at regular intervals. The redraw tries to move the falling piece downwards, and if the physics says no, it adds a new piece at the top instead. (Bug: that should eventually fail with a full pile of pieces.) GLibTimeout sets up the timer callback handler which invokes .QueueDraw. The default interval is 300 milliseconds, and if the game engine wants to speed that up, it can adjust $newInterval which will replace the GLibTimeout on the next tick (sorry about the line wrap):

my $oldInterval = 300;
my $newInterval = 300;
...
GLibTimeout.Add($newInterval, &TimeoutEvent);
...
sub TimeoutEvent()
{
    $drawingarea.QueueDraw;
    my $intervalSame = ($newInterval == $oldInterval);
    unless $intervalSame { GLibTimeout.Add($newInterval, &TimeoutEvent); }
    return $intervalSame; # True means continue calling this timeout handler
}

Thanks to the excellent way Gtk handles typing, the keystroke event handler is fairly self documenting. The Piece subroutines do the physics ($colorindex 4 is the square block that does not rotate):

$drawingarea.add_KeyPressEvent(&KeyPressEvent);
...
sub KeyPressEvent($sender, $eventargs) #OK not used
{
    given $eventargs.Event.Key {
        when 'Up' { if $colorindex != 4 { TryRotatePiece() } }
        when 'Down' { while CanMovePiece(0,1) {++$pieceY;} }
        when 'Left' { if CanMovePiece(-1,0) {--$pieceX;} }
        when 'Right' { if CanMovePiece( 1,0) {++$pieceX;} }
    }
    return True; # means this keypress is now handled
}

With a bit more glue added, here is the result:

This post has glossed over other details such as the drawing of the graphics, because a later Perl 6 Advent post will cover that, even showing off some beautiful fractals, so keep following this blog! The above software was presented at the London Perl Workshop 2011.