Design Patterns and Video Games

2D Strategy Game (6): Game logic

We start the design and implementation of the game logic using the Command pattern. Then, we use it to fill areas:

This post is part of the 2D Strategy Game series

Advanced Command pattern

We implement game logic with a more advanced Command pattern:

Advanced Command pattern

The Command interface has three methods, some providing a new feature:

Implementation: the addCommand() method of the Logic class adds the command to the command list. The executeCommands() method runs all scheduled commands:

def executeCommands(self):
    commands = self.__commands.copy()
    priorities = sorted(commands.keys())
    for priority in priorities:
        command = commands[priority]
        if not command.check(self):

Lines 2-3 copy the list in a variable and clear the attribute. We must proceed this way if the commands schedule commands. Thanks to this copy, we add commands to the attribute, not the list currently processed.

Line 4 sorts the priority levels. The key() method returns a list of dictionary keys, and the sorted() function returns a sorted list. As a result, priorities contains the priority levels from the lowest to the highest.

Line 5 iterates through these levels, and line 6 gets the command corresponding to the current priority. Then, if the check() method returns False, we end the current iteration (lines 7-8). Finally, we run the command (line 9).


We create three command classes to update cells in each layer type (ground, impassable, and objects). These classes inherit a SetLayerValueCommand class with shared features:

Commands for updating the world cells

Commands priority

The priority() method of the SetLayerValueCommand class return a priority level that depends on the cell to update:

def priority(self) -> int:
    return WORLD_PRIORITY + self._coords[0] + self._coords[1] * WORLD_MAX_WIDTH

The main idea is to compute a unique integer value for each cell and for the update case. The expression self._coords[0] + self._coords[1] * WORLD_MAX_WIDTH is the usual conversion from 2D to 1D coordinates x + y * width. We assume that WORLD_MAX_WIDTH contains the highest with of the world; this is something we should check when creating or loading a level.

The WORLD_PRIORITY value is set to ensure that the priority values don't intersect with others. Right now, these commands are the only ones, so we don't have to worry about that: it will be useful later.

Note that all layer command classes share this priority() method: it means that we can only update a cell for a single layer. It is a design choice; if we want to update a cell on several layers simultaneously, we need to create a different set of priority values for each layer.

Command check

The check() method of the SetGroundValueCommand() returns True if the command leads to an update:

def check(self, logic: Logic) -> bool:
    value = self._value
    if not checkCellValue("ground", value):
        return False
    coords = self._coords
    world =
    if not world.contains(coords):
        return False

    value = self._value
    groundValue = world.ground.getValue(coords)
    if value == groundValue:  # Value already set
        return False

    if value == CellValue.GROUND_SEA:  # Sea case
        impassableValue = world.impassable.getValue(coords)
        if impassableValue != CellValue.NONE:
            return False
        objectsValue = world.objects.getValue(coords)
        if objectsValue != CellValue.NONE:
            return False

    return True

Lines 2-4 ensure that the value we want to set is correct. For instance, for the ground layer, the possible values are 101 and 102. As before, we don't want to add these values in the code: we collect them in a specific place (the CellValue class). Moreover, we also put the logic that checks these values in the same python file with the checkCellValue() function:

CellValueRanges = {
    "ground": (101, 103),
    "impassable": (201, 204),
    "objects": (301, 311)
def checkCellValue(layer: str, value: CellValue):
    if layer != "ground" and value == CellValue.NONE:
        return True
    valueRange = CellValueRanges[layer]
    return valueRange[0] <= value < valueRange[1]

Note that we also try to write compact code even for implementing this function. For example, we use the CellValueRanges dictionary in place of a large if...elif block to get the value ranges.

The remaining lines of the check() method ensure that we don't try to remove ground under an existing element in another layer.

The implementation of the check() method in the other command classes is similar.

Command execution

The execute() method of the SetGroundValueCommand() sets the value and schedule new commands if we ask for a fill:

def execute(self, logic: Logic):
    coords = self._coords
    value = self._value
    ground =
    ground.setValue(coords, value)

    if self._fill:
        x, y = coords[0], coords[1]
        logic.addCommand(SetGroundValueCommand((x + 1, y), value, True))
        logic.addCommand(SetGroundValueCommand((x - 1, y), value, True))
        logic.addCommand(SetGroundValueCommand((x, y + 1), value, True))
        logic.addCommand(SetGroundValueCommand((x, y - 1), value, True))

Lines 8-12 add a command in each direction around the current cell at (x,y). The game logic will execute these commands during the next game epoch. Consequently, if you run the program and click with the middle button, the area around the mouse cursor is slowly filled with a random object. It is because it runs at the pace of the game epoch update, which is 30 times per second (see the end of the main game loop in the run() method of UserInterface).

Final program

Download code and assets

In the next post, I add a cache to reduce rendering time.