Create a systemΒΆ

In the previous tutorial, we created a component that holds data regarding the physical properties of an object (mass and drag).

In this tutorial we will implement a system that visits the entity, reads this data and performs various tasks. In particular we will implement a gravity system that when applied overrides the mass of the object to zero.

Below you can see the GravitySystem we created

class GravitySystem(System):

    def __init__(self, name=None, type=None, id=None):
        super().__init__(name, type, id)

    def applyGravityEffect(self, rigidBody: RigidBody):

        #check if the visitor visits a node that it should not
        if (isinstance(rigidBody,RigidBody)) == False:
            return #in Python due to duck typing we need to check this!
        print(self.getClassName(), ": applyGravityEffect called")

        rigidBody.mass = 0

        print('New mass is: ' + str(rigidBody.mass))

Pay attention to the following points:

  1. We inherit the System class.

  2. No need to implement any abstract methods

  3. We implemented the method applyGravityEffect() that we ll invoke it each time we visit an entity that has a rigidBody component. This is applied from the visitor pattern. This method makes the mass of the body zero and prints an ack message.

Finally, we need to link the system with the corresponding component. We are doing this in the accept() method of the rigidBody. As you see when the component gets accepted it calls the applyGravityEffect() method from the GravitySystem.

class RigidBody(Component):

#...

def accept(self, system: System, event = None):
    system.applyGravityEffect(self)

#...

We can test our system with the code below. We create a new GravitySystem and make our rigidBody to accept it.

gravitySystem = GravitySystem("gravityUpdate", "GravitySystem", "002")
rigidBody.accept(gravitySystem)

Note

Usually if our entities are held in another structure (e.g scenegraph) we will have a method that traverses the graph and applies the systems to the nodes. By doing this each node will call the corresponding functions from the systems.