Adding Physics and Collision Detection to Your Game - Beginning Swift Games Development for iOS - page 47

The first thing to look at in this method is the parameter being passed to the method. Here
you have a contact parameter with a type of SKPhysicsContact . The SKPhysic-
sContact contains all of the information you need to handle a node contact. The
SKPhysicsContact class contains five properties to help you determine the character-
istics of the contact. Each of these is defined in Table 3-2 .

Table 3-2 . The SKPhysicsContact Properties

Method Purpose

The bodyA property, an SKPhysicsBody , represents first body in the con-
tact. (This will be the playerNode .)

bodyA

The bodyB property, an SKPhysicsBody , represents second body in the
contact. (This will be the orbNode .)

bodyB

The contactPoint , a CGPoint , represents the contact point between the
two physics bodies in the scene coordinates.

contactPoint

The contactImpulse , a CGFloat , specifies how hard these two bodies
struck each other using newton-seconds as the unit of measure.

collisionImpulse

The collisionNormal , a CGVector , specifies the direction of the colli-
sion.

collisionNormal

You are interested only in the second of these properties, bodyB . Getting back to the
didBeginContact() method, notice the second line of the method. Here you are ref-
erencing the contact's bodyB property, which as I mentioned earlier is an SKPhysic-
sContact . Once you have this reference, you can then get the SKNode instance from
the bodyB.node property, which represents the second node in the collision.
Once you have the second SKNode in the collision, you can check to see whether it has a
name of POWER_UP_ORB . If it does, then you invoke the node's removeFromPar-
ent() method, which, as it sounds, removes the orbNode from the scene.
Go ahead and replace the contents of the current didBeginContact() with this ver-
sion and run the app one last time. This time, when the playerNode collides with the
orbNode , the orbNode disappears from the scene.
At the end of this chapter, the new GameScene.swift should look like Listing 3-2 .

Listing 3-2 . GameScene.swift: The SuperSpaceMan GameScene with Collison Detection