Physics & Collisions - Learn SpriteBuilder for iOS Game Development - page 118

Upon closer inspection, you'll notice that all four collision callback methods take the
same parameters as input, and two of them return a BOOL. Following is a list that ex-
plains under which circumstances and in which order each of these methods is called:
Begin methods are called when two bodies first come into contact. It's al-
ways the first method called in a collision event between two bodies. By re-
turning NO , the collision is ignored, bodies are allowed to pass through each
other, and PreSolve and PostSolve methods for this collision event
will not be called.
PreSolve methods get called repeatedly for as long as two bodies are in con-
tact, and before the collision has been resolved. This allows you to tweak the
collision response by (temporarily) modifying the contacting body's proper-
ties, such as friction or restitution. Returning NO will not resolve the colli-
sion so that the two bodies are allowed to penetrate for this particular point
in time.
PostSolve methods get called after the collision has been resolved, meaning
the bodies have been moved apart and their velocities updated. PostSolve
can be used to reset any temporarily modified properties from the
PreSolve step. But more importantly, other collision-based properties like
totalKineticEnergy and totalImpulse have now been calculated.
This allows you to break joints or play sounds as necessary, perhaps when
the impact force exceeds a given threshold.
Separate methods get called the moment two previously contacting bodies
are no longer in contact.
Tip A Begin method is always followed by a corresponding Separate
method, even if the Begin method returned NO . You can rely on counting the
number of Begin and End calls to determine how many, or if any, two bodies
of the given collision types are currently in contact with each other.
Collision Types and Callback Parameter Names
The collision callback methods in Listing 4-12 all take the same parameters as input. The
names of the collisionTypeA and collisionTypeB parameters actually play an