Designing Games with Game Maker - YoYo Games

Persistent objects
Secondly, you can make an object persistent. A persistent object will continue existing when
you move from one room to the next. It only disappears when you explicitly destroy it. So you
only need to put an instance of the object in the first room and then it will remain available in
all rooms. This is great when you have a main character that moves from room to room. Using
persistent objects is a powerful mechanism but also one that easily leads to errors.
Parents
Every object can have a parent object. When an object has a parent, it inherits the behavior of
the parent. Stated differently, the object is a sort of special case of the parent object. For
example, if you have 4 different balls, named ball1, ball2, ball3 and ball4, which all behave the
same but have a different sprite, you can make ball1 the parent of the other three. Now you
only need to specify events for ball1. The others will inherit the events and behave exactly the
same way. Also, when you apply actions to instances of the parent object they will also be
applied to the children. So, for example, if you destroy all ball1 instances the ball2, ball3, and
ball4 instances will also be destroyed. This saves a lot of work.
Often, objects should behave almost identically but there will be some small differences. For
example, one monster might move up and down and the other left and right. For the rest they
have exactly the same behavior. In this case almost all events should have the same actions
but one or two might be different. Again we can make one object the parent of the other. But
in this case we also define certain events for the child object. These events "override" the
parent events. So whenever an event for the child object contains actions, these are executed
instead of the event of the parent. If you also want to execute the parent event you can call
the so-called "inherited" event using the appropriate action.
It is actually good practice in such cases to create one base object. This base object contains
all the default behavior but is never used in the game. All actual objects have this base object
as parent. Parent objects can again have parents, and so on. (Obviously you are not allowed
to create cycles.) In this way you can create an object hierarchy. This is extremely useful to
keep your game structured and you are strongly advised to learn to use this mechanism.
There is also a second use of the parent object. It also inherits the collision behavior for other
objects. Let us explain this with an example. Assume you have four different floor objects.