Designing Games with Game Maker - YoYo Games

There are a number of actions with which you can control which other actions are performed.
Most of these actions ask a question, for example whether a position is empty. When the
answer is yes (true) the next action is executed, otherwise it is skipped. If you want multiple
actions to be executed or skipped based on the outcome you can put them in a block by
putting start block and end block actions around them. There can also be an else part which is
executed when the answer is no. So a question typically looks as follows:
Here the question is asked whether a position for the current instance is collision free. If so,
the instance starts moving in a given direction. If not, the instance jumps to a given position.
For all questions there is a field labeled NOT. If you check this field, the result of the question
is reversed. That is, if the result was true it becomes false and if it was false, it becomes true.
This allows you to perform certain actions when a question is not true.
For many questions you can indicate that they should apply to all instances of a particular
object. In this case the result is true only if it is true for all instances of the object. For
example, you can check whether for all balls the position slightly to the right is collision free.
The following questions and related actions are available. (Note that they all have a differently
shaped icon and a different background color so that they can more easily be distinguished
from other actions.)
Check Empty
This question returns true if the current instance, placed at the indicated position does not
generate a collision with an object. You can specify the position as either absolute or relative.
You can also indicate whether only solid, or all objects, should be taken into account. This
action is typically used to check whether the instance can move to a particular position.