This read-only variable returns whether or not the
instance is currently "sleeping" (true) or not
(false), A "sleeping" instance is one that is not actively
engaged in any physical simulation. GameMaker Studio 2 will
put objects to sleep to save simulation cycles when an instance is
at rest and not in collision with another instance.
instance_destroy(); // 销毁实例
The above code checks to see if the object is being actively simulated or not and if it is not it is destroyed.