Create an object when there is none in a specific area

I want to create a random object outside the layout on which bullet behavior is enabled which moves it down. I am trying to perform it in this order,

Create object which moves down.

When it enters the layout, check if it have traveled the distance that equals its own height.

When it meets the condition, create another object (once) outside the layout, that will move same as first created.

I have compared Y with Height but of no use. Any suggestions?

If its a scrolling background you want to make, there are quite few examples around.

Anyway,

spawn object on y -10

System, compare 2 values: object.bullet.distance traveled > WindowHeight + 10

action spawn another on y - 10

I have created a family of objects with different heights, so when an object will be created it will be of different height. I want to compare the height with distance, like if object's height was 100px I want to check if Y+100 is traveled then create next object above which will have a different height, say 250px, so how to automatically alter the condition?

If its a scrolling background you want to make, there are quite few examples around.

Anyway,

spawn object on y - Object.Height

System, compare 2 values: object.bullet.distance traveled > WindowHeight + Object.Height

action spawn another on y - Object.Height