Factorio modding questions

[ProfoundDisputes]

I have a few question that I could not get the answer to through various searches. Sorry, if they are obvious but I am not familiar with modding games or programming them. I have mostly programming experience in C, C++ and Visual Basic. These questions pertain to things I keep running into while creating my mod:

1) Ran into issues with stuff such as "LuaEquipment API call when LuaEquipment was invalid" Also with LuaItemstacks. What does it mean for things to be invalid in the game? Figured out that it is a reference to an object that has been removed by the game engine.

2) What is a force? I can't tell if I understand it correctly. Is a force simply something that autonomy in the game? Like a bitters, players and such?

3) Entity is anything placed in the game world? So this means assemblers, player, inserters or items dropped on the ground?

4)The event calls are called after the action has been performed or before the action has been performed?

Those are just the ones I can remember now. Thank You in advance!

[darkfrei]

2) What is a force? I can't tell if I understand it correctly. Is a force simply something that autonomy in the game? Like a bitters, players and such?

Force is something as category for players, their buildings and researches.

3) Entity is anything placed in the game world? So this means assemblers, player, inserters or items dropped on the ground?

Yes, all what you can see are entities. But floor is tiles. Item-on-ground is also entity. The ghost is entity too. Items on belts are not entities I think. Some special case for item stack, but without stacking anymore.

4)The event calls are called after the action has been performed or before the action has been performed?

If the event has prefix "on_pre_" then it will be called before event.