Thank you all for your answers,
Vanilla's Obsidian Blocks was just an idea, if it's simpler to do that with my own blocks I'll go this way.
I actually don't want to make my structures to check every tick if the structure is full, if that's possible of course ( onNeighorChange something like that I guess ) it prevent lags on servers when a lot of structures are placed on the world.
It'd look like some old bricks furnace. Also, flame & smoke animation.
I should override this on the blocks I'll use ? It's ok to use this if the structures has 3 different blocks ?
I've made an item that I'd like to use to make the structure appear. Dunno how to make this work properly, maybe, onItemRightClick then check if the structure is full.
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So, first of all, I'd make a method that check when all blocks are placed in the TE and an other when the structure is complete. Then, in my blocks class, override the 2 methods
"hasTileEntity" "getTileEntity"