Opening the sign GUI and storing its string

So I want to create a custom sign item, but not in the matter of actually placing a block. I'm currently using the onItemRightClick function which works like a charm, but it's when I want to implement the sign mechanics things start getting tricky. Checking out the ItemSign class,  i found it uses the player.openEditSign to open the sign GUI, but it seem to require that the player places a block. Additionally I fail to recognize where it puts the information typed into the sign. Am I missing to look at a class, or completely missing the concept of how one can use the sign class?