Writable blocks (Signs)

I think with making they have GUI that has text input. But im not sure can it be made saving the text, so then it will be like sign.

You can, if you make a variable and by clicking a button in the gui it give the variable the value of the text input but idk if you can display it on the block side

How to display a text on a block's side?

If you don't need your block to store items, NBT or energy/fluid, you can try this steps:

1. Enable tile entity of your block and do not change any other inventory settings;
2. Make the block open a GUI with text label and "Ready" button;
3. Create a procedure for that button that contains Set text variable textVar to: text inside textfield procedure block and custom code snippet - the code for it is:
   

   world.getTileEntity(new BlockPos((int)x, (int)y, (int)z)).setText(2, new StringTextComponent(textVar));

4. Lock code of your block, remove CustomTileEntity class, replace all occurrences of CustomTileEntity with SignTileEntity, reformat and save code.

i confused on step 4...

WHY IS EVERYTHING GUIS??