Add an API for custom formspec elements

API.md

@@ -61,6 +61,47 @@ Removes all recipe filters and adds a new one.
 
 Returns a map of recipe filters, indexed by name.
 
+### Custom formspec elements
+
+#### `craftguide.add_formspec_element(name, def)`
+
+Adds a formspec element to the current formspec.
+Supported types: `box`, `label`, `image`, `button`, `tooltip`, `item_image`, `image_button`, `item_image_button`
+
+Example:
+
+```lua
+craftguide.add_formspec_element("export", {
+	type = "button",
+	element = function(data)
+		-- Should return a table of parameters according to the formspec element type.
+		-- Note: for all buttons, the 'name' parameter *must not* be specified!
+		if data.recipes then
+			return {
+				data.iX - 3.7,   -- X
+				sfinv_only and 7.9 or 8, -- Y
+				1.6,             -- W
+				1,               -- H
+				ESC(S("Export")) -- label
+			}
+		end
+	end,
+	-- Optional.
+	action = function(player, data)
+		-- When the button is pressed.
+		print("Exported!")
+	end
+})
+```
+
+#### `craftguide.remove_formspec_element(name)`
+
+Removes the formspec element with the given name.
+
+#### `craftguide.get_formspec_elements()`
+
+Returns a map of formspec elements, indexed by name.
+
 ### Miscellaneous
 
 #### `craftguide.show(player_name, item, show_usages)`