add on_replace function (thanks wuzzy)

api.lua

@@ -944,12 +944,23 @@ local replace = function(self, pos)
 
 -- print ("replace node = ".. minetest.get_node(pos).name, pos.y)
 
-		minetest.set_node(pos, {name = with})
+		local oldnode = {name = what}
+		local newnode = {name = with}
+		local on_replace_return
 
-		-- when cow/sheep eats grass, replace wool and milk
-		if self.gotten == true then
-			self.gotten = false
-			self.object:set_properties(self)
+		if self.on_replace then
+			on_replace_return = self.on_replace(self, pos, oldnode, newnode)
+		end
+
+		if on_replace_return ~= false then
+
+			minetest.set_node(pos, {name = with})
+
+			-- when cow/sheep eats grass, replace wool and milk
+			if self.gotten == true then
+				self.gotten = false
+				self.object:set_properties(self)
+			end
 		end
 	end
 end
@@ -2599,6 +2610,7 @@ minetest.register_entity(name, {
 	replace_what = def.replace_what,
 	replace_with = def.replace_with,
 	replace_offset = def.replace_offset or 0,
+	on_replace = def.on_replace,
 	timer = 0,
 	env_damage_timer = 0, -- only used when state = "attack"
 	tamed = false,

api.txt

@@ -1,5 +1,5 @@
 
-MOB API (2nd July 2017)
+MOB API (4th July 2017)
 
 The mob api is a function that can be called on by other mods to add new animals or monsters into minetest.
 
@@ -105,12 +105,21 @@ This functions registers a new mob as a Minetest entity.
             'explode' sound when exploding
             'distance' maximum distance sounds are heard from (default is 10)
 
-Mobs can look for specific nodes as they walk and replace them to mimic eating
+Mobs can look for specific nodes as they walk and replace them to mimic eating.
 
         'replace_what' group if items to replace e.g. {"farming:wheat_8", "farming:carrot_8"}
         'replace_with' replace with what e.g. "air" or in chickens case "mobs:egg"
         'replace_rate' how random should the replace rate be (typically 10)
         'replace_offset' +/- value to check specific node to replace
+       	'on_replace(self, pos, oldnode, newnode)' gets called when mob is about to replace a node
+	    self: ObjectRef of mob
+	    pos: Position of node to replace
+	    oldnode: Current node
+	    newnode: What the node will become after replacing
+
+	    If false is returned, the mob will not replace the node.
+
+	    By default, replacing sets self.gotten to true and resets the object properties.
 
 The 'replace_what' has been updated to use tables for what, with and y_offset e.g.