Fixed an error that appears when trying to delete an instance that is already deleted. (#109)

* Prevent deleting already removed instances.

* Prehashed empty hash.

* Use go.exists instead of pcall, when detecting non existing instances.

README_API.md

@@ -222,6 +222,13 @@ IMPORTANT! If you provide a function you must also make sure to call `monarch.on
 * `fn_or_url` (function) - The function to call or URL to send message to
 
 
+## monarch.has_missing_resources(screen_id)
+Check if a screen has any missing resources. If the screen is loaded by a collection proxy with Load Dynamically this will use `collectionproxy.missing_resources()`. For other cases this function will always return false.
+
+**RETURN**
+* `result` (boolean) - true if the screen has missing resources
+
+
 ## monarch.debug()
 Enable verbose logging of the internals of Monarch.
 

monarch/monarch.lua

@@ -430,7 +430,9 @@ local function unload(screen, force)
 	elseif screen.factory then
 		log("unload() factory", screen.id)
 		for id, instance in pairs(screen.factory_ids) do
-			go.delete(instance)
+			if go.exists(instance) then
+				go.delete(instance)
+			end
 		end
 		screen.factory_ids = nil
 		if screen.auto_preload and not force then
@@ -462,8 +464,7 @@ local function preload(screen)
 	screen.preloading = true
 	if screen.proxy then
 		log("preload() proxy")
-		local missing_resources = collectionproxy.missing_resources(screen.proxy)
+		if M.has_missing_resources(screen.id) then
-		if #missing_resources > 0 then
 			local error_message = ("preload() collection proxy %s is missing resources"):format(tostring(screen.id))
 			log(error_message)
 			screen.preloading = false
@@ -1131,6 +1132,20 @@ function M.preload(id, options, cb)
 	return true -- return true for legacy reasons (before queue existed)
 end
 
+--- Check if a screen has missing resources, always returns false for factory
+-- @param id (string|hash) - Id of the screen to preload
+function M.has_missing_resources(id)
+	assert(id, "You must provide a screen id")
+	id = tohash(id)
+	assert(screens[id], ("There is no screen registered with id %s"):format(tostring(id)))
+
+	local screen = screens[id]
+	if screen.proxy then
+		local missing_resources = collectionproxy.missing_resources(screen.proxy)
+		return #missing_resources > 0
+	end
+	return false
+end
 
 --- Unload a preloaded monarch screen
 -- @param id (string|hash) - Id of the screen to unload
@@ -1361,8 +1376,10 @@ function M.on_post(id, fn_or_url)
 	end
 end
 
+local empty_hash = hash("")
+
 local function url_to_key(url)
-	return (url.socket or hash("")) .. (url.path or hash("")) .. (url.fragment or hash(""))
+	return (url.socket or empty_hash) .. (url.path or empty_hash) .. (url.fragment or empty_hash)
 end