Using ChunkStays
++ A plugin may need to manipulate data in arbitrary chunks, and it needs a way to make the server + guarantee that the chunks are available in memory.
+ +The problem
++ Usually when plugins want to manipulate larger areas of world data, they need to make sure that the + server has the appropriate chunks loaded in the memory. When the data being manipulated can be further + away from the connected players, or the data is being manipulated from a console handler, there is a + real chance that the chunks are not loaded.
++ This gets even more important when using the cBlockArea class for reading + and writing. Those functions will fail when any of the required chunks aren't valid. This means that + either the block area has incomplete data (Read() failed) or incomplete data has been written to the + world (Write() failed). Recovery from this is near impossible - you can't simply read or write again + later, because the world may have changed in the meantime.
+ +The solution
++ The naive solution would be to monitor chunk loads and unloads, and postpone the operations until all + the chunks are available. This would be quite ineffective and also very soon it would become very + difficult to maintain, if there were multiple code paths requiring this handling.
++ An alternate approach has been implemented, accessible through a single (somewhat hidden) function + call: cWorld:ChunkStay(). All that this call basically does is, it tells the + server "Load these chunks for me, and call this callback function once you have them all." And the + server does exactly that - it remembers the callback and asks the world loader / generator to provide + the chunks. Once the chunks become available, it calls the callback function for the plugin.
++ There are a few gotcha-s, though. If the code that was requesting the read or write had access to some + of the volatile objects, such as cPlayer or + cEntity objects, those cannot be accessed by the callback anymore, because + they may have become invalid in the meantime - the player may have disconnected, the entity may have + despawned. So the callback must use the longer way to access such objects, such as calling + cWorld:DoWithEntityByID() or + cWorld:DoWithPlayer().
+ +The example
+
+ As a simple example, consider a theoretical plugin that allows a player to save the immediate
+ surroundings of the spawn into a schematic file. The player issues a command to initiate the save, and
+ the plugin reads a 50 x 50 x 50 block area around the spawn into a cBlockArea and saves it on the disk
+ as "
+ The first attempt shows the naive approach. It simply reads the block area and saves it, then sends the + message. I'll repeat once more, this code is the wrong way to do it!
+
+function HandleCommandSaveSpawn(a_Split, a_Player)
+ -- Get the coords for the spawn:
+ local SpawnX = a_Player:GetWorld():GetSpawnX()
+ local SpawnY = a_Player:GetWorld():GetSpawnY()
+ local SpawnZ = a_Player:GetWorld():GetSpawnZ()
+ local Bounds = cCuboid(SpawnX - 25, SpawnY - 25, SpawnZ - 25, SpawnX + 25, SpawnY + 25, SpawnZ + 25)
+ Bounds:ClampY(0, 255)
+
+ -- Read the area around spawn into a cBlockArea, save to file:
+ local Area = cBlockArea()
+ local FileName = a_Player:GetName() .. "_spawn.schematic"
+ Area:Read(a_Player:GetWorld(), Bounds, cBlockArea.baTypes + cBlockArea.baMetas)
+ Area:SaveToSchematicFile(FileName)
+
+ -- Notify the player:
+ a_Player:SendMessage(cCompositeChat("The spawn has been saved", mtInfo))
+ return true
+end
+
+ + Now if the player goes exploring far and uses the command to save their spawn, the chunks aren't + loaded, so the BlockArea reading fails, the BlockArea contains bad data. Note that the plugin fails to + do any error checking and if the area isn't read from the world, it happily saves the incomplete data + and says "hey, everything's right", althought it has just trashed any previous backup of the spawn + schematic with nonsense data.
++
+ The following script uses the ChunkStay method to alleviate chunk-related problems. This is the + right way of doing it:
+
+function HandleCommandSaveSpawn(a_Split, a_Player)
+ -- Get the coords for the spawn:
+ local SpawnX = a_Player:GetWorld():GetSpawnX()
+ local SpawnY = a_Player:GetWorld():GetSpawnY()
+ local SpawnZ = a_Player:GetWorld():GetSpawnZ()
+ local Bounds = cCuboid(SpawnX - 25, SpawnY - 25, SpawnZ - 25, SpawnX + 25, SpawnY + 25, SpawnZ + 25)
+ Bounds:ClampY(0, 255)
+
+ -- Get a list of chunks that we need loaded:
+ local MinChunkX = math.floor((SpawnX - 25) / 16)
+ local MaxChunkX = math.ceil ((SpawnX + 25) / 16)
+ local MinChunkZ = math.floor((SpawnZ - 25) / 16)
+ local MaxChunkZ = math.ceil ((SpawnZ + 25) / 16)
+ local Chunks = {}
+ for x = MinChunkX, MaxChunkX do
+ for z = MinChunkZ, MaxChunkZ do
+ table.insert(Chunks, {x, z})
+ end
+ end -- for x
+
+ -- Store the player's name and world to use in the callback, because the a_Player object may no longer be valid:
+ local PlayerName = a_Player:GetName()
+ local World = a_Player:GetWorld()
+
+ -- This is the callback that is executed once all the chunks are loaded:
+ local OnAllChunksAvailable = function()
+ -- Read the area around spawn into a cBlockArea, save to file:
+ local Area = cBlockArea()
+ local FileName = PlayerName .. "_spawn.schematic"
+ if (Area:Read(World, Bounds, cBlockArea.baTypes + cBlockArea.baMetas)) then
+ Area:SaveToSchematicFile(FileName)
+ Msg = cCompositeChat("The spawn has been saved", mtInfo)
+ else
+ Msg = cCompositeChat("Cannot save the spawn", mtFailure)
+ end
+
+ -- Notify the player:
+ -- Note that we cannot use a_Player here, because it may no longer be valid (if the player disconnected before the command completes)
+ World:DoWithPlayer(PlayerName,
+ function (a_CBPlayer)
+ a_CBPlayer:SendMessage(Msg)
+ end
+ )
+ end
+
+ -- Ask the server to load our chunks and notify us once it's done:
+ World:ChunkStay(Chunks, nil, OnAllChunksAvailable)
+
+ -- Note that code here may get executed before the callback is called!
+ -- The ChunkStay says "once you have the chunks", not "wait until you have the chunks"
+ -- So you can't notify the player here, because the saving needn't have occurred yet.
+
+ return true
+end
+
+ + Note that this code does its error checking of the Area:Read() function, and it will not overwrite the + previous file unless it actually has the correct data. If you're wondering how the reading could fail + when we've got the chunks loaded, there's still the issue of free RAM - if the memory for the area + cannot be allocated, it cannot be read even with all the chunks present. So we still do need that + check.
+ +The conclusion
++ Although it makes the code a little bit longer and is a bit more difficult to grasp at first, the + ChunkStay is a useful technique to add to your repertoire. It is to be used whenever you need access to + chunks that may potentially be inaccessible, and you really need the data.
+Possibly the biggest hurdle in using the ChunkStay is the fact that it does its work in the + background, thus invalidating all cPlayer and cEntity objects your function may hold, so you need to + re-acquire them from their IDs and names. This is the penalty for using multi-threaded code.
+ +