* CraftBlockState#getWorldHandle() would previously run into a NPE when being invoked for a non-placed BlockState. It now returns null in this case.
* CraftChest#getInventory() would previously encounter this NPE when being called for a non-placed BlockState. It will now automatically forward the call to #getBlockInventory() when it is being called for either a non-placed BlockState, or during world generation.
* CraftStructureBlock#applyTo was able to run into a NPE when being called for non-placed BlockStates (this is for example called by #getSnapshotNBT()).
* Replaced all no-world-generation preconditions with a general 'ensureNoWorldGeneration' method.
* Removed a few superfluous #isPlaced() calls. If getWorldHandle() reutrns a World, this already implies that the BlockState is placed.
## **Current API**
The current world generation API is very old and limited when you want to make more complex world generation. Resulting in some hard to fix bugs such as that you cannot modify blocks outside the chunk in the BlockPopulator (which should and was per the docs possible), or strange behavior such as SPIGOT-5880.
## **New API**
With the new API, the generation is more separate in multiple methods and is more in line with Vanilla chunk generation. The new API is designed to as future proof as possible. If for example a new generation step is added it can easily also be added as a step in API by simply creating the method for it. On the other side if a generation step gets removed, the method can easily be called after another, which is the case with surface and bedrock. The new API and changes are also fully backwards compatible with old chunk generators.
### **Changes in the new api**
**Extra generation steps:**
Noise, surface, bedrock and caves are added as steps. With those generation steps three extra methods for Vanilla generation are also added. Those new methods provide the ChunkData instead of returning one. The reason for this is, that the ChunkData is now backed by a ChunkAccess. With this, each step has the information of the step before and the Vanilla information (if chosen by setting a 'should' method to true). The old method is deprecated.
**New class BiomeProvider**
The BiomeProvider acts as Biome source and wrapper for the NMS class WorldChunkManager. With this the underlying Vanilla ChunkGeneration knows which Biome to use for the structure and decoration generation. (Fixes: SPIGOT-5880). Although the List of Biomes which is required in BiomeProvider, is currently not much in use in Vanilla, I decided to add it to future proof the API when it may be required in later versions of Minecraft.
The BiomeProvider is also separated from the ChunkGenerator for plugins which only want to change the biome map, such as single Biome worlds or if some biomes should be more present than others.
**Deprecated isParallelCapable**
Mojang has and is pushing to a more multi threaded chunk generation. This should also be the case for custom chunk generators. This is why the new API only supports multi threaded generation. This does not affect the old API, which is still checking this.
**Base height method added**
This method was added to also bring the Minecraft generator and Bukkit generator more in line. With this it is possible to return the max height of a location (before decorations). This is useful to let most structures know were to place them. This fixes SPIGOT-5567. (This fixes not all structures placement, desert pyramids for example are still way up at y-level 64, This however is more a vanilla bug and should be fixed at Mojangs end).
**WorldInfo Class**
The World object was swapped for a WorldInfo object. This is because many methods of the World object won't work during world generation and would mostly likely result in a deadlock. It contains any information a plugin should need to identify the world.
**BlockPopulator Changes**
Instead of directly manipulating a chunk, changes are now made to a new class LimitedRegion, this class provides methods to populated the chunk and its surrounding area. The wrapping is done so that the population can be moved into the place where Minecraft generates decorations. Where there is no chunk to access yet. By moving it into this place the generation is now async and the surrounding area of the chunk can also be used.
For common methods between the World and LimitedRegion a RegionAccessor was added.
* Actually capture all the data of TileEntities. This is done by creating a copy of the TileEntity. The methods of BlockState which currently directly access the TileEntity reference will modify the data of that TileEntity-snapshot instead.
* With the call to BlockState.update, the captured TileEntity data gets applied to the current TileEntity in the world.
* Methods which trigger block specific actions will use the current TileEntity from the world.
* CraftBlockState does not hand out the wrapped or the snapshot TileEntity directly. Instead, it provides an applyTo method to copy the data to a given TileEntity and a method to directly get a copy of the TileEntity NBT data represented by the BlockState. CraftMetaBlockState was updated to make use of that.
* Added #getSnapshotInventory() to bukkit which allows modifiying the captured inventory snapshots of containers.
* Tried to clarify which methods only work if the BlockState is placed, which methods require the block in the world to still be of the same type (methods which trigger actions), and that .getInventory() directly modifies the inventory of the block in the world if the BlockState is placed and becomes invalid if the block type is changed.
Backwards compatibility
* If the BlockState acts as InventoryHolder, getInventory() will still return the inventory directly backed by the TileEntity in the world (like before), and not the snapshot inventory. This compromise should reduce the potential of these changes to break existing plugins, or craftbukkit's own use of BlockState.
* The snapshot's inventory can be accessed by a new method getSnapshotInventory()
* In case the BlockState is not placed (if it was retrieved from the MetaBlockState of an item), the getInventory() method will however return the snapshot inventory. So that when the BlockState gets applied back to the item, the inventory changes are properly included.
* With the changes to CraftMetaBlockState it is no longer required to call the update method before passing a modified BlockState to the CraftMetaBlockState. For backwards compatibility the update method will simply return true for a non-placed BlockState, without actually doing anything.
Impact on plugins
* Restoring blocks now actually works as expected, properly restoring the TileEntity data, reglardless if the block changed its type in the meantime.
* Plugins are now consistently required to call the update method in order to apply changes to blocks. Though, regarding the Javadoc they should have been required to do so anyways.
* New feature: Plugins can take and modify inventory snapshots.
* Breaking change: If a plugin gets the BlockState of a block in the world, modifies the inventory returned by .getInventory(), and then tries to use the same BlockState to apply the TileEntity data to an ItemStack block meta, the ItemStack will use the snapshot inventory, disregarding the changes made to the inventory returned by .getInventory(). This is the compromise of .getInventory() returning the inventory directly backed by the TileEntity in the world.
Other fixes related to BlockState:
* TileEntityContainer#getLocation() will run into a NPE if the TileEntity is non-placed (ex. when getting the BlockState from a CraftMetaBlockState).
* Beacon.getEntitiesInRange() would previously throw a NPE if called for a non-placed BlockState. It was changed to now require to be placed and use the current TileEntity in the world. If the TileEntity in the world is no longer a beacon, it will return an empty list.
* EndGateway now supports setting and getting the exit location even for non-placed EndGateways (inside BlockStateMeta) by using / returning a location with world being null.
The javadocs state that a null may be used to remove the currently
playing sound, however this causes a NullPointerException.
It also doesn't process registering the record correctly, along with
processing non-valid items.
