Block update

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A block update is an in-game mechanism that occurs when a block is modified in some way. Limited to the computing power of a computer, the game can not process all blocks at all times, which is why the game needs a mechanism such as a block update. When a block changes (due to a player, an entity, or a chunk tick, etc.), it notifies nearby blocks that they should check if they need to respond to the change.

Java Edition[edit]

In Java Edition, there are three types of block updates: NC update, PP update, and comparator update.

Priorities[edit]

When a block changes, it can sends different block updates that have different priorities:

Priority Description Method in code
-3 Updates before the change. N/A
-2 Some updates after the change. Blockstate.onRemove()
-1 Some updates after the change. Blockstate.onPlace()
0 Genaral NC updates. Level.blockUpdated()
1 Genaral comparator updates. Level.updateNeighbourForOutputSignal()
2 Genaral PP updates. Level.updateNeighbourShapes()
3 Later updates. N/A

PP update[edit]

A PP update (i.e. update post placement, update neighbour shapes, update shape) is the most common type of block updates, mainly used for connected blocks changing their shapes (e.g. stairs, fences) and attached blocks dropping into items (e.g. torches, cactus, wheat). It almost always has a priority of 2.

Sending[edit]

Range of General PP updates (Green wool)
General PP updates
When a block is placed, destroyed, replaced, or its blockstate is changed, the game sends PP update to its immediate neighbors, in order of west, east, north, south, down, up.
Exception
However, there are some exceptions:
  • Lighting a nether portal does not send PP update.
  • A nether portal generating does not send PP update.
  • Changing block state with debug stick does not send PP update.
  • Shulker boxes sends general PP updates when beginning to open/close or complete openning/closing.
  • When a redstone wire is placed, destroyed, replaced, or its blockstate is changed, for any block that the redstone wire points horizontally to, if it is not a redstone wire, then the game also sends PP updates to the block above it and the block below it. However, it does not send PP update to an observer.
  • Pistons and beds have more complex updates.

Receiving[edit]

When a block receives a PP update, the direction of the PP update may be considered. For example, a wheat accepts only PP updates from above.

Only following blocks can receive PP updates:

Block What to do when receiving a PP update
Attached blocks (e.g. torches, snow, carpet, doors, fires, etc.) Tries to drop as an item.
Connective blocks (e.g. fences, stairs, chests, redstone wire, nether portal, etc.) Tries to changes its shape.
Redstone repeater Checks for being locked or unlocked.
Note block Tries to updates its instrument type.
Leaves Calculates the distance to logs.
Concrete powder Solidifies if it is contacting with water or lava.
Water and Waterlogged blocks Requests a scheduled tick for flowing.
Observer Requests a scheduled tick for pulsing.
Gravity-affected blocks Requests a scheduled tick for checking for falling.
Grass path and Farmland Requests a scheduled tick for checking whether it is covered with a solid block.
Cactus Requests a scheduled tick for checking for dropping.
Coral Requests a scheduled tick for checking whether it is outside of water.
Magma block Tries to request a scheduled tick to generate bubble columns.

NC update[edit]

NC update (i.e. neighbor changed, update neighbors, redstone update) is the most widely known type of block updates, mainly used for redstone components.

Sending[edit]

Range of General NC updates (Green wool)
General NC updates
Most blocks send NC updates to their immediate neighbors when they are placed, destroyed, or replaced, or their blockstates are changed, in order of west, east, down, up, north, south.
Exception
Some changes do not produce General NC updates:
Other NC updates
Some blocks (mainly redstone components) sends Sends more NC updates other than general NC updates when changing.
Block Change Priority Range and order Schematic diagram
Lever / Button / Pressure plate / Tripwire hook Destroy an activated lever/button/pressure plate/tripwire hook. -2 Their immediate neighbors (in order of west, east, down, up, north, south). Then the immediate neighbors (in order of west, east, down, up, north, south) of the block they're attached to.
Active or deactive them.
Change pressure plate's signal.
Tripwire hook connects to a tripwire.
3
Repeater and Comparator Place or its blockstate changes. -1 The block they're facing. Then the immediate neighbors (without the repeater/comparator itself. In order of west, east, down, up, north, south) of the block they're facing.
Remove. -2
Comparator's signal changes. 3
Chest Begin to be open/closed N/A (the block is not changed) Its immediate neighbors (in order of west, east, down, up, north, south).
Trapped chest Begin to be open/closed N/A (the block is not changed) Its immediate neighbors (in order of west, east, down, up, north, south). Then the immediate neighbors (in order of west, east, down, up, north, south) of the block below it.
Observer Lit or unlit. 3 The block it's facing. Then the immediate neighbors (without the observer itself. In order of west, east, down, up, north, south) of the block it's facing.
Destroy or replace an activated observer. -2
Place an activated observer. -1 Set itself to unactivated without any update. Then update the block it's facing. Then the immediate neighbors (without the observer itself. In order of west, east, down, up, north, south) of the block it's facing.
Redstone torch Place or its blockstate changes. -1 The immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of the torch.
Destroy, replace, or its blockstate changes. -2
Redstone wire Destroy or replace. -2 The immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of the wire. Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of adjacent redstone wires (in order of north, east, south, west). Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of redstone wires above the solid blocks adjacent to the redstone wire (in order of north, east, south, west). Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of redstone wires below the non-solid blocks adjacent to the redstone wire (in order of north, east, south, west).
R
When removing "R"
R
When removing "R"
Place. -1 Updates the immediate neighbors (in order of west, east, down, up, north, south) of the block above the wire. Then updates the immediate neighbors (in order of west, east, down, up, north, south) of the block below the wire. Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of adjacent redstone wires (in order of north, east, south, west). Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of redstone wires above the solid blocks adjacent to the redstone wire (in order of north, east, south, west). Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of redstone wires below the non-solid blocks adjacent to the redstone wire (in order of north, east, south, west).
R
When removing "R"
R
When removing "R"
Its signal changes 3 Update the immediate neighbors (in order of west, east, down, up, north, south) of the datum points. Datum points include the wire and the immediate neighbors of the wire, in random order.
Changes between dot and cross 3 Update the immediate neighbors (without the wire itself, and in order of west, east, down, up, north, south) of the solid block next to the wire (in order of north, east, south, west).
Rails [more information needed]
Any block Placed with a command. 3 Update the immediate neighbors (in order of west, east, down, up, north, south).

Receiving[edit]

When a block receives a NC update, the source of the NC update may be considered. For example, a rail accepts only NC updates from power components.

Only following blocks can receive NC updates:

Block What to do when receiving an NC update
Frosted ice Tries to melt.
Redstone wire Tries to change its state.
Redstone repeater
Redstone comparator
Redstone torch
Rail
Detector rail
Activator rail
Powered rail
Command blocks
Dropper
Dispenser
Note block
Redstone lamp
TNT
Piston
Trapdoors
Fence gates
Doors
Water Tries to make itself in the right state.
Lava
Soul sand Request a scheduled tick to generate bubble columns.
Sponge Tries to absorb water.

Comparator update[edit]

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Comparator updates

Redstone comparators can use certain blocks and entities as power sources, and in turn, the blocks or entities send comparator updates (i.e. Block entity updates, Update neighbour for output signal) when changing.

When a block/entity that can be detected by comparators changes, it sends updates to surrounding comparators (include comparators separated by a solid block), in order of north, east, south, and west.

Only comparators can receive these updates. When receiving, they calculates and changes their signal.

Special blocks[edit]

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Pistons and beds have complex behavior.

Block updates and DFS[edit]

Block updates follow the principle of DFS.

For example, when Block A changes, it maybe sends block updates to B and C one by one. Receiving the update, B maybe changes and sends updates to D and E. Receiving update from B, D changes and sends updates to some block. Then E receives the update from B. Then C receives the update from A.

Self update[edit]

When a block is placed, it maybe checks and adapts itself to its surroundings, in a priority of -1. Strictly speaking, this isn't a block update. However , there is a significant bug: When a block is placed/changed, if it changes again when self update, then the former change does not produce any updates that is in a priority of 0, 1, or 2. That's why self update is mentioned on this page.

Here is a list that self updates that can cause the bug.

  • Hopper
    • When being placed, if it should be powered, becomes powered.
  • Wet sponge
    • If in the nether, it turns into an dry sponge.
  • Sponge
    • When being placed, it absorbs water and turns into an wet sponge.
  • Target and Observer
    • When a powered target or observer is placed, it turns into an unpowered state.
  • Tripwire
    • When being placed, it updates the signal and connection.
  • Fire
    • When being placed, if it should be extinguished, removes itself.
  • TNT
    • When being placed, if it should explode, removes itself.
  • Rail, Detector rail, Activator rail, Powered rail
    • When being placed, it updates its shape.
  • Detector rail
    • When being placed, it checks whether it is pressed and updates its state.
  • Carved pumpkin
    • When being placed, if it should turn into a golem, removes itself.

Bedrock Edition[edit]

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History[edit]

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