Slowing or boosting entities on a block

A Block Part changes how fast, or how high, entities move on the blocks it is attached to, as a block that overrides NeoForge's Block.getSpeedFactor() and Block.getJumpFactor() does. Both Part methods get the loop context of the side that ticks the entity (no Host) and an event with the block state and the factor to edit, which starts at the block's own value (honey is 0.4 speed and 0.5 jump, soul sand 0.4 speed, every other block 1.0).

struct Mud;

impl mod_api::BlockPart for Mud {
    fn block_speed_factor(&self, _: &dyn mod_api::LoopContext, event: &mut mod_api::BlockSpeedFactorEvent) {
        event.factor = 0.25;
    }
    fn block_jump_factor(&self, _: &dyn mod_api::LoopContext, event: &mut mod_api::BlockJumpFactorEvent) {
        event.factor = 0.5;
    }
}

// In `register`: let mud = r.block_key("mymod:mud"); r.block_part(mud, Mud);

The entity reads the block at its feet and, when that gives 1.0 (and, for speed, the block is not water or a bubble column), the block below it. A part's factor is the block's factor in that rule, so 1.0 from your part lets the block below decide, exactly as for a vanilla block.

The player, every mob's ground and jump path, boats and minecarts read the factors where vanilla does. Both hooks fire on both sides and on the client's replayed ticks: register the mod on both and keep each method a pure function of its inputs, or prediction and the server disagree. A mod that wants every block uses living_speed and living_jump_power instead.

See the hook notes for the cost and the details.