1) AnvilDamageEvent.java

package net.minecraftforge.event.anvil;

import net.minecraft.block.BlockState;
import net.minecraftforge.eventbus.api.Cancelable;
import net.minecraftforge.eventbus.api.Event;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;

@Cancelable
public class AnvilDamageEvent extends Event
{
    /**
     * A simple event fired whenever code is fired from the FallingBlockEntity to damage the Anvil.
     * If the event is canceled, vanilla behaviour will not run and the Anvil's BlockState will not change (It won't be damaged).
     * If the event is not canceled, vanilla behaviour will run and the Anvil will be damaged.
     */
    public static class Pre extends AnvilDamageEvent
    {
        /**
         * The current blockstate beneath the FallingBlockEntity for the Anvil.
         */
        @Nullable
        private final BlockState groundState;

        public Pre(@Nonnull BlockState currentState, @Nullable BlockState groundState) {
            super(currentState);
            this.groundState = groundState;
        }

        @Nullable
        public BlockState getGroundState() {
            return groundState;
        }
    }

    /**
     * The current state of the Anvil pre-damage
     */
    @Nonnull
    private final BlockState currentState;

    /**
     * A simple event fired whenever code is fired to attempt to damage the Anvil.
     * If the event is canceled, vanilla behaviour will not run and the Anvil's BlockState will not change (It won't be damaged).
     * If the event is not canceled, vanilla behaviour will run and the Anvil will be damaged.
     */
    public AnvilDamageEvent(@Nonnull BlockState currentState)
    {
        this.currentState = currentState;
    }

    @Nonnull
    public BlockState getCurrentState() { return currentState; }
}

2) ForgeHooks.java

    public static boolean onAnvilDamagePre(BlockState currentState, BlockState groundState) {
        AnvilDamageEvent.Pre e = new AnvilDamageEvent.Pre(currentState, groundState);
        return MinecraftForge.EVENT_BUS.post(e);
    }

    public static boolean onAnvilDamage(BlockState currentState) {
        AnvilDamageEvent e = new AnvilDamageEvent(currentState);
        return MinecraftForge.EVENT_BUS.post(e);
    }

3) AnvilBlock.java

   @Nullable
   public static BlockState damage(BlockState state) {
      if (net.minecraftforge.common.ForgeHooks.onAnvilDamage(state)) return state;
      if (state.isIn(Blocks.ANVIL)) {
         return Blocks.CHIPPED_ANVIL.getDefaultState().with(FACING, state.get(FACING));
      } else {
         return state.isIn(Blocks.CHIPPED_ANVIL) ? Blocks.DAMAGED_ANVIL.getDefaultState().with(FACING, state.get(FACING)) : null;
      }
   }

4) FallingBlockEntity.java

   public boolean onLivingFall(float distance, float damageMultiplier) {
      if (this.hurtEntities) {
         int i = MathHelper.ceil(distance - 1.0F);
         if (i > 0) {
            List<Entity> list = Lists.newArrayList(this.world.getEntitiesWithinAABBExcludingEntity(this, this.getBoundingBox()));
            boolean flag = this.fallTile.func_235714_a_(BlockTags.ANVIL);
            DamageSource damagesource = flag ? DamageSource.ANVIL : DamageSource.FALLING_BLOCK;

            for(Entity entity : list) {
               entity.attackEntityFrom(damagesource, (float)Math.min(MathHelper.floor((float)i * this.fallHurtAmount), this.fallHurtMax));
            }

            if (flag && (double)this.rand.nextFloat() < (double)0.05F + (double)i * 0.05D && !net.minecraftforge.common.ForgeHooks.onAnvilDamagePre(this.fallTile, this.onGround ? this.getEntityWorld().getBlockState(this.getPositionUnderneath()) : null)) {
               BlockState blockstate = AnvilBlock.damage(this.fallTile);
               if (blockstate == null) {
                  this.dontSetBlock = true;
               } else {
                  this.fallTile = blockstate;
               }
            }
         }
      }

      return false;
   }