Cancel block drop on ExplosionPrimeEvent?

How?

I don't think you can do that. You can however set the block list in EntityExplodeEvent.
Edit: Maybe you can cancel the ExplosionPrimeEvent, make a new Explosion class that extends pocketmine\level\Explosion, which also has a new explodeB() method which does what you want without dropping blocks.
Example:

PHP:

ExplosionWithoutBlockdrops.php

class ExplosionWithoutBlockdrops extends Explosion{
   public function explodeB(){
      $send = [];
      $updateBlocks = [];
      $source = (new Vector3($this->source->x, $this->source->y, $this->source->z))->floor();
      $yield = (1 / $this->size) * 100;
      if($this->what instanceof Entity){
         $this->level->getServer()->getPluginManager()->callEvent($ev = new EntityExplodeEvent($this->what, $this->source, $this->affectedBlocks, $yield));
         if($ev->isCancelled()){
            return false;
         }else{
            $yield = $ev->getYield();
            $this->affectedBlocks = $ev->getBlockList();
         }
      }
      $explosionSize = $this->size * 2;
      $minX = Math::floorFloat($this->source->x - $explosionSize - 1);
      $maxX = Math::ceilFloat($this->source->x + $explosionSize + 1);
      $minY = Math::floorFloat($this->source->y - $explosionSize - 1);
      $maxY = Math::ceilFloat($this->source->y + $explosionSize + 1);
      $minZ = Math::floorFloat($this->source->z - $explosionSize - 1);
      $maxZ = Math::ceilFloat($this->source->z + $explosionSize + 1);
      $explosionBB = new AxisAlignedBB($minX, $minY, $minZ, $maxX, $maxY, $maxZ);
      $list = $this->level->getNearbyEntities($explosionBB, $this->what instanceof Entity ? $this->what : null);
      foreach($list as $entity) {
         $distance = $entity->distance($this->source) / $explosionSize;
         if ($distance <= 1) {
            $motion = $entity->subtract($this->source)->normalize();
            $impact = (1 - $distance) * ($exposure = 1);
            $damage = (int)((($impact * $impact + $impact) / 2) * 8 * $explosionSize + 1);
            if ($this->what instanceof Entity){
               $ev = new EntityDamageByEntityEvent($this->what, $entity, EntityDamageEvent::CAUSE_ENTITY_EXPLOSION, $damage);
            }elseif($this->what instanceof Block){
               $ev = new EntityDamageByBlockEvent($this->what, $entity, EntityDamageEvent::CAUSE_BLOCK_EXPLOSION, $damage);
            }else{
               $ev = new EntityDamageEvent($entity, EntityDamageEvent::CAUSE_BLOCK_EXPLOSION, $damage);
            }
               $entity->attack($ev->getFinalDamage() , $ev);
               $entity->setMotion($motion->multiply($impact));
            }
         }
         $air = Item::get(Item::AIR);
         foreach($this->affectedBlocks as $block) {
            if ($block->getId() === Block::TNT) {
               $mot = (new Random())->nextSignedFloat() * M_PI * 2;
               $tnt = Entity::createEntity("PrimedTNT", $this->level->getChunk($block->x >> 4, $block->z >> 4) , new Compound("", ["Pos" => new Enum("Pos", [new Double("", $block->x + 0.5) , new Double("", $block->y) , new Double("", $block->z + 0.5) ]) , "Motion" => new Enum("Motion", [new Double("", -sin($mot) * 0.02) , new Double("", 0.2) , new Double("", -cos($mot) * 0.02) ]) , "Rotation" => new Enum("Rotation", [new Float("", 0) , new Float("", 0) ]) , "Fuse" => new Byte("Fuse", mt_rand(10, 30)) ]));
$tnt->spawnToAll();
         }elseif (mt_rand(0, 100) < $yield) {
            /*foreach($block->getDrops($air) as $drop) {
               $this->level->dropItem($block->add(0.5, 0.5, 0.5) , Item::get( . . . $drop));
            }*/
            //this is where we stop the blocks dropping
         }
         $this->level->setBlockIdAt($block->x, $block->y, $block->z, 0);
         $pos = new Vector3($block->x, $block->y, $block->z);
         for ($side = 0; $side < 5; $side++) {
            $sideBlock = $pos->getSide($side);
            if (!isset($this->affectedBlocks[$index = Level::blockHash($sideBlock->x, $sideBlock->y, $sideBlock->z) ]) and !isset($updateBlocks[$index])) {
               $this->level->getServer()->getPluginManager()->callEvent($ev = new BlockUpdateEvent($this->level->getBlock($sideBlock)));
               if (!$ev->isCancelled()) {
                  $ev->getBlock()->onUpdate(Level::BLOCK_UPDATE_NORMAL);
               }
               $updateBlocks[$index] = true;
            }
         }
         $send[] = new Vector3($block->x - $source->x, $block->y - $source->y, $block->z - $source->z);
      }
      $pk = new ExplodePacket();
      $pk->x = $this->source->x;
      $pk->y = $this->source->y;
      $pk->z = $this->source->z;
      $pk->radius = $this->size;
      $pk->records = $send;
      $this->level->addChunkPacket($source->x >> 4, $source->z >> 4, $pk);
      return true;
   }
}

EventListener.php
class EventListener implements Listener{
   public function ExplosionPrimeEvent(ExplosionPrimeEvent $event){
      $event->setCancelled();
      $explosion = new ExplosionWithoutBlockDrops($event->getEntity(), $event->getForce(), $event->getEntity());
      $explosion->explodeA(); // not sure what this does
      $explosion->explodeB();
   }
}

You can also foreach the array you got from $event->getBlockList() and set those blocks to air and cancel EntityExplodeEvent (No damage to players) or $event->setBlockList([]); (Damages nearby players)

That is indeed a much better idea.

Ugh... That isn't example... I can make a plugin with that xD jk

PHP:

      public function ExplosionPrimeEvent(ExplosionPrimeEvent $e){
          $e->setBlockBreaking(false);
      }

Once again, please be reminded to follow PHP code conventions to starat class method names with lowercase so that others can read more easily.

Thanks, that's a good idea!

This always makes me wonder if PEMapModder have returned...