VillagerInventory.php

<?php
namespace villagertrade;

use pocketmine\entity\Villager;
use pocketmine\inventory\BaseInventory;
use pocketmine\item\Item;
use pocketmine\nbt\NetworkLittleEndianNBTStream;
use pocketmine\nbt\tag\{ByteTag, CompoundTag, IntTag, ListTag};
use pocketmine\network\mcpe\protocol\UpdateTradePacket;
use pocketmine\Player;

class VillagerInventory extends BaseInventory {

    /** @var Villager */
    private $villager;

    public function __construct(Villager $villager)
    {
        $this->villager = $villager;
        parent::__construct();
    }

    public function getName() : string
    {
        return "Villager Inventory";
    }

    public function getDefaultSize() : int
    {
        return 2;
    }

    public function getVillager() : Villager
    {
        return $this->villager;
    }

    public function onOpen(Player $player) : void
    {
        parent::onOpen($player);

        $pk = new UpdateTradePacket();
        $pk->windowId = $player->getWindowId($this);
        $pk->varint1 = 0;//could be career?
        $pk->varint2 = 0;//could be careerlevel?
        $pk->isWilling = false;
        $pk->traderEid = $this->villager->getId();
        $pk->playerEid = $player->getId();
        $pk->displayName = $this->getName();

        $writer = new \pocketmine\nbt\NetworkLittleEndianNBTStream();
        $writer->setData(
            new CompoundTag("Offers", [
                new ListTag("Recipes", [
                    new CompoundTag("", [
                        new ByteTag("rewardExp", 1),
                        new IntTag("maxUses", mt_rand(2, 12)),
                        new IntTag("uses", 0),
                        Item::get(Item::DIAMOND)->nbtSerialize(-1, "buy"),
                        Item::get(Item::PAPER)->nbtSerialize(-1, "sell")
                    ])
                ])
            ])
        );

        $pk->offers = $writer->write();
        $player->dataPacket($pk);
    }
}