Some base work for forms API

src/pocketmine/Player.php

@@ -71,6 +71,7 @@
 use pocketmine\event\TextContainer;
 use pocketmine\event\Timings;
 use pocketmine\event\TranslationContainer;
+use pocketmine\forms\BaseForm;
 use pocketmine\inventory\BigCraftingGrid;
 use pocketmine\inventory\CraftingGrid;
 use pocketmine\inventory\Inventory;
@@ -119,6 +120,7 @@
 use pocketmine\network\mcpe\protocol\LoginPacket;
 use pocketmine\network\mcpe\protocol\MapInfoRequestPacket;
 use pocketmine\network\mcpe\protocol\MobEquipmentPacket;
+use pocketmine\network\mcpe\protocol\ModalFormRequestPacket;
 use pocketmine\network\mcpe\protocol\MovePlayerPacket;
 use pocketmine\network\mcpe\protocol\PlayerActionPacket;
 use pocketmine\network\mcpe\protocol\PlayerHotbarPacket;
@@ -296,6 +298,11 @@ public static function isValidUserName(string $name) : bool{
 	/** @var int|null */
 	protected $lineHeight = null;
 
+	/** @var int */
+	protected $formIdCounter = 0;
+	/** @var BaseForm[] */
+	protected $forms = [];
+
 	/**
 	 * @return TranslationContainer|string
 	 */
@@ -3271,6 +3278,35 @@ public function sendWhisper(string $sender, string $message){
 		$this->dataPacket($pk);
 	}
 
+	public function sendForm(BaseForm $form) : void{
+		$id =  $this->formIdCounter++;
+
+		$pk = new ModalFormRequestPacket();
+		$pk->formId = $id;
+		$pk->formData = json_encode($form);
+		$this->dataPacket($pk);
+
+		$this->forms[$id] = $form;
+	}
+
+	public function onFormSubmit(int $formId, $responseData) : bool{
+		$form = $this->forms[$formId] ?? null;
+
+		if($form === null){
+			return false;
+		}
+
+		try{
+			$form->handleResponse($this, $responseData);
+		}catch(\Throwable $e){
+			$this->server->getLogger()->logException($e);
+		}
+
+		unset($this->forms[$formId]);
+
+		return true;
+	}
+
 	/**
 	 * Note for plugin developers: use kick() with the isAdmin
 	 * flag set to kick without the "Kicked by admin" part instead of this method.

src/pocketmine/forms/BaseForm.php

@@ -0,0 +1,69 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+*/
+
+declare(strict_types=1);
+
+namespace pocketmine\forms;
+
+use pocketmine\network\mcpe\protocol\ModalFormRequestPacket;
+use pocketmine\network\mcpe\protocol\ModalFormResponsePacket;
+use pocketmine\Player;
+
+abstract class BaseForm implements \JsonSerializable{
+
+	const TYPE_MODAL = "modal";
+	const TYPE_FORM = "form";
+	const TYPE_CUSTOM_FORM = "custom_form";
+
+	/**
+	 * Returns the type used to show this form to clients
+	 * @return string
+	 */
+	abstract public function getType() : string;
+
+	/**
+	 * Handles a modal form response from a player
+	 *
+	 * @param Player $player
+	 * @param mixed  $packet
+	 */
+	abstract public function handleResponse(Player $player, $data) : void;
+
+	/**
+	 * Serializes the form to JSON for sending to clients.
+	 *
+	 * @return array
+	 */
+	final public function jsonSerialize() : array{
+		$jsonBase = [
+			"type" => $this->getType()
+		];
+
+		return array_merge($jsonBase, $this->serializeFormData());
+	}
+
+	/**
+	 * Serializes additional data needed to show this form to clients.
+	 * @return array
+	 */
+	abstract protected function serializeFormData() : array;
+
+}

src/pocketmine/forms/ModalForm.php

@@ -0,0 +1,86 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+*/
+
+declare(strict_types=1);
+
+namespace pocketmine\forms;
+
+use pocketmine\network\mcpe\protocol\ModalFormResponsePacket;
+use pocketmine\Player;
+
+/**
+ * This form type present a simple "yes/no" dialog with two buttons.
+ */
+abstract class ModalForm extends BaseForm{
+
+	/** @var string */
+	private $title;
+	/** @var string */
+	private $content;
+	/** @var string */
+	private $button1;
+	/** @var string */
+	private $button2;
+
+	/**
+	 * @param string $title Text to put on the title of the dialog.
+	 * @param string $text Text to put in the body.
+	 * @param string $button1Text Text to show on the "Yes" button. Defaults to client-translated "Yes" string.
+	 * @param string $button2Text Text to show on the "No" button. Defaults to client-translated "No" string.
+	 */
+	public function __construct(string $title, string $text, string $button1Text = "gui.yes", string $button2Text = "gui.no"){
+		$this->title = $title;
+		$this->content = $text;
+		$this->button1 = $button1Text;
+		$this->button2 = $button2Text;
+	}
+
+	public function getType() : string{
+		return self::TYPE_MODAL;
+	}
+
+	final public function handleResponse(Player $player, $data) : void{
+		if(!is_bool($data)){
+			throw new \UnexpectedValueException("Expected bool, got " . gettype($data));
+		}
+
+		$this->onSubmit($player, $data);
+	}
+
+	/**
+	 * Called when a player submits this form. Plugins should extend the class and implement this method to handle form
+	 * responses.
+	 *
+	 * @param Player $player The player who submitted the form.
+	 * @param bool   $responseValue True if the player clicked button1, false if button2.
+	 */
+	abstract public function onSubmit(Player $player, bool $responseValue) : void;
+
+	public function serializeFormData() : array{
+		return [
+			"title" => $this->title,
+			"content" => $this->content,
+			"button1" => $this->button1,
+			"button2" => $this->button2
+		];
+	}
+
+}

src/pocketmine/network/mcpe/PlayerNetworkSessionAdapter.php

@@ -232,7 +232,7 @@ public function handlePlayerSkin(PlayerSkinPacket $packet) : bool{
 	}
 
 	public function handleModalFormResponse(ModalFormResponsePacket $packet) : bool{
-		return false; //TODO: GUI stuff
+		return $this->player->onFormSubmit($packet->formId, json_decode($packet->formData, true));
 	}
 
 	public function handleServerSettingsRequest(ServerSettingsRequestPacket $packet) : bool{