How do I assign variables from BlockBase object?

Question

I want to create a Block-Module for drupal 8 (beta 12). To display the Content I want to use a twig-template (mytest-block.html.twig).

But the Twig-Template needs some Data to Display, So I need a Method to assign Some Variables from MytestBlock.php to the Template.

I know how to assign Variables from mytest.module, but I have no idea how to assign Variables from the Block-Plugin MytestBlock.php.

Or is it possible to access the instance of MytestBlock.php and therefore call a method which would return the variables as an array?

My MWE-Module consists of following files:

mytest/
├── mytest.info.yml
├── mytest.module
├── src
│   └── Plugin
│       └── Block
│           └── MytestBlock.php
└── templates
    └── mytest-block.html.twig

File Content mytest.info.yml:

name: mytest
description: A Testmodule
core: 8.x
package: Custom
version: 0.1
dependencies:
    - block
type: module

File Content mytest.module:

<?php

function mytest_theme() {
    return array('mytest_block' =>
        array('variables' => array('test' => 'hello')));
}

function template_preprocess_mytest_block(&$variables) {
    $variables["test2"] = "hello2";

    //a call to get_variables() would also be an option, e.g.:
    //$variables = array_merge($variables, $A_INSTANCE->get_variables());
}

File Content mytest-block.html.twig:

Template
{{ test }}
{{ test2 }}
{{ test3 }}
{{ test4 }}

File Content MytestBlock.php:

<?php

namespace Drupal\mytest\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Block\BlockPluginInterface;

/**
 * Provides a Test-Block
 *
 * @Block(
 *   id = "MytestBlock",
 *   admin_label = @Translation("My Testblock"),
 * )
 */
class MytestBlock extends BlockBasMytestBlock.phpe implements BlockPluginInterface {
  public function build() {
    return array(
        '#theme' => 'mytest_block',
        /*here I want to assign some variables: e.g.:
         * test3 = hello3 */
    );
  }

  public function get_variables() {
    retrun array("test4" => "hello4");
  }
}

In this example, only test and test2 are assigned, but I want, that test3 and test4 are assigned too.

Answer 1

Via try and error, I found out, that declaring the variables-array in the function mytest_theme() makes it possible, to set the variable in the MytestBlock-Class:

Changes in mytest.module:

function mytest_theme() {
    return array('mytest_block' =>
        array('variables' => array('test' => 'hello', 'test3' => null)));
}

Changes in MytestBlock.php:

class MytestBlock extends BlockBase implements BlockPluginInterface {
  public function build() {
    return array(
        '#theme' => 'mytest_block',
        '#test3' => 'hello3',
    );
  }
}

But I still have no idea how to access the Block-Plugin Instance in mytest.module.