validateContexts example

// Try to get a value of a valid context, while this value has not been set.     try {
      $plugin->getContextValue('user');
    }
    catch (ContextException $e) {
      $this->assertSame("The 'entity:user' context is required and not present.", $e->getMessage(), 'Requesting a non-set value of a required context should throw a context exception.');
    }

    // Try to pass the wrong class type as a context value.     $plugin->setContextValue('user', $node);
    $violations = $plugin->validateContexts();
    $this->assertNotEmpty($violations, 'The provided context value does not pass validation.');

    // Set an appropriate context value and check to make sure its methods work     // as expected.     $user = User::create(['name' => $name]);
    $plugin->setContextValue('user', $user);

    $this->assertEquals($user->getAccountName(), $plugin->getContextValue('user')->getAccountName());
    $this->assertEquals($user->label(), $plugin->getTitle());

    // Test Optional context handling.