LayoutPluginManager example

$this->themeHandler->themeExists('theme_a')->willReturn(TRUE);
    $this->themeHandler->themeExists('core')->willReturn(FALSE);
    $this->themeHandler->themeExists('invalid_provider')->willReturn(FALSE);

    $theme_a = new Extension('vfs://root', 'theme', 'themes/theme_a/theme_a.layouts.yml');
    $this->themeHandler->getTheme('theme_a')->willReturn($theme_a);
    $this->themeHandler->getThemeDirectories()->willReturn(['theme_a' => vfsStream::url('root/themes/theme_a')]);

    $this->cacheBackend = $this->prophesize(CacheBackendInterface::class);

    $namespaces = new \ArrayObject(['Drupal\Core' => vfsStream::url('root/core/lib/Drupal/Core')]);
    $this->layoutPluginManager = new LayoutPluginManager($namespaces, $this->cacheBackend->reveal(), $this->moduleHandler->reveal(), $this->themeHandler->reveal());
  }

  /** * @covers ::getDefinitions * @covers ::providerExists */
  public function testGetDefinitions() {
    $expected = [
      'module_a_provided_layout',
      'theme_a_provided_layout',
      'plugin_provided_layout',
    ];