class TestForm

class TestForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['test_table'] = [
            '#type' => 'table',
            '#header' => [
                'Column 1',
                'Column 2',
                'Column 3',
            ],
            'row_1' => [
                'col_1' => [
                    '#plain_text' => 'foo',
                ],
                'col_2' => [
                    '#plain_text' => 'bar',
                ],
                'col_3' => [
                    '#plain_text' => 'baz',
                ],
            ],
            'row_2' => [
                'col_1' => [
                    '#plain_text' => 'one',
                ],
                'col_2' => [
                    '#plain_text' => 'two',
                ],
                'col_3' => [
                    '#plain_text' => 'three',
                ],
            ],
        ];
        $form['name'] = [
            '#type' => 'textfield',
            '#title' => 'Name',
            '#default_value' => 'Test name',
        ];
        $form['checkbox_enabled'] = [
            '#type' => 'checkbox',
            '#title' => 'Checkbox enabled',
            '#default_value' => TRUE,
        ];
        $form['checkbox_disabled'] = [
            '#type' => 'checkbox',
            '#title' => 'Checkbox disabled',
            '#default_value' => FALSE,
        ];
        $form['description'] = [
            '#type' => 'textfield',
            '#title' => 'Description',
            '#default_value' => '',
        ];
        $form['options'] = [
            '#type' => 'select',
            '#title' => 'Options',
            '#options' => [
                1 => 'one',
                2 => 'two',
                3 => 'three',
            ],
            '#default_value' => 2,
        ];
        $form['duplicate_button'] = [
            '#type' => 'submit',
            '#name' => 'duplicate_button',
            '#value' => 'Duplicate button 1',
        ];
        $form['duplicate_button_2'] = [
            '#type' => 'submit',
            '#name' => 'duplicate_button',
            '#value' => 'Duplicate button 2',
        ];
        $form['test_textarea_with_newline'] = [
            '#type' => 'textarea',
            '#title' => 'Textarea with newline',
            '#default_value' => "Test text with\nnewline",
        ];
        $form['save'] = [
            '#type' => 'submit',
            '#value' => $this->t('Save'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'test_page_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // Empty on purpose, we just want to test the rendered form elements.
    }

}