How to create custom modal ui form component with multiple fieldset and field

Question

i have layout handle like frontname_index_index

<referenceContainer name="content">
   <container name="button.actions.container" label="Container">
     all button action here .. such as save, back reset  
   </container>
   <container name="form" label="Form"><!-- this modal form -->
    <block class="Namespace\Module\Block\Adminhtml\FormWrap\Form" >
       <uiComponent name="ui_form"/>
     </block>
   </container>
</referenceContainer>

Possible to use ui modifiers to add dynamic button to specific spot? When i pressed button it will open modal ui_form ability save , edit like normal form

Answer 1

[UPDATED MY ANSWER]
etc/adminhtml/di.xml
Add modifier to add custom form modal

<virtualType name="Vendor\Module\Ui\DataProvider\Item\Form\Modifier\Pool" type="Magento\Ui\DataProvider\Modifier\Pool">
    <arguments>
        <argument name="modifiers" xsi:type="array">
            <item name="item-form-modal" xsi:type="array">
                <item name="class" xsi:type="string">Vendor\Module\Ui\DataProvider\Item\Form\Modifier\Item</item>
                <item name="sortOrder" xsi:type="number">10</item>
            </item>
        </argument>
    </arguments>
</virtualType>

Button action for render modal

return [
        'label' => __('Add New Item'),
        'class' => 'primary button-add-item',
        'data_attribute' => [
            'mage-init' => [
                'Magento_Ui/js/form/button-adapter' => [
                    'actions' => [
                        [
                            'targetName' => 'item_form.item_form.item_form_modal',
                            'actionName' => 'toggleModal'
                        ],
                        [
                            'targetName' => 'item_form.item_form.item_form_modal.insert_item_form',
                            'actionName' => 'render'
                        ]
                    ]
                ]
            ]
        ],
        'on_click' => '',
        'sort_order' => 20
    ];
}

File Modifier where i defination form

return $this->arrayManager->set(
        'item_form_modal',
        $meta,
        [
            'arguments' => [
                'data' => [
                    'config' => [
                        'isTemplate' => false,
                        'componentType' => 'modal',
                        'options' => [
                            'title' => __('New Item'),
                        ],
                        'imports' => [
                            'state' => '!index=insert_item_form:responseStatus'
                        ],
                    ],
                ],
            ],
            'children' => [
                'insert_item_form' => [
                    'arguments' => [
                        'data' => [
                            'config' => [
                                'label' => '',
                                'componentType' => 'container',
                                'component' => 'Magento_Ui/js/form/components/insert-form',
                                'dataScope' => '',
                                'update_url' => $this->urlBuilder->getUrl('mui/index/render'),
                                'render_url' => $this->urlBuilder->getUrl(
                                    'mui/index/render_handle',
                                    [
                                        'handle' => 'megamenuadmin_item_form',
                                        'store' => $this->_storeManager->getStore()->getId(),
                                        'buttons' => 1
                                    ]
                                ),
                                'autoRender' => false,
                                'ns' => 'item_add_form',
                                'externalProvider' => 'item_add_form.item_add_form_data_source',
                                'toolbarContainer' => '${ $.parentName }',
                                'formSubmitType' => 'ajax',
                            ],
                        ],
                    ]
                ]
            ]
        ]
    );

item_form_modal has child element form ui insert_item_form insert via ajax. Handle layout megamenuadmin_item_form contain ui-form. Between children [] you can add any components such as: fieldsets, fields, selects, inputs v.v or ui-form like i do

layout megamenuadmin_item_form

 <page>
    <referenceContainer name="content">
     <uiComponent name="item_add_form"/>
   </referenceContainer>
 </page>

form ui-component item_add_form

<?xml version="1.0" encoding="UTF-8"?>

<form xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Ui:etc/ui_configuration.xsd">
<argument name="data" xsi:type="array">
    <item name="js_config" xsi:type="array">
        <item name="provider" xsi:type="string">item_add_form.item_add_form_data_source</item>
        <item name="deps" xsi:type="string">item_add_form.item_add_form_data_source</item>
        <item name="namespace" xsi:type="string">item_add_form</item>
    </item>
    <item name="buttons" xsi:type="array">
        <item name="cancel" xsi:type="string">Vendor\Module\Block\Adminhtml\Item\Button\Cancel</item>
        <item name="save" xsi:type="string">Vendor\Module\Block\Adminhtml\Item\Button\Save</item>
    </item>
    <item name="template" xsi:type="string">templates/form/collapsible</item>
    <item name="config" xsi:type="array">
        <item name="ajaxSaveType" xsi:type="string">simple</item>
    </item>
</argument>
<dataSource name="item_add_form_data_source">
    <argument name="dataProvider" xsi:type="configurableObject">
        <argument name="class" xsi:type="string">Vendor\Module\Model\Item\DataProvider</argument>
        <argument name="name" xsi:type="string">item_add_form_data_source</argument>
        <argument name="primaryFieldName" xsi:type="string">id</argument>
        <argument name="requestFieldName" xsi:type="string">menu_id</argument>
        <argument name="data" xsi:type="array">
            <item name="config" xsi:type="array">
                <item name="submit_url" xsi:type="url" path="megamenuadmin/item/save"/>
            </item>
        </argument>
    </argument>
    <argument name="data" xsi:type="array">
        <item name="js_config" xsi:type="array">
            <item name="component" xsi:type="string">Magento_Ui/js/form/provider</item>
        </item>
    </argument>
</dataSource>
<fieldset name="filedset1">
    Fields go here
</fieldset>
<fieldset name="fieldset2">


</fieldset>
</form>