im_extender.php

<?php
$thisfile = basename(__FILE__, '.php');

register_plugin(
    $thisfile,
    'ItemManager\'s Extension',
    '0.1',
    'Your name',
    'https://your-site.com',
    'An ItemManger admin extension.',
    '',
    ''
);

add_action('ImActivated', 'im_action');

function im_action($manager) {

    if(!class_exists('AdminExtended'))
    {
        class AdminExtended extends Admin
        {
            protected function callModelMethod($method, $args)
            {
                if($method == 'saveItem') {

                    $this->beforeItemSave($args);
                    parent::callModelMethod($method, $args);

                } elseif($method == 'deleteItem') {

                    $this->beforeItemDelete($args[0], $args[1]);
                    parent::callModelMethod($method, $args);

                } else {

                    parent::callModelMethod($method, $args);

                }

            }

            /**
             * Here you can override native item-editor view of the ItemManager
             * admin area.
             */
            protected function buildItemEditor()
            {
                // Get the item ID
                $id = !empty($this->input['edit']) ? (int) $this->input['edit'] :
                    (isset($this->input['iid']) ? (int) $this->input['iid'] : null);
                // Get current category
                $category = $this->manager->getCategoryMapper()->getCategory($this->manager->cp->currentCategory());

                // Run a native IM method if this isn't an item of a different category
                if(!isset($category) || $category->name != 'Countries') {
                    return parent::buildItemEditor();
                }

                ////////////////////////////////////////////////////////////////////////////////////////////
                //
                // OK, this seems to be an item of the category where you want to show your dropdown field.
                // Below you could write your own code to show the Admin-Editor for the items, but I will
                // just copy most of the code from the original function and only partly add my own logic
                // to it.
                //
                ////////////////////////////////////////////////////////////////////////////////////////////

                // Get our templates
                $itemeditor = $this->tpl->getTemplates('itemeditor');
                // We comment default 'form' template out, since we will use our own - See below 'getEditorWrapper()'
                // method.
                //$form = $this->tpl->getTemplate('form', $itemeditor);
                $fieldarea = $this->tpl->getTemplate('fieldarea', $itemeditor);
                $infotext = $this->tpl->getTemplate('infotext', $itemeditor);
                $required = $this->tpl->getTemplate('required', $itemeditor);

                $categoryid = $category->id;
                $backpage = !empty($this->input['page']) ? intval($this->input['page']) : 1;
                $stamp = !empty($this->input['timestamp']) ? $this->input['timestamp'] : time();

                $ic = $this->manager->getItemMapper();
                $ic->limitedInit($categoryid, $id);
                $curitem = !empty($ic->items[$id]) ? $ic->items[$id] : null;

                // Is a new item
                if(!$curitem) {
                    $curitem = new Item($categoryid);
                    $active = $this->manager->config->backend->itemactive;
                } else {
                    $active = $curitem->active;
                }

                //
                // Let's start building our dropdown right here:
                // First we get all items from the category 'Cities' to display them in a dropdown field.
                //
                $citiesCategory = $this->manager->getCategoryMapper()->getCategory('name=Cities');
                $ic->alloc($citiesCategory->id);
                // Get the value of the hidden field 'data' to retrieve already selected cities
                $selected = !empty($curitem->fields->data->value) ?
                    unserialize(base64_decode($curitem->fields->data->value)) : [];
                
                // Now, we're going through all the cities and build our dropdown list.
                $options = '';
                if($ic->simpleItems) {
                    foreach($ic->simpleItems as $city) {
                        $options .= '<option value="'.$city->id.'"'.
                            (array_key_exists($city->id, $selected) ? ' selected': '').'>'.$city->name.'</option>';
                    }
                }

                // Initialize the category fields
                $fc = new FieldMapper();
                $fc->init($categoryid);
                $fields = $fc->filterFields('position', 'asc');

                $tplfields = '';
                if($fields)
                {
                    foreach($fields as $fieldname => $field)
                    {
                        // Input
                        $inputClassName = 'Input'.ucfirst($field->type);
                        $InputType = new $inputClassName($fields[$fieldname]);
                        $output = $InputType->prepareOutput();

                        // Field
                        $fieldClassName = 'Field'.ucfirst($field->type);
                        $fieldType = new $fieldClassName($this->tpl);
                        $fieldType->name = $fieldname;
                        $fieldType->id = $fieldType->name;

                        if(!empty($field->configs))
                        {
                            foreach($field->configs as $key => $val)
                            {
                                $fieldType->configs->$key = $val;
                            }
                        }

                        // Hidden
                        if($field->type == 'hidden') continue;

                        // Dropdown
                        if($field->type == 'dropdown') $fieldType->options = $field->options;

                        // File upload
                        if($field->type == 'fileupload') {
                            $fieldType->categoryid = $categoryid;
                            $fieldType->itemid = $id;
                            $fieldType->realid = $field->id;
                            $fieldType->timestamp = $stamp;
                        }

                        foreach($output as $outputkey => $outputvalue)
                        {
                            $fieldType->class = empty($field->fieldclass) ? $field->type.'-field' : $field->fieldclass;

                            if(!empty($field->fieldcss))
                                $fieldType->style = $field->fieldcss;

                            if(is_array($outputvalue))
                            {
                                $fieldType->$outputkey = array();
                                $counter = 0;
                                if(!isset($curitem->fields->$fieldname->$outputkey))
                                    continue;

                                foreach($curitem->fields->$fieldname->$outputkey as $arrkey => $arrval)
                                {
                                    $fieldType->{$outputkey}[] =
                                        (string) $curitem->fields->$fieldname->{$outputkey}[$counter];
                                    $counter++;
                                }
                            } else
                                $fieldType->$outputkey = !empty($curitem->fields->$fieldname->$outputkey) ?
                                    (string) $curitem->fields->$fieldname->$outputkey : '';

                            if(MsgReporter::isError())
                                $fieldType->$outputkey = !empty($this->input[$fieldType->name]) ?
                                    $this->input[$fieldType->name] : '';
                        }

                        // Set default field values
                        if(empty($fieldType->value) && !empty($field->default))
                            $fieldType->value = (string) $field->default;

                        $tplinfotext = '';
                        if(!empty($field->info))
                            $tplinfotext = $this->tpl->render($infotext, array('infotext' => $field->info));

                        $tplrequired = '';
                        if(!empty($field->required) && $field->required == 1)
                            $tplrequired = $this->tpl->render($required, array());


                        if($field->type != 'chunk')
                        {
                            $tplfields .= $this->tpl->render($fieldarea, array(
                                    'fieldid' =>  $field->name,
                                    'label' => $field->label,
                                    'infotext' => $tplinfotext,
                                    'area-class' => !empty($field->areaclass) ? $field->areaclass : 'fieldarea',
                                    'area-style' => !empty($field->areacss) ? ' style="'.$field->areacss.'"' : '',
                                    'label-class' => !empty($field->labelclass) ? $field->labelclass : '',
                                    'label-style' => !empty($field->labelcss) ? ' style="'.$field->labelcss.'"' : '',
                                    'required' => $tplrequired,
                                    'field' => $fieldType->render())
                            );
                        } else
                        {
                            $tplfields .= $fieldType->render();
                        }
                    }
                }

                //
                // Take a look at this, here we use our modified wrapper template.
                //
                return $this->getEditorWrapper([
                        'action_edit' => !is_null($id) ? $id : '',
                        'action_page' => $backpage,
                        'item-id' => $id,
                        'position' => !empty($curitem->position) ? $curitem->position : '',
                        'checked' => ($active > 0) ? ' checked ' : '',
                        'back-page' => $backpage,
                        'timestamp' => $stamp,
                        'infotext' => '',
                        'currentcategory' => $this->manager->cp->currentCategory(),
                        'itemname' => !empty($curitem->name) ? $curitem->name : '',
                        'options' => $options,
                        'custom-fields' => $tplfields,
                        'created' => ((int) $curitem->created > 0) ?
                            date((string) $this->manager->config->backend->timeformat, (int) $curitem->created) : '',
                        'updated' => ($curitem->updated > 0) ?
                            date((string) $this->manager->config->backend->timeformat, (int) $curitem->updated) : ''
                    ]
                );
            }

            /**
             * Here you can write your own method that will be executed after
             * someone saves an item via ItemManager's backend.
             *
             */
            protected function beforeItemSave(& $input)
            {
                // First check if this is the right category we want to edit, if not, we skip the function.
                $category = $this->manager->getCategoryMapper()->getCategory($this->manager->cp->currentCategory());
                if(!isset($category) || $category->name != 'Countries') return;
                // Now, catch the value of our dropdown field and
                // prepare its contents for saving into the hidden field.
                $selected = [];
                if(isset($input['cities']) && !empty($input['cities'])) {
                    // Get our cities, because we also want to adopt the city names in our array
                    $mapper = $this->manager->getItemMapper();
                    $citiesCategory = $this->manager->getCategoryMapper()->getCategory('name=Cities');
                    if(!$citiesCategory) return;
                    $mapper->alloc($citiesCategory->id);

                    if($mapper->simpleItems) {
                        foreach($mapper->simpleItems as $city) {
                            if(in_array($city->id, $input['cities'])) {
                                $selected[$city->id] = $city->name;
                            }
                        }
                    }
                }

                // Serialize the $selected arry to save it as hidden field value.
                $input['data'] = base64_encode(serialize($selected));
            }

            /**
             * Here you can write your own method that will be executed before
             * someone deletes an item via ItemManager's backend.
             *
             * Something troublesome is if we want to delete a city from our cities list,
             * since we now have to go through all our countries to remove the deleted
             * city ID from the selected cities array again.
             *
             * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
             * MOTE: If there are many items (Countries), it can slow down the script
             * extremely, because we have to unserialize every single data field value
             * to check its contents
             * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
             */
            protected function beforeItemDelete($id, $catid)
            {
                // First check if this is the right category we want to edit, if not, we skip the function.
                $category = $this->manager->getCategoryMapper()->getCategory($this->manager->cp->currentCategory());
                if(!isset($category) || $category->name != 'Cities') return;
                // Now let's retrieve all the countries and in case there is the deleted
                // city there, we will delete this out there.
                $countryCategory = $this->manager->getCategoryMapper()->getCategory('name=Countries');

                if(!$countryCategory) return;

                $mapper = $this->manager->getItemMapper();
                $mapper->init($countryCategory->id);
                $mapper->alloc($countryCategory->id);
                $countries = $mapper->items;

                if(!$countries) return;

                foreach($countries as $country) {

                    $selected = !empty($country->fields->data->value) ?
                        unserialize(base64_decode($country->fields->data->value)) : [];

                    // Remove deletet city from the array
                    if(array_key_exists($id, $selected))
                    {
                        unset($selected[$id]);
                        $country->fields->data->value = base64_encode(serialize($selected));

                        $country->save();

                        $mapper->simplify($country);
                        $mapper->save();
                    }
                }

            }



            private function getEditorWrapper(array $data = [])
            {
                ob_start();
                ?>
                <div class="manager-wrapper">
                    <h3 class="menuglava">Create / Edit Item</h3>
                    <form class="largeform" action="load.php?id=imanager&edit=<?php echo $data['action_edit'];
                    ?>&page=<?php echo $data['action_page']; ?>" method="post" enctype="multipart/form-data"
                          accept-charset="utf-8">
                        <input name="iid" type="hidden" value="<?php echo $data['item-id']; ?>">
                        <input name="page" type="hidden" value="<?php echo $data['back-page']; ?>" >
                        <input name="categoryid" type="hidden" value="<?php echo $data['currentcategory']; ?>">
                        <input name="timestamp" type="hidden" value="<?php echo $data['timestamp']; ?>">
                        <div>
                            <div class="fieldarea">
                                <label for="catid" class="im-left">ID</label>
                                <p id="catid" class="im-cat-info"><?php echo $data['item-id']; ?></p>
                            </div>
                            <div class="fieldarea">
                                <label for="title">Title</label>
                                <div class="field-wrapper">
                                    <input id="title" class="im-title" name="name" type="text"
                                            value="<?php echo $data['itemname']; ?>">
                                </div>
                            </div>
                            <!-- [ DROPDOWN ] -->
                            <div class="fieldarea">
                                <label for="title">Cities</label>
                                <div class="field-wrapper">
                                    <select name="cities[]" style="background:none" multiple>
                                        <?php echo $data['options']; ?>
                                    </select>
                                </div>
                            </div>
                            <!-- [ /DROPDOWN ] -->

                            <div class="fieldarea">
                                <label for="position">Position</label>
                                <div class="field-wrapper">
                                    <input id="position" name="itempos" type="number" value="<?php
                                    echo $data['position']; ?>">
                                </div>
                            </div>
                            <div class="fieldarea">
                                <label for="active">Enabled</label>
                                <div class="field-wrapper">
                                    <input id="active" name="active" type="checkbox" value="1" <?php
                                    echo $data['checked']; ?>>
                                </div>
                            </div>
                            <?php echo $data['custom-fields']; ?>
                            <div class="fieldarea">
                                <label for="itemcreated" >Created date</label>
                                <div class="field-wrapper"><?php echo $data['created']; ?></div>
                            </div>

                            <div class="fieldarea">
                                <label for="itemcreated" >Modified</label>
                                <div class="field-wrapper"><?php echo $data['updated']; ?></div>
                            </div>

                            <p><input name="submit" type="submit" class="submit" value="Save"></p>
                        </div>
                    </form>
                </div>
                <?php return ob_get_clean();
            }
        }

        $newbackend = new \AdminExtended;
        $manager->setAdmin($newbackend);
    }
}