A function for sending Wordpress Advanced Custom Fields (ACF) Repeater Field, Flexible Content Field, Gallery Field data with Thermal API.

functions.php

<?php

function north_cast_api_data($content) {
	if (is_numeric($content)) $content = intval($content);
	else {
		$unserialized_content = @unserialize($content);
		// we got serialized content
		if ($unserialized_content !== false) {
			// make sure that integers are represented as such, instead of str
			foreach ($unserialized_content as $fn => &$c) {
				if (is_numeric($c)) $c = intval($c);
			}
			$content = $unserialized_content;
		}
	}
	return $content;
}

add_filter( 'thermal_post_entity', function($data, &$post, $state ) {
	if( $state === 'read' ){

		// get all of the post's metadata
		$custom_fields_raw = (object) get_post_meta($post->ID);

		// uncomment the following to include the raw data for testing
		//$data->meta->custom_fields_raw = $custom_fields_raw;

		// create an object for storing all the post data
		$sections = new StdClass;
		$custom_fields_additional = array();

		foreach ($custom_fields_raw as $fieldName => $content) {
			// remove any fields that start with an underscore, as it's a private one
			if (substr((string)$fieldName, 0, 1) == '_') continue;

			// if the string matches the format string_X_string, where X is any
			// integer then we need to put it into an object
			if (preg_match('_\d+_', (string)$fieldName, $indexes) > 0) {
				// we need a type of field, an index of that field, and an item
				$index = (int) $indexes[0];
				$keys = explode('_'.$index.'_', (string)$fieldName);
				$type = $keys[0];
				$item = $keys[1];

				// we need the type of this item, which is only stored in that type's
				// serialized content
				$type_ref = &$custom_fields_raw->$type;
				$type_info = unserialize($type_ref[0]);

				// now we need to create an array for that type
				if (!$sections->$type) $sections->$type = array();
				$object = &$sections->$type;
				// add the object's type, if we have one.
				if ($type_info[$index]) $object[$index]->type = $type_info[$index];

				// check if we've got an integer or serialized data, and massage to the
				// right type accordingly
				$content = north_cast_api_data($content[0]);

				// now add the content to the data array
				$object[$index]->data->$item = $content;
			}
			// if we didn't match that format, this may be an additional custom field 
			// that needs to be included.
			else {
				$custom_fields_additional[$fieldName] = north_cast_api_data($content[0]);
			}
		}

		$data->meta->sections = $sections;
		$data->meta->custom_fields = (object) $custom_fields_additional;
	}
	return $data;
}, 10, 3);

Will this ever work with category specific custom fields? The meta field is empty for ACF fields set for taxonomies