Example how to use Simple Fields together with WP API. Just a quick example, modify to suit your own needs.

gistfile1.php

<?php

function simple_fields_wp_api_json_prepare_post( $_post, $post, $fields ) {

  // we dont't get the post id 	
	$post_id = $_post["ID"];
	
	$field_values = simple_fields_get_all_fields_and_values_for_post($post_id);
	$field_values_for_json = array(
		"field_groups" => array()
	);
	
	foreach ( $field_values["field_groups"] as $one_field_group ) {
		$field_values_for_json["field_groups"][ $one_field_group["slug"] ] = array(
			"fields" => array()
		);
		
		foreach ( $one_field_group["fields"] as $one_field ) {
		
			$field_values_for_json["field_groups"][ $one_field_group["slug"] ]["fields"][ $one_field["slug"] ] = $one_field["saved_values"];
			
		}
		
	}
	
	$_post["simple_fields"] = $field_values_for_json;
	
	return $_post;

}
// 		return apply_filters( 'json_prepare_post', $_post, $post, $fields );
add_filter( 'json_prepare_post', "simple_fields_wp_api_json_prepare_post" );

?>