The OpenLayers module supports layer types, which are ctools plugins. Layer types are related to layers: layers are exportables which contain per-layer configuration, like map server URLs and parameters. Layer types are plugins which provide the forms necessary to create new layers and the functionality of those layers on a map. For instance, the openlayers_views layer type contains the method required to pull data from views and insert it into its own features array.

For instance, the code of the MapBox layer type is

class openlayers_layer_type_mapbox extends openlayers_layer_type {
  /**
   * Provide initial values for options.
   */
  function options_init() {
    return array(
      'maxExtent' => array(-20037508.34,-20037508.34,20037508.34,20037508.34),
      'baselayer' => TRUE,
      'serverResolutions' => openlayers_get_resolutions('900913'),
      'resolutions' => openlayers_get_resolutions('900913'),
      'maxExtent' => openlayers_get_extent('900913'),
      'projection' => array('900913'),
      'baselayer' => TRUE,
      'layer_handler' => 'MapBox',
    );
  }

  /**
   * Options form which generates layers
   */
  function options_form() {
    return array(
      'layername' => array(
        '#type' => 'textfield',
        '#title' => t('Layer Name'),
        '#default_value' => $this->data['layername']
      ),
      'layer_type' => array(
        '#type' => 'hidden',
        '#value' => 'openlayers_layer_type_mapbox'
      ),
      'projection' =>
        array(
         '0' => array(
          '#type' => 'hidden',
          '#value' => '900913',
        ),
      ),
    );
  }

  /**
   * Render.
   */
  function render(&$map) {
    drupal_add_js(drupal_get_path('module', 'mapbox') .'/includes/layer_types/mapbox.js');
    return $this->options;
  }
}

Details

render() is called when the map is built + displayed, and it can set details of the layer by referencing $this

options_form() is called when the user chooses to add a new layer of this type.

settings_form() can be defined to allow users to set per-layer-type settings, like API keys, which will be automatically stored as variables.

options_init() stores properties of the layer which are not configurable by default.

Layer Type Javascript

Layer types typically include javascript files that are factories for OpenLayers.Layer.* objects. For example, the javascript file for TMS layers is

Drupal.openlayers.layer.tms = function (name, map, options) {
  var styleMap = Drupal.openlayers.getStyleMap(map, options.name);
    if (options.maxExtent !== undefined) {
      options.maxExtent = new OpenLayers.Bounds.fromArray(options.maxExtent);
    }
    if (options.type === undefined){
      options.type = "png";
    }
    options.projection = new OpenLayers.Projection('EPSG:'+options.projection);
    var layer = new OpenLayers.Layer.TMS(name, options.base_url, options);
    layer.styleMap = styleMap;
    return layer;
};

and the TMS layer type specifies that its data should be run through this function by specifying 'layer_handler' => 'tms' in its options_init() function. For each layer, if the layer handler is defined, then it is run.

      if (options.layer_handler !== undefined && Drupal.openlayers.layer[options.layer_handler] !== undefined) {
        var layer = Drupal.openlayers.layer[options.layer_handler](name, map, options);

Implementing New Layer Types

See openlayers.module, where openlayers_openlayers_layer_types defines arrays of ctools plugin info specifying the classes and files associated with layer plugins.