<?php namespace Craft;

class SamplePlugin extends BasePlugin {

    function getName() { ... }
    function getVersion() { ... }
    function getDeveloper() { ... }
    function getDeveloperUrl() { ... }

    public function getSettingsHtml()
    {
        return craft()->templates->render('sample/settings', [
            'settings' => $this->getSettings()
        ]);
    }

    public function prepSettings($settings)
    {
        // I'm currently doing my 'custom' validation here. But if
        // there's an error, how do I display errors on the settings page?

        return $settings;
    }

    public function rules()
    {
        // Tried this, but since SamplePlugin isn't a Model, it does nothing.
    }

    protected function defineSettings()
    {
        return [
            'mySetting' => [
                AttributeType::String,
                'required' => true,
                // Can custom validation rules be set here?
            ]
        ];
    }

}