The after plugin, as its name suggests, runs after the observed method.

When writing the after plugin, there are a few key points to remember:

1. The first parameter coming into the plugin is an observed type instance.
2. The second parameter coming into the plugin is the result of the observed method, often called $result or called after the variable returned from the observed method (as in the following example: $data).
3. All other parameters are parameters of the observed method.
4. The plugin must return the same $result|$data variable of the same type, as we are free to modify the value.

Let's take a look at one of Magento's after plugin implementations, the one specified in the module-catalog/etc/di.xml file:

<type name="MagentoIndexerModelConfigData">
    <plugin name="indexerProductFlatConfigGet" 
        type="MagentoCatalogModelIndexerProductFlatPluginIndexerConfigData" />
</type>

The original method this plugin is targeting is the get method of the MagentoIndexerModelConfigData class:

public function get($path = null, $default = null) {
    // The rest of the code...
    return $data;
}

The implementation of the after plugin is provided via the afterGet method of the MagentoCatalogModelIndexerProductFlatPluginIndexerConfigData class, as per the following partial example:

public function afterGet(MagentoIndexerModelConfigData, $data, $path = null, $default = null) {
    // The rest of the code...
    return $data;
}

Special care should be taken when using plugins. While they provide great flexibility, they also make it easy to induce bugs, performance bottlenecks, and other less obvious types of instabilities – even more so if several plugins are observing the same method.