gistfile1.php

<?php

public function getInstaller($type)
{
    // Plugin API 1.1
    foreach ($pm->getCapabilities('Composer\Plugin\Capability\Installable') as $installCap) {
        if ($installCap->supports($type)) {
            return $installCap;
        }
    }

    // Plugin API 1.2
    foreach ($pm->getCapabilities('Composer\Plugin\Capability\Installable2') as $capability) {
        // some other setup
    }   
}

public function getInstaller($type)
{
    /**
     * Plugin API 5.0
     * Composer drops support for "Installable".
     * "Installable2" is renamed to "Installable"
     */
    foreach ($pm->getAllPluginCapabilities('Composer\Plugin\Capability\Installable') as $installCap) {
        // some setup
    }     
}