Output data like WP’s plugin installer/updater

I want to update my plugin and have proper feedback, like the WordPress update/upgrade mechanism. How is this effect done?

Essentially I have a script I want to run that has lots of processing, and with each run through the loop, output another line to the screen. Is this done with the output buffer?

Yes, WordPress uses output buffering for displaying these messages. There’s a nifty function you can use within your loop called show_message() which utilizes wp_ob_end_flush_all();

function show_message($message) {
    if ( is_wp_error($message) ){
        if ( $message->get_error_data() )
            $message = $message->get_error_message() . ': ' . $message->get_error_data();
        else
            $message = $message->get_error_message();
    }
    echo "<p>$message</p>\n";
    wp_ob_end_flush_all();
    flush();
}

You might wish to abstract this to your own function as there is a feature in the queue to migrate this to a method within WP_Error. It’s possible this function will become deprecated in the future.