At what point during the Wordpress Lifecycle can I use wp_get_current_user?

I'm writing an integration pie开发者_如何学运维ce with a third party system and need to check certain values when a user logs in.

I've tried several different hooks: wp_init, wp_login and wp_loaded, but wp_get_current_user always returns null.

E.g.: To determine if there is a user currently logged in, do this:

<?php
wp_get_current_user();
if ( 0 == $current_user->ID ) {
    // Not logged in.
} else {
    // Logged in.
}
?>

default usage The call to wp_get_current_user() return WP_User object.

<?php
    wp_get_current_user();
    /**
     * @example Safe usage: $current_user = wp_get_current_user();
     * if ( !($current_user instanceof WP_User) )
     *     return;
     */
    echo 'Username: ' . $current_user->user_login . '<br />';
    echo 'User email: ' . $current_user->user_email . '<br />';
    echo 'User level: ' . $current_user->user_level . '<br />';
    echo 'User first name: ' . $current_user->user_firstname . '<br />';
    echo 'User last name: ' . $current_user->user_lastname . '<br />';
    echo 'User display name: ' . $current_user->display_name . '<br />';
    echo 'User ID: ' . $current_user->ID . '<br />';
?>

more on this page http://codex.wordpress.org/Function_Reference/wp_get_current_user

I was able to accomplish this using http://codex.wordpress.org/Function_Reference/get_user_by.

function CheckWordPressLogin($User) {
  //$User contains login user name.
  $CurrentUser = get_user_by('login', $User); //$CurrentUser is a WP_User object.
}
add_action('wp_login', 'CheckWordPressLogin');