Where is the admin bar instantiated?

Question

There are so many resources on how to enable, disable, change and fix the admin bar, but per a question from a student, we just want to follow the path to how it's made.

What we know: 1. The trail starts with footer.php -> wp_footer(); 2. The trail ends with wp_includes/adminbar.php

What's the trail in the middle?

Answer 1

Here is the backtrace of the adminbar rendering in the home page of the backend for WordPress 4.9.2

array (
  'file' => 'wp-admin/index.php',
  'line' => 100,
  'function' => 'include',
)
array (
  'file' => 'wp-admin/admin-header.php',
  'line' => 219,
  'function' => 'do_action',
)
array (
  'file' => 'wp-includes/plugin.php',
  'line' => 457,
  'function' => 'do_action',
  'class' => 'WP_Hook',
  'type' => '->',
)
array (
  'file' => 'wp-includes/class-wp-hook.php',
  'line' => 310,
  'function' => 'apply_filters',
  'class' => 'WP_Hook',
  'type' => '->',
)
array (
  'file' => 'wp-includes/class-wp-hook.php',
  'line' => 286,
  'function' => 'wp_admin_bar_render',
)
array (
  'file' => 'wp-includes/admin-bar.php',
  'line' => 92,
  'function' => 'render',
  'class' => 'WP_Admin_Bar',
  'type' => '->',
)
array (
  'file' => 'wp-includes/class-wp-admin-bar.php',
  'line' => 263,
  'function' => '_render',
  'class' => 'WP_Admin_Bar',
  'type' => '->',
)
array (
  'file' => 'wp-includes/class-wp-admin-bar.php',
  'line' => 413,
)

and here the path on the home page

array (
  'file' => 'index.php',
  'line' => 17,
  'function' => 'require',
)
array (
  'file' => 'wp-blog-header.php',
  'line' => 19,
  'function' => 'require_once',
)
array (
  'file' => 'wp-includes/template-loader.php',
  'line' => 74,
  'function' => 'include',
)
array (
  'file' => 'wp-content/themes/storefront/page.php',
  'line' => 38,
  'function' => 'get_footer',
)
array (
  'file' => 'wp-includes/general-template.php',
  'line' => 76,
  'function' => 'locate_template',
)
array (
  'file' => 'wp-includes/template.php',
  'line' => 647,
  'function' => 'load_template',
)
array (
  'file' => 'wp-includes/template.php',
  'line' => 688,
  'function' => 'require_once',
)
array (
  'file' => 'wp-content/themes/storefront/footer.php',
  'line' => 36,
  'function' => 'wp_footer',
)
array (
  'file' => 'wp-includes/general-template.php',
  'line' => 2630,
  'function' => 'do_action',
)
array (
  'file' => 'wp-includes/plugin.php',
  'line' => 457,
  'function' => 'do_action',
  'class' => 'WP_Hook',
  'type' => '->',
)
array (
  'file' => 'wp-includes/class-wp-hook.php',
  'line' => 310,
  'function' => 'apply_filters',
  'class' => 'WP_Hook',
  'type' => '->',
)
array (
  'file' => 'wp-includes/class-wp-hook.php',
  'line' => 286,
  'function' => 'wp_admin_bar_render',
)