How to detect the usage of Gutenberg

The new editor called Gutenberg is here as plugin in 4.9, and as core functionality called Block Editor, in 5.0. Regarding to it, it is often needed to determine programmatically which editor is used to edit post or page in the site console. How to do it?

Update: There are number of outdated answers to similar question:

• gutenberg_post_has_blocks() – this function exists only in Gutenberg plugin, and not in 5.0 Core
• is_gutenberg_page() – the same
• the_gutenberg_project() – the same
• has_blocks() – does not work (returns false) when Classic Editor is on and its option “Default editor for all users” = “Block Editor”
• answer simply produces fatal error Call to undefined function get_current_screen()

So, before commenting this question and answer, please take a work to check what do you propose. Check it now, with 4.9 and current version of WordPress, and all possible combinations of Classic Editor and Gutenberg/Block Editor. I will be happy to discuss tested solution, not links to something.

There are several variants:

• WordPress 4.9, Gutenberg plugin is not active
• WordPress 4.9, Gutenberg plugin is active
• WordPress 5.0, Block Editor by default
• WordPress 5.0, Classic Editor plugin is active
• WordPress 5.0, Classic Editor plugin is active, but in site console in “Settings > Writing” the option “Use the Block editor by default…” is selected

All the mentioned variants can be processed by the following code:

/**
 * Check if Block Editor is active.
 * Must only be used after plugins_loaded action is fired.
 *
 * @return bool
 */
function is_active() {
    // Gutenberg plugin is installed and activated.
    $gutenberg = ! ( false === has_filter( 'replace_editor', 'gutenberg_init' ) );

    // Block editor since 5.0.
    $block_editor = version_compare( $GLOBALS['wp_version'], '5.0-beta', '>' );

    if ( ! $gutenberg && ! $block_editor ) {
        return false;
    }

    if ( is_classic_editor_plugin_active() ) {
        $editor_option       = get_option( 'classic-editor-replace' );
        $block_editor_active = array( 'no-replace', 'block' );

        return in_array( $editor_option, $block_editor_active, true );
    }

    return true;
}

/**
 * Check if Classic Editor plugin is active.
 *
 * @return bool
 */
function is_classic_editor_plugin_active() {
    if ( ! function_exists( 'is_plugin_active' ) ) {
        include_once ABSPATH . 'wp-admin/includes/plugin.php';
    }

    if ( is_plugin_active( 'classic-editor/classic-editor.php' ) ) {
        return true;
    }

    return false;
}

Function returns true if block editor is active by any means, and false – in the case if classic editor is here. This function must only be used after plugins_loaded action is fired.

P.S. Due release of version 1.2 of Classic Editor plugin, code is updated, as classic-editor-replace options now takes values not replace and no-replace, but classic and block.