BREADCRUMBS(3)              RSX Framework Manual              BREADCRUMBS(3)

NAME
    breadcrumbs - Progressive breadcrumb navigation with caching

SYNOPSIS
    Layout Integration:

    class My_Spa_Layout extends Spa_Layout {
        on_action(url, action_name, args) {
            if (this._breadcrumb_cancel) this._breadcrumb_cancel();
            this._breadcrumb_cancel = Rsx_Breadcrumb_Resolver.stream(
                this.action, url,
                (data) => this._render_breadcrumbs(data)
            );
        }
    }

    Action Methods:

    class Contacts_View_Action extends Spa_Action {
        async page_title() { return this.data.contact.name; }
        async breadcrumb_label() { return this.data.contact.name; }
        async breadcrumb_label_active() { return 'View Contact Details'; }
        async breadcrumb_parent() {
            return Rsx.Route('Contacts_Index_Action');
        }
    }

DESCRIPTION
    The RSX breadcrumb system provides progressive breadcrumb resolution with
    caching for instant SPA navigation. Breadcrumbs render immediately using
    cached data while validating against live action data in the background.

    Key characteristics:
    - Instant render from cache when all chain links are cached
    - Progressive label updates for uncached pages (shows "------" placeholders)
    - Active page always validates against live action data
    - In-memory cache persists for session (cleared on page reload)

    Resolution Phases:
    1. Immediate Cached Render - If ALL chain links cached, render instantly
    2. Active Action Resolution - Await live action's breadcrumb methods
    3. Chain Walk - Walk parents using cache or detached action loads
    4. Render Chain - Emit chain structure (may have null labels)
    5. Progressive Updates - Update labels as they resolve

CORE API
    Rsx_Breadcrumb_Resolver.stream(action, url, callback)

        Streams breadcrumb data as it becomes available.

        Parameters:
            action   - The active SPA action instance
            url      - Current URL (pathname + search)
            callback - Called with breadcrumb data on each update

        Returns:
            Function - Call to cancel streaming (e.g., on navigation)

        Callback receives:
            {
                title: "Page Title" | null,
                chain: [
                    { url: "/path", label: "Label" | null,
                      is_active: false, resolved: true },
                    { url: "/path/sub", label: null,
                      is_active: true, resolved: false }
                ],
                fully_resolved: true | false
            }

        When label is null, the Breadcrumb_Nav component displays "------"
        in muted text as a placeholder.

ACTION METHODS
    page_title()
        Returns the page title for document.title. Called once when action
        renders. Used for browser tab title.

        async page_title() {
            return 'Contacts';  // Root page
        }

        async page_title() {
            return this.data.contact.name;  // Detail page
        }

        async page_title() {
            return this.data.is_edit
                ? `Edit: ${this.data.contact.name}`
                : 'New Contact';  // Add/Edit page
        }

    breadcrumb_label()
        Returns the label shown when this action appears as a parent in
        another action's breadcrumb chain. Typically returns the entity
        name or short identifier.

        async breadcrumb_label() {
            return this.data.contact.name;
        }

        async breadcrumb_label() {
            return 'Projects';
        }

        Not needed for pages that are never parents (leaf nodes like edit
        forms). Default returns page_title() if not defined.

    breadcrumb_label_active()
        Returns the label shown for the current active page. This can be
        more descriptive than breadcrumb_label() since it's not constrained
        by breadcrumb width.

        // Root pages - descriptive text
        async breadcrumb_label_active() {
            return 'Manage your contact database';
        }

        // Detail pages - entity identifier
        async breadcrumb_label_active() {
            return `View ${this.data.contact.name}`;
        }

        // Edit pages - action context
        async breadcrumb_label_active() {
            return this.data.is_edit ? 'Edit Contact' : 'New Contact';
        }

    breadcrumb_parent()
        Returns the URL of the parent action for building the breadcrumb
        chain. The layout walks up this chain calling breadcrumb_label()
        on each parent until reaching null.

        // Root pages - no parent
        async breadcrumb_parent() {
            return null;
        }

        // View page - parent is list
        async breadcrumb_parent() {
            return Rsx.Route('Contacts_Index_Action');
        }

        // Edit page - parent depends on context
        async breadcrumb_parent() {
            if (this.data.is_edit) {
                return Rsx.Route('Contacts_View_Action', {
                    id: this.args.id
                });
            }
            return Rsx.Route('Contacts_Index_Action');
        }

LAYOUT INTEGRATION
    The layout uses Rsx_Breadcrumb_Resolver.stream() to progressively render
    breadcrumbs as data becomes available.

    Complete Example:
        class My_Spa_Layout extends Spa_Layout {
            on_create() {
                this._breadcrumb_cancel = null;
            }

            on_action(url, action_name, args) {
                // Cancel any previous breadcrumb stream
                if (this._breadcrumb_cancel) {
                    this._breadcrumb_cancel();
                }

                // Stream breadcrumbs progressively
                this._breadcrumb_cancel = Rsx_Breadcrumb_Resolver.stream(
                    this.action,
                    url,
                    (data) => this._render_breadcrumbs(data)
                );
            }

            _render_breadcrumbs(data) {
                // Update title (null = still loading)
                if (data.title !== null) {
                    this.$sid('header_title').html(data.title);
                }

                // Update breadcrumbs
                this.$sid('header_breadcrumbs').component('Breadcrumb_Nav', {
                    crumbs: data.chain
                });
            }
        }

    The callback is called multiple times:
    - First with cached data (if full cache hit)
    - Then with validated active action data
    - Then progressively as parent labels resolve

BREADCRUMB_NAV COMPONENT
    Template:
        <Define:Breadcrumb_Nav tag="nav" aria-label="breadcrumb">
            <ol class="breadcrumb mb-0">
                <% for (let i = 0; i < this.args.crumbs.length; i++) {
                    const crumb = this.args.crumbs[i];
                    const is_last = i === this.args.crumbs.length - 1;
                %>
                    <% if (is_last || !crumb.url) { %>
                        <li class="breadcrumb-item active" aria-current="page">
                            <% if (crumb.label === null) { %>
                                <span class="breadcrumb-placeholder">------</span>
                            <% } else { %>
                                <%= crumb.label %>
                            <% } %>
                        </li>
                    <% } else { %>
                        <li class="breadcrumb-item">
                            <a href="<%= crumb.url %>">
                                <% if (crumb.label === null) { %>
                                    <span class="breadcrumb-placeholder">------</span>
                                <% } else { %>
                                    <%= crumb.label %>
                                <% } %>
                            </a>
                        </li>
                    <% } %>
                <% } %>
            </ol>
        </Define:Breadcrumb_Nav>

    Uses Bootstrap 5 breadcrumb classes for consistent styling.

PATTERNS BY PAGE TYPE
    Index/Root Pages:
        Actions at the top of a hierarchy with no parent.

        class Contacts_Index_Action extends Spa_Action {
            async page_title() { return 'Contacts'; }
            async breadcrumb_label() { return 'Contacts'; }
            async breadcrumb_label_active() {
                return 'Manage your contact database';
            }
            async breadcrumb_parent() { return null; }
        }

        Result: Contacts / Manage your contact database
        (single breadcrumb with descriptive text)

    View/Detail Pages:
        Actions showing a single record with a parent list page.

        class Contacts_View_Action extends Spa_Action {
            async page_title() { return this.data.contact.name; }
            async breadcrumb_label() { return this.data.contact.name; }
            async breadcrumb_label_active() { return 'View Contact'; }
            async breadcrumb_parent() {
                return Rsx.Route('Contacts_Index_Action');
            }
        }

        Result: Contacts > John Smith / View Contact

    Edit Pages:
        Actions for editing existing or creating new records.

        class Contacts_Edit_Action extends Spa_Action {
            async page_title() {
                if (this.data.is_edit) {
                    return `Edit: ${this.data.contact.name}`;
                }
                return 'New Contact';
            }

            async breadcrumb_label_active() {
                return this.data.is_edit ? 'Edit Contact' : 'New Contact';
            }

            async breadcrumb_parent() {
                if (this.data.is_edit) {
                    return Rsx.Route('Contacts_View_Action', {
                        id: this.args.id
                    });
                }
                return Rsx.Route('Contacts_Index_Action');
            }
        }

        Edit result: Contacts > John Smith > View Contact / Edit Contact
        Add result: Contacts / New Contact

    Nested Pages (Sublayouts):
        Actions within sublayouts like settings sections.

        class Settings_Profile_Display_Action extends Spa_Action {
            async page_title() { return 'Display Preferences'; }
            async breadcrumb_label() { return 'Display Preferences'; }
            async breadcrumb_label_active() {
                return 'Configure display settings';
            }
            async breadcrumb_parent() {
                return Rsx.Route('Dashboard_Index_Action');
            }
        }

        Result: Dashboard > Display Preferences / Configure display settings

DOCUMENT TITLE
    The layout sets document.title using page_title():

        document.title = 'RSX - ' + await this.action.page_title();

    Format: "AppName - PageTitle"

    Examples:
    - RSX - Contacts
    - RSX - John Smith
    - RSX - Edit: John Smith

CACHING
    The breadcrumb resolver maintains an in-memory cache keyed by URL:

    Cache Structure:
        { label, label_active, parent_url }

    Cache Behavior:
        - Parent links: Use cache if available, skip detached action load
        - Active link: Show cached value immediately, then validate with
          fresh data from live action
        - Fresh values overwrite cached values

    Cache Invalidation:
        - Visiting a page updates cache for that URL only
        - Window reload clears entire cache
        - No TTL - cache persists for session duration
        - Parent links are never re-validated (only active page validates)

    Example Timeline (first visit to /contacts/123):
        t=0ms   Cache miss for /contacts/123, skip Phase 1
        t=50ms  Active action resolves, update cache
        t=51ms  Walk chain: /contacts is NOT cached, load detached
        t=100ms /contacts resolves, cache and render chain
        t=101ms fully_resolved: true

    Example Timeline (second visit to /contacts/123):
        t=0ms   Full cache hit, render immediately
        t=1ms   User sees breadcrumbs instantly
        t=50ms  Active action validates (values unchanged)
        t=51ms  No re-render needed

PERFORMANCE CONSIDERATIONS
    Progressive Loading:
        The resolver doesn't wait for all data before rendering. It emits
        updates as each piece of data becomes available, showing "------"
        placeholders for pending labels.

    Detached Action Cleanup:
        The resolver automatically stops detached actions after extracting
        breadcrumb data to prevent memory leaks.

DEFAULT METHODS
    If an action doesn't define breadcrumb methods, defaults apply:

        page_title()             Returns class name (e.g., "Contacts_View_Action")
        breadcrumb_label()       Returns page_title()
        breadcrumb_label_active() Returns page_title()
        breadcrumb_parent()      Returns null (no parent)

    Override only the methods you need. Most actions need at minimum:
    - page_title()
    - breadcrumb_label_active() (for descriptive root page text)
    - breadcrumb_parent() (for non-root pages)

MIGRATION FROM HARDCODED BREADCRUMBS
    If converting from inline breadcrumb components:

    Old Pattern (template-based):
        <Page_Header>
            <Page_Header_Left>
                <Page_Title>Edit Contact</Page_Title>
                <Breadcrumb>
                    <Breadcrumb_Item $href="...">Dashboard</Breadcrumb_Item>
                    <Breadcrumb_Item $href="...">Contacts</Breadcrumb_Item>
                    <Breadcrumb_Item $active=true>Edit</Breadcrumb_Item>
                </Breadcrumb>
            </Page_Header_Left>
        </Page_Header>

    New Pattern (action methods):
        // Remove from template, keep only:
        <Page_Header>
            <Page_Header_Right>
                <!-- action buttons -->
            </Page_Header_Right>
        </Page_Header>

        // Add to action class:
        async page_title() { return 'Edit Contact'; }
        async breadcrumb_label_active() { return 'Edit Contact'; }
        async breadcrumb_parent() {
            return Rsx.Route('Contacts_View_Action', { id: this.args.id });
        }

SEE ALSO
    spa(3) - SPA routing and layouts
    spa(3) DETACHED ACTION LOADING - Spa.load_detached_action() details
    spa(3) SUBLAYOUTS - Nested layout handling