Data Observer
- Introduction
- Why Use an Observer?
- Marker Reference
- Purpose & Boundaries
- Purpose
- Boundaries
- Single Observer Principle
- Traditional JavaScript Module Approach
- Why Single Observer?
- Composition Example
- Advantages
- State (Mutation) Management
- Example Usage in HTML
- Runtime Adaptation
- Initial State Recognition
- Common DOs and DON’Ts
- Relationship to Agents (data-agent)
- Key Takeaways
A Data Observer provides block-level behavioral sovereignty through its data-observer
attribute, ensuring one observer per block and granting that observer full control over the block’s lifecycle, states, and resource management.
Core Principle: A block’s Observer is its single controller, unifying lifecycle events, mutation states, and resource usage in one place.
Introduction
Copy link to this sectionA Data Observer is a behavioral pattern that initializes like a module to govern a block’s interactions, states, and lifecycle. This approach bridges the gap between static HTML and dynamic interactions by providing a centralized solution for managing all complex behaviors in web applications.
- Single Observer Per Block: No second observer can attach to the same block. This prevents collisions or confusion.
- Observer-First: All block-level logic (mutations, resource usage, event handling) belongs in the observer.
- Optional Composition: When you need extra specialized behaviors (e.g., analytics or logs tracking), you embed them inside the main observer, maintaining a one-observer rule in the DOM.
Why Use an Observer?
Copy link to this section- Consolidates logic for state transitions (
--mutation
) and resource usage into one place. - By disallowing multiple observers on the same block, overlapping lifecycle management is avoided.
- Observers act as a single “source of truth,” simplifying debugging, iteration, and generative AI-driven expansions.
Observation Point:
data-observer
is the block-level marker that attaches an “owner” class to manage the entire block. This design separates structure from behavior while preserving clear relationships between components and dynamic functionality.
Marker Reference
Copy link to this section<!-- Basic usage -->
<article class="product_card"
data-observer="MyCustomObserver">
<!-- MyCustomObserver JS class now governs this block’s lifecycle -->
</article>
- Syntax:
data-observer="ObserverName"
- ObserverName: Typically, the ES6 class or function name representing block logic (e.g.,
MediaPlayerObserver
,ProductObserver
).
Purpose & Boundaries
Copy link to this sectionPurpose
Copy link to this section- Lifecycle Control: Initialize, update, and destroy block resources.
- Mutation Management: Toggling or handling block states (
--featured
,--active
), ensuring consistent transitions. - Resource Governance: Manage network calls, event listeners, or local data relevant to this block.
Boundaries
Copy link to this section- Scope: Observers operate only at the block level (L3 in Blocktail).
- Sovereignty: Only controls the block it’s mounted on.
- Layer Rules:
- Invalid at L1 (Matrix)
- Invalid at L2 (Context)
- Invalid at L5 (Tail)
An example of an invalid attempt would be:
<!-- WRONG: Observers can't attach to a Context -->
<main class="context--catalog"
data-observer="CatalogObserver"> <!-- Invalid -->
</main>
Single Observer Principle
Copy link to this sectionTraditional JavaScript Module Approach
Copy link to this sectionOutside of Blocktail, developers often rely on “js-*
” selectors or ad-hoc modules for dynamic behaviors:
<!-- Traditional Pattern -->
<div class="js-slider js-animated js-lazy">
<div class="js-slider-content">
<div class="js-slider-item">
<!-- Pattern confusion & multiplication -->
</div>
</div>
</div>
A few key issues with the approach:
- Pattern Degradation: Classes and scripts accumulate, losing clarity over time
- Unclear Behavioral Ownership: Different modules may override each other’s logic
- Scattered State Management: States (e.g., active, lazy) spread across multiple classes
- Unreliable Resource Cleanup: Hard to track who sets or removes event listeners
Why Single Observer?
Copy link to this sectionIn Blocktail, one data-observer
per block ensures a single source of truth for all relevant lifecycle events, state management (mutations), resource usage, and child elements. If additional specialized behaviors—like analytics or performance tracking—are required, you can:
- Embed them as internal modules within the observer’s code, or
- Invoke Agents (
data-agent
), either mounted in the HTML or dynamically called from the observer for cross-layer tasks
Key Point: Centralizing all block logic in a single observer prevents collisions and confusion. The HTML should reflect one block owner, ensuring immediate clarity about “who controls this block.”
Composition Example
Copy link to this sectionclass AnalyticsModule {
constructor(parentObserver) {
this.parentObserver = parentObserver;
}
init() { /* Analytics setup */ }
trackEvent(eventName) { /* Logging logic */ }
cleanup() { /* Remove listeners */ }
}
class PerformanceModule {
constructor(parentObserver) {
this.parentObserver = parentObserver;
}
init() { /* Performance metrics init */ }
monitor() { /* Track memory usage, CPU stats, etc. */ }
cleanup() { /* Dispose of resources */ }
}
class MediaPlayerObserver {
constructor(element) {
this.block = element;
// Internal modules for specialized tasks
this.analytics = new AnalyticsModule(this);
this.performance = new PerformanceModule(this);
}
init() {
// Main block setup
this.initializePlayer();
// Initialize internal modules
this.analytics.init();
this.performance.init();
}
onPlay() {
this.player.play();
// Delegate specialized logic to internal modules
this.analytics.trackEvent('play');
this.performance.monitor();
// Alternatively, if cross-layer or repeated logic is needed:
// AgentRegistry.invoke('ParallaxScroll', { element: this.block });
}
destroy() {
// Cleanup internal modules first
this.analytics.cleanup();
this.performance.cleanup();
// Then finalize main observer tasks
}
}
MediaPlayerObserver
is the single observer attached in the DOM:
<article class="media_player"
data-observer="MediaPlayerObserver">
- AnalyticsModule and PerformanceModule are internal modules. They do not appear as second observers.
- Agents can also be invoked from code if the block needs cross-layer or repeated behaviors (e.g., parallax effect).
Advantages
Copy link to this section- Single Observer in the DOM
- No second
data-observer="Xyz"
on the same block—only one observer claims sovereignty.
- No second
- No Sovereignty Confusion
- Internal modules or agent invocations never create additional block owners.
- Reusability
- If you need the same analytics logic in another block, simply reuse the same internal module or agent invocation in that block’s observer.
- Reduced HTML Noise
- The markup has only one observer attribute, protecting us from “JS keyword searches” for finding all controlling scripts.
State (Mutation) Management
Copy link to this sectionObservers typically handle all block-specific states (mutations). For instance:
class ProductObserver {
constructor(element) {
this.block = element;
this.mutations = this.getCurrentMutations();
}
getCurrentMutations() {
return {
isFeatured: this.block.classList.contains('--featured'),
isOnSale: this.block.classList.contains('--on_sale')
};
}
init() {
if (this.mutations.isFeatured) {
this.activateFeaturedState();
}
if (this.mutations.isOnSale) {
this.activateOnSaleState();
}
}
setMutation(mutation, active) {
const className = `--${mutation}`;
this.block.classList.toggle(className, active);
this.mutations[`is${mutation[0].toUpperCase() + mutation.slice(1)}`] = active;
if (active) {
this[`activate${mutation[0].toUpperCase() + mutation.slice(1)}State`]();
} else {
this[`deactivate${mutation[0].toUpperCase() + mutation.slice(1)}State`]();
}
}
}
- Localize block-specific states within the observer.
- Methods like
activateFeaturedState()
maintain clarity, avoiding random toggles throughout the code.
Example Usage in HTML
Copy link to this section<article class="product_card --featured --on_sale"
data-observer="ProductObserver">
<h2 class="-title">Sample Product</h2>
<div class="-price_container">
<span class="-original">$99</span>
<span class="-sale">$79</span>
</div>
</article>
- The
product_card
block has a singledata-observer="ProductObserver"
. - States
--featured
,--on_sale
are toggled by that observer’s code.
Runtime Adaptation
Copy link to this sectionWhen mounted, an observer
acknowledges pre-existing mutations from server-side rendering or static compilation and then maintains control over subsequent state changes.
Initial State Recognition
Copy link to this section
class MediaPlayerObserver {
constructor(element) {
this.block = element;
// Capture pre-loaded states
this.mutations = {
isLightMode: this.block.classList.contains('--light_mode'),
isAutoplay: this.block.classList.contains('--autoplay'),
isPreferred: this.block.classList.contains('--preferred'),
isVideo: this.block.classList.contains('--video'),
isAudio: this.block.classList.contains('--audio')
/* ... */
};
}
init() {
// Initialize based on existing mutations
if (this.mutations.isVideo) {
this.initializeVideoPlayer();
} else if (this.mutations.isAudio) {
this.initializeAudioPlayer();
}
if (this.mutations.isAutoplay) {
this.setupAutoplay();
}
}
}
- Initial State: Observers respect mutations present before initialization, such as server-rendered personalization.
- State Management: After initialization, mutation changes are handled through observer methods to ensure consistency.
Common DOs and DON’Ts
Copy link to this sectionDO:
- Compose additional logic internally for advanced features.
- Manage block states (mutations) in the observer.
- Centralize block lifecycle (init, transitions, destroy) in the observer while leveraging Agents for cross-layer utility tasks.
DON’T:
- Attach an observer to L1 (matrix), L2 (context), or L5 (tail).
- Mount multiple observers on the same block.
- Manage another block’s states from your observer (no cross-block meddling, please).
- Scatter resource usage or events in multiple watchers—centralize them in the single observer.
Relationship to Agents (data-agent)
Copy link to this section- Agents can appear at any layer (L1–L5) for cross-layer or reusable tasks (e.g., lazy loading, infinite scroll).
- Unlike an Observer, an Agent never manages the block’s lifecycle or states.
- An Observer and an Agent can coexist on the same block:
<article class="product_card --featured"
data-observer="ProductObserver"
data-agent="RevealAnimation">
<!-- Observer = lifecycle owner, Agent = animation effect -->
</article>
- Observers remain sovereign over the block, while Agents handle small “addon” like behaviors or repeated utility tasks.
Key Takeaways
Copy link to this section- Additional sub-observers (analytics, performance, etc.) can be embedded to handle specialized logic—all within the main observer.
- Observers operate at L3 only, ensuring alignment with Blocktail’s layering principles.
- Observers unify our block logic, simplifying debugging and iterative expansions.
- Cross-layer or repeated behaviors are attached as Agents, but never overshadow the observer’s control.
For a top-level view of how Observers (B1) and Agents (B2) fit into the entire Behavioral Layer System—including dynamic invocation vs. mounting—see the Behavioral Layer System page. Next, continue reading about Data Agent, which handles cross-layer or repeated behaviors without interfering in block-level sovereignty.