Hierarchical State Machines (EFS) in Sparklet: Structuring Embedded UI Behavior the Right Way

Designing embedded user interfaces (UIs) that respond fluidly to user interactions and system events often requires more than static screens and hard-coded logic. To handle complexity — such as dynamic screen transitions, timed events, and conditional behavior — developers need structured mechanisms that encapsulate UI logic cleanly, predictably, and maintainably. That’s where Hierarchical State Machines (HSMs) in Sparklet UI Designer come into play.

In this blog, we’ll take a deep dive into HSMs in Sparklet, explain what they are, why they matter for embedded UI design, how they work within Sparklet projects, real-world use cases, and best practices for implementation — all in a way that’s accessible for engineers and designers working on resource-constrained embedded systems.

What Is a Hierarchical State Machine?

A state machine is a model of behavior where an entity can be in one of several defined states and transitions between those states in response to events. A hierarchical state machine (HSM) extends this concept by allowing nested states — states that contain other states — enabling more structured and scalable modeling of complex logic. In an HSM:

1. State represents a particular mode of operation.
2. Event is something that triggers a potential transition.
3. Transition defines what happens when events occur in specific states.
4. Actions are tasks or commands executed when entering, exiting, or during transitions.

This structure is more powerful than flat state machines because it helps avoid duplicated logic, supports logical grouping of behaviors, and improves readability and maintainability — especially in large embedded UI projects with many screens and interactions.

Why State Machines Matter in Embedded UI Design

Embedded graphical user interfaces — such as automotive instrument clusters, industrial human-machine interfaces (HMIs), or consumer appliance displays — must respond to different inputs, error conditions, user interactions, and timed sequences. Unlike traditional software UIs, embedded UIs often run on microcontrollers (MCUs) or resource-constrained processors where performance and determinism are critical.

Hierarchical state machines are valuable because they:

1. Provide structured control over UI flow — defining screens and transitions in a predictable manner.
2. Make logic easy to visualize and debug — states and transitions form a clear map of behavior.
3. Encapsulate event handling — simplifying the complexity of reacting to user input, timeouts, and system changes.
4. Reduce code complexity — designers can rely on declarative state logic instead of complex programmatic conditions.

In Sparklet UI Designer, state machines are represented visually and stored as State Machine Files (EFS) used by the Sparklet runtime to manage UI logic at runtime.

How Sparklet’s Hierarchical State Machines Work

Sparklet’s state machine implementation is inspired by UML (Unified Modeling Language) state machine concepts, which are widely used in software and embedded design. The basic flow when working with an HSM in Sparklet UI Designer follows these steps:

1. Define the State Machine: Each project that requires dynamic UI logic includes an HSM resource file. This file defines all the states, events that can occur, and actions to take on events. This file lives in the project hierarchy under your Sparklet application.
2. Create States: States represent screens or logical modes of the UI. For example, a device might have states like init_screen, main_menu, settings_screen, or error_state.
3. Assign Events and Transitions: Each state can receive events such as timers, button presses, or system events. When an event occurs, the state machine defines how to transition to another state or perform an action within the same state.
4. Define Entry and Exit Actions: States may have actions that occur when the machine enters or exits them — for example, showing or hiding views, initializing variables, or triggering animations.

This structured logic ensures that UIs remain predictable and responsive. For example, a button press in one visual state might lead to a specific sequence of UI changes without requiring inline code for each case.

Mapping HSM to UI Behavior

One practical application of state machines in Sparklet is navigating between screens or views in your project. Sparklet UI Designer uses placeholder files — containers that can hold multiple views — and the state machine controls which view is shown or hidden based on the current state.

For example:

1. The init_screen state may set up a placeholder and initialize the first screen.
2. On a timed event (e.g., a splash timeout), the state machine transitions to main_screen.
3. A button press event in main_screen might trigger a transition to settings_screen.

By including view transitions in the state machine actions, the UI becomes tightly coupled with state logic, simplifying design and reducing the need for manual wiring of screens.

Event Types and Actions in Sparklet HSM

Sparklet UI Designer’s state machine editor supports different event groups that drive transitions and actions:

1. Internal – initial setup or housekeeping actions.
2. Timers and Delays – transitions triggered after a timed delay.
3. Key Input – events triggered by hardware keyboard keys or physical input keys mapped to the system.
4. Widget Events – interaction-based events triggered by widgets, such as button press, release, toggle, swipe, or other widget-specific actions. Sparklet also allows custom events to be defined, enabling precise control over state transitions and UI behavior.

Each event can be programmed to execute certain actions — such as showing a new view, hiding the current one, setting UI states, or changing data values.

This general event-action mapping brings two major benefits:

1. You can control UI changes based on UI events (e.g., button press).
2. You can respond to time-based or internal logic without writing code.

Real-World Use Cases for Hierarchical State Machines

1. Automotive Dashboards: Instrument clusters often have complex behaviors: startup splash screens, navigation between different clusters, warnings, and mode changes. Using HSMs, designers can define states like:boot_screen, normal_operation, warning_popup. Each transition — such as from normal operation to a warning — can be triggered by events and handled consistently.
2. Industrial Control Panels: Industrial HMIs frequently navigate between multiple views representing different machine states. An HSM can define states such as: idle, running, paused, error. Transitions between these states are straightforward to manage, and HSMs help avoid scattered logic across the UI layout.
3. Consumer Electronics: For consumer devices like smart appliances or wearable devices with limited screens, state machines offer a clean way to handle: Power-on sequence, Settings navigation, Alerts and notifications. An HSM can manage each feature mode and event without requiring logic in dozens of separate event handlers.

Benefits of Using HSMs in Sparklet UI Designer

Implementing state machines in embedded UI projects with Sparklet provides:

1. Clarity: State machines make behavior explicit and visual, improving comprehension for the entire team.
2. Reduced Code Maintenance: State logic is pulled into design files, making the UI project easier to maintain than distributed conditional code.
3. Predictable Runtime Behavior: Since HSM transitions and actions are predefined, the UI reacts consistently to events.
4. Reusable Logic Blocks: Common states and transitions can be reused across different screens or projects.
5. Zero Code UI Logic: Designers can manage complex UI flows without writing C or scripting code — reducing development time.

Best Practices for Designing HSMs in Sparklet

To get the most out of hierarchical state machines in Sparklet UI Designer:

1. Define meaningful state names — meaningful labels improve understanding.
2. Organize states hierarchically — avoid flat lists in large projects.
3. Use timed transitions where appropriate — to automate splash screens or delayed behavior.
4. Document events and actions clearly — in complex projects, documentation saves debugging time.
5. Test with a simulation — Sparklet UI Designer allows you to simulate state transitions early in the design phase, which helps identify logic issues before deployment.

Conclusion

Hierarchical state machines are a powerful, structured method for managing complex UI logic in embedded systems. In Sparklet UI Designer, they provide a visual, declarative way to define states, events, and transitions — bridging the gap between UI design and event-driven logic without writing code. By integrating HSMs into your embedded interface projects, you gain clarity, predictability, and maintainability — vital for high-quality products in automotive, industrial, medical, and consumer embedded markets.

Whether you’re switching screens, reacting to input events, or orchestrating complex interactions, Sparklet’s hierarchical state machine empowers you to build robust, scalable embedded UIs with confidence.

https://sparkletui.com/documentation/