package ThermostatStateMachine {
private import ScalarValues::*;
// 1. Structural Component
part def ThermostatController {
    attribute currentTemp : Real;
    attribute targetTemp : Real;
    
    // 2. State-Based Behavior (Inline Usage)
    state thermostatControl {
        
        // Initial Transition
        entry; then idleState;

        // State Blocks
        state idleState {
            entry action; 
            do action;    
            exit action;
        }

        state heatingState {
            entry action;
            do action;
            exit action;
        }

        state coolingState {
            entry action;
            do action;
            exit action;
        }

        // 3. Transitions & Guards
        transition idleToHeating first idleState if currentTemp < (targetTemp - 1.5) then heatingState;
        transition heatingToIdle first heatingState if currentTemp >= targetTemp then idleState;
        transition idleToCooling first idleState if currentTemp > (targetTemp + 1.5) then coolingState;
        transition coolingToIdle first coolingState if currentTemp <= targetTemp then idleState;
    }
}
}

🧠 Key Architecture Concepts## Definition vs. Inline Usage

• Definitions (state def) create standalone blueprints.

• Definitions cannot read external part attributes.

• Usages (state) live inside specific parts.

• Usages read component attributes directly.

• Use inline usage for data-driven guards.

The Initial Node Pattern

• SysML v2 drops the classic graphical circle.

• It uses the entry; then ; syntax.

• This forces immediate execution on system boot.

📝 Syntax Element Breakdown## 1. State Actions

Every state can manage three distinct operational phases:

• entry action;: Fires instantly upon entering the state.

• do action;: Executes continuously while inside the state.

• exit action;: Triggers right before leaving the state.

2. Transition Chains

Transitions dictate system pathing via a strict keyword sequence. They must follow this precise order:
$$\ text{transition} \rightarrow \text{first [Source]} \rightarrow \text{if [Guard]} \rightarrow \text{then [Target]}$$

• first: Declares the active origin state.

• if: Validates a Boolean conditional guard.

• then: Defines the destination state target.

⚠️ Vital Compilation Quick-Tips

• Keep transitions on one line. Multi-line breaks confuse the compiler.

• Never leave actions empty. Use the action; keyword placeholder.

• Check your closing braces. Do not close states prematurely.

Conclusion