AN: Capstone Dive Computer Example - Quantum Leaps

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Application Note: Capstone Dive Computer Example www.state-machine.com (2) B1_DOWN_SIG = Q_USER_SIG, /**< button B1 depressed */ B1_UP_SIG, B2_DOWN_SIG, /**< button B1 released */ /**< button B2 depressed */ B2_UP_SIG, /**< button B2 released */ (3) MAX_PUB_SIG, /**< the last published signal */ ASCENT_RATE_ADC_SIG, /**< raw ascent rate ADC reading */ HEARTBEAT_SIG, /**< heartbeat of the Capstone scuba diving computer */ DT_TTS_SIG, /**< signal to alternate dive time/time to surface display */ ALARM_REQUEST_SIG, ALARM_SILENCE_SIG, /**< alarm request to AlarmMgr */ /**< alarm silence to AlarmMgr */ TIMEOUT_SIG, /**< timeout for playing a note in AlarmMgr */ (4) MAX_SIG /**< the last signal */ }; (5) typedef struct ADCEvtTag { (6) QEvent super; /* derives from QEvent */ (7) uint16_t raw; /* raw ADC reading */ } ADCEvt; (8) typedef struct AlarmEvtTag { QEvent super; /* derives from QEvent */ (9) uint8_t alarm_type; /* alarm type */ } AlarmEvt; (10) enum AlarmTypes { /* arranged in ascending order of alarm priority */ ALL_ALARMS, DEPTH_ALARM, ASCENT_RATE_ALARM, OUT_OF_AIR_ALARM, MAX_ALARM /* keep always last */ }; /*..........................................................................*/ (11) void Capstone_ctor(void); (12) void AlarmMgr_ctor(void); (13) extern QActive * const AO_Capstone; /* "opaque" pointer to Capstone AO */ (14) extern QActive * const AO_AlarmMgr; /* "opaque" pointer to AlarmMgr AO */ #endif /* capstone_h */ Listing 1 Signals and events used in the Dive Computer (file capstone.h) (1) For smaller applications, such as the Dive Computer, all signals can be defined in one enumeration (rather than in separate enumerations or, worse, as preprocessor #define macros). An enumeration automatically guarantees the uniqueness of signals. (2) Note that the user signals must start with the offset Q_USER_SIG to avoid overlapping the reserved QEP signals. (3) The globally published signals are grouped at top of the enumeration. The MAX_PUB_SIG enumeration automatically keeps track of the maximum published signals in the application. (4) The MAX_SIG enumeration automatically keeps track of the total number of signals used in the application. (5-7) Every event with parameters, such as the ADCEvt derives from the QEvent base structure. Any number of event parameters can be added after the first member super (see also the sidebar below). Copyright © Quantum Leaps, LLC. All Rights Reserved. 10 of 29
Application Note: Capstone Dive Computer Example www.state-machine.com Single Inheritance in C Inheritance is the ability to derive new structures based on existing structures in order to reuse and organize code. You can implement single inheritance in C very simply by literally embedding the base structure as the first member of the derived structure. For example, the following diagram (a) shows the structure ADCEvt derived from the base structure QEvent by embedding the QEvent instance as the first member of ADCEvt. To make this idiom better stand out, QP always names the base structure member super. Derivation of structures in C (a), memory alignment (b), the UML class diagram (c). As shown in panel (b) of the figure above, such nesting of structures always aligns the first data member super at the beginning of every instance of the derived structure. This alignment is guaranteed by the C standard, such as WG14/N1124, Section 6.7.2.1.13, which says: “… A pointer to a structure object, suitably converted, points to its initial member. There may be unnamed padding within a structure object, but not at its beginning” [ISO/IEC 9899:TC2]. In particular, this alignment lets you treat a pointer to the derived ADCEvt structure as a pointer to the QEvent base structure. Consequently, you can always safely pass a pointer to ADCEvt to any C function that expects a pointer to QEvent. (To be strictly correct in C, you should explicitly cast this pointer. In OOP such casting is called upcasting and is always safe.) Therefore, all functions designed for the QEvent structure are automatically available to the ScoreEvt structure as well as other structures derived from QEvent. Panel (c) in the figure above shows the UML class diagram depicting the inheritance relationship between ADCEvt and QEvent structures. QP uses single inheritance quite extensively not just for derivation of events with parameters, but also for derivation of state machines and active objects. Of course, the C++ version of QP uses the native C++ support for class inheritance rather than “derivation of structures”. (8-9) The AlarmEvt structure describes the ALARM(alarm_type) event. (10) The enumeration AlarmTypes enumerates all alarm types in the order of priority. The capstone.h header file shows how to keep the code and data structure of every active object strictly encapsulated within its own C-file. For example, all code and data for the active object Capstone are encapsulated in the file capstone.c, with the external interface consisting of the function Capstone_ctor() and the pointer AO_Capstone. (11-12) These functions perform an early initialization of the active objects in the system. They play the role of static “constructors”, which in C you need to call explicitly, typically at the beginning of main(). Copyright © Quantum Leaps, LLC. All Rights Reserved. 11 of 29
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