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H4 is a simple way of doing multiple things / "tasks" / "processes" at the same time on ESP8266 and ESP32. It is the base of any IOT app you want to build if the app needs to to do more than one thing at a time - which most do!
Technically-speaking, it is a "Functional scheduler" in the form of an ArduinoIDE library providing timers for ESP8266 / ESP32 which can call:
- Normal functions
- Class functions
- Lambda functions
- Any C++ "functor"
Think of it as "Ticker on steroids". H4 has many more options:
- every
- everyRandom
- nTimes
- nTimesRandom
- once
- onceRandom
- queueFunction
- randomTimes
- randomTimesRandom
- repeatWhile
- repeatWhileEver
These allow you to run multiple simultaneous* tasks without worrying about any other tasks or the watchdog timer.
The following youtube videos may prove helpful:
General Introduction / concepts
The "gray series" H4 plain and simple
Watch the video! * Why you need it
Successfully running multiple simultaneous* tasks on most embedded MCUs is notoriously difficult. It usually requires either a ready-made RTOS (e.g. freeRTOS) or a great deal of experience of low-level C/C++ programming and an intimate knowledge of the MCU timer architecture. Both have an intimidating learning curve and most beginners have little knowledge of either.
To make matters worse, the vast majority of examples available are simple one-function "proof of concept" demos written without any concept of resource sharing. Trying to combine two such examples that require co-operation but are written with none is a recipe that leads inevitably to crashes, reboots and "random" failures, often dispiritng and/or deterring the newcomer.
The final "passion killer" is that very few beginners understand the difference between synchronous (a.k.a. "blocking") and asynchronous (a.k.a "non-blocking") code. Successfully running simultaneous* tasks absolutley requires that they do. Most common examples run blocking code, some don't. Mixing the two willy-nilly (because you never knew you shouldn't) is another source of problems and questions regularly seen in support forums.
If - for example - you want MQTT and a webserver and some fast-changing sensors / actuators, then you are in for a lot of hard work and many failures unless you are already familiar with all of the above.
The Arduino core goes some way to help with the Ticker library, which is a great start and allows the user to run "background" independent timer tasks but is limited in several ways:
- It has only two methods: single-shot timer and steady repeating timer (H4 has 11 types)
- It can only call "normal" bare static functions, i.e. it cannot call class methods, lambdas or functional objects, which more advanced code requires, especially in event-driven programming.
- Because it is run in an interrupt-like context, the "safe" work that can be done inside the timer callback is limited - again many "newbies" don't know this and pack it with "prohibited" code that causes problems. In other words while it helps, it does not (and cannot) remove the need for detailed knowledge and experience of sync vs async programming.
H4 is a library that addresses all of these problems and provides a simple interface for the beginner that hides all of these issues that the user never knew they needed to know. Now they really don't. It also provides sophisticated features for the more experienced programmer to greatly reduce development time. It is - if you like - a "poor man's OS" but with minimal size and a much smaller learning curve by comparison with a full RTOS. In truth it is just a scheduler with some fancy timing functions that can call:
- "Normal" functions
- Class methods
- Lambdas
- Functional objects
... but it is the difference between weeks of frustration and a few short hours of joy when developing a new project.
H4 is the foundation that allows for the simple addition of modules (or "plugins") which leverage the safe, "synchronous event loop" model to handle WiFi, MQTT, Webserver, NTP, GPIO handling and pretty much anything else you'd ever want to do with your MCU / IOT "Thing".
For more information or to download the optional H4Plugins library, go to H4Plugins
(* on a single-core MCU no two tasks are ever truly simultaneous - H4 just makes it look like they are and allows you to code as if they were)
H4 allows serialisation of asynchronous events into a synchronous event queue running "on the main loop". On its own, these are generated by its own timer functions, but when using the H4Plugins system, all events e.g. buttons, sensors, timers, incoming web request, MQTT messages are sent to the queue.
If you don't have a clue what that means, don't worry: it is exactly the point of H4 - you don't need to. Without H4, you most definitely would, but here's a picture anyway...
All you need to know is that you no longer write your sketch with setup and loop functions and decide what order to do things. H4 controls all of that for you and H4 alone decides when parts of your code will run and that is only when they won't interfere of break other things that need to be done "behind the scenes".
Those parts of your code are known as "callback functions" or more simply "callbacks". You define what happens inside the callback and H4 decides when to run it.
Your sketch will simply be a series of short callback functions that do what you need to do and a function h4setup where you tell H4 about what type of external events you want to manage and which of your callback functions should be called when one of those events occurs.
The things you do in h4setup are the same kind of thing you used to do in the standard setup but since there is no loop, all the things you used to do there are now in your callback function(s). The good thing is that you don't have to mess with millis() or Tickeror worry about what code works best where and what is not allowed in a Ticker etc because all you code is called from loop by H4.
All the code you write is "normal" code that does not need to worry about any of your other code interfering with it (unless you make it, of course! H4 is clever, but it's not magic - you can still easily crash the MCU with bad code!).
The one thing you will never need to do is call delay or yield. Ever. Repeat after me: "I will never call delay or yield in an H4 sketch".
In case you didn't quite "get" that:
DO NOT EVER CALL THE DELAY FUNCTION OR THE YIELD FUNCTION
If you think you need to, or if you think you need to call millis or micros or delayMicroseconds then quite simply: you are using H4 incorrectly and not "thinking async". The only support answer you will get if you call any of those things is: "don't do it then" :)
H4 invokes a user-defined callback when an "event" occurs. In this basic library, all of those events are caused by timers. When you use H4Plugins those events can also be:
- A GPIO pin changing state
- An incoming web request
- An incoming MQTT request
- A WiFi disconnection or reconnection
- An MQTT server disconnection or reconnection
- A "presence detection" event, e.g. a mobile device joining/leaving the network
- An Alexa voice command
- An absolute time or calendar event
Many of you will only be familiar with the first one of the options listed below: if that is the case, stick with it but try to learn about the others - they will make your life so much easier and allow you to write much better code. "Lambda" functions in particular are incredibly useful and something you should know about for the future. There are many examples provided to show how easy it is to use them. For now, just stick with "normal" functions.
- "Normal" void function with no parameters - just like any other you have ever written,
- Lambda function
- Class function
- Any C++ functor with an
operator()()
#include<H4.h>
H4 h4(115200); // Automatically starts Serial for you if speed provided
void myCallback(){
digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN)); // invert pin state
}
void h4setup(){ // do the same type of thing as the standard setup
pinMode(LED_BUILTIN,OUTPUT);
h4.every(1000,myCallback); // All times are milliseconds, 1000=1 second
}This tells H4 to call your function myCallback every 1000 milliseconds. Some of you might say: "well that's not much different or simpler than the usual way" - and you'd be right, but you'd be missing the point. Watch this, which uses a Lambda function to make life easier:
#include<H4.h>
H4 h4(115200); // Automatically starts Serial for you if speed provided
#define LED1 2 // these numbers will depend on your specific board
#define LED2 D1 // Do not use the same values without knowing why!
#define LED3 D2
void myCallback(uint8_t pin){
digitalWrite(pin, !digitalRead(pin)); // invert pin state
}
void h4setup(){ // do the same type of thing as the standard setup
pinMode(LED1,OUTPUT);
pinMode(LED2,OUTPUT);
pinMode(LED3,OUTPUT);
// using "Lambda" functions to pass value of pin to myCallback when the event occurs
h4.every(1000,[](){ myCallback(LED1); }); // All times are milliseconds, 1000=1 second
h4.every(2000,[](){ myCallback(LED2); }); // All times are milliseconds, 1000=1 second
h4.every(3000,[](){ myCallback(LED3); }); // All times are milliseconds, 1000=1 second
}Voila: Three different LEDs, all running at different speeds at the same time Now think what "normal" code you would have to write to do that. Now do it with 5 LEDs...using H4 it's trivial. The "usual" way is a lot longer, a lot harder, more error-prone and looks awful.
void h4UserLoop(void); // called once per main loop: ONLY USE THIS IF YOU KNOW EXACTLY WHAT YOU ARE DOING!
void onReboot(void); // called just prior to reboot to allow user to clean up etc. see function hookRebootAll timers return a "handle" (type H4_TIMER - see footnote*) which can be used to subsequently cancel the task. It can mostly be ignored.
With one exception (queueFunction) all tasks start after the first specified time interval and not immediately. Using the infinite task "every(1000..." will invoke the first instance of user callback at Tstart + 1sec (1000 mS).
All callbacks take an optional "chain" function (type H4_FN_VOID) which is called on completion of the timer (if ever). "Completion" can occur one of three ways:
- Naturally-expiring (e.g. "
onceXXX") task exits - Free-running (e.g. "
everyXXX") task is cancelled by user code - (Rarely) Task can terminate itself arbitrarily
Tasks which only "make sense" if they are unique (e.g. a system "ticker") can declare themselves on creation as a "singleton". Any existing task with the same type and ID will be cancelled and replaced by the new instance, ensuring only one copy is ever running at a time.
It is important to understand that no task will actually start running until you exit h4setup: you are only declaring / defining them. Once you exit h4setup, H4 takes over the loop and starts processing whatever it finds in the queue.
( * footnote: or H4_TASK_PTR - they are identical)
After #include<H4.h> at top of sketch, user should instantiate an H4 object at global scope, naming it h4
H4 h4; // can also provide a value for Serial speed and/or length of Q, default=20, e.g. H4 h4(115200,13);
// for advanced users:
h4.context // contains the H4_TASK_PTR (or H4_TIMER - same thing) of the currently scheduled task
(or H4::context - choose the style you prefer)H4(uint32_t baud=0,size_t qSize=H4_Q_CAPACITY);
/*
baud = Serial output speed, e.g. 115200. If not specified, Serial will not be started, so you won't see any messages.
qSize = maximum length of task queue. It cannot be set below 5.
Setting it higher than, say, 10 just wastes space and slows things down.
Don't mess with this: 5 is *usually* plenty. Leave it alone till you know more about H4.
*//*
every // run task every t milliseconds
everyRandom // run task continuously rescheduling at random time nMin < t < nMax milliseconds
nTimes // run task n times at intervals of t milliseconds
nTimesRandom // run task n times rescheduling at random time nMin < t < nMax milliseconds
once // have a guess
onceRandom // have another guess
queueFunction // run task NOW* - no initial interval t.[* on next schedule: MCU-dependent, but ~uSec]
randomTimes // run task any number of times nMin < n < nMax at intervals of t
randomTimesRandom // run task random number of time nMin < n < nMax at random intervals of nMin < t < nMax milliseconds
repeatWhile // run task until user-defined "countdown" function (type H4_FN_COUNT) returns zero then cancel
repeatWhileEver // run task until user-defined "countdown" function (type H4_FN_COUNT) returns zero then reschedule [ See Note 1]
common parameters:
fn = your callback function
fnc = your "chain" callback function called on timer completion (if ever)
msec = time in milliseconds
Rmin = a random value in milliseconds for random timers
Rmax = a random value in milliseconds for random timers
s = true makes this a Singleton, leave as false for normal timers
tmax = maximum number of times to run
tmin = minimum number of times to run
u = unique ID: leave as zero or see "Advanced Topics"
*/
H4_TIMER every(uint32_t msec, H4_FN_VOID fn, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
H4_TIMER everyRandom(uint32_t Rmin, uint32_t Rmax, H4_FN_VOID fn, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
H4_TIMER nTimes(uint32_t n, uint32_t msec, H4_FN_VOID fn, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
H4_TIMER nTimesRandom(uint32_t n, uint32_t msec, uint32_t Rmax, H4_FN_VOID fn, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
H4_TIMER once(uint32_t msec, H4_FN_VOID fn, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
H4_TIMER onceRandom(uint32_t Rmin, uint32_t Rmax, H4_FN_VOID fn, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
H4_TIMER queueFunction(H4_FN_VOID fn, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
H4_TIMER randomTimes(uint32_t tmin, uint32_t tmax, uint32_t msec, H4_FN_VOID fn, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
H4_TIMER randomTimesRandom(uint32_t tmin, uint32_t tmax, uint32_t msec, uint32_t Rmax, H4_FN_VOID fn, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
H4_TIMER repeatWhile(H4_FN_COUNT w, uint32_t msec, H4_FN_VOID fn = []() {}, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
H4_TIMER repeatWhileEver(H4_FN_COUNT w, uint32_t msec, H4_FN_VOID fn = []() {}, H4_FN_VOID fnc = nullptr, uint32_t u = 0,bool s=false);
Note 1
The user MUST reset, cancel, stop < whatever > the condition that causes the countdown expiry, otherwise an infinite loop will occur and probably crash the MCU. Since repeatWhile cancels itself after the first occurrence of countdown expiry this is not an issue, but repeatWhileEver reschedules itself, so if the countdown function has not reset itself, it will still be "expired" and so repeatWhileEver reschedules itself, so if the countdown function has not reset itself, it will still be "expired" and so repeatWhileEver... You get the picture?
/*
99% of the time you will just need 'cancel'
cancel // with immediate effect, do not run current instance or chain function, pass "GO" or collect $200
cancelAll // You really don't ever want to do this, but it's there...
cancelSingleton // have a guess
cancelSingleton(initializer_list<uint32_t> l) // have several guesses in one call
finishEarly // jump to chain function after current schedule then quit
finishNow // quit after current schedule, do not run chain function
finishIf // "finishEarly" if user-supplied termination function (type H4_FN_TIF) returns true
*/
void cancel(initializer_list<H4_TASK_PTR> l); // cancel multiple timers in a single call
H4_TASK_PTR cancel(H4_TASK_PTR t = context);
void cancelAll(H4_FN_VOID fn = nullptr);
void cancelSingleton(uint32_t s);
void cancelSingleton(initializer_list<uint32_t> l);
uint32_t finishEarly(H4_TASK_PTR t = context);
uint32_t finishNow(H4_TASK_PTR t = context);
bool finishIf(H4_TASK_PTR t, H4_FN_TIF f);The library has been tested using the following firmware. Please do not even think about raising anhy issues unless you have the following correctly installed.
N.B.
Note that PlatformIO is not in the above list. Many folk do use it, but you will need to create your own installation configuration. I am currently in discussions to add a PIO install to the standard H4 Installer. If you are able to help / contribute to this, please get in touch!
Simply download the zip of this repository and install as an Arduino library: Sketch/Include Library/Add .ZIP Library...
This section is for experts only.
Call h4reboot to ...er... reboot the device
Call hookReboot(H4_FN_VOID f) to add a function to be called just prior to reboot - this is useful for "cleaning up" / closing files, writing buffers etc. By default it will call onReboot so there is no need to explicilty hook that function.
addTaskNames// adds user task names to enable easier identification task/Q dumpsdumpTask// get info per Q itemdumpQ// call dumpTask for all Q itemsgetTaskType// human-readable timer typegetTaskName// human-readable timer name
#if H4_HOOK_TASKS
void _hookTask(H4_FN_TASK f); // replace default task dumper with your own (see below)
static std::string dumpTask(task* t,uint32_t faze); // called once per Q item to return formatted string of task details
static void dumpQ(); // call dumpTask for every item currently scheduled in the Q
static void addTaskNames(H4_INT_MAP names); // adds users own task names to any existing names
static std::string getTaskType(uint32_t t); // returns human-readable timer type (see below)
static const char* getTaskName(uint32_t t); // returns task name or "ANON" if none
H4_FN_TASK H4::taskHook=[](task* t,uint32_t faze=0){ Serial.printf("%s\n",dumpTask(t,faze).data()); };
// faze is an index into :
const char* taskPhase[]={"IDL","NEW","SCH","OOQ","DIE"};
// IDL no event has occured, item is just being dimped by dumpQ(); its just "in the Q"
// NEW on task creation
// SCH in task being rescheduled
// OOQ task has dropped out-of-q ( next event will be DEL)
// DEL task is destroyed
H4_INT_MAP taskTypes={
{1,"link"}, // future use
{3,"evry"}, // h4.every(...
{4,"evrn"}, // h4.everyTandom(...
{5,"ntim"}, // h4.nTimes(...
{6,"ntrn"}, // h4.nTimesRandom(...
{7,"once"}, // h4.once(...
{8,"1xrn"}, // h4.onceRandom(...
{9,"qfun"}, // h4.queueFunction(...
{10,"rntx"}, // h4.randomTimes(...
{11,"rnrn"}, // h4.randomTimesRandom(...
{12,"rptw"}, // h4.repeatWhile(...
{13,"rpwe"}, // h4.repeatWhileEver(...
{99,"work"} // chunker
};
#endifSome libraries require that you call a "loop" / "handle" / "keepalive" function at regular intervals, usually in the main loop - but there isn't one any more...
So, for these (and only) these, the optional userLoop callback exists. Define it and put the library handler function(s) inside it.
If you have any other code inside the userLoop, you are using H4 incorrectly and will not get support.
All usage should be performed with care and only if you really know what you are doing!
"Worker threads" or tasks that have a big job to do "in the background" can "chunk up" the job by saving intermediate values in an area managed by H4 and preserved between schedules. A simple example might be to do something with a very large array. The user might create an "every" task. On the first pass ,create an iterator, do(X) and store the iterator in "partial". On subsequent passes, retrieve the iterator from "partials", increment it, do(X) and store it etc. When the iterator "runs off the end", the task cancels itself. H4 will do any cleanup.
h4.context->createPartial(void* d,size_t l); // create a block of data of length l in partial results - will be automatically deleted on task end
h4.context->getPartial(void* d); // fill a block of data (user-defined) into d from partials results
h4.context->putPartial(void* d); // save a lump of data d in partial results
// If d is created by malloc etc, you MUST MUST MUST deallocate it at some point. static data is safer
// if you can afford the memory.As you may have guessed by now, h4.context is simply a task pointer to an item in the Queue. Most methods and members are public, so TAKE CARE.
h4.context->partial is a pointer to the partial results and can be used IN READ ONLY MODE in preference to getPartial(). DO NOT run off the end of it: h4.context->len contains its size in byes saved from storePartial(). DO NOT CHANGE THE VALUE OF len!!!
That is just an example: the h4Chunker template function already does it for you:
template<typename T>
static void h4Chunker(T &x,std::function<void(typename T::iterator)> fn,uint32_t lo=H4_JITTER_LO,uint32_t hi=H4_JITTER_HI,H4_FN_VOID final=nullptr){
/*
Takes a data structure of type T and calls f with increasing values of an iterator until the iterator == T::end();
f should just manage the single instance of the T sub-item and exit
The millisecond time between consecutive calls is randomised to spread the load and can be changed by tweaking these values in H4.h:
#define H4_JITTER_LO 100
#define H4_JITTER_HI 350
final is a function to allow the user to cleanup, summarise results etc
*/
The public cancellation methods map directly onto the following task functions. Since we are talking about code already inside a task, you can save a function call by calling these directly on the context pointer, e.g. h4.context->endK(); will do the same as h4.cancel(); Make this the last call, you cannot rely on anything after it has been called, just get out while you still can. Run! Run!
uint32_t endF(); // Finalise: finishEarly
uint32_t endU(); // Unconditional finishNow;
uint32_t endC(H4_FN_TIF); // Conditional
uint32_t endK(); // Kill, chop, die, abort, terminate with extreme prejudice etcThere are other functions, but here's the golden rule: If I haven't mentioned it and explained it here: DON'T CALL IT!
These control H4's scheduling so DO NOT CHANGE THEM!. A lot of things will break if you do, including probably your whole app. They are public and writeable for three reasons:
- They allow some "clever" functionality: its good to know which iteration you are on in an nTimes task (see the "10 Green Bottles" example)
- Some H4Plugins need access and for efficiency they are public to avoid "getters and setters"
- I'm lazy
Don't try to use them to "cheat" the system, you will fail. I often do, and I wrote it! Use the public calls for safety.
H4_FN_VOID f; // "It". The task. Your callback that gets called on each schedule
uint32_t rmin=0; // (usually) the controlling time in mS OR the minimum random time [ see Note 2 ]
uint32_t rmax=0; // the maximum random time [ see Note 2 ]
H4_FN_COUNT reaper; // when this function returns zero, the Grim Reaper calls and the task dies a horrible death
H4_FN_VOID chain; // you guessed it...
uint32_t uid=0; // unique ID of task
bool singleton=false; // sigh...
size_t len=0; // length of partial data
uint32_t at; // "due" time [ see note 3 ]
uint32_t nrq=0; // number of times this task has been RE-queued. Note the "RE-" [ See Note 4 ]
void* partial=NULL; // ptr -> Your partial resultsNote 2
rmin and rmax work together. If it's not a random task, rmin is functionally equivalent to the mSec parameter which controls the timer, e.g. once(30000,... will have rmin = 30000 and rmax=0. For randomly-timed tasks, they hold the min and max values of the randomness. In queueFunction, they are both zero, since it never has to wait for any time before it runs f(). There is a special case: when they are equal (but not both zero) then the "due" time is set to T=1000 x 60 x 60 x 24 or a whole day's worth of milliseconds, causing the task to be rescheduled at the exact same time tomorrow... allowing a "daily(..." task or an at(clock_time... task - see the H4P_TimeKeeper plugin
Note 3
The core of the scheduler runs on an architecture-dependent clock which returns a number of milliseconds T. Whatever T is, H4 adds rmin to it and sets that as the "due" time, i.e. run this at time T+rmin. On every loop, the task at the head of the queue is checked and if at is > T+rmin, i.e. the time at which is was due to run has passed, f() is called and the task then makes its reschedule decision.
If the reaper returns zero, the task cleans up, calls the chain (if any) and deletes itself. If it needs to reschedule, it adds rmin (or randomRange(rmin,rmax)) to the due time at and copies itself back into the queue.
Note 4
At the end of a task that has run once, it has been scheduled but it has not been RE-queued hence nrq==0. For all task types except queueFunction the value of nrq+1 is the number of times that f() has been run, being 1 schedule + nrq RE-schedules.
(c) 2021 Phil Bowles h4plugins@gmail.com