add documentation to scheduled functions (#6234)

* add documentation to scheduled functions

cores/esp8266/Schedule.cpp

@@ -98,12 +98,9 @@ bool schedule_recurrent_function_us (const std::function<bool(void)>& fn, uint32
     item->mFunc = fn;
 
     if (rFirst)
-    {
         item->mNext = rFirst;
-        rFirst = item;
+
-    }
+    rFirst = item;
-    else
-        rFirst = item;
 
     return true;
 }

cores/esp8266/Schedule.h

@@ -5,6 +5,21 @@
 
 #define SCHEDULED_FN_MAX_COUNT 32
 
+// The purpose of scheduled functions is to trigger, from SYS stack (like in
+// an interrupt or a system event), registration of user code to be executed
+// in user stack (called CONT stack) without the common restrictions from
+// system context.  Details are below.
+
+// The purpose of recurrent scheduled function is to independantly execute
+// user code in CONT stack on a regular basis.
+// It has been introduced with ethernet service in mind, it can also be used
+// for all libraries in the need of a regular `libdaemon_handlestuff()`.
+// It allows these services to be run even from a user loop not going back
+// to `loop()`.
+// Ticker + scheduled function offer the same service but recurrent
+// scheduled function happen more often: every yield() (vs every loop()),
+// and time resolution is microsecond (vs millisecond). Details are below.
+
 // scheduled functions called once:
 //
 // * internal queue is FIFO.
@@ -17,27 +32,34 @@
 // * There is no mechanism for cancelling scheduled functions.
 // * `yield` can be called from inside lambdas.
 // * Returns false if the number of scheduled functions exceeds
-//   SCHEDULED_FN_MAX_COUNT.
+//   SCHEDULED_FN_MAX_COUNT (or memory shortage).
 // * Run the lambda only once next time.
+// * A scheduled function can schedule a function.
 
 bool schedule_function (const std::function<void(void)>& fn);
 
 // Run all scheduled functions.
-// Use this function if your are not using `loop`, or `loop` does not return
+// Use this function if your are not using `loop`,
-// on a regular basis.
+// or `loop` does not return on a regular basis.
 
 void run_scheduled_functions();
 
 // recurrent scheduled function:
 //
-// * internal queue if not FIFO.
+// * Internal queue may not be a FIFO.
 // * Run the lambda periodically about every <repeat_us> microseconds until
 //   it returns false.
 // * Note that it may be more than <repeat_us> microseconds between calls if
 //   `yield` is not called frequently, and therefore should not be used for
 //   timing critical operations.
-// * There is no mechanism for cancelling recurrent scheduled functions.
+// * Please ensure variables or instances used from inside lambda will exist
-// * long running operations or yield() or delay() are not allowed in the lambda.
+//   when lambda is later called.
+// * There is no mechanism for externally cancelling recurrent scheduled
+//   functions.  However a user function returning false will cancel itself.
+// * Long running operations or yield() or delay() are not allowed in the
+//   recurrent function.
+// * A recurrent function currently must not schedule another recurrent
+//   functions.
 
 bool schedule_recurrent_function_us (const std::function<bool(void)>& fn, uint32_t repeat_us);