Denislam Valeev / Mbed OS Nucleo_rtos_basic

1

mbed-os/events/README.md@0:e056ac8fecf8, 2018-03-13 (annotated)

Committer:
valeyev
Date:
Tue Mar 13 07:17:50 2018 +0000
Revision:
0:e056ac8fecf8
looking for...

Who changed what in which revision?

UserRevisionLine numberNew contents of line
valeyev 0:e056ac8fecf8 1 ## The mbed-events library ##
valeyev 0:e056ac8fecf8 2
valeyev 0:e056ac8fecf8 3 The mbed-events library provides a flexible queue for scheduling events.
valeyev 0:e056ac8fecf8 4
valeyev 0:e056ac8fecf8 5 ``` cpp
valeyev 0:e056ac8fecf8 6 #include "mbed_events.h"
valeyev 0:e056ac8fecf8 7 #include <stdio.h>
valeyev 0:e056ac8fecf8 8
valeyev 0:e056ac8fecf8 9 int main() {
valeyev 0:e056ac8fecf8 10 // creates a queue with the default size
valeyev 0:e056ac8fecf8 11 EventQueue queue;
valeyev 0:e056ac8fecf8 12
valeyev 0:e056ac8fecf8 13 // events are simple callbacks
valeyev 0:e056ac8fecf8 14 queue.call(printf, "called immediately\n");
valeyev 0:e056ac8fecf8 15 queue.call_in(2000, printf, "called in 2 seconds\n");
valeyev 0:e056ac8fecf8 16 queue.call_every(1000, printf, "called every 1 seconds\n");
valeyev 0:e056ac8fecf8 17
valeyev 0:e056ac8fecf8 18 // events are executed by the dispatch method
valeyev 0:e056ac8fecf8 19 queue.dispatch();
valeyev 0:e056ac8fecf8 20 }
valeyev 0:e056ac8fecf8 21 ```
valeyev 0:e056ac8fecf8 22
valeyev 0:e056ac8fecf8 23 The mbed-events library can be used as a normal event loop, or it can be
valeyev 0:e056ac8fecf8 24 backgrounded on a single hardware timer or even another event loop. It is
valeyev 0:e056ac8fecf8 25 both thread and irq safe, and provides functions for easily composing
valeyev 0:e056ac8fecf8 26 independent event queues.
valeyev 0:e056ac8fecf8 27
valeyev 0:e056ac8fecf8 28 The mbed-events library can act as a drop-in scheduler, provide synchronization
valeyev 0:e056ac8fecf8 29 between multiple threads, or just act as a mechanism for moving events out of
valeyev 0:e056ac8fecf8 30 interrupt contexts.
valeyev 0:e056ac8fecf8 31
valeyev 0:e056ac8fecf8 32 ### Usage ###
valeyev 0:e056ac8fecf8 33
valeyev 0:e056ac8fecf8 34 The core of the mbed-events library is the [EventQueue](EventQueue.h) class,
valeyev 0:e056ac8fecf8 35 which represents a single event queue. The `EventQueue::dispatch` function
valeyev 0:e056ac8fecf8 36 runs the queue, providing the context for executing events.
valeyev 0:e056ac8fecf8 37
valeyev 0:e056ac8fecf8 38 ``` cpp
valeyev 0:e056ac8fecf8 39 // Creates an event queue enough buffer space for 32 Callbacks. This
valeyev 0:e056ac8fecf8 40 // is the default if no argument was provided. Alternatively the size
valeyev 0:e056ac8fecf8 41 // can just be specified in bytes.
valeyev 0:e056ac8fecf8 42 EventQueue queue(32*EVENTS_EVENT_SIZE);
valeyev 0:e056ac8fecf8 43
valeyev 0:e056ac8fecf8 44 // Events can be posted to the underlying event queue with dynamic
valeyev 0:e056ac8fecf8 45 // context allocated from the specified buffer
valeyev 0:e056ac8fecf8 46 queue.call(printf, "hello %d %d %d %d\n", 1, 2, 3, 4);
valeyev 0:e056ac8fecf8 47 queue.call(&serial, &Serial::printf, "hi\n");
valeyev 0:e056ac8fecf8 48
valeyev 0:e056ac8fecf8 49 // The dispatch function provides the context for the running the queue
valeyev 0:e056ac8fecf8 50 // and can take a millisecond timeout to run for a fixed time or to just
valeyev 0:e056ac8fecf8 51 // dispatch any pending events
valeyev 0:e056ac8fecf8 52 queue.dispatch();
valeyev 0:e056ac8fecf8 53 ```
valeyev 0:e056ac8fecf8 54
valeyev 0:e056ac8fecf8 55 The EventQueue class provides several call functions for posting events
valeyev 0:e056ac8fecf8 56 to the underlying event queue. The call functions are thread and irq safe,
valeyev 0:e056ac8fecf8 57 don't need the underlying loop to be running, and provide an easy mechanism
valeyev 0:e056ac8fecf8 58 for moving events out of interrupt contexts.
valeyev 0:e056ac8fecf8 59
valeyev 0:e056ac8fecf8 60 ``` cpp
valeyev 0:e056ac8fecf8 61 // Simple call function registers events to be called as soon as possible
valeyev 0:e056ac8fecf8 62 queue.call(doit);
valeyev 0:e056ac8fecf8 63 queue.call(printf, "called immediately\n");
valeyev 0:e056ac8fecf8 64
valeyev 0:e056ac8fecf8 65 // The call_in function registers events to be called after a delay
valeyev 0:e056ac8fecf8 66 // specified in milliseconds
valeyev 0:e056ac8fecf8 67 queue.call_in(2000, doit_in_two_seconds);
valeyev 0:e056ac8fecf8 68 queue.call_in(300, printf, "called in 0.3 seconds\n");
valeyev 0:e056ac8fecf8 69
valeyev 0:e056ac8fecf8 70 // The call_every function registers events to be called repeatedly
valeyev 0:e056ac8fecf8 71 // with a period specified in milliseconds
valeyev 0:e056ac8fecf8 72 queue.call_every(2000, doit_every_two_seconds);
valeyev 0:e056ac8fecf8 73 queue.call_every(400, printf, "called every 0.4 seconds\n");
valeyev 0:e056ac8fecf8 74 ```
valeyev 0:e056ac8fecf8 75
valeyev 0:e056ac8fecf8 76 The call functions return an id that uniquely represents the event in the
valeyev 0:e056ac8fecf8 77 the event queue. This id can be passed to `EventQueue::cancel` to cancel
valeyev 0:e056ac8fecf8 78 an in-flight event.
valeyev 0:e056ac8fecf8 79
valeyev 0:e056ac8fecf8 80 ``` cpp
valeyev 0:e056ac8fecf8 81 // The event id uniquely represents the event in the queue
valeyev 0:e056ac8fecf8 82 int id = queue.call_in(100, printf, "will this work?\n");
valeyev 0:e056ac8fecf8 83
valeyev 0:e056ac8fecf8 84 // If there was not enough memory necessary to allocate the event,
valeyev 0:e056ac8fecf8 85 // an id of 0 is returned from the call functions
valeyev 0:e056ac8fecf8 86 if (id) {
valeyev 0:e056ac8fecf8 87 error("oh no!");
valeyev 0:e056ac8fecf8 88 }
valeyev 0:e056ac8fecf8 89
valeyev 0:e056ac8fecf8 90 // Events can be cancelled as long as they have not been dispatched. If the
valeyev 0:e056ac8fecf8 91 // event has already expired, cancel has no side-effects.
valeyev 0:e056ac8fecf8 92 queue.cancel(id);
valeyev 0:e056ac8fecf8 93 ```
valeyev 0:e056ac8fecf8 94
valeyev 0:e056ac8fecf8 95 For a more fine-grain control of event dispatch, the `Event` class can be
valeyev 0:e056ac8fecf8 96 manually instantiated and configured. An `Event` represents an event as
valeyev 0:e056ac8fecf8 97 a C++ style function object and can be directly passed to other APIs that
valeyev 0:e056ac8fecf8 98 expect a callback.
valeyev 0:e056ac8fecf8 99
valeyev 0:e056ac8fecf8 100 ``` cpp
valeyev 0:e056ac8fecf8 101 // Creates an event bound to the specified event queue
valeyev 0:e056ac8fecf8 102 EventQueue queue;
valeyev 0:e056ac8fecf8 103 Event<void()> event(&queue, doit);
valeyev 0:e056ac8fecf8 104
valeyev 0:e056ac8fecf8 105 // The event can be manually configured for special timing requirements
valeyev 0:e056ac8fecf8 106 // specified in milliseconds
valeyev 0:e056ac8fecf8 107 event.delay(10);
valeyev 0:e056ac8fecf8 108 event.period(10000);
valeyev 0:e056ac8fecf8 109
valeyev 0:e056ac8fecf8 110 // Posted events are dispatched in the context of the queue's
valeyev 0:e056ac8fecf8 111 // dispatch function
valeyev 0:e056ac8fecf8 112 queue.dispatch();
valeyev 0:e056ac8fecf8 113
valeyev 0:e056ac8fecf8 114 // Events can also pass arguments to the underlying callback when both
valeyev 0:e056ac8fecf8 115 // initially constructed and posted.
valeyev 0:e056ac8fecf8 116 Event<void(int, int)> event(&queue, printf, "received %d and %d\n");
valeyev 0:e056ac8fecf8 117
valeyev 0:e056ac8fecf8 118 // Events can be posted multiple times and enqueue gracefully until
valeyev 0:e056ac8fecf8 119 // the dispatch function is called.
valeyev 0:e056ac8fecf8 120 event.post(1, 2);
valeyev 0:e056ac8fecf8 121 event.post(3, 4);
valeyev 0:e056ac8fecf8 122 event.post(5, 6);
valeyev 0:e056ac8fecf8 123
valeyev 0:e056ac8fecf8 124 queue.dispatch();
valeyev 0:e056ac8fecf8 125 ```
valeyev 0:e056ac8fecf8 126
valeyev 0:e056ac8fecf8 127 Event queues easily align with module boundaries, where internal state can
valeyev 0:e056ac8fecf8 128 be implicitly synchronized through event dispatch. Multiple modules can
valeyev 0:e056ac8fecf8 129 use independent event queues, but still be composed through the
valeyev 0:e056ac8fecf8 130 `EventQueue::chain` function.
valeyev 0:e056ac8fecf8 131
valeyev 0:e056ac8fecf8 132 ``` cpp
valeyev 0:e056ac8fecf8 133 // Create some event queues with pending events
valeyev 0:e056ac8fecf8 134 EventQueue a;
valeyev 0:e056ac8fecf8 135 a.call(printf, "hello from a!\n");
valeyev 0:e056ac8fecf8 136
valeyev 0:e056ac8fecf8 137 EventQueue b;
valeyev 0:e056ac8fecf8 138 b.call(printf, "hello from b!\n");
valeyev 0:e056ac8fecf8 139
valeyev 0:e056ac8fecf8 140 EventQueue c;
valeyev 0:e056ac8fecf8 141 c.call(printf, "hello from c!\n");
valeyev 0:e056ac8fecf8 142
valeyev 0:e056ac8fecf8 143 // Chain c and b onto a's event queue. Both c and b will be dispatched
valeyev 0:e056ac8fecf8 144 // in the context of a's dispatch function.
valeyev 0:e056ac8fecf8 145 c.chain(&a);
valeyev 0:e056ac8fecf8 146 b.chain(&a);
valeyev 0:e056ac8fecf8 147
valeyev 0:e056ac8fecf8 148 // Dispatching a will in turn dispatch b and c, printing hello from
valeyev 0:e056ac8fecf8 149 // all three queues
valeyev 0:e056ac8fecf8 150 a.dispatch();
valeyev 0:e056ac8fecf8 151 ```
valeyev 0:e056ac8fecf8 152
valeyev 0:e056ac8fecf8 153