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mbed-os/events/README.md@0:6bf0743ece18, 2020-03-28 (annotated)

Committer:
demayer
Date:
Sat Mar 28 15:28:19 2020 +0000
Revision:
0:6bf0743ece18
IMU Thread with an event-queue running parallel to handle tasks like a 5 times blinking LED. Button with interrupt detected.

Who changed what in which revision?

UserRevisionLine numberNew contents of line
demayer 0:6bf0743ece18 1 ## The mbed-events library ##
demayer 0:6bf0743ece18 2
demayer 0:6bf0743ece18 3 The mbed-events library provides a flexible queue for scheduling events.
demayer 0:6bf0743ece18 4
demayer 0:6bf0743ece18 5 ``` cpp
demayer 0:6bf0743ece18 6 #include "mbed_events.h"
demayer 0:6bf0743ece18 7 #include <stdio.h>
demayer 0:6bf0743ece18 8
demayer 0:6bf0743ece18 9 int main() {
demayer 0:6bf0743ece18 10 // creates a queue with the default size
demayer 0:6bf0743ece18 11 EventQueue queue;
demayer 0:6bf0743ece18 12
demayer 0:6bf0743ece18 13 // events are simple callbacks
demayer 0:6bf0743ece18 14 queue.call(printf, "called immediately\n");
demayer 0:6bf0743ece18 15 queue.call_in(2000, printf, "called in 2 seconds\n");
demayer 0:6bf0743ece18 16 queue.call_every(1000, printf, "called every 1 seconds\n");
demayer 0:6bf0743ece18 17
demayer 0:6bf0743ece18 18 // events are executed by the dispatch method
demayer 0:6bf0743ece18 19 queue.dispatch();
demayer 0:6bf0743ece18 20 }
demayer 0:6bf0743ece18 21 ```
demayer 0:6bf0743ece18 22
demayer 0:6bf0743ece18 23 The mbed-events library can be used as a normal event loop, or it can be
demayer 0:6bf0743ece18 24 backgrounded on a single hardware timer or even another event loop. It is
demayer 0:6bf0743ece18 25 both thread and irq safe, and provides functions for easily composing
demayer 0:6bf0743ece18 26 independent event queues.
demayer 0:6bf0743ece18 27
demayer 0:6bf0743ece18 28 The mbed-events library can act as a drop-in scheduler, provide synchronization
demayer 0:6bf0743ece18 29 between multiple threads, or just act as a mechanism for moving events out of
demayer 0:6bf0743ece18 30 interrupt contexts.
demayer 0:6bf0743ece18 31
demayer 0:6bf0743ece18 32 ### Usage ###
demayer 0:6bf0743ece18 33
demayer 0:6bf0743ece18 34 The core of the mbed-events library is the [EventQueue](EventQueue.h) class,
demayer 0:6bf0743ece18 35 which represents a single event queue. The `EventQueue::dispatch` function
demayer 0:6bf0743ece18 36 runs the queue, providing the context for executing events.
demayer 0:6bf0743ece18 37
demayer 0:6bf0743ece18 38 ``` cpp
demayer 0:6bf0743ece18 39 // Creates an event queue enough buffer space for 32 Callbacks. This
demayer 0:6bf0743ece18 40 // is the default if no argument was provided. Alternatively the size
demayer 0:6bf0743ece18 41 // can just be specified in bytes.
demayer 0:6bf0743ece18 42 EventQueue queue(32*EVENTS_EVENT_SIZE);
demayer 0:6bf0743ece18 43
demayer 0:6bf0743ece18 44 // Events can be posted to the underlying event queue with dynamic
demayer 0:6bf0743ece18 45 // context allocated from the specified buffer
demayer 0:6bf0743ece18 46 queue.call(printf, "hello %d %d %d %d\n", 1, 2, 3, 4);
demayer 0:6bf0743ece18 47 queue.call(&serial, &Serial::printf, "hi\n");
demayer 0:6bf0743ece18 48
demayer 0:6bf0743ece18 49 // The dispatch function provides the context for the running the queue
demayer 0:6bf0743ece18 50 // and can take a millisecond timeout to run for a fixed time or to just
demayer 0:6bf0743ece18 51 // dispatch any pending events
demayer 0:6bf0743ece18 52 queue.dispatch();
demayer 0:6bf0743ece18 53 ```
demayer 0:6bf0743ece18 54
demayer 0:6bf0743ece18 55 The EventQueue class provides several call functions for posting events
demayer 0:6bf0743ece18 56 to the underlying event queue. The call functions are thread and irq safe,
demayer 0:6bf0743ece18 57 don't need the underlying loop to be running, and provide an easy mechanism
demayer 0:6bf0743ece18 58 for moving events out of interrupt contexts.
demayer 0:6bf0743ece18 59
demayer 0:6bf0743ece18 60 ``` cpp
demayer 0:6bf0743ece18 61 // Simple call function registers events to be called as soon as possible
demayer 0:6bf0743ece18 62 queue.call(doit);
demayer 0:6bf0743ece18 63 queue.call(printf, "called immediately\n");
demayer 0:6bf0743ece18 64
demayer 0:6bf0743ece18 65 // The call_in function registers events to be called after a delay
demayer 0:6bf0743ece18 66 // specified in milliseconds
demayer 0:6bf0743ece18 67 queue.call_in(2000, doit_in_two_seconds);
demayer 0:6bf0743ece18 68 queue.call_in(300, printf, "called in 0.3 seconds\n");
demayer 0:6bf0743ece18 69
demayer 0:6bf0743ece18 70 // The call_every function registers events to be called repeatedly
demayer 0:6bf0743ece18 71 // with a period specified in milliseconds
demayer 0:6bf0743ece18 72 queue.call_every(2000, doit_every_two_seconds);
demayer 0:6bf0743ece18 73 queue.call_every(400, printf, "called every 0.4 seconds\n");
demayer 0:6bf0743ece18 74 ```
demayer 0:6bf0743ece18 75
demayer 0:6bf0743ece18 76 The call functions return an id that uniquely represents the event in the
demayer 0:6bf0743ece18 77 the event queue. This id can be passed to `EventQueue::cancel` to cancel
demayer 0:6bf0743ece18 78 an in-flight event.
demayer 0:6bf0743ece18 79
demayer 0:6bf0743ece18 80 ``` cpp
demayer 0:6bf0743ece18 81 // The event id uniquely represents the event in the queue
demayer 0:6bf0743ece18 82 int id = queue.call_in(100, printf, "will this work?\n");
demayer 0:6bf0743ece18 83
demayer 0:6bf0743ece18 84 // If there was not enough memory necessary to allocate the event,
demayer 0:6bf0743ece18 85 // an id of 0 is returned from the call functions
demayer 0:6bf0743ece18 86 if (id) {
demayer 0:6bf0743ece18 87 error("oh no!");
demayer 0:6bf0743ece18 88 }
demayer 0:6bf0743ece18 89
demayer 0:6bf0743ece18 90 // Events can be cancelled as long as they have not been dispatched. If the
demayer 0:6bf0743ece18 91 // event has already expired, cancel has no side-effects.
demayer 0:6bf0743ece18 92 queue.cancel(id);
demayer 0:6bf0743ece18 93 ```
demayer 0:6bf0743ece18 94
demayer 0:6bf0743ece18 95 For a more fine-grain control of event dispatch, the `Event` class can be
demayer 0:6bf0743ece18 96 manually instantiated and configured. An `Event` represents an event as
demayer 0:6bf0743ece18 97 a C++ style function object and can be directly passed to other APIs that
demayer 0:6bf0743ece18 98 expect a callback.
demayer 0:6bf0743ece18 99
demayer 0:6bf0743ece18 100 ``` cpp
demayer 0:6bf0743ece18 101 // Creates an event bound to the specified event queue
demayer 0:6bf0743ece18 102 EventQueue queue;
demayer 0:6bf0743ece18 103 Event<void()> event(&queue, doit);
demayer 0:6bf0743ece18 104
demayer 0:6bf0743ece18 105 // The event can be manually configured for special timing requirements
demayer 0:6bf0743ece18 106 // specified in milliseconds
demayer 0:6bf0743ece18 107 event.delay(10);
demayer 0:6bf0743ece18 108 event.period(10000);
demayer 0:6bf0743ece18 109
demayer 0:6bf0743ece18 110 // Posted events are dispatched in the context of the queue's
demayer 0:6bf0743ece18 111 // dispatch function
demayer 0:6bf0743ece18 112 queue.dispatch();
demayer 0:6bf0743ece18 113
demayer 0:6bf0743ece18 114 // Events can also pass arguments to the underlying callback when both
demayer 0:6bf0743ece18 115 // initially constructed and posted.
demayer 0:6bf0743ece18 116 Event<void(int, int)> event(&queue, printf, "recieved %d and %d\n");
demayer 0:6bf0743ece18 117
demayer 0:6bf0743ece18 118 // Events can be posted multiple times and enqueue gracefully until
demayer 0:6bf0743ece18 119 // the dispatch function is called.
demayer 0:6bf0743ece18 120 event.post(1, 2);
demayer 0:6bf0743ece18 121 event.post(3, 4);
demayer 0:6bf0743ece18 122 event.post(5, 6);
demayer 0:6bf0743ece18 123
demayer 0:6bf0743ece18 124 queue.dispatch();
demayer 0:6bf0743ece18 125 ```
demayer 0:6bf0743ece18 126
demayer 0:6bf0743ece18 127 Event queues easily align with module boundaries, where internal state can
demayer 0:6bf0743ece18 128 be implicitly synchronized through event dispatch. Multiple modules can
demayer 0:6bf0743ece18 129 use independent event queues, but still be composed through the
demayer 0:6bf0743ece18 130 `EventQueue::chain` function.
demayer 0:6bf0743ece18 131
demayer 0:6bf0743ece18 132 ``` cpp
demayer 0:6bf0743ece18 133 // Create some event queues with pending events
demayer 0:6bf0743ece18 134 EventQueue a;
demayer 0:6bf0743ece18 135 a.call(printf, "hello from a!\n");
demayer 0:6bf0743ece18 136
demayer 0:6bf0743ece18 137 EventQueue b;
demayer 0:6bf0743ece18 138 b.call(printf, "hello from b!\n");
demayer 0:6bf0743ece18 139
demayer 0:6bf0743ece18 140 EventQueue c;
demayer 0:6bf0743ece18 141 c.call(printf, "hello from c!\n");
demayer 0:6bf0743ece18 142
demayer 0:6bf0743ece18 143 // Chain c and b onto a's event queue. Both c and b will be dispatched
demayer 0:6bf0743ece18 144 // in the context of a's dispatch function.
demayer 0:6bf0743ece18 145 c.chain(&a);
demayer 0:6bf0743ece18 146 b.chain(&a);
demayer 0:6bf0743ece18 147
demayer 0:6bf0743ece18 148 // Dispatching a will in turn dispatch b and c, printing hello from
demayer 0:6bf0743ece18 149 // all three queues
demayer 0:6bf0743ece18 150 a.dispatch();
demayer 0:6bf0743ece18 151 ```
demayer 0:6bf0743ece18 152
demayer 0:6bf0743ece18 153