examples/usercmodule: Add example of a native C class.

This shows how ports can add their own custom types/classes.

It is part of the unix coverage build, so we can use it for tests too.

Signed-off-by: Laurens Valk <laurens@pybricks.com>

docs/develop/cmodules.rst

@@ -95,9 +95,12 @@ A MicroPython user C module is a directory with the following files:
 Basic example
 -------------
 
-This simple module named ``cexample`` provides a single function
-``cexample.add_ints(a, b)`` which adds the two integer args together and returns
-the result. It can be found in the MicroPython source tree
+The ``cexample`` module provides examples for a function and a class. The
+``cexample.add_ints(a, b)`` function adds two integer args together and returns
+the result. The ``cexample.Timer()`` type creates timers that can be used to
+measure the elapsed time since the object is instantiated.
+
+The module can be found in the MicroPython source tree
 `in the examples directory <https://github.com/micropython/micropython/tree/master/examples/usercmodule/cexample>`_
 and has a source file and a Makefile fragment with content as described above::
 
@@ -272,3 +275,13 @@ can now be accessed in Python just like any other builtin module, e.g.
     import cexample
     print(cexample.add_ints(1, 3))
     # should display 4
+
+.. code-block:: python
+
+    from cexample import Timer
+    from time import sleep_ms
+
+    watch = Timer()
+    sleep_ms(1000)
+    print(watch.time())
+    # should display approximately 1000

examples/usercmodule/cexample/examplemodule.c

@@ -1,6 +1,9 @@
 // Include MicroPython API.
 #include "py/runtime.h"
 
+// Used to get the time in the Timer class example.
+#include "py/mphal.h"
+
 // This is the function which will be called from Python as cexample.add_ints(a, b).
 STATIC mp_obj_t example_add_ints(mp_obj_t a_obj, mp_obj_t b_obj) {
     // Extract the ints from the micropython input objects.
@@ -13,6 +16,59 @@ STATIC mp_obj_t example_add_ints(mp_obj_t a_obj, mp_obj_t b_obj) {
 // Define a Python reference to the function above.
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(example_add_ints_obj, example_add_ints);
 
+// This structure represents Timer instance objects.
+typedef struct _example_Timer_obj_t {
+    // All objects start with the base.
+    mp_obj_base_t base;
+    // Everything below can be thought of as instance attributes, but they
+    // cannot be accessed by MicroPython code directly. In this example we
+    // store the time at which the object was created.
+    mp_uint_t start_time;
+} example_Timer_obj_t;
+
+// This is the Timer.time() method. After creating a Timer object, this
+// can be called to get the time elapsed since creating the Timer.
+STATIC mp_obj_t example_Timer_time(mp_obj_t self_in) {
+    // The first argument is self. It is cast to the *example_Timer_obj_t
+    // type so we can read its attributes.
+    example_Timer_obj_t *self = MP_OBJ_TO_PTR(self_in);
+
+    // Get the elapsed time and return it as a MicroPython integer.
+    mp_uint_t elapsed = mp_hal_ticks_ms() - self->start_time;
+    return mp_obj_new_int_from_uint(elapsed);
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(example_Timer_time_obj, example_Timer_time);
+
+// This represents Timer.__new__ and Timer.__init__, which is called when
+// the user instantiates a Timer object.
+STATIC mp_obj_t example_Timer_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args) {
+    // Allocates the new object and sets the type.
+    example_Timer_obj_t *self = m_new_obj(example_Timer_obj_t);
+    self->base.type = (mp_obj_type_t *)type;
+
+    // Initializes the time for this Timer instance.
+    self->start_time = mp_hal_ticks_ms();
+
+    // The make_new function always returns self.
+    return MP_OBJ_FROM_PTR(self);
+}
+
+// This collects all methods and other static class attributes of the Timer.
+// The table structure is similar to the module table, as detailed below.
+STATIC const mp_rom_map_elem_t example_Timer_locals_dict_table[] = {
+    { MP_ROM_QSTR(MP_QSTR_time), MP_ROM_PTR(&example_Timer_time_obj) },
+};
+STATIC MP_DEFINE_CONST_DICT(example_Timer_locals_dict, example_Timer_locals_dict_table);
+
+// This defines the type(Timer) object.
+MP_DEFINE_CONST_OBJ_TYPE(
+    example_type_Timer,
+    MP_QSTR_Timer,
+    MP_TYPE_FLAG_NONE,
+    make_new, example_Timer_make_new,
+    locals_dict, &example_Timer_locals_dict
+    );
+
 // Define all properties of the module.
 // Table entries are key/value pairs of the attribute name (a string)
 // and the MicroPython object reference.
@@ -21,6 +77,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_2(example_add_ints_obj, example_add_ints);
 STATIC const mp_rom_map_elem_t example_module_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_cexample) },
     { MP_ROM_QSTR(MP_QSTR_add_ints), MP_ROM_PTR(&example_add_ints_obj) },
+    { MP_ROM_QSTR(MP_QSTR_Timer),    MP_ROM_PTR(&example_type_Timer) },
 };
 STATIC MP_DEFINE_CONST_DICT(example_module_globals, example_module_globals_table);
 

tests/misc/cexample_class.py

@@ -0,0 +1,20 @@
+# test custom native class
+
+try:
+    import cexample
+    import time
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+t = cexample.Timer()
+
+print(t)
+print(t.time() <= 1)
+
+time.sleep_ms(100)
+
+elapsed = t.time()
+
+if not (99 <= elapsed <= 110):
+    print("Elapsed time should be approx. 100ms but it is", elapsed)

tests/misc/cexample_class.py.exp

@@ -0,0 +1,2 @@
+<Timer>
+True