Access values

The truth objects (tm_tt_id_t) are immutable objects unless you explicitly make them writable. Therefore you do not have to be afraid of accidentally changing a value when reading from an object property.

To read from an object property we need access to the correct Truth Instance as well as to an object id. We also need to know what kind of property we want to access. That is why we always want to define our properties in a Header-File. Which allows us and others to find quickly our type definitions. A good practice is to comment on what kind of data type property contains.

Let us assume our object is of type TM_TT_TYPE__RECT:

enum {
    TM_TT_PROP__RECT__X, // float
    TM_TT_PROP__RECT__Y, // float
    TM_TT_PROP__RECT__W, // float
    TM_TT_PROP__RECT__H, // float

When we know what we want to access, we call the correct function and access the value. In our example we want to get the width of an object. The width is stored in TM_TT_PROP__RECT__W.

The function we need to call:

void (*get_float)(tm_the_truth_o *tt,const tm_the_truth_object_o *obj, uint32_t property);

With this knowledge we can assemble the following function that logs the width of an object:

void log_with(tm_the_truth_o *tt, tm_tt_id_t my_object){   
	const float width = tm_the_truth_api->get_float(tt,tm_tt_read(tt,my_object),TM_TT_PROP__RECT__W);
    TM_LOG("the width is %f",width);

Make the code robust

To ensure we are actually handling the right type we should check this at the beginning of our function. If the type is not correct we should early out and log a warning.

All we need to do is compare the tm_tt_type_t's of our types. Therefore we need to obtain the type id from the object id and from our expected type. From a tm_tt_id_t we can obtain the type by calling tm_tt_type() on them. tm_the_truth_api->object_type_from_name_hash(tt, TM_TT_TYPE_HASH__MY_TYPE); will give us back the object type from a given hash. After that we can do our comparison.

void log_with(tm_the_truth_o *tt, tm_tt_id_t my_object) {
  const tm_tt_type_t type = tm_tt_type(my_object);
  const tm_tt_type_t expected_type =
      tm_the_truth_api->object_type_from_name_hash(tt, TM_TT_TYPE_HASH__RECT);

  if (type.u64 != expected_type.u64) {
    TM_LOG("The provided type does not mmatch! %p{tm_tt_type_t} != "
           &type, &expected_type);

  const float width = tm_the_truth_api->get_float(tt, tm_tt_read(tt, my_object),
  TM_LOG("the width is %f", width);

Note: Check out the logger documentation for more information on it. log.h