eo: implement class overriding (+unit tests)

this enables an app or a platform to add an override for a given class,
then return a different object when that class is created. the benefit is
that a class can be internally customized by the app without needing to
modify upstream versions of that class

@feature
fix T7516

Reviewed-by: Marcel Hollerbach <marcel-hollerbach@t-online.de>
Differential Revision: https://phab.enlightenment.org/D7702

src/lib/eo/Eo.h

@@ -1516,6 +1516,33 @@ EAPI Eo * _efl_add_internal_start(const char *file, int line, const Efl_Class *k
  */
 EAPI void efl_del(const Eo *obj);
 
+/**
+ * @brief Set an override for a class
+ *
+ * This can be used to override a class with another class such that when @p klass is added
+ * with efl_add(), an object of type @p override is returned.
+ *
+ * @param[in] klass The class to be overridden
+ * @param[in] override The class to override with; must inherit from or implement @p klass
+ * @return Return @c true if the override was successfully set
+ *
+ * @ingroup Efl_Object
+ */
+EAPI Eina_Bool efl_class_override_register(const Efl_Class *klass, const Efl_Class *override);
+
+/**
+ * @brief Unset an override for a class
+ *
+ * This is used to unset a previously-set override on a given class. It will only succeed if
+ * @p override is the currently-set override for @p klass.
+ *
+ * @param[in] klass The class to unset the override from
+ * @param[in] override The class override to be removed
+ * @return Return @c true if the override was successfully unset
+ *
+ * @ingroup Efl_Object
+ */
+EAPI Eina_Bool efl_class_override_unregister(const Efl_Class *klass, const Efl_Class *override);
 /**
  * @brief Get a pointer to the data of an object for a specific class.
  *

src/lib/eo/eo.c

@@ -44,6 +44,7 @@ int _eo_log_dom = -1;
 Eina_Thread _efl_object_main_thread;
 static unsigned int efl_del_api_generation = 0;
 static Efl_Object_Op _efl_del_api_op_id = 0;
+static Eina_Hash *class_overrides;
 
 typedef enum _Eo_Ref_Op {
    EO_REF_OP_NONE,
@@ -868,6 +869,12 @@ _efl_add_internal_start(const char *file, int line, const Efl_Class *klass_id, E
 
    if (is_fallback) fptr = _efl_add_fallback_stack_push(NULL);
 
+   if (class_overrides)
+     {
+        const Efl_Class *override = eina_hash_find(class_overrides, &klass_id);
+        if (override) klass_id = override;
+     }
+
    EO_CLASS_POINTER_GOTO_PROXY(klass_id, klass, err_klass);
 
    // Check that in the case of efl_add we do pass a parent.
@@ -2596,6 +2603,33 @@ efl_compatible(const Eo *obj, const Eo *obj_target)
    return EINA_FALSE;
 }
 
+EAPI Eina_Bool
+efl_class_override_register(const Efl_Class *klass, const Efl_Class *override)
+{
+   EINA_SAFETY_ON_NULL_RETURN_VAL(klass, EINA_FALSE);
+   EINA_SAFETY_ON_NULL_RETURN_VAL(override, EINA_FALSE);
+   EINA_SAFETY_ON_TRUE_RETURN_VAL(!efl_isa(override, klass), EINA_FALSE);
+   if (!class_overrides)
+     class_overrides = eina_hash_pointer_new(NULL);
+   EINA_SAFETY_ON_NULL_RETURN_VAL(class_overrides, EINA_FALSE);
+
+   eina_hash_set(class_overrides, &klass, override);
+   return EINA_TRUE;
+}
+
+EAPI Eina_Bool
+efl_class_override_unregister(const Efl_Class *klass, const Efl_Class *override)
+{
+   const Efl_Class *set;
+   EINA_SAFETY_ON_NULL_RETURN_VAL(klass, EINA_FALSE);
+   EINA_SAFETY_ON_NULL_RETURN_VAL(override, EINA_FALSE);
+   if (!class_overrides) return EINA_TRUE;
+
+   set = eina_hash_find(class_overrides, &klass);
+   if (set != override) return EINA_FALSE;
+   return eina_hash_del_by_key(class_overrides, &klass);
+}
+
 EAPI Eina_Bool
 efl_destructed_is(const Eo *obj_id)
 {

src/tests/eo/suite/eo_test_general.c

@@ -1798,11 +1798,38 @@ EFL_START_TEST(efl_object_size)
 }
 EFL_END_TEST
 
+EFL_START_TEST(eo_test_class_replacement)
+{
+   Eo *obj;
+
+   /* test basic override */
+   ck_assert(efl_class_override_register(SIMPLE_CLASS, SIMPLE3_CLASS));
+   obj = efl_add_ref(SIMPLE_CLASS, NULL);
+   fail_if(!obj);
+   ck_assert_ptr_eq(efl_class_get(obj), SIMPLE3_CLASS);
+   efl_unref(obj);
+
+   /* test overriding with non-inheriting class */
+   ck_assert(!efl_class_override_register(SIMPLE_CLASS, SIMPLE2_CLASS));
+
+   /* test removing an invalid override */
+   ck_assert(!efl_class_override_unregister(SIMPLE_CLASS, SIMPLE2_CLASS));
+
+   /* test removing an override */
+   ck_assert(efl_class_override_unregister(SIMPLE_CLASS, SIMPLE3_CLASS));
+   obj = efl_add_ref(SIMPLE_CLASS, NULL);
+   fail_if(!obj);
+   ck_assert_ptr_eq(efl_class_get(obj), SIMPLE_CLASS);
+   efl_unref(obj);
+}
+EFL_END_TEST
+
 void eo_test_general(TCase *tc)
 {
    tcase_add_test(tc, eo_simple);
    tcase_add_test(tc, eo_singleton);
    tcase_add_test(tc, efl_object_override_tests);
+   tcase_add_test(tc, eo_test_class_replacement);
    tcase_add_test(tc, eo_signals);
    tcase_add_test(tc, efl_data_fetch);
    tcase_add_test(tc, efl_data_safe_fetch);