Properly link flecs library

engine/libs/flecs/examples/cpp/observers/basics/src/main.cpp

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+#include <basics.h>
+#include <iostream>
+
+// Observers provide a mechanism for responding to builtin and user defined
+// events. They are similar to systems, in that they have the same callback
+// signature and use the same query interface to match with entities, but 
+// instead of a phase they have an event kind.
+//
+// The most commonly used builtin events are:
+//  - flecs::OnAdd: a component was added
+//  - flecs::OnRemove: a component was removed
+//  - flecs::OnSet: a component's value was changed
+//
+// The OnAdd and OnRemove events are only thrown when a component is 
+// actually added or removed. If an application invokes add and the entity
+// already has the component, no event is emitted. Similarly, if an application
+// invokes remove for a component the entity doesn't have, no event is 
+// emitted. That is in contrast to OnSet, which is invoked each time set 
+// or modified is invoked.
+//
+// Observers are different from component hooks in a number of ways:
+//  - A component can only have one hook, whereas it can match many observers
+//  - A hook matches one component, whereas observers can match complex queries
+//  - Hooks are for add/set/remove events, observers can match custom events.
+
+struct Position {
+    double x, y;
+};
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // Create an observer for three events
+    ecs.observer<Position>()
+        .event(flecs::OnAdd)
+        .event(flecs::OnRemove)
+        .event(flecs::OnSet)
+        .each([](flecs::iter& it, size_t i, Position& p) {
+            if (it.event() == flecs::OnAdd) {
+                // No assumptions about the component value should be made here. If 
+                // a ctor for the component was registered it will be called before
+                // the EcsOnAdd event, but a value assigned by set won't be visible.
+                std::cout << " - OnAdd: " << it.event_id().str() << ": "
+                    << it.entity(i).name() << "\n";
+            } else {
+                // EcsOnSet or EcsOnRemove event
+                std::cout << " - " << it.event().name() << ": " 
+                    << it.event_id().str() << ": "
+                    << it.entity(i).name() 
+                    << ": {" << p.x << ", " << p.y << "}\n";
+            }
+        });
+
+    // Create entity, set Position (emits EcsOnAdd and EcsOnSet)
+    flecs::entity e = ecs.entity("e")
+        .set<Position>({10, 20});
+
+    // Remove component (emits EcsOnRemove)
+    e.remove<Position>();
+
+    // Remove component again (no event is emitted)
+    e.remove<Position>();
+
+    // Output
+    //   - OnAdd: Position: e
+    //   - OnSet: Position: e: {10, 20}
+    //   - OnRemove: Position: e: {10, 20}
+}