Properly link flecs library

engine/libs/flecs/examples/cpp/systems/mutate_entity/src/main.cpp

@@ -0,0 +1,70 @@
+#include <mutate_entity.h>
+#include <iostream>
+#include <iomanip>
+
+struct Timeout { 
+    double value;
+};
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // System that deletes an entity after a timeout expires
+    ecs.system<Timeout>()
+        .each([](flecs::iter& it, size_t index, Timeout& t) {
+            t.value -= static_cast<double>(it.delta_time());
+            if (t.value <= 0) {
+                // Delete the entity
+
+                // To make sure that the storage doesn't change while a system
+                // is iterating entities, and multiple threads can safely access
+                // the data, mutations (like a delete) are added to a command
+                // queue and executed when it's safe to do so.
+
+                // When the entity to be mutated is not the same as the entity
+                // provided by the system, an additional mut() call is required.
+                // See the mutate_entity_handle example.
+                flecs::entity e = it.entity(index);
+                e.destruct();
+                std::cout << "Expire: " << e.name() << "deleted!\n";
+            }
+        });
+
+    // System that prints remaining expiry time
+    ecs.system<const Timeout>()
+        .each([](flecs::entity e, const Timeout& t) {
+            std::cout << "PrintExpire: " << e.name() << " has " << std::fixed
+                << std::setprecision(2) << t.value << " seconds left\n";
+        });
+
+    // Observer that triggers when entity is actually deleted
+    ecs.observer<Timeout>()
+        .event(flecs::OnRemove)
+        .each([](flecs::entity e, Timeout&) {
+            std::cout << "Expired: " << e.name() << " actually deleted\n";
+        });
+
+
+    flecs::entity e = ecs.entity("MyEntity")
+        .set<Timeout>({3.0});
+
+    ecs.set_target_fps(1);
+
+    while (ecs.progress()) {
+        // If entity is no longer alive, exit
+        if (!e.is_alive()) {
+            break;
+        }
+
+        std::cout << "Tick...\n";
+    }
+
+    // Output
+    //   PrintExpire: MyEntity has 2.00 seconds left
+    //   Tick...
+    //   PrintExpire: MyEntity has 0.99 seconds left
+    //   Tick...
+    //   Expire: MyEntitydeleted!
+    //   PrintExpire: MyEntity has -0.02 seconds left
+    //   Expired: MyEntity actually deleted
+}