Change detection

The world counts frames in ticks. world.tick() ends one tick and starts the next. Three query filters look at what happened during the current tick:

Filter	Matches entities that, this tick...
Added(T)	were given a T they didn't have before
Changed(T)	had their T replaced with addComponent, or marked with markChanged
Removed(T)	lost their T and are still alive

try world.addComponent(player, Health{ .hp = 100 });

var fresh = world.query(.{clutch.Added(Health)});
while (fresh.next()) |view| { ... } // visits `player`

world.tick(); // next tick: `player` no longer matches Added(Health)

The single-entity versions are wasAdded, wasChanged and wasRemoved.

Marking changes

Clutch doesn't notice writes made through pointers. After changing a component through get or view.get, call markChanged if anything filters on Changed:

world.get(player, Health).?.hp -= 10;
world.markChanged(player, Health);

Despawned entities never match Removed. Neither does a new entity that reuses a despawned entity's id.

Opting out

Tracking costs a little memory and time on every add and remove. Components that nothing filters on can opt out:

const Particle = struct {
    pub const track_changes = false;
    life: f32,
};

Adding and removing such a component is about 20% faster. Using Added, Changed, Removed or markChanged with it becomes a compile error that names the reason.