Handler events

Every handler in the library takes one event object, never a positional argument list:

<Button onPress={({ player }) => player.sendMessage('hi')} />
<Slot onInsert={({ stack, host }) => count(host, stack)} />
<Form onSubmit={({ values }) => save(values)} />

Type	Fields	Used by
UiEvent	player, host?, container?	Form.onCancel
PressEvent	player, host?, container?	Button.onPress
ContainerEvent	player, host, container	Container.onOpen / onClose
SlotEvent	player, host, container, stack	Slot.onInsert / onRemove
SubmitEvent	player, values	Form.onSubmit

player is always the player the event is about: the viewer on a form, and on a container screen the player who moved the item. host is the entity or block that owns the screen, so it is present exactly on screens a host owns — always on a container screen, never on a form. container travels with host: it is that screen's own cells as the engine's own Container, addressed by index or by name, so a handler written inside the component reaches them without importing the screen object — see Reaching the cells. What a handler receives can gain a field without breaking a single call site, which is why it is an object rather than arguments.

Next steps

• Button — onPress and the press event
• Form — onSubmit and onCancel
• Container screens — where host comes from