on: → Slot / @connect

An on:click={go} looks, in Svelte, like a function attribute. In libwasm it is a small graph. The element becomes its own struct. That struct mixes in Slot!"click", which is an EventEmitter!(). It declares @callback!"click" void onClick(MouseEvent ev) that this.emit(click). The parent declares @child GoButton goButton and @connect!"goButton.click" void on_goButton_click() that calls the author’s go(). The author’s method is still named go. The printer added the button struct and the connect path around it.

struct GoButton {
  mixin Slot!"click";
  @callback!"click" void onClick(MouseEvent ev) { this.emit(click); }
  mixin NodeDef!"button";
}
struct ClickField {
  @child GoButton goButton;
  @connect!"goButton.click" void on_goButton_click() { go(); }
  void go() { /* author body */ }
}

Empty Slot!"click" is EventEmitter!(). libwasm add appends (~=). Assigning a delegate into that Vector does not compile — an older Slot assigned (cbs = del) and broke against Vector. If you write mixin Slot by hand, append. Do not assign.

@connect!"a.b" is a member path on the compiled struct. Dots stay dots. It is not a CSS selector and not a kit path. @connect!("list.items","link.clicker") is the list form: the first path is the HTMLArray, the second is the event on item type T with dots rewritten to underscores (link_clicker). The connected method’s first parameter must be size_t. That integer is the item address (or index via getIndexInArray). Forgetting it is a compile error in the generated D, not a runtime miss.

Component events on:done={go} connect to @connect!"panel.done" — the child’s Slot, not a new DOM listener. Several on: on one node become several listeners. Modifiers are not a second event system: preventDefault, stopPropagation, once, trusted (isTrusted), and self are applied in the callback. <form on:submit> uses nodeHandle because HTMLFormElement.opDispatch is not a template; UFCS .handle is the wrong lookup.

Author mixin Slot!("name", T) that already this.emit(name) is not re-emitted. The printer will not double-wrap a Slot you already wrote. on:click|self|trusted is ComboMedia. Drag, copy, scroll, contextmenu, pointer, focus, wheel, and touch are the same IR with different event names (ComboMisc, ComboNest, ComboSem).