UXNotification

UXNotification is what an observer’s callback receives when a notification is delivered. It has four fields and no behaviour.

#use <UXKit>            // or #import "UXNotificationCenter.xc"

Overview

class UXNotification {
    u8*     name;       // the notification that was posted
    Object* object;     // who posted it; 0 if anonymous
    i32     a;          // payload
    i32     b;
}

void onResize(UXNotification* note) {
    i32 width  = note.a;
    i32 height = note.b;
    UXWindow* win = (UXWindow* ?)note.object;
}

Two integers, not a dictionary

a and b carry a small generic payload. This covers most notifications (a resize is width and height, a selection is an index and a count) without allocating a userInfo dictionary for every post.

When two integers are not enough, make object the thing being announced and let the observer ask it:

nc.post((u8*)"doc.changed", (Object*)document);
// observer: ((Document* ?)note.object).title()

The notification stays transient and the data stays where it already lives, instead of being copied into the post.

`object` is a strong reference

The observer registration holds its observer weakly to avoid a retain cycle. The notification’s object is strong, as NSNotification’s is.

This is safe because a notification is transient: created, delivered and discarded within one post. It cannot form a cycle, and nothing needs to stay alive beyond the call.

Reading `object` safely

object is 0 for an anonymous post, so a cast needs a check unless you know every poster of that name:

void onChanged(UXNotification* note) {
    Document* d = (Document* ?)note.object;
    if (d == (Document*)0) { return; }      // anonymous, or not a Document
    …
}

The checked cast also returns null on a type mismatch, which matters when a name is posted by more than one kind of sender.

Fields

name

u8* name

The notification that was posted. Check it when one callback serves several registrations. A method per notification is usually clearer.

object

Object* object

The poster, or 0. The centre also filters on it: an observer registered against a specific object hears only posts whose object is that one. See UXNotificationCenter.

a / b

i32 a;
i32 b;

The payload, with a meaning defined by the notification. Document what yours carry: they are untyped, so the name is the only contract.