UXNotificationCenter

UXNotificationCenter is a neutral publish/subscribe bus, shaped like NSNotificationCenter. One object posts a named notification, and any number of others are called back. The observers need not know the poster exists.

It is the decoupled sibling of target/action: an action goes from one sender to one known receiver; a notification goes from one sender to N unknown receivers.

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

Overview

UXNotificationCenter* nc = UXNotificationCenter.shared();

nc.addObserver((Object*)self, &self.onChanged, (u8*)"doc.changed", (Object*)0);
nc.postWith((u8*)"doc.changed", (Object*)0, 42, 7);

void onChanged(UXNotification* note) {
    Stdio.printf("%s a=%d b=%d\n", note.name, note.a, note.b);
}

Posting a name nobody observes is not an error; it is not delivered. A library can post freely without knowing whether anyone is listening.

Lifetime: why this is not a callback list

A naive observer list keeps its observers alive, which creates a retain cycle: window → centre → observer → window. The centre therefore owns neither half of a registration:

the observer is held weak:
the callback never owns its receiver

A dead observer’s callback reads false, so dispatch skips it and the next sweep prunes the entry.

The notification’s `object` is strong, deliberately

UXNotification.object (the poster) is a strong reference, like NSNotification’s. A notification is transient (created, delivered and discarded within one post), so it cannot form a cycle and nothing needs to stay alive beyond the call.

It must not be weak. A weak register/unregister on the sender for every post churns the sender’s weak list, which accumulates and eventually corrupts. The symptom is a crash inside the runtime’s weak bookkeeping, far from the post that caused it.

Filtering by sender

The fourth argument to addObserver is the object to hear from:

nc.addObserver((Object*)w, &w.onChanged, (u8*)"doc.changed", (Object*)docA);

`object`	meaning
`0`	any sender — every post of that name
an object	only posts whose sender is that object

An observer registered with 0 hears posts from every sender, including ones a filtered observer ignores. Both kinds coexist on the same name. A document-per-window app uses this: a global status line listens to all documents, and each window listens only to its own.

Topics

shared · addObserver · removeObserver · post · postWith · postNotification

shared

static UXNotificationCenter* shared(void)

The process-wide centre, made on first use. It has the same lazy-singleton shape as UXNull.null() and UXLog.shared().

You can also make your own centre for a private bus.

addObserver

void addObserver(Object* observer, callback method void(UXNotification* note),
                 u8* name, Object* object)

Subscribes. observer is held weakly and is what removeObserver matches on; method is the callback; name is the notification; object filters by sender (see above).

removeObserver

void removeObserver(Object* observer)

Drops every registration for that observer.

post

void post(u8* name, Object* object)

Posts with no payload.

postWith

void postWith(u8* name, Object* object, i32 a, i32 b)

Posts with a small generic payload. a and b are two integers, which cover most toolkit notifications (a resize carries width and height) without allocating a userInfo dictionary for every post.

When two integers are not enough, define a notification whose object is the thing being announced, and let the observer ask it.

postNotification

void postNotification(UXNotification* n)

Posts a pre-built UXNotification, when you are forwarding one or want to construct it yourself.

Example

post 'doc.changed':
  left  heard 'doc.changed' (a=42 b=7)
  right heard 'doc.changed' (a=42 b=7)
post 'doc.saved' (no observers):
post from a DIFFERENT sender:
  left  heard 'doc.changed' (a=1 b=1)
  right heard 'doc.changed' (a=1 b=1)
post from the watched sender:
  left  heard 'doc.changed' (a=2 b=2)
  right heard 'doc.changed' (a=2 b=2)
  watch heard 'doc.changed' (a=2 b=2)
after removeObserver(right):
  left  heard 'doc.changed' (a=9 b=9)

left and right registered with object == 0, so they hear every post; watch registered against one sender and hears only that one. The program is website/site/examples/uxkit/notify.xc and the doc-examples gate compiles it.

Conforms to

A plain class (not an Object subclass)