static void pointer in AVFoundation.Framework in sample code

Question

I am just going through the sample code of AVFoundation.Framework -> AVSimpleEditoriOS & I found following line which I could not understand.

static void *AVSEPlayerItemStatusContext = &AVSEPlayerItemStatusContext;
static void *AVSEPlayerLayerReadyForDisplay = &AVSEPlayerLayerReadyForDisplay;

consider following

static void *AVSEPlayerItemStatusContext = nil;
static void *AVSEPlayerLayerReadyForDisplay = nil;

Above two lines, I can figure out that those are 2 static void/generic pointers with some fancy name.

Now back to those 2 lines, I am pasting it here again,

static void *AVSEPlayerItemStatusContext = &AVSEPlayerItemStatusContext;
static void *AVSEPlayerLayerReadyForDisplay = &AVSEPlayerLayerReadyForDisplay;

Does above mean, 2 static void/generic pointers storing reference of it's own & why would be it needed in what sense?

I just need little guide to learn such coding pattern. Awaiting for knowledge.

Answer 1

A self-referencing pointer

static void *foo = &foo;

is just a method to create a unique pointer at compile time.

In that "AVSimpleEditoriOS" sample project, these pointers are later used as context parameter for

[self addObserver:self forKeyPath:@"player.currentItem.status" options:NSKeyValueObservingOptionNew context:AVSEPlayerItemStatusContext];

and

[self addObserver:self forKeyPath:@"playerLayer.readyForDisplay" options:NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew context:AVSEPlayerLayerReadyForDisplay];

The actual value of the context parameter does not matter at all, it is just some unique value that is passed to

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
     if (context == AVSEPlayerItemStatusContext) {
        // Notification for @"player.currentItem.status"
        // ...
     } else if (context == AVSEPlayerLayerReadyForDisplay) {
        // Notification for @"playerLayer.readyForDisplay"
        // ...
     } else {
        // Something else, pass to superclass:
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

(Alternatively, one could check the keyPath parameter in observeValueForKeyPath.) See @Bavarious' comment below for why unique context pointers are generally preferred over key path strings.