Example for DISPATCH_SOURCE_TYPE_MEMORYPRESSURE (Mac only, since 10.9) ... Since we share code between iOS and mac, I'm trying to be a good system citizen and reimplement the equivalent of UIApplicationDidReceiveMemoryWarningNotification on the Mac.

PSPDFEnvironment.m

NSString *const PSPDFApplicationDidReceiveMemoryWarningNotification = @"PSPDFApplicationDidReceiveMemoryWarningNotification";

// Test with sudo memory_pressure -l critical. Don't forget to re-set afterwards!
__attribute__((constructor)) static void PSPDFInstallLowMemoryNotificationWarningMac(void) {
    static dispatch_source_t source;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        source = dispatch_source_create(DISPATCH_SOURCE_TYPE_MEMORYPRESSURE, 0, DISPATCH_MEMORYPRESSURE_WARN|DISPATCH_MEMORYPRESSURE_CRITICAL, dispatch_get_main_queue());
        dispatch_source_set_event_handler(source, ^{
            dispatch_source_memorypressure_flags_t pressureLevel = dispatch_source_get_data(source);
            [NSNotificationCenter.defaultCenter postNotificationName:PSPDFApplicationDidReceiveMemoryWarningNotification object:nil userInfo:@{@"pressure": @(pressureLevel)}];
        });
        dispatch_resume(source);
    });
}