Using specific time and day (NOT date) to activate 'if' function in Xcode - xcode

Question

I'm current piecing together a streaming radio app for a show I work on and so far, it's okay - the streaming works once you push the 'Listen Live!' button and since that's the main aim, I'm happy. However, I'm trying to get clever now and set up an on-screen display that shows when the show is on the air; it's only on for two hours a week, so I thought it'd be nice to show when it's on and off via an on-screen display in the app. Nothing fancy... here's what I've got on screen so far:
1) A counting clock taken from here - EDIT: http://www.youtube.com/watch?v=pNVWST15pyc – that's really simple. Can't believe I didn't realise that changing the hh to HH makes it 24-hour... god, I'm so dumb.
2) A label to show just the day of the week, not the date or month, taken from here - NSCalendar to Display Weekday - as the app only needs to know whether it's Sunday (the on-air day) or not. EDIT: Made this work properly now because I'm an idiot who didn't link it up properly. :(
What I really want to do, however, is have the app set up two settings for an image view - an 'on air' and 'off air' switch - so that when it's between 8pm and 10pm on Sunday night, it shows the 'on air' image and when it's any other time, it shows the 'off air' one. I'm pretty sure that's an if statement but I'm not sure where to start trying to combine the clock and weekday bits to make that work. Can anyone make some suggestions please?
Also, since those times are GMT, I want to lock the app into GMT regardless of where in the world the user is so it relates directly to the show. I'm guessing I can do that using the timeZone:nil bit of the clock code by changing the 'nil' bit to 'Europe/London', but doing so just makes the whole thing crash out in spectacular fashion (I've tried it in various forms with no success). Again, suggestions would be massively appreciated.
Apologies for asking what may seem like simple questions - aside from some adult learning courses on Xcode, I'm a bit of a novice. :D

Answer 1

You should use the NSDateComponents object, which has a -weekday instance method (1 being Sunday for the gregorian calendar). Details on how to get started are in Apple's Date and Time Programming Guide. Here's a code sample that relates to what you are trying to do:
NSDate *today = [NSDate date];
NSCalendar *gregorian = [[NSCalendar alloc]
initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *weekdayComponents =
[gregorian components:(NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:today];
NSInteger day = [weekdayComponents day];
NSInteger weekday = [weekdayComponents weekday];
NSDateComponents will also let you get and set (through so-called accessor methods) hour, minute, second, and timezone.
For the problem you described above, I would write a method like this:
- (BOOL)isShowOnAir {
BOOL onAir = NO;
NSDate *now = [NSDate date];
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *components = [gregorian components:(NSWeekdayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSTimeZoneCalendarUnit) fromDate:now];
[components setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
NSInteger day = [components day]; // Sunday == 1
if (day == 1) {
NSInteger hour = [components hour];
//NSInteger minute = [components minute]; -- not used but here's how to access it
if (hour == 20 || hour == 21) { // covers 8:00 - 9:59 PM
onAir = YES;
}
}
return onAir;
}