If you are a developer you may want the ability to nudge users to update your app a bit more forcefully, without having to update other apps in the process. To do so is not that hard and with iOS 5 it became even easier.

You can get the current version of the app from the bundle's infoDictionary:

float currentVersion=[[[[NSBundle mainBundle] infoDictionary] objectForKey:@"CFBundleVersion"]floatValue];

For the lastest version of the app you need to check the Internet. Unfortunately Apple does not expose an API where you can check what the current version of your app is, so you have to create your own. Just create a plist file and host it on your own website or webservice. My file looks like:

{
	"current Version" = 1.0;
}

I am using the human readable plist format, but you can use XML if you prefer. Then upload it to favorite server. I use a public dropbox folder, but you can use s3, or your own server, or generate the content dynamically. Then when the app loads check if the current version is less than then lastest version, and if so display a UIAlertView to the user to update.

Before iOS 5 it was a big pain to download something from the interent. There is ASIHTTPRequest, but it is no longer supported and in my opinion got too bloated. You could use NSURLConnection in iOS 4, but it required a lot of delegate methods and to store the data in an NSMutableData as it came it. You could use a UIWebview to load a page with a few javascript variables and when the webview finished check the variables with stringByEvaluatingJavaScriptFromString:, but it was not very elegant or efficient.

With iOS 5 there is now a very convient function in NSURLConnection to download a request asynchronously without a lot of overhead - sendAsynchronousRequest:queue:completionHandler:. You just pass it a block to deal with the data (or the error) when it is done.

The code below is in my app delegate.

-(void) alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex{
    if (buttonIndex==1) {
        NSURL* appStoreURL=[NSURL URLWithString:@"http://itunes.apple.com/us/app/URL_TO_APP"];
        if ([[UIApplication sharedApplication] canOpenURL:appStoreURL]) {
            [[UIApplication sharedApplication] openURL:appStoreURL];
        }
    }
}
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
     NSURLRequest* request=[NSURLRequest requestWithURL:[NSURL URLWithString:@"http://URL_TO_YOUR_PLIST_FILE.plist"]];

    [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *request, NSData *data, NSError *error) {
        if (!error) {
            NSDictionary* dict=[NSPropertyListSerialization propertyListWithData:data options:NSPropertyListImmutable format:NULL error:NULL];
            float lastestVersion=[[dict objectForKey: @"current Version"] floatValue];
            float currentVersion=[[[[NSBundle mainBundle] infoDictionary] objectForKey:@"CFBundleVersion"]floatValue];

            if (currentVersion < lastestVersion)
            {
                 UIAlertView* alert=[[UIAlertView alloc] initWithTitle:nil
                                                              message:NSLocalizedString(@"An update is available. Would you like to update now?",@"")
                                                             delegate:self
                                                    cancelButtonTitle:NSLocalizedString( @"No",@"")
                                                    otherButtonTitles:NSLocalizedString(@"Yes",@""), nil];
                [alert show];
            }
        }
    }];
    return YES;
}

The above code uses Automatic Reference Counting (ARC) and storyboards, so your code may look a bit different if you have to load UIViewControllers or add a few "autorelease"'s but the idea is the same. If you use the code remember to change the plist URL and your app download URL.

If you have a custom popup view for the app you can have the app delegate tell the current view to display that. You can also add more variables to your plist file, such as a custom message to display, or lastest news related to your app.

If you want to not annoy your users too much, you can save the time the user pressed "No don't Update" in NSUserDefaults and suppress the alert for the week or two.

Of everything in iOS5 (ARC and UIStoryboards included) I think that this update to NSURLConnection is the most useful improvement. What do you think?

If you know what Jewish calendar date you are looking for it is pretty easy, for example Passover is the 15th of Nisan, every year without exception. Create a NSDateComponents with those values (Nisan is the 8th month) and create a seconds NSDateComponents the current hebrew year, month and day. If Passover hasn't happend yet set the year of the NSDateComponents to the current Hebrew Year current year otherwise set it to next year. Now just use NSCalendar's dateFromComponents: and you now know when the next Passover is - or at least how many seconds since 1970 it will be.

The code below can easily be made into an NSDate Extension.

+(NSDate*) nextHebrewDateWithMonth:(int) month Day:(int) day{
    NSCalendar *hebrew = [[[NSCalendar alloc] initWithCalendarIdentifier:NSHebrewCalendar] autorelease];
	NSUInteger unitFlags = NSDayCalendarUnit | NSMonthCalendarUnit|NSYearCalendarUnit;

	NSDateComponents *ComponentsOfToday=[hebrew components:unitFlags fromDate:[NSDate date]];
    NSDateComponents *componentsOfFutureDate=[[[NSDateComponents alloc] init] autorelease];
    componentsOfFutureDate.month=month;
    componentsOfFutureDate.day=day;

    BOOL AlreadyPassedDate=(componentsOfFutureDate.month < ComponentsOfToday.month
                            ||
                            (componentsOfFutureDate.month == ComponentsOfToday.month && componentsOfFutureDate.day < ComponentsOfToday.day ));
    if  (AlreadyPassedDate){
        componentsOfFutureDate.year=ComponentsOfToday.year+1;
    }else{
        componentsOfFutureDate.year=ComponentsOfToday.year;
    }

    return [hebrew dateFromComponents:componentsOfFutureDate];
}

An interesting to note is there there no Gregorian calendar used at all. No where in the code is the Gregorian calendar even implied. All of the NSDateComponents are for Hebrew calendar components. This is part of what I meant when I said in a previous post that IOS does not give preferential treatment to the Gregorian calendar. In some languages the gregorian calendar would HAVE to be used because it is part of how the date object works; here it am simply looking up a point of time on a hebrew calendar.

Once you have the date of the holiday you may want to do different things. If you want a count down you can use NSCalendar components:fromDate:toDate:options:. The calendar you create may be significate or may not be, depending on how the count down works. If you set the unitflags of second, minute, hour, day it will be the same in any calendar, but if you are counting down months it will be slightly different for different calendars.

If you want to know the gregorian date of passover you will need to create a Gregorian NSCalendar. Then you can get the month, day, year with components:fromDate:.

If you want to print the date as a string you can use NSDateFormatter. The formatter does need a calendar to translate a point in time into a date, but it will use the user's default calendar if you don't specify. This can be very cool as will make a different string for different users, each according to his locale, and preferences.

NSDate: NSDate represents a single point in time. This instant takes place simultaneously across the world. It is measured in seconds (with double precision) since January 1, 1970. Apple calls these seconds NSTimeInterval - but it is just a fancy way of saying seconds measured in double precision floating point. NSDate is remarkable for how little it does. It does not know what day of the week or what year this point in time is on. For that you need...

NSCalendar: iOS does not give preferential treatment to the gregorian calendar. This can be very frustrating, as 95% of the time you are dealing with a gregorian calendar. We generally don't think about how a single point in time doesn't have a date - the calendar gives it a date. Once you have a calendar you can interpret a NSDate (a single point in time) to a day, month, year and exact calendar date. However NSCalendar sometimes needs some extra information to be able to interpret an NSDate correctly.

NSTimeZone: even if you have a calendar and point in time, there are about 24 different possibilities for what the date and time is depending on where you are in the world. To complicate matters some countries (such as Israel) don't have a regular Day Light Saving time patterns but are debated each year in the parliament. NSTimeZone has the annoying property of having + localTimeZone, + defaultTimeZone, and + systemTimeZone which I have never really seen a difference between.

NSDateFormatter: Even when you have a point in time, a calendar and a timezone there are still literally hundreds of different ways to present that data as a string. In Europe January 3, 2012 is 3-1-2012 in the United States it is 1-3-2011. You may wish you print the time, or day of the week or the seconds. So you ussally need a NSDateFormatter whenever you deal with dates and strings to tell the systems how to print a date or how to interpret a string that represents a date.

NSDateComponents: besides printing strings sometimes you want directly access what day, month, year, or date of the week it is. To do this you get an NSDateComponents from an NSCalendar. These components can represent either a point in time or distance between two times. When you get differences in time you need to set NSCalendarUnit flags. These flags are just bit shifted int enumerated type that can be OR'ed together to list which values you want. For example if you set (years | days | seconds) then the highest the day value can be 365, but if you set (months | days | seconds) then days could at most be 31.

NSLocale: creeps into a lot of these classes. One of the main reasons for this is that in western countries Sunday is the first day of the week, but in eastern countries Monday is the first day of the week.

Ray Wenderlich has a great turtorial for how to use this layer is an UIView (http://www.raywenderlich.com/6063/uikit-particle-systems-in-ios-5-tutorial ). I don't have much to add on a technical side as he does an excellent job, but I do want to share my fire animation.

I found a Fire animation implimeneted in mac at http://lists.ximian.com/pipermail/mono-osx/2011-March/004196.html where you can also find the fire and smoke pngs I use for this animation. It took a little tweaking make it look right on IOS but here it is:

-(void)awakeFromNib
{
 CAEmitterLayer* fireEmitter = (CAEmitterLayer*)self.layer; ;
    fireEmitter.emitterPosition = CGPointMake(self.bounds.size.width/2, self.bounds.size.height);
    fireEmitter.emitterMode =   kCAEmitterLayerOutline;
    fireEmitter.emitterShape = kCAEmitterLayerLine;

    fireEmitter.emitterSize = CGSizeMake(self.bounds.size.width/5,self.bounds.size.width/5 );

    float gas=.7;
    // Create the fire emitter cell
    CAEmitterCell *fire = [CAEmitterCell emitterCell];
    fire.emissionLongitude = M_PI;
    fire.emissionLatitude = -M_PI/2 ;
    fire.BirthRate = 0;//100*gas;
    fire.lifetime=gas;
    fire.lifetimeRange=gas*0.35;
    fire.Velocity = 80;
    fire.VelocityRange = 21;
    fire.EmissionRange = 1.1f;
    fire.yAcceleration = -200;
    fire.ScaleSpeed = 0.3f;
    fire.color=[[UIColor colorWithRed:0.8 green:0.4 blue:0.2 alpha:0.1] CGColor];
    fire.contents = (id)[[UIImage imageNamed:@"fire.png"] CGImage];
    [fire setName:@"fire"];

    CAEmitterCell *smoke = [CAEmitterCell emitterCell];
    smoke.BirthRate = 0;//11;
    smoke.emissionLongitude = -M_PI ;
    smoke.emissionLatitude=-M_PI;
    smoke.lifetime = gas*4;
    smoke.Velocity = 40;
    smoke.VelocityRange = 20;
    smoke.emissionRange = M_PI / 4;
    smoke.Spin = 1;
    smoke.SpinRange = 6;
    smoke.yAcceleration = -160;
    smoke.Scale = 0.1f;
    smoke.AlphaSpeed = -0.22f;
    smoke.ScaleSpeed = 0.7f;
    smoke.color=[[UIColor colorWithRed:1 green:1 blue:1 alpha:0.3*gas] CGColor];

    smoke.contents = (id)[[UIImage imageNamed:@"smoke.png"] CGImage];
    [smoke setName:@"smoke"];

    fireEmitter.emitterCells = [NSArray arrayWithObjects:smoke,fire, nil];
}

I used this fire animation to create a Virtual Menorah which will (hopefully) be released soon.You can download Hanukkah Menorah on the app store now for FREE. It supports both the iPad and iPhone. It also has some In-App-Purchases to change from the default menorah.