Question

I’d like to create a simple pills reminder iOS application (let’s leave notifications aside for the moment).
I want to create a single DB record for a pill that has repetitions and check if a specific day intersects date “generated” by the repetition.

Example:
I set a pills period that starts from April 12 and ends April 20, with repetition every 2 days , at 3.00 pm.
So this pills is valid for this dates :

• April 12 at 3.00pm
• April 14 at 3.00pm
• April 16 at 3.00pm
• April 18 at 3.00pm
• April 20 at 3.00pm

Questions:

• Which type of data can describe the information “Every 2 days”? NSDateInterval would be a good solution ?

• How can i verify that a specific day agrees with my repetition scheme ? (i.e. Check if April 13 is a valid date for the previous example and get “NO” as answer)

Answer 1

Here is an example class which should meet your criteria:

main.m

#import <Foundation/Foundation.h>
#import "DateChecker.h"

int main(int argc, const char * argv[])
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    // Specify a period in days
    NSInteger period = 2;

    // Set the start/end/current days.  These can be retrieved from a database or elsewhere
    NSDate *startDate = [NSDate dateWithString:@"2012-04-12 00:00:00 +0000"];
    NSDate *endDate = [NSDate dateWithString:@"2012-04-20 00:00:00 +0000"];
    NSDate *currentDate = [NSDate dateWithString:@"2012-04-14 00:00:00 +0000"];

    // Actually do the checking.  This could alternatively be offloaded to a function in DateChecker
    if([DateChecker date:currentDate isBetweenDate:startDate andDate:endDate])
    {
        if([DateChecker date:currentDate fallsOnStartDate:startDate withInterval:period])
        {
            NSLog(@"The date %@ falls in the specified date period & is on the interval date.", currentDate);
        }
        else
        {
            NSLog(@"The date %@ falls in the specified date period & is NOT on the interval date.", currentDate);
        }
    }
    else
    {
        NSLog(@"The date %@ does not fall in the specified date period.", currentDate);
    }

    [pool release];

    return 0;
}

DateChecker.h

#import <Foundation/Foundation.h>

@interface DateChecker : NSObject
{
}
+ (BOOL)date:(NSDate*)date fallsOnStartDate:(NSDate*)beginDate withInterval:(NSInteger)daysInterval;
+ (BOOL)date:(NSDate*)date isBetweenDate:(NSDate*)beginDate andDate:(NSDate*)endDate;

@end

DateChecker.m

#import "DateChecker.h"

@implementation DateChecker

+ (BOOL)date:(NSDate*)date fallsOnStartDate:(NSDate*)beginDate withInterval:(NSInteger)daysInterval
{
    NSDateComponents *components;
    NSInteger days;

    // Get the number of days since the start date
    components = [[NSCalendar currentCalendar] components: NSDayCalendarUnit fromDate: beginDate toDate: date options: 0];
    days = [components day];

    // If the number of days passed falls on the interval, then retrun YES
    if(days % daysInterval == 0)
    {
        return YES;
    }
    else
    {
        return NO;
    }
}

+ (BOOL)date:(NSDate*)date isBetweenDate:(NSDate*)beginDate andDate:(NSDate*)endDate
{
    if ([date compare:beginDate] == NSOrderedAscending)
        return NO;

    if ([date compare:endDate] == NSOrderedDescending) 
        return NO;

    return YES;
}

@end

That should give you an idea on how to write a class that can handle your requirements. I ran this as a command line app and it seemed to work alright. The function

+ (BOOL)date:(NSDate*)date isBetweenDate:(NSDate*)beginDate andDate:(NSDate*)endDate

was graciously obtained from How to Check if an NSDate occurs between two other NSDates.