Question

I have one Class called “System” it contains some variables and two arrays and I need to access this from two other classes which should be able to read and write that variables
Im a total Beginner so its pretty possible that i already did some mistakes.

System.h

@interface System : UIViewController{

float length_of_one_hour; 
float length_of_first_break;
float length_of_second_break;
float length_of_lunch_break;
float length_of_shortned_hour;
float school_begin;

int school_end[5];
float school_length[5]; 

}

About_now.m

- (void)read_school_end_monday{

school_end_label.text=[NSString stringWithFormat:@"%i", school_end[0]];

}

Settings.m

- (IBAction)set_school_end_monday{

school_end[0]= [school_end_on_mondays_textfield.text intValue];

}

But i don’t know what to write in System.h and About_now.m that the variables are saved in System class and can be accessed from anywhere. And Yes I already tried @public and extern.
BTW I need to have an array for the school_end because I’ll calculate it (in use of length of an hour and when school actually starts etc.) with a function which already works but i need to access the variables from the About_now class afterwards.

Hope there is someone who can help me. Thanks

Answer 1

A common way to share data across classes in iOS apps is by following the singleton pattern.

SystemModel.h

#import <Foundation/Foundation.h>

@interface SystemModel : NSObject {
    @public
    float length_of_one_hour; 
    float length_of_first_break;
    float length_of_second_break;
    float length_of_lunch_break;
    float length_of_shortned_hour;
    float school_begin;

    int school_end[5];
    float school_length[5]; 
}
+(SystemModel*)instance;
@end

SystemModel.m

@implementation SystemModel
static SystemModel* _instance= nil;

+(SystemModel*)instance {
    @synchronized([SystemModel class]) {
        if (!_instance)
            [[self alloc] init];
        return _instance;
    }
    return nil;
}

+(id)alloc {
    @synchronized([SystemModel class]) {
        return (_instance = [super alloc]);
}
    return nil;
}

-(id)init {
    self = [super init];
    if (self != nil) {
        // your init code
    }
    return self;
}

@end

Now you can use your instance like this:

 float tmp = [SystemModel instance]->length_of_one_hour;

You could also convert instance variables to properties, and use the dot syntax. It does not matter for floats and arrays, but for id-based objects using properties is preferred.