CODEWITHSUNDEEP
ios
Janusz Chudzynski
Janusz Chudzynski

Reputation: 2710

NSQueueOperation weird behavior

I am adding multiple instances of subclass of NSOperation in a for loop:

NSMutableArray * operations = [[NSMutableArray alloc]initWithCapacity:0];
     for(int i =1; i<81;i++){
         [operations addObject:[[PDFGenerator alloc]initWithPageNumber:i andPDFParser:pdf]];
     }
    [_queue addOperations:operations waitUntilFinished: NO];

in PDFGenerator I have a variable that stores the current page number for the operation.

@implementation PDFGenerator
int pageCounter;

In the main method of the PDFGenerator I am logging the current page number and it prints 80 for ALL the operations!

I already fixed by using @property for the current page count, but I am trying to understand why it's happening. Any ideas? thanks!

Upvotes: 0

Views: 96

Answers (1)

Ben Flynn
Ben Flynn

Reputation: 18932

When you just use:

int pageCounter;

You are creating a global variable. Presuming you set this at each iteration, then refer to it in your PDFGenerator methods, it will always use the last value it was set to.

Example:

// Bar.h
@interface Bar : NSObject
FOUNDATION_EXPORT int someThing;
@end

// Bar.m
@implementation Bar
int someThing;
@end

// Foo.m
#import "Foo.h"
#import "Bar.h"

@implementation Foo
- (void)doSomething 
{
    ++someThing;
}
@end

That's totally valid code, and calls to [Foo doSomething] increment someThing.

If you wanted an instance variable, it would look like this:

@interface Bar()
{
    int someThing;
}

@end

@implementation Bar
- (void)doSomething
{
    ++someThing;
}
@end

In this case someThing is defined as an instance variable (not a global variable). It is an accessible part of objects of the Bar.

Upvotes: 1

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