Question

I’ve developed a Category that gives the NSOperation the ability to be executed in the background at timed intervals. I would really appreciate getting some feedback on this, especially any potential problems with this approach that I’m not thinking of.

Thank you!

Here’s the code:

NSOperation+Repeat.h

#import <Foundation/Foundation.h>

@interface NSOperation (repeat)

@property (readonly, nonatomic) NSTimeInterval repeatInterval;
@property (readonly, nonatomic) NSOperationQueue *repeatOperationQueue;

- (void)performUsingOperationQueue:(NSOperationQueue *)operationQueue;
- (void)performAtRepeatingInterval:(NSTimeInterval)interval usingOperationQueue:(NSOperationQueue *)operationQueue;

@end

NSOperation+Repeat.m

#import "NSOperation+repeat.h"
#import <objc/runtime.h>

static char const * const RepeatPropertiesKey = "RepeatProperties";

@implementation NSOperation (repeat)

@dynamic repeatInterval;
@dynamic repeatOperationQueue;

static NSString * RepeatIntervalKey = @"interval";
static NSString * RepeatOperationQueueKey = @"operationQueue";
static NSString * RepeatTimerKey = @"timer";

- (NSMutableDictionary *)repeatProperties {
    NSMutableDictionary * properties = objc_getAssociatedObject(self, RepeatPropertiesKey);
    if (properties == nil) {
        properties = [NSMutableDictionary new];
        objc_setAssociatedObject(self, RepeatPropertiesKey, properties, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    return properties;
}

- (NSTimeInterval)interval {
    NSNumber * interval = [[self repeatProperties] objectForKey:RepeatIntervalKey];
    return [interval doubleValue];
}

- (NSOperationQueue *)repeatOperationQueue {
    NSOperationQueue * operationQueue = [[self repeatProperties] objectForKey:RepeatOperationQueueKey];
    return operationQueue;
}

- (void)performUsingOperationQueue:(NSOperationQueue *)operationQueue {
    [operationQueue addOperation:[self copy]];
}

- (void)performAtInterval:(NSTimer *)timer {
    [self performUsingOperationQueue:self.repeatOperationQueue];
}

- (void)performAtRepeatingInterval:(NSTimeInterval)interval usingOperationQueue:(NSOperationQueue *)operationQueue {
    // Save interval and operationQueue in repeatProperties
    [self.repeatProperties setValue:[NSNumber numberWithDouble:interval] forKey:RepeatIntervalKey];
    [self.repeatProperties setValue:operationQueue forKey:RepeatOperationQueueKey];

    // Create timer to call performAtInterval on self
    NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:(interval*60)
                                                      target:self
                                                    selector:@selector(performAtInterval:)
                                                    userInfo:nil
                                                     repeats:YES];

    // Save the timer in repeatProperties
    [self.repeatProperties setValue:timer forKey:RepeatTimerKey];

    [self performUsingOperationQueue:operationQueue];
}

@end

Here’s an example of a NSOperation subclass that can repeat:

MSScheduleImportOperation.h

#import <Foundation/Foundation.h>
#import "NSOperation+Repeat.h"

@interface MSScheduleImportOperation : NSOperation <NSCopying>

@property (readonly, strong, nonatomic) NSString* employeeId;

- (id)initWithEmployeeId:(NSString *)employeeId;

@end

MSScheduleImportOperation.m

#import "MSScheduleImportOperation.h"

@implementation MSScheduleImportOperation

@synthesize employeeId = __employeeId;

- (id)initWithEmployeeId:(NSString *)employeeId     {
    self = [super init];
    __employeeId = [employeeId copy];
    return self;
}

- (id)copyWithZone:(NSZone *)zone {
    MSScheduleImportOperation* copy = [[MSScheduleImportOperation alloc] initWithEmployeeId:self.employeeId];
    return copy;
}

- (void)main
{
 ...   
}


@end

Answer 1

The Apple documentation says:

An operation object is a single-shot object—that is, it executes its task once and cannot be used to execute it again.

So the first problem is that there might be internals that stop it from working. Although, I see you try to get around the problem by making a copy.

This leads us to the other problem is that NSOperation is not advertised to conform to NSCopying.

[operationQueue addOperation:[self copy]];

This line should throw an exception.