NSTimer with block -- am I doing it right?

Question

The following is my Objective-C category on NSTimer to do block-based firing of NSTimers. I can't see anything wrong with it, but what I am getting is that the block I pass into the schedule... method is being deallocated despite me calling copy on it.

What am I missing?

typedef void(^NSTimerFiredBlock)(NSTimer *timer);

@implementation NSTimer (MyExtension)

+ (void)timerFired:(NSTimer *)timer 
{
    NSTimerFiredBlock blk = timer.userInfo;
    if (blk != nil) {
        blk(timer);
    }
}

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)seconds 
                                    repeats:(BOOL)repeats 
                                   callback:(NSTimerFiredBlock)blk 
{
    return [NSTimer scheduledTimerWithTimeInterval:seconds
                                            target:self
                                          selector:@selector(timerFired:)
                                          userInfo:[blk copy]
                                           repeats:repeats];
}

@end

Hanway

I think (void)timerFired:(NSTimer *)timer should be an instance method -, not a class method +. The same probably applies to scheduledTimerWithTimeInterval, but I am less sure about that. – Erythrocytometer

That would not be the case. NSTimer scheduled methods are class methods. – Hanway

Passing self as the target strongly suggests an instance method. All code samples of NSTimer that I've seen use instance methods for selectors as well. – Erythrocytometer

Class objects are first class objects in Objective-C. No reason why "self" can't point to a class. That you have not seen it does not mean it is not supported. See here. – Hanway

Just curious, if you pass something other than a block (say, an instance of your own object with dealloc) for userInfo, does your dealloc gets called? – Erythrocytometer

I tried your code, and it works like a charm. I put a [NSTimer scheduledTimerWithTimeInterval:1 repeats:YES callback:^(NSTimer* t){ NSLog(@"I'm here"); }]; call in my application:didFinishLaunchingWithOptions:, and I am getting the I'm here logs every second. – Erythrocytometer

Works great, definitely odd calling with self, but it is actually fine. – Fabrizio

Answer 1

I found this code over at http://orion98mc.blogspot.ca/2012/08/objective-c-blocks-for-fun.html

Great work

NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:0.7
      target:[NSBlockOperation blockOperationWithBlock:^{ /* do this! */ }]
      selector:@selector(main)
      userInfo:nil
      repeats:NO
];

Clingstone

Wow, this is great. From experimentation, if I don't hold a reference to the NSTimer, it looks it lives until it fires and then gets cleaned up. – Giorgia

Answer 2

You have a project on github that does the job !

Cocoapod BlocksKit, allow you to Blockify a bunch of Classes...

#import "NSTimer+BlocksKit.h"
[NSTimer bk_scheduledTimerWithTimeInterval:1.0 block:^(NSTimer *time) {
       // your code
    } repeats:YES];

Naarah

Answer 3

Here is the Swift version of Mc.Stever's answer:

NSTimer.scheduledTimerWithTimeInterval(0.7, target: NSBlockOperation(block: {
    /* do work */
}), selector: "main", userInfo: nil, repeats: false)

Rudbeckia

Swift 2.2: NSTimer.scheduledTimerWithTimeInterval(0.7, target: NSBlockOperation(block: { /* do work */ }), selector: #selector(NSOperation.main), userInfo: nil, repeats: false) – Boehmite

Answer 4

What you're missing is that if the block you're passing in is on the stack then copy will do exactly what the name says — it'll create a copy of the block over on the heap. You'd therefore expect no change in the behaviour of the one you passed in; nobody new is retaining it. The copy will stay alive while the original is deallocated.

(aside: if you're not using ARC you'll also want to autorelease the copy; you're meant to pass a non-owning reference as userInfo:. Otherwise the copy will never be deallocated)

Drapery

Answer 5

try this

typedef void(^NSTimerFiredBlock)(NSTimer *timer);

@interface NSObject (BlocksAdditions)

- (void)my_callBlock:(NSTimer *)timer;

@end

@implementation NSObject (BlocksAdditions)

- (void)my_callBlock:(NSTimer *)timer {
    NSTimerFiredBlock block = (id)self;
    block(timer);
}

@implementation NSTimer (MyExtension)

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)seconds 
                                    repeats:(BOOL)repeats 
                                   callback:(NSTimerFiredBlock)blk 
{
    blk = [[blk copy] autorelease];
    return [NSTimer scheduledTimerWithTimeInterval:seconds
                                            target:blk
                                          selector:@selector(my_callBlock:)
                                          userInfo:nil
                                           repeats:repeats];
}

@end

Tantalite

Actually, I tried this solution and I m still getting the same problem. I don't think it's that selectors are not supported on class methods. Class objects are also first class objects in Objective-C. – Hanway

selectors definitely support class methods. your problem is block get deallocated before timer fired but i cannot see why this is happening. what is the error message? – Tantalite