NSTimer Category + Blocks implementation to replace selector

Question

I am quite new to blocks and objective-c, and i am trying to write my first category using both. My idea is to create a category on NSTimer that will receive a block as a parameter and this block will be used in the selector call. Right now I have this.

// NSTimer+Additions.h

#import <Foundation/Foundation.h>

typedef void (^VoidBlock)();

@interface NSTimer (NSTimer_Additions)


+ (NSTimer *)scheduleTimerWithTimeInterval:(NSTimeInterval)theSeconds repeats:(BOOL)repeats actions:(VoidBlock)actions;
@end

#import "NSTimer+Additions.h"

static VoidBlock _voidBlock;

@interface NSTimer (AdditionsPrivate) // Private stuff
- (void)theBlock;
@end


@implementation NSTimer (NSTimer_Additions)


+ (NSTimer *)scheduleTimerWithTimeInterval:(NSTimeInterval)theSeconds repeats:(BOOL)repeats actions:(VoidBlock)actions {

    [_voidBlock release];
    _voidBlock = [actions copy];

    NSTimer* timer = [[NSTimer alloc] initWithFireDate:[NSDate date] 
                                          interval:theSeconds
                                            target:self 
                                          selector:@selector(theBlock) 
                                          userInfo:nil 
                                           repeats:repeats];
    [timer fire];

    return [timer autorelease];
}


- (void)theBlock {
    _voidBlock();
}

@end

Gist for the code: https://gist.github.com/1065235

Everything compiles fine but i have the following error:

2011-07-05 14:35:47.068 TesteTimer[37716:903] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[NSTimer theBlock]: unrecognized selector sent to class 0x7fff70bb0a18'

How can I make this category work?

Answer 1

Your major flaw besides the wrong target is your use of a static variable. You won't be able to support beyond a single timer.

Using block as argument to the invoked method.

@interface NSTimer (AdditionsPrivate) // Private stuff
- (void)theBlock:(VoidBlock)voidBlock;
@end


@implementation NSTimer (NSTimer_Additions)

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)theSeconds repeats:(BOOL)repeats actions:(VoidBlock)actions {
    NSInvocation * invocation = [NSInvocation invocationWithMethodSignature:[self instanceMethodSignatureForSelector:@selector(theBlock:)]];
    NSTimer * timer = [NSTimer scheduledTimerWithTimeInterval:theSeconds
                                                   invocation:invocation
                                                      repeats:repeats];
    [invocation setTarget:timer];
    [invocation setSelector:@selector(theBlock:)];

    Block_copy(actions);
    [invocation setArgument:&actions atIndex:2];
    Block_release(actions);

    return timer;
}


- (void)theBlock:(VoidBlock)voidBlock {
    voidBlock();
}

@end

The problem with using associative references was the leak as there was no good point to release the block.

---

Earlier Approach using associative references

You can use associative references to attach the block to that particular instance of NSTimer.

@implementation NSTimer (NSTimer_Additions)

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)theSeconds repeats:(BOOL)repeats actions:(VoidBlock)actions {
    NSInvocation * invocation = [NSInvocation invocationWithMethodSignature:[self instanceMethodSignatureForSelector:@selector(theBlock)]];
    NSTimer * timer = [NSTimer scheduledTimerWithTimeInterval:theSeconds
                                                   invocation:invocation
                                                      repeats:repeats];
    [invocation setTarget:timer];
    [invocation setSelector:@selector(theBlock)];

    objc_setAssociatedObject(timer, @"Block", actions, OBJC_ASSOCIATION_COPY);

    return timer;
}


- (void)theBlock {
    VoidBlock _voidBlock = (VoidBlock)objc_getAssociatedObject(self, @"Block");
    _voidBlock();
}

@end

Answer 2

What about leveraging userInfo to carry your block? (this is done with ARC)

void (^callback)(void) = ^{
    NSLog(@"do stuff");
}

NSTimer *timer = [NSTimer timerWithTimeInterval:10.0 target:self selector:@selector(handleTimeout:) userInfo:[NSDictionary dictionaryWithObject:[callback copy] forKey:@"block"] repeats:NO];
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];

And then add the static selector of:

+ (void)handleTimeout:(NSTimer *)timer
{
    void (^callback)(void) = [timer.userInfo objectForKey:@"block"];
    callback();

    [timer invalidate];
    timer = nil;
};

Answer 3

This should work:

NSTimer* timer = [[NSTimer alloc] initWithFireDate:[NSDate date] 
                                          interval:theSeconds
                                            target:timer
                                          selector:@selector(theBlock) 
                                          userInfo:nil 
                                           repeats:repeats];

The problem is that you're setting the target of the new NSTimer instance to be self. However, in the context of + scheduleTimerWithTimeInterval:repeats:actions: (notice the +), self is NSTimer, and not (as you probably thought) your newly-created NSTimer instance.

As you can see from the error message, your app is crashing because NSTimer doesn't respond to the class method + theBlock, which is of course correct since you only defined the instance method - theBlock.