NSTimer with safe release. Credits to Effective Objective-C 2.0.

main.m

int main(int argc, char *argv[]) {
    @autoreleasepool {
        TimerExample *timer = [TimerExample new];
        [timer startTicking];
        
        NSDate *timeout = [NSDate dateWithTimeIntervalSinceNow:5];
        while ([timeout timeIntervalSinceNow]>0) {
            [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode 
                                     beforeDate:[NSDate dateWithTimeIntervalSinceNow:1]];
        }
        
        [timer stopTicking];
    }
}

NSTimer+(BlockSupport).m

#import <Foundation/Foundation.h>

@interface NSTimer (BlockSupport)

+ (NSTimer*)scheduledTimerWithTimeInterval:(NSTimeInterval)interval
                                     block:(void(^)())block
                                   repeats:(BOOL)repeats;
                            
@end

@implementation NSTimer (BlockSupport)
+ (NSTimer*)scheduledTimerWithTimeInterval:(NSTimeInterval)interval
                                     block:(void(^)())block
                                   repeats:(BOOL)repeats
{
    return [self scheduledTimerWithTimeInterval:interval
                                       // The class will be retained, not the instance.
                                       // The Class object is a singleton so it doesn't matter.
                                         target:self 
                                       selector:@selector(blockInvoke:)
                                       // userInfo is retained while the block is valid
                                       userInfo:[block copy]  // make it a heap block
                                        repeats:repeats];
}

+ (void)blockInvoke:(NSTimer*)timer {
    void (^block)() = timer.userInfo;
    if (block) {
        block();
    }
}

@end

TimerExample.m

// This example class retains NSTimer as an instance variable.
// NSTimer only retains self inside the block and only during the block duration.
// If the instance of the class is released, the timer is invalidated and released, 
// and the class is deallocated as soon as the block is not running.

// If timer is not invalidated in the class dealloc:
//   - the class and its NSTimer instance gets released,
//   - the timer is not released because retainCount won't be 0 while it is valid,
//   - the timer won't call anything because the __weak variable will be nil.

@interface TimerExample : NSObject
- (void)startTicking;
- (void)stopTicking;
@end

@implementation TimerExample {
    NSTimer *_tickTimer;
}

- (id)init {
    return [super init];
}

- (void)dealloc {
    [_tickTimer invalidate];
}

- (void)stopTicking {
    [_tickTimer invalidate];
    _tickTimer = nil;
}

- (void)startTicking {
    __weak TimerExample *weakSelf = self; // avoids retain
    _tickTimer = [NSTimer 
        scheduledTimerWithTimeInterval:1.0
                                 block:^{ 
                                     // Creates a strong reference to keep the 
                                     // instance alive for the duration of the block.
                                     // If class is deallocated and timer is not invalidated, weakSelf will be nil.
                                     TimerExample *strongSelf = weakSelf;
                                     [strongSelf tick];
                                 }    
                               repeats:YES];
}

- (void)tick {
    NSLog(@"poll");
}

@end