iPhone dev — performSelector:withObject:afterDelay

Question

To repeat a method call (or message send, I guess the appropriate term is) every x seconds, is it better to use an NSTimer (NSTimer's scheduledTimerWithTimeInterval:target:selector:userInfo:repeats:) or to have the method recursively call itself at the end (using performSelector:withObject:afterDelay)? The latter doesn't use an object, but maybe its less clear/readable? Also, just to give you an idea of what I'm doing, its just a view with a label which counts down to 12:00 midnight, and when it gets to 0, it will blink the time (00:00:00) and play a beep sound forever.

Thanks.

Edit: also, what would be the best way to repeatedly play a SystemSoundID (forever) ? Edit: I ended up using this to play the SystemSoundID forever:

// Utilities.h
#import <Foundation/Foundation.h>
#import <AudioToolbox/AudioServices.h>


static void soundCompleted(SystemSoundID soundID, void *myself);

@interface Utilities : NSObject {

}

+ (SystemSoundID)createSystemSoundIDFromFile:(NSString *)fileName ofType:(NSString *)type;
+ (void)playAndRepeatSystemSoundID:(SystemSoundID)soundID;
+ (void)stopPlayingAndDisposeSystemSoundID;

@end


// Utilities.m
#import "Utilities.h"


static BOOL play;

static void soundCompleted(SystemSoundID soundID, void *interval) {
    if(play) {
        [NSThread sleepForTimeInterval:(NSTimeInterval)interval];
        AudioServicesPlaySystemSound(soundID);
    } else {
        AudioServicesRemoveSystemSoundCompletion(soundID);
        AudioServicesDisposeSystemSoundID(soundID);
    }

}

@implementation Utilities

+ (SystemSoundID)createSystemSoundIDFromFile:(NSString *)fileName ofType:(NSString *)type {
    NSString *path = [[NSBundle mainBundle] pathForResource:fileName ofType:type];
    SystemSoundID soundID;

    NSURL *filePath = [NSURL fileURLWithPath:path isDirectory:NO];

    AudioServicesCreateSystemSoundID((CFURLRef)filePath, &soundID);
    return soundID;
}

+ (void)playAndRepeatSystemSoundID:(SystemSoundID)soundID interval:(NSTimeInterval)interval {
    play = YES
    AudioServicesAddSystemSoundCompletion(soundID, NULL, NULL,
                                          soundCompleted, (void *)interval);
    AudioServicesPlaySystemSound(soundID);
}

+ (void)stopPlayingAndDisposeSystemSoundID {
    play = NO
}

@end

Seems to work fine.. And for the label blinking I'll use an NSTimer I guess.

Answer 1

Since your application depends on time accuracy (i.e. it needs to execute once per second), the NSTimer would be better. It takes some time for the method itself to execute, and an NSTimer would be fine with that (as long as your method takes less than 1 second, if it's called every second).

To repeatedly play your sound, you can set a completion callback and replay the sound there:

SystemSoundID tickingSound;

...

AudioServicesAddSystemSoundCompletion(tickingSound, NULL, NULL, completionCallback, (void*) self);

...

static void completionCallback(SystemSoundID mySSID, void* myself) {
  NSLog(@"completionCallback");

  // You can use this when/if you want to remove the completion callback
  //AudioServicesRemoveSystemSoundCompletion(mySSID);

  // myself is the object that called set the callback, because we set it up that way above
  // Cast it to whatever object that is (e.g. MyViewController, in this case)
  [(MyViewController *)myself playSound:mySSID];
}

Answer 2

Just to add a bit to the other answers, the case for a recursive call would be when the call might take an unknown amount of time - say you are calling a web service repeatedly with small amounts of data until you are finished. Each call may take some unknown amount of time so you have the code do nothing until the web call returns, then the next batch is sent out until no more data remains to be sent and the code does not call itself again.

Answer 3

A timer is more suited to a strictly defined interval. You will lose accuracy if you have your function call itself with a delay because its not really synced to a time interval. There's always the time taken to run the actual method itself as well which puts the interval out.

Stick with an NSTimer, I'd say.