Most efficient way of implementing an alarm on iPhone

I have implemented a simple clock like so:

- (void)runTimer {

    timer = [NSTimer scheduledTimerWithTimeInterval:1.0 
                                             target:self 
                                           selector:@selector(showActivity) 
                                           userInfo:nil 
           开发者_C百科                                 repeats:YES];

}

- (void)showActivity {

    NSDateFormatter *df = [[[NSDateFormatter alloc] init] autorelease];
    NSDate *date = [NSDate date];

    [df setDateFormat:@"HH:mm"];

    [clock setFont:digitalFont];

    [clock setText:[df stringFromDate:date]];

}

It's basiclly just a task that is run every second to update the time in a UILabel. Now what if I would like to extend this clock to react on certain times, just like a alarm clock. Continously checking the value of NSDate seems battery intensive. How would I do this?

The simplest way to achieve this? Just have another NSTimer that is set to fire at the time of the alarm.

NSTimer* myTimer = [[NSTimer alloc] initWithFireDate:(NSDate *)date
                                            interval:(NSTimeInterval)seconds
                                              target:(id)target
                                            selector:(SEL)aSelector
                                            userInfo:(id)userInfo
                                             repeats:(BOOL)repeats];

You can use initWithFireDate method