iOS 7 Programming Cookbook

7.14. Creating Timers

Problem

You would like to perform a specific task repeatedly with a certain delay. For instance, you want to update a view on your screen every second that your application is running.

Solution

Use a timer:

- (void) paint:(NSTimer *)paramTimer{
    /* Do something here */
    NSLog(@"Painting");
}

- (void) startPainting{
    
    self.paintingTimer = [NSTimer
                          scheduledTimerWithTimeInterval:1.0
                          target:self
                          selector:@selector(paint:)
                          userInfo:nil
                          repeats:YES];
    
}

- (void) stopPainting{
    if (self.paintingTimer != nil){
        [self.paintingTimer invalidate];
    }
}

- (void)applicationWillResignActive:(UIApplication *)application{
    [self stopPainting];
}

- (void)applicationDidBecomeActive:(UIApplication *)application{
    [self startPainting];
}

The invalidate method will also release the timer, so that we don’t have to do that manually. As you can see, we have defined a property called paintingTimer that is declared in this way in the implementation file (.m file):

#import "AppDelegate.h"

@interface AppDelegate ()
@property (nonatomic, strong) NSTimer *paintingTimer;
@end

@implementation AppDelegate

Discussion

A timer is an object that fires an event at specified intervals. A timer must be scheduled in a run loop. Defining an NSTimer object creates a nonscheduled timer that does nothing but is available to the program when you want to schedule it. Once you issue a call, e.g. scheduledTimerWithTimeInterval:target:selector:userInfo:repeats:, the time becomes a scheduled timer and will fire the event you ...