How do I use NSTimer?

Question

How do I use an NSTimer? Can anyone give me step by step instructions?

Answer 1

Firstly I'd like to draw your attention to the Cocoa/CF documentation (which is always a great first port of call). The Apple docs have a section at the top of each reference article called "Companion Guides", which lists guides for the topic being documented (if any exist). For example, with NSTimer, the documentation lists two companion guides:

• Timer Programming Topics for Cocoa
• Threading Programming Guide

For your situation, the Timer Programming Topics article is likely to be the most useful, whilst threading topics are related but not the most directly related to the class being documented. If you take a look at the Timer Programming Topics article, it's divided into two parts:

• Timers
• Using Timers

For articles that take this format, there is often an overview of the class and what it's used for, and then some sample code on how to use it, in this case in the "Using Timers" section. There are sections on "Creating and Scheduling a Timer", "Stopping a Timer" and "Memory Management". From the article, creating a scheduled, non-repeating timer can be done something like this:

[NSTimer scheduledTimerWithTimeInterval:2.0     target:self     selector:@selector(targetMethod:)     userInfo:nil     repeats:NO]; 

This will create a timer that is fired after 2.0 seconds and calls targetMethod: on self with one argument, which is a pointer to the NSTimer instance.

If you then want to look in more detail at the method you can refer back to the docs for more information, but there is explanation around the code too.

If you want to stop a timer that is one which repeats, (or stop a non-repeating timer before it fires) then you need to keep a pointer to the NSTimer instance that was created; often this will need to be an instance variable so that you can refer to it in another method. You can then call invalidate on the NSTimer instance:

[myTimer invalidate]; myTimer = nil; 

It's also good practice to nil out the instance variable (for example if your method that invalidates the timer is called more than once and the instance variable hasn't been set to nil and the NSTimer instance has been deallocated, it will throw an exception).

Note also the point on Memory Management at the bottom of the article:

Because the run loop maintains the timer, from the perspective of memory management there's typically no need to keep a reference to a timer after you’ve scheduled it. Since the timer is passed as an argument when you specify its method as a selector, you can invalidate a repeating timer when appropriate within that method. In many situations, however, you also want the option of invalidating the timer—perhaps even before it starts. In this case, you do need to keep a reference to the timer, so that you can send it an invalidate message whenever appropriate. If you create an unscheduled timer (see “Unscheduled Timers”), then you must maintain a strong reference to the timer (in a reference-counted environment, you retain it) so that it is not deallocated before you use it.

Answer 2

there are a couple of ways of using a timer:

1) scheduled timer & using selector

NSTimer *t = [NSTimer scheduledTimerWithTimeInterval: 2.0                       target: self                       selector:@selector(onTick:)                       userInfo: nil repeats:NO]; 

• if you set repeats to NO, the timer will wait 2 seconds before running the selector and after that it will stop;
• if repeat: YES, the timer will start immediatelly and will repeat calling the selector every 2 seconds;
• to stop the timer you call the timer's -invalidate method: [t invalidate];

As a side note, instead of using a timer that doesn't repeat and calls the selector after a specified interval, you could use a simple statement like this:

[self performSelector:@selector(onTick:) withObject:nil afterDelay:2.0]; 

this will have the same effect as the sample code above; but if you want to call the selector every nth time, you use the timer with repeats:YES;

2) self-scheduled timer

NSDate *d = [NSDate dateWithTimeIntervalSinceNow: 60.0]; NSTimer *t = [[NSTimer alloc] initWithFireDate: d                               interval: 1                               target: self                               selector:@selector(onTick:)                               userInfo:nil repeats:YES];  NSRunLoop *runner = [NSRunLoop currentRunLoop]; [runner addTimer:t forMode: NSDefaultRunLoopMode]; [t release]; 

• this will create a timer that will start itself on a custom date specified by you (in this case, after a minute), and repeats itself every one second
  
  

3) unscheduled timer & using invocation

NSMethodSignature *sgn = [self methodSignatureForSelector:@selector(onTick:)]; NSInvocation *inv = [NSInvocation invocationWithMethodSignature: sgn]; [inv setTarget: self]; [inv setSelector:@selector(onTick:)];  NSTimer *t = [NSTimer timerWithTimeInterval: 1.0                       invocation:inv                        repeats:YES]; 

and after that, you start the timer manually whenever you need like this:

NSRunLoop *runner = [NSRunLoop currentRunLoop]; [runner addTimer: t forMode: NSDefaultRunLoopMode]; 

And as a note, onTick: method looks like this:

-(void)onTick:(NSTimer *)timer {    //do smth }