Question

I’ve been trying to figure out how to setup an NSTimer to allow me to print the current time in a UILabel within a View, and have it update every second (no finer resolution required – just a simple clock).

At first, I wasn’t using a NSRunLoop, but if I try and include one, the execution just “spins” inside the loop, blocking further execution. I have posted my code below.

-(id) printCurrentTime {

now = [NSDate date];
dateFormat = [[NSDateFormatter alloc] init];

[dateFormat setTimeStyle:NSDateFormatterMediumStyle];

NSString *nowstr = [dateFormat stringFromDate:now];
[dateFormat release];
NSLog(@"Current time is: %@",nowstr);

return nowstr;
}

And in the ViewController source file, I execute as per:

TimeStuff *T = [[TimeStuff alloc] init];
NSString *thetime = [T printCurrentTime];
[timelabel setText:thetime];
[T release];
[self.view addSubview:timelabel];


NSTimer *timmeh = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(printCurrentTime) userInfo:nil repeats:YES];

[[[NSRunLoop currentRunLoop] addTimer:timmeh forMode:NSDefaultRunLoopMode] run];

The “TimeStuff” class is effectively an empty class, save for the printCurrentTime function.

Questions:

1) Should I be including the RunLoop in the AppDelegate class? I am having trouble visualising how this all should hang together, as in – what are the steps to achieving a Loop based on Timer to update a text label with the up-to-second time. Pretty stumped.

2) In the event that I should use a NSThread, should that also be in it’s own class / the Delegate class.

3) Is the ViewController class totally out of bounds for looping/timers, and simply the “eye candy” class, with callbacks to functions in the Delegate class?

Thank you for your time and patience.

Answer 1

You don’t need to deal with run loops at all.

This line :

NSTimer *timmeh = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(printCurrentTime) userInfo:nil repeats:YES];

will create a timer and attach it to the current thread’s run loop for you. You don’t need the [NSRunLoop addTimer:forMode:] call at all – you can delete that line.

PS You certainly don’t need to go as far as NSThreads!

---

EDIT Regarding your comment :

You will need to make an instance of your TimeStuff class for the timer to use if that’s where your printCurrentTime method is. i.e.

@interface MyViewController : UIViewcontroller {
    TimeStuff *timeStuff
}

and in your viewDidLoad method:

- (void)viewDidLoad {
    [super viewDidLoad];

    ...

    // Create our timestuff if we don't have one already
    if (nil == timeStuff)
        timeStuff = [[TimeStuff alloc] init];

    // Start the timer
    [NSTimer scheduledTimerWithTimeInterval:1.0 target:timeStuff selector:@selector(printCurrentTime) userInfo:nil repeats:YES];

and not forgetting dealloc

- (void)dealloc {
    [timeStuff release];
    ...
    [super dealloc];
}

Passing in the timeStuff as the target for the timer tells it where to look for the printCurrentTime method!

Hope that helps,

PS All the line @class TimeStuff does is tell the compiler that there is a class called TimeStuff. It has no idea that you want to use it for your timer’s selector!