Question

I’m pretty sure this is really simple, and I’m just missing something obvious. I have an app that needs to download data from a web service for display in a UITableView, and I want to display a UIAlertView if the operation takes more than X seconds to complete. So this is what I’ve got (simplified for brevity):

MyViewController.h

@interface MyViewController : UIViewController
        <UITableViewDelegate, UITableViewDataSource> {
    NSTimer *timer;
}

@property (nonatomic, retain) NSTimer *timer;

MyViewController.m

@implementation MyViewController

@synthesize timer;

- (void)viewDidLoad {
    timer = [NSTimer scheduledTimerWithTimeInterval:20
          target:self
        selector:@selector(initializationTimedOut:)
        userInfo:nil
         repeats:NO];

    [self doSomethingThatTakesALongTime];
    [timer invalidate];
}

- (void)doSomethingThatTakesALongTime {
    sleep(30); // for testing only
    // web service calls etc. go here
}

- (void)initializationTimedOut:(NSTimer *)theTimer {
    // show the alert view
}

My problem is that I’m expecting the [self doSomethingThatTakesALongTime] call to block while the timer keeps counting, and I’m thinking that if it finishes before the timer is done counting down, it will return control of the thread to viewDidLoad where [timer invalidate] will proceed to cancel the timer. Obviously my understanding of how timers/threads work is flawed here because the way the code is written, the timer never goes off. However, if I remove the [timer invalidate], it does.

Answer 1

I think there is a problem with scheduling a timer and doing a blocking call on the same thread. Until the blocking call is completed, the run-loop cannot fire the timer.

I suggest you to detach a thread to perform the long operation. Once the long operation is finished, call back on the main thread to invalidate the timer.

Note: it is important to invalidate the timer on the same thread it was scheduled.

- (void)viewDidLoad {
    timer = [NSTimer scheduledTimerWithTimeInterval:20
          target:self
        selector:@selector(initializationTimedOut:)
        userInfo:nil
         repeats:NO];

    [NSThread detachNewThreadSelector:@selector(doSomethingThatTakesALongTime:) toTarget:self withObject:nil];
}

- (void)doSomethingThatTakesALongTime:(id)arg {
    sleep(30); // for testing only
    // web service calls etc. go here
    [self performSelectorOnMainThread:@selector(invalidate) withObject:nil waitUntilDone:NO];
}

- (void)invalidate {
    [timer invalidate];
}

- (void)initializationTimedOut:(NSTimer *)theTimer {
    // show the alert view
}