iOS : How to refresh view in a function? - ios

Question

I'm very very new to iOS programming but I already have a big issue I can't solve. It seems so easy.
I have a button, I click on it to change a label called message
Here is the code:
- (IBAction)react:(id)sender {
int hasard ;
hasard=3;
message.text=[NSString stringWithFormat:#"1 %d",hasard];
sleep (1);
message.text=[NSString stringWithFormat:#"2 %d",hasard];
}
It works well but I don't see the first message.text change.
When I click the button, I have to wait one second and I see 2 3
I thought I could see 1 3, wait a second and then see 2 3.
What is missing? It seems so obvious.

Answer 1

sleep() will suspend the current threads execution which is the main thread so you are blocking all UI operations until the method completes. You need to schedule the second assignment to run after the specified time without blocking the current run loop. This can be achieved with GCD.
- (IBAction)react:(id)sender {
int hasard ;
hasard=3;
message.text=[NSString stringWithFormat:#"1 %d",hasard];
int64_t delayInSeconds = 1.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
message.text=[NSString stringWithFormat:#"2 %d",hasard];
});
}

Answer 2

You are blocking the main thread (well, rather sleep() blocks it). If you do so, committed changes to the UI won't appear - you'll only see the final result. You have to do something else (blocking the UI is a very bad idea in terms of user experience, by the way). You can try using a timer, for example:
int hasard = 3;
- (void)react:(id)sender
{
message.text = [NSString stringWithFormat:#"1 %d", hasard];
[NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:#selector(timer:) userInfo:nil repeats:NO];
}
- (void)timer:(NSTimer *)tmr
{
message.text = [NSString stringWithFormat:#"2 %d", hasard];
}