Issue updating label in iOS when iterating over array - ios

Question

I am new to iOS programming, and I could not find an answer out there already.
In Xcode 5, I am iterating over an array, and attempting to update a label with the values as they change.
here is the .h file...
#import <UIKit/UIKit.h>
#interface ViewController : UIViewController
#property (strong, nonatomic) NSArray *currentNumber;
#property (strong, nonatomic) IBOutlet UILabel *showLabel;
- (IBAction)start;
#end
here is the main part of the .m file...
#import "ViewController.h"
#interface ViewController ()
#end
#implementation ViewController
- (void)viewDidLoad
{
[super viewDidLoad];
self.currentNumber = [NSArray arrayWithObjects:#"1", #"2", #"3", #"4", nil];
}
This is where it gets tricky...
The following works perfectly...
- (IBAction)start {
self.showLabel.text = [NSString stringWithFormat:#"new text"];
}
#end
As does this...
- (IBAction)start {
for (NSString *p in self.currentNumber) {
NSLog(#"%#", p);
sleep(3);
}
}
#end
But when I replace the NSLog with setting the .text attribute, it "fails". The timing still happens, and the label updates with the last item in the array after...
- (IBAction)start {
for (NSString *p in self.currentNumber) {
self.showLabel.text = [NSString stringWithFormat:#"%#", p];
sleep(3);
}
}
#end
And the last bit of weirdness, if I use the NSLog, and try to change the .text attribute before the "for" loop is called, the text change is ignored until AFTER the loop completes...
- (IBAction)start {
self.showLabel.text = [NSString stringWithFormat:#"5"];
for (NSString *p in self.currentNumber) {
NSLog(#"%#", p);
sleep(3);
}
}
#end
What am I missing?
(If you want to see the source files, you can get them at https://github.com/lamarrg/iterate

Answer 1

As you've realized, the UI will only update when the main thread is processing events. In a loop, it won't be.
There's a couple ways around this.
The simplest is to perform your loop in a background thread. There's a wrinkle, though: This will allow the user to continue to interact with your UI. And also, the UI can only be updated from the main thread.
You'll want to dispatch your work to the background, then have the background dispatch your work back to the main thread.
This sounds complicated, and it is. Thankfully, Apple added blocks and Grand Central Dispatch to Objective-C. You can use those to break down the chunks of code and make sure they're executed on the correct thread.
- (IBAction)start {
[self disableMyUI];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_NORMAL, 0), ^{
// this code will be executed "later", probably after start has returned.
// (in all cases, later should be considered "soon but not immediately.")
for (NSString *p in self.currentNumber) {
dispatch_async(dispatch_get_main_queue(),^{
// this code will be executed "later" by the main loop.
// You may have already moved on to the next thing, and even
// dispatched the next UI update.
// Don't worry; the main queue does things in order.
self.showLabel.text = [NSString stringWithFormat:#"%#", p];
});
sleep(3); // do your heavy lifting here, but keep in mind:
// you're on a background thread.
}
dispatch_async(dispatch_get_main_queue,^{
// this occurs "later," but after other all other UI events queued
// to the main queue.
[self enableMyUI];
});
}
// this line of code will run before work is complete
}
You'll have to write disableMyUI and enableMyUI; make sure they disable everything (including the back button if you're using navigation, the tab bar if you're using a tab bar controller, etc).
Another way around this is to use a NSTimer. However, if you do this you're still doing your work on the main thread. It'll work if you can split your work into predictable, small pieces, but you're better off doing it on a background thread.
One thing to keep in mind: Although you're not likely to run into problems while developing, doing heavy work on the main thread will lead to user crashes. On iOS there is a process that watches if applications are responding to events, such as drawing updates. If an application isn't responding to events in a timely fashion, it will be terminated. So living with the lack of UI updates isn't an option for you; you need to only do time consuming operations from background thread.
See also:
Programming with Objective-C: Working with Blocks

Answer 2

If you want to update the label periodically, don't use sleep. If you call it on the main thread you'll be blocking the UI, which is not very desirable.
Use a NSTimer instead, making it fire every N seconds.
Something like this will do:
- (void)startUpdatingLabel {
[NSTimer scheduledTimerWithTimeInterval:0 target:self selector:#selector(updateLabelWithIndex:) userInfo:#0 repeats:NO];
}
- (void)updateLabel:(NSTimer *)timer {
NSInteger index = [timer.userInfo integerValue];
if (index >= self.currentNumber.count) {
return;
}
self.showLabel.text = [NSString stringWithFormat:#"%#", self.currentNumber[index]];
[NSTimer scheduledTimerWithTimeInterval:3 target:self selector:#selector(updateLabelWithIndex:) userInfo:#(index+1) repeats:NO];
}
Every time updateLabel: is invoked it schedules a new timer which will call it again in 3 seconds. Each time the index value is increased and passed along.