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jk7
Mar 31, 2008, 12:40 AM
I'm writing an app for the iPhone and I'm finding that the UILabel is not updating. A worker thread updates a string and calls a method on the view that changes the text in a UILabel subview.

I'm not seeing the new values appear in the UILabel.

Here's the code for the view, the updateDisplay method is called by the worker thread.

Anybody know why the UILabel doesn't get updated?

I attached the whole project.

Thx

#import "MyView.h"

@implementation MyView

@synthesize label;

-(id)initWithFrame:(CGRect)aRect
{
if (![super initWithFrame:aRect]) return nil;

self.backgroundColor = [UIColor whiteColor];

CGRect labelFrame = CGRectInset(aRect, 90, 90);

self.label = [[UILabel alloc] initWithFrame:labelFrame];
self.label.contentMode = UIViewContentModeRedraw;
[self addSubview: self.label];
[self bringSubviewToFront:self.label];
self.label.text = @"initial value";

return self;
}

-(void) updateDisplay:(NSString *)str
{
self.label.text = str;
[self.label setNeedsDisplay];
//[self setNeedsDisplay];
//[self.window setNeedsDisplay];
}
@end



jk7
Mar 31, 2008, 11:40 AM
I added a setNeedsDisplay after I noticed that the UILabel was not updating.

Also if anybody runs that project be sure to exit the application by pressing the home button.

I've crashed my mbp a number of times debugging that project and think it might be runaway threads.

therevolution
Apr 3, 2008, 10:49 PM
I am actually having a similar problem with cells in a UITableView. Updating a UILabel might be similar. Did you get it resolved, or does anyone else know the trick to updating the UI?

It'll update the contents if I touch the screen, but not if it's left alone. I've tried several variations of [table reloadData], [table setNeedsLayout], [table setNeedsDisplay], doing the same to each of the cells... no luck so far.

jk7
Apr 3, 2008, 11:14 PM
This code is working for me in the above project, currently having problems crashing mbp so wouldn't recomend running the project attached to this thread

-(void) updateDisplay : ( NSString *)str
{
[self.label performSelectorOnMainThread : @ selector(setText : ) withObject:str waitUntilDone:YES];
}

therevolution
Apr 3, 2008, 11:36 PM
Awesome, thanks. That did it. You get a gold star.

jared_kipe
Jul 31, 2009, 04:04 PM
You shouldn't need to call self.label.text, since label is an ivar of your own class.

label.text = str;

SHOULD be fine. I've never created a view like this programatically, I always use xib files and then move or change properties as I need to. I wouldn't think this would change how the UIView subclasses like UILabel or UIImageView redraw themselves when changed.

It will not redraw until the method that makes the change returns though.
I use similar methods to update the main view after changing triangles in the Right Triangle Tutor app I have on the store currently.

Bagnaria
Nov 10, 2010, 07:23 PM
One thing to be aware of with iPhone applications is that one can not rely on UI elements that where initialized from a nib file. The reason is that Cocoa Touch tries to save as much memory as possible and only instantiates the UI elements of a nib file when they are visible. Then it tosses them only to re-instantiate them should the interface become visible again. So while the "Your"-ViewController class is instantiated you can not rely on the IBOutlets to be there. An easy check in the Debugger is that the label (or other UI element) will be null/nil. This has a lot to do with timing so what works in one place may not work in another. Which why this is so hard to catch.

The solution:
While the IBOutlets may not always be instantiated, all other instance variables will be persistent. So in this example I would simply add an NSString ivar, let's call it "labelString". The "labelString" can be set as soon as your controller has been initialized (or right then). When it is showtime the viewDidLoad method will be called. It should look similar to this like this:

- (void)viewDidLoad {
uiLabel.text = labelString; // set the label when it is actually shown

[super viewDidLoad];
}

Hope this helps. It is a behaviour that can be very frustrating, but once you are aware of it you will be much happier.