UIImageView setImage does not change how image looks - ios

Question

I am writing a tone generator UI and I need to change UIImageView image every time generator starts generating and finishes generating.
To do so I have
#property (weak, nonatomic) IBOutlet UIImageView *headphonesImage;
in my UIViewController. And I add default image to it like so
- (void) viewDidLoad
{
...
headphonesImagesArray = [NSArray arrayWithObjects:
[UIImage imageNamed: #"Both Headphones"],
[UIImage imageNamed: #"Both Headphones Playing"],
nil];
[self.headphonesImage setAlpha: 0.5];
[self.headphonesImage setImage: headphonesImagesArray[0]];
...
}
My tone generator sends messages into this method
- (void) toneGeneratorControllerStateHasChangedWithNotification: (NSNotification *) notification
{
if([notification.name isEqualToString: ToneGenerationHasFinished])
[self.headphonesImage setImage: headphonesImagesArray[0]];
else
[self.headphonesImage setImage: headphonesImagesArray[1]];
}
The problem is that though headphonesImage image changes to [1] and then nothing happens. I can see in variables inspector that headphonesImage image is changed every time the method is invoked, but these changes are not present on screen of neither simulator nor iPhone. I can't even hide this damn UIImageView. setHidden: true does nothing.
Please help!

Answer 1

"My tone generator sends messages into this method"
If you are writing the tone generator, it sounds like it is running on a background thread. You should send notifications on the main thread, especially if they trigger UI changes (such as setting an image), e.g.:
dispatch_async(dispatch_get_main_queue(),^{
[[NSNotificationCenter defaultCenter] postNotificationName:#"toneGeneratorControllerStateHasChanged"
object:nil
userInfo:imageDict];
});
Alternatively, you can send the image change to the main thread as Igor suggests. The disadvantage there is having to do any time you use such notifications, as opposed to doing it in one place.

Answer 2

Do this:-
- (void) toneGeneratorControllerStateHasChangedWithNotification: (NSNotification *) notification
{
if([notification.name isEqualToString: ToneGenerationHasFinished]) {
self.headphonesImage.image = [UIImage imageNamed:#"Both Headphones"];
}
else {
self.headphonesImage.image = [UIImage imageNamed:#"Both Headphones Playing"];
}
}
instead of setImage.

Answer 3

in VDL
headphonesImagesArray = [NSArray arrayWithObjects:
#"Both Headphones",
#"Both Headphones Playing",
nil];
And do this :
- (void) toneGeneratorControllerStateHasChangedWithNotification: (NSNotification *) notification
{
if([notification.name isEqualToString: ToneGenerationHasFinished]) {
self.headphonesImage.image = [UIImage imageNamed: headphonesImagesArray[0]];
}
else {
self.headphonesImage.image = [UIImage imageNamed:headphonesImagesArray[1]];
}
}

Answer 4

It looks very shady, but it seems that I've found the solution:
- (void) toneGeneratorControllerStateHasChangedWithNotification: (NSNotification *) notification
{
BOOL finished = FALSE;
if([notification.name isEqualToString: ToneGenerationHasFinished])
finished = TRUE;
dispatch_async(dispatch_get_main_queue(),
^{
if(finished)
[self.headphonesImage setImage: headphonesImagesArray[0]];
else
[self.headphonesImage setImage: headphonesImagesArray[1]];
});
}
As far as I know all the UI drawing happens in the main thread. Looks like notification method does not work in the main thread unless you explicitely tell it to do so...