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POiOS: Is there a way to determine when an UIView did appear, after calling view.hidden = NO
 

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  1. POiOS: Is there a way to determine when an UIView did appear, after calling view.hidden = NO
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    <p>I have an UIView with 8 to 10 large images (each loaded into an UIImageView) as subviews. Those images are aligned next to each other, so that they compose a real long horizontal image. This view is put into an UIScrollView, so that the user can scroll along this long horizontal image.</p> <p>Loading all those images (from the web) takes quite some time, so I decided to put that loading code into a NSOperation subclass. Inside the operation's main method each ImageView gets added to the container view.</p> <p>The view is hidden during this process and instead there is a small rotating image (pretty much like an UIActivityIndicatorView). Here is some code:</p> <p>CategoryViewController.m</p> <pre><code>- (id)initWithFrame:(CGRect)frame { self = [super init]; if(self) { // do some unrelated stuff [...] loadIndicator = [[UIImageView alloc] initWithImage:img]; [loadIndicator setFrame:CGRectMake((frame.size.width - width) / 2, (frame.size.height - height) / 2, width, height)]; [loadIndicator setHidden:YES]; // this is the view which will contain those large images backgroundView = [[UIView alloc] init]; backgroundView.hidden = YES; scrollView = [[UIScrollView alloc] initWithFrame:frame]; [scrollView setDelegate:(id&lt;UIScrollViewDelegate&gt;)self]; [scrollView addSubview:backgroundView]; [self.view setFrame:frame]; } return self; } - (void)viewDidLoad { // do some unrelated stuff [...] [self.view addSubview:loadIndicator]; [self.view addSubview:scrollView]; // setup a custom activity indicator loadIndicator.hidden = NO; [UIView beginAnimations:nil context:NULL]; [UIView setAnimationDelegate:self]; [UIView setAnimationCurve:UIViewAnimationCurveLinear]; loadIndicatorRotation = 0; rotationTimer = [NSTimer scheduledTimerWithTimeInterval: 0.01 target: self selector:@selector(updateRotation:) userInfo: nil repeats: YES]; [UIView commitAnimations]; // setup the operation to load all background images operationQueue = [[NSOperationQueue alloc] init]; LoadBackgroundImagesOperation* operation = [[LoadBackgroundImagesOperation alloc] initWithImages:bgImageNames View:backgroundView Frame:frame]; // register to recieve the finish notification [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(onFinishedLoadingBgImages) name:@"BgImagesFinishedLoading" object:operation]; // Add the operation to the queue [operationQueue addOperation:operation]; [operation release]; [super viewDidLoad]; } </code></pre> <p>main method of LoadBackgroundImagesOperation.m</p> <pre><code>- (void)main { int left = 0; int width, height; int i; for(i = 0; i &lt; imageNames.count; i++) { if([self isCancelled]) { [[NSNotificationCenter defaultCenter] postNotificationName:@"BgImagesCancelled" object:self]; return; } // code to retrieve the url of current image [...] NSData* data = [[NSData alloc] initWithContentsOfURL:imgURL]; UIImage* image = [[UIImage alloc] initWithData:data]; [data release]; double scalingFactor = frame.size.height / image.size.height; width = image.size.width * scalingFactor; height = image.size.height * scalingFactor; UIImageView* imgView = [[UIImageView alloc] initWithImage:image]; [image release]; [imgView setFrame:CGRectMake(left, 0, width, height)]; [background addSubview:imgView]; [imgView release]; left += width; [background setFrame:CGRectMake(0, 0, left, height)]; } [[NSNotificationCenter defaultCenter] postNotificationName:@"BgImagesFinishedLoading" object:self]; } </code></pre> <p>When the operation is finished I hide the rotating image and show the view, which contains the large images.</p> <p>(back in CategoryViewController.m again)</p> <pre><code>- (void) onFinishedLoadingBgImages { [scrollView setContentSize:backgroundView.frame.size]; backgroundView.hidden = NO; loadIndicator.hidden = YES; if(operationQueue != nil) { [operationQueue cancelAllOperations]; [operationQueue release]; operationQueue = nil; } [rotationTimer invalidate]; rotationTimer = nil; } </code></pre> <p>The problem is that it takes around 2 seconds for the simulator and up to 5 seconds on the real device untill the images are actually visible after calling <code>backgroundView.hidden = NO</code></p> <p>I would just continue showing the activity indicator during that time, but I don't know how to detect that <code>backgroundview</code> did appear in order to hide the indicator again.</p> <p>I tried to put <code>backgroundview</code> in its own UIViewController and notify my parent view controller when viewDidAppear is called, but that doesn't work. (I read that in nested view controllers I would have to call viewDidAppear manually, but that is not an option, because i don't know when it did appear in the first place).</p> <p>Also, I can't just do the dirty trick and change the order of <code>backgroundView</code> and <code>loadIndicator</code> to hide it, because most of the images have some transparency in them, so the user would still see the indicator.</p> <p>So I really hope, that any of you guys have a suggestion on how to detect when <code>backgroundview</code> is actually visible.</p> <p>Thanks, Chris</p> <p>Oh, and sorry if some of the code is somewhat unrelated to the problem. Just wanted to give you the big picture of what I am doing.</p>
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