Simple layer problem [iphone]

Question

Basically I'm sorting through objects in an array by y value (The furthest down the page is at the start of the array) but I'm having a bit of trouble. I assign all of the UIImageViews in an array a value:

for (UIImageView *Blocky in objectsArray){
  [Blocky.layer setValue:[NSString stringWithFormat: @"%f",
                                     Blocky.center.y] forKey:@"value"];
}

Because it's a UIImageView I have to put layer after "Blocky" otherwise I get the following error:

*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<UIImageView 0x3d44880> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key value.'

My problem is when I'm sorting it I don't know where to put the ".layer" so I get the same problem because UIImageViews can't deal with keys on there own. Here is my sorting code:

NSSortDescriptor *sortDescriptor =
  [[NSSortDescriptor alloc] initWithKey:@"value" ascending:YES];
[objectsArray sortUsingDescriptors:[NSArray
                                    arrayWithObject:sortDescriptor]];
[sortDescriptor release];

Thanks in advance, Ozzie

Answer 1

Are you sure CALayer is supposed to accept arbitrary KVC keys? This looks wrong to me.

Shouldn't you extract the heights to a buffer of structures and qsort that?

eg

I think it would be something like this:

//
// before @implementation sections
//
typedef struct __MySortEntry {
  UIImageView* image;
  CGFloat height;
} MySortEntry, *PMySortEntry;

static int compareMySortEntry (const void * a, const void * b)
{
  CGFloat temp =( ((PMySortEntry)a)->height - ((PMySortEntry)b)->height );
  return (temp<0)?-1:(temp>0)?1:0;
}

//
// in @implementation section somewhere
//
NSMutableData* mutable = [[NSMutableData alloc] initWithLength:
  sizeof(MySortEntry)*objectsArray.count];
PMySortEntry entries = (PMySortEntry)mutable.mutableBytes;
for (int c = 0; c < objectsArray.count; c++) {
  entries[c].image = (UIImageView*)[objectsArray objectAtIndex:c];
  entries[c].height = entries[c].image.center.y;
}

qsort(entries, objectArray.count, sizeof(MySortEntry), compareMySortEntry);

for (int c=0; c < objectArray.count; c++) {
  UIImage* imageInSequence = entries[c].image;
  // do something with images **in sequence**
}
[mutable release];

Edit: Changed center.y to size.height.

Edit: Changed size.height back to center.y. Oops.

Edit: Changed UIImage to UIImageView.

Answer 2

Use the NSMutableArray sortUsingFunction:context: method:

// top of file
static NSInteger compareImageHeights(id image1, id image2, void* context) {
  CGFloat temp = ((UIImageView*)image2).center.y - ((UIImageView*)image1).center.y;
  return (temp>0)?NSOrderedAscending:(temp<0)?NSOrderedDescending:NSOrderedSame;
}
// within file somewhere
[objectsArray sortUsingFunction:compareImageHeights context:NULL];

The reason the function is required (and you can't just use sortUsingSelector: or sortUsingSortDescriptor:) is that you can't specify a KVC key to identify just the y component only as a sort variable.

Edit: Changed center.y to size.height.

Edit: Oops. Changed size.height back to center.y. Fixed type to UIImageView rather than UIImage. It seems KVC objects can in theory support arbitrary keys, but some will and some won't.

Answer 3

Sorry guys. By height I meant which image is furthest down the page rather than which is the tallest. Thanks for the help though. It's a shame because i had the problem that you have solved before and it never got solved! Does anyone know how to order imageviews by which is furthest down the page?

Answer 4

Why not set the tag property on each view as you create them based on the height and retrieve them by tag?