Problem with NSCollectionView

Question

I'm quite a rookie when it comes to Cocoa programming, so I hope some experts can hint me on the right direction with a problem I'm facing.

I have a NSCollectionView that is bound to a NSArrayController (A) of elements having two properties: an URL to a picture and a NSArray (B) of elements of another class. For every element inside the NSArrayController (A), I load a subview with a NSImageView that should display the image in the URL and a NSTableView that is bound to the elements of the NSArray (B).

Everything works fine, except for one very important thing: the URL to the picture is not immediately available when I create the subview, it becomes available later, but when it does, I don't see the picture loading in the NSImageView. I would think of a problem in the KVC/KVO implementation, but the strange thing is that the image loads correctly if, when the URL becomes available, the subview is not visible (eg in a portion of the scrollview that is not displayed).

Example: The NSScrollview size is such that it can display only two subviews at a time. I add three elements to the NSArrayController (A): the first two images don't load, if I scroll down the scrollview to see the third element, I find the image loaded correctly.

Any ideas about what could cause such a strange behaviour? Thank you in advance

Luca

series.h

@interface Series : NSObject {
@private
NSMutableString * banner;
}
-(Series*)initWithEpisode:(Episode*)ep;
-(void)setBanner:(NSString*)_banner;
@property (retain, readwrite) NSMutableString *banner;
@end

series.m

@implementation Series
@synthesize banner;
-(Series*)initWithEpisode:(Episode*)ep
{
self = [super init];
if(self){
banner = [[NSMutableString alloc]initWithString:@"some invalid URL"];
}
-(void) setBanner:(NSString*)_banner
{
[banner setString:[NSString stringWithFormat:@"some root path/%@", _banner];
}
-(void)dealloc
{
[super dealloc];
[banner release];
}

SeriesListViewController.m

-(void)VideoChangedStatus:(Episode*)ep{
//This is the delegate called by the object Episode when it retrieves the URL via asynchronous call to a webservice
Series *tmp = [[Series alloc]initWithEpisode:ep];
[[seriesList objectAtIndex:[seriesList indexOfObject:tmp]]setBanner:[ep banner]];
}

The binding is done in the subview nib file, to the NSImageView: I set File's Owner of type NSCollectionViewItem and then bind Valueurl to representedObject.banner I didn't subclass NSCollectionView nor NSCollectionViewItem

Answer 1

After days of trying I found a solution that works: apparently it's not enough to use the setString method, I need to re-intialize the property inside the setBanner method

-(void) setBanner:(NSString*)_banner  
 {
     banner = [NSMutableString[NSString stringWithFormat:@"some root path/%@", _banner]];  
 }  

Still, I'd be very glad to know if someone has an explanation of why setString was causing that strange (to me) problem and why this solution works.

Thank you Luca

Answer 2

I'm not sure why you've declared banner to be a mutable string — it looks like an immutable string would suffice.

At any rate, when you write a custom setter method you need to send -willChangeValueForKey: and -didChangeValueForKey: to ensure KVO (and hence bindings) compliance:

-(void) setBanner:(NSString*)_banner
{
    [self willChangeValueForKey:@"banner"];
    [banner setString:[NSString stringWithFormat:@"some root path/%@", _banner];
    [self didChangeValueForKey:@"banner"];

}