Convert array of NSData to array of UIImage

Question

Here is the thing. At a point in my app, I want to show on a CollectionView the photos stored in Core Data (yes, I use External Storage). To get only the photos in my Entity I do the following:

NSManagedObjectContext *managedObjectContext = [self managedObjectContext];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"SmallThing"];
[fetchRequest setResultType:NSDictionaryResultType];
[fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:@"smallphoto", nil]];
self.photos = [[managedObjectContext executeFetchRequest:fetchRequest error:nil]mutableCopy];

That seemed to work fine, it returns the data for the photos and store it on the array. Then I created a method to convert it to UIImage so I could store on another array as following:

- (NSMutableArray *)convertDataToImage:(UIImage *)image {
for (NSData *data in self.photos) {
    image = [[UIImage alloc] initWithData:data];
    [self.images addObject:image];
}
return self.images;
}

When I get to the line image = [[UIImage alloc] initWithData:data it crashes and I get the following error:

[16697:2367280] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSKnownKeysDictionary1 length]: unrecognized selector sent to instance 0x7fae0c117cc0'

I don't know what else I can do. Any ideas ?

Thanks in advance

This means data is really an NSKnownKeysDictionary1, not an NSData object. self.photos is not an array of NSData like your code is assuming. — rmaddy
– rmaddy, Commented Aug 18, 2015 at 23:44
Yes, sir, you were quite right. Now I'm having problem on adding the image to the images array. For some reason it's returning null — Leonardo Rangel
– Leonardo Rangel, Commented Aug 19, 2015 at 0:42

Carl Veazey · Accepted Answer · 2015-08-19 00:02:22Z

1

You are explicitly telling the fetch request to return dictionaries with the line:

[fetchRequest setResultType:NSDictionaryResultType];

Which means the array returned by executeFetchRequest... contains NSDictionary objects, with key-value pairs corresponding to the properties you specified.

Your code in convertDataToImage should be along the lines of:

for (NSDictionary *dictionary in self.photos) {
    NSData *data = dictionary[@"smallphoto"];
    ...
}

If you specified a different property to fetch other than smallphoto you would of course substitute that string there as appropriate.

answered Aug 19, 2015 at 0:02

Carl Veazey

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4 Comments

Leonardo Rangel Over a year ago

Very well! The error disappeared, but the image is returning nil therefore the images array is nil as well.

Leonardo Rangel Over a year ago

I mean the array is returning nil. It's like the images are not being added to it

Carl Veazey Over a year ago

@LeonardoRangel I don't understand what you mean by that. Which value is nil and when? What keys & values do the NSDictionarys in the fetched results have?

Leonardo Rangel Over a year ago

I figured it out, it was a stupid thing I was missing, I never initialized the images array so it was always nill. It's working just fine now. Thank you very much

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