Accessing properties from Object within NSMutableArray not working - ios

Question

I have the class LearnfestItem.h :
#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
#interface LearnfestItem : NSObject
#property (strong, nonatomic) NSString * itemId;
#property (strong, nonatomic) NSString * itemTitle;
#property (strong, nonatomic) NSString * itemDescription;
#property (strong, nonatomic) NSString * itemContent;
#property (strong, nonatomic) NSString * itemType;
#property (strong, nonatomic) UIImage * itemImage;
#property (strong, nonatomic) NSDate * itemRegistered;
-(id)initWithData:(NSDictionary *)data andImage:(UIImage *)image;
#end
& Object.m :
#import "LearnfestItem.h"
#import "Defaults.h"
#implementation LearnfestItem
-(id)init
{
self = [self initWithData:nil andImage:nil];
return self;
}
-(id)initWithData:(NSDictionary *)data andImage:(UIImage *)image
{
self = [super init];
self.itemId = data[ITEM_ID];
self.itemTitle = data[ITEM_TITLE];
self.itemDescription = data[ITEM_DESCRIPTION];
self.itemContent = data[ITEM_CONTENT];
self.itemType = data[ITEM_TYPE];
self.itemImage = image;
self.itemRegistered = data[ITEM_REGISTERED];
return self;
}
#end
In my UIViewController I have a UITableView that creates a NSMutableArray of LearnfestItems within the cellForRowAtIndexPath method:
LearnfestItem * createLearnfestItem = [[LearnfestItem alloc] initWithData:learnfestItemDictionary andImage:learnfestItemImage];
NSLog(#"Insert learnfest item with id: %# at index %li", createLearnfestItem.itemId, (long)row);
[self.learnfestItemObjects insertObject:createLearnfestItem atIndex:row];
On didSelectRowAtIndexPath I want to receive the LearnfestItem from the NSMutableArray I do this by calling:
self.selectedLearnfestItem = [self.learnfestItemObjects objectAtIndex:indexPath.row];
Then I want to send it to another view controller to present the data I do this in the prepareForSegue segement:
LearnfestItemViewController * learnfestVC = [segue destinationViewController];
NSLog(#"Sending learnfest item with id: %#", self.selectedLearnfestItem.itemId);
learnfestVC.item = self.selectedLearnfestItem;
When I try to access my LearnfestItem's properties within cellForRowAtIndexPath. All I get is null... and so forth in my other table view delegate methods.. Can anyone spot what i'm doing wrong? Thanks

Answer 1

Code you show is missing all error and validity checking. When using NSAssert() and item.length/item.count checks, you will know what's going on.
However, based on your code I'd suggest two things:
All objects which also have mutable version should use "copy" properties. Now you have "strong" pointer to data received via init method, but if you reset the originally given data variable to point elsewhere (e.g. reusing one data variable inside for loop to init multiple items), then... I don't know what your LearnfestItem item properties will point to.
Make sure your LearnfestItemViewController *item is a copy, too.
My guess is that everything is initialised correctly, but the data disappears afterwards.
For example:
#property (nonatomic, copy) NSString * itemId; // Safe
#property (nonatomic, strong) NSString * itemId; // Not safe
When your property class has mutable variations (e.g. NSString vs. NSMutableString vs. MyMutableString), copy is safer. Using strong will create a pointer to original data, which could have been a mutable instance and could be modified afterwards. Using strong will always point to original data, even after it has been changed.
Second part:
learnfestVC.item = self.selectedLearnfestItem;
Your LearnfestItemViewController contains some property related to LearnfestItem class. Make sure it's a copy, too. When using segues, the calling object quite often just disappears. Make sure your new controller has local copy of all needed data (or use a protocol delegate, but that's a longer discussion)

Answer 2

Adding error and data validity checking will make your task a lot easier. Instead of trying to figure out afterwards why something doesn't work, you'll get notifications when something isn't as you're expecting.
Here's quick and dirty "maintenance" for your code. What you should get out of this is some ideas what to check, where and how. In normal situations this is overkill, but now you have a mysterious problem and need to find it. It can be hard and monotonous work.
#import Foundation;
#import UIKit;
#interface LearnfestItem : NSObject
#property (copy, nonatomic) NSString *itemId;
#property (copy, nonatomic) NSString *itemTitle;
#property (copy, nonatomic) NSString *itemDescription;
#property (copy, nonatomic) NSString *itemContent;
#property (copy, nonatomic) NSString *itemType;
#property (strong, nonatomic) UIImage *itemImage;
#property (strong, nonatomic) NSDate *itemRegistered;
- (instancetype)initWithData:(NSDictionary *)data andImage:(UIImage *)image;
#end
Object.m :
#import "Defaults.h"
#import "LearnfestItem.h"
#implementation LearnfestItem
- (instancetype)init
{
self = [self initWithData:nil andImage:nil];
return self;
}
- (instancetype)initWithData:(NSDictionary *)data andImage:(UIImage *)image
{
NSAssert(data.length, #"My Assert: missing data");
NSAssert(image, #"My Assert: missing image");
if ((self = [super init]))
{
// TODO: nil ok, if doesn't exist?
_itemId = data[ITEM_ID];
_itemTitle = data[ITEM_TITLE];
_itemDescription = data[ITEM_DESCRIPTION];
_itemContent = data[ITEM_CONTENT];
_itemType = data[ITEM_TYPE];
_itemImage = image;
_itemRegistered = data[ITEM_REGISTERED];
}
return self;
}
#end
"(original text) In my UIViewController I have a UITableView that creates a NSMutableArray of LearnfestItems within the cellForRowAtIndexPath method:"
NSAssert(learnfestItemDictionary.count, #"My Assert: missing dict");
NSAssert(learnfestItemImage, #"My Assert: missing image");
LearnfestItem *createLearnfestItem = [[LearnfestItem alloc] initWithData:learnfestItemDictionary andImage:learnfestItemImage];
NSLog(#"Insert learnfest item with id: %# at index %#", createLearnfestItem.itemId, #(row));
NSAssert(createLearnfestItem, #"My Assert: missing item");
NSAssert(self.learnfestItemObjects.count > row, #"My Assert: learnfestItemObjects");
self.learnfestItemObjects[row] = createLearnfestItem;
"(original text) On didSelectRowAtIndexPath I want to receive the LearnfestItem from the NSMutableArray I do this by calling:"
NSAssert(self.learnfestItemObjects.count > indexPath.row, #"My Assert: learnfestItemObjects");
self.selectedLearnfestItem = self.learnfestItemObjects[indexPath.row];
"(original text) Then I want to send it to another view controller to present the data I do this in the prepareForSegue segment:"
LearnfestItemViewController *learnfestVC = (LearnfestItemViewController *)[segue destinationViewController];
NSLog(#"Sending learnfest item with id: %#", self.selectedLearnfestItem.itemId);
learnfestVC.item = self.selectedLearnfestItem;