UICollectionView：didSelectItemAtIndexPath：如何使用基于选择的数据填充新视图？

Question

Here's the deal:

I have a UICollectionView which populates itself from an array of objects. I want to be able to tap one of the views representing an object in my UICollection view and be taken to a new view displaying details of the object I selected in the UICollectionView (like the selected object's ID for example).

Unfortunately, whatever I try to set is always null!

Here is the object:

@interface obj : NSObject

    @property (nonatomic, strong) NSString *objId;
    ...
    @property (nonatomic, strong) NSString *imageURL;

@end

Here is the main view controller:

@interface ViewController : UIViewController <UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout>

    @property (weak, nonatomic) IBOutlet UICollectionView *badgesCollectionView;
    @property (strong, nonatomic) NSArray *objArray;
    ...

@end

Here is the detail view:

@interface DetailViewController : UIViewController

    @property (weak, nonatomic) IBOutlet UILabel *objId;

@end

Here is where I'm trying to populate the detail view (in my main ViewController.m)

// Implement action for object selection
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath{

    DetailViewController *detailView = [[DetailViewController alloc] init];

    detailView.objIdLabel.text = [[self.objArray objectAtIndex:indexPath.row] objId];

    [self.navigationController pushViewController:detailView animated:YES];

    NSLog(@"Object Selection: Report");
    NSLog(@"Label Text      : %@", detailView.objId.text);
    NSLog(@"Id of object    : %@", [[self.objArray objectAtIndex:indexPath.row] objId]);

}

When I select an object in UICollectionView, I get a transition to a detailView without anything populated. Here is what's logged:

Object Selection: Report
Label Text      : (null)
Id of object    : f0f47920-f449-4fd0-a74e-804546905437

Any insight into this would be fantastic. Thanks!

Answer 1

The problem can come from the DetailViewController implementation.

DetailViewController *detailView = [[DetailViewController alloc] init];

only init the Controller

but you declare

@interface DetailViewController : UIViewController

    @property (weak, nonatomic) IBOutlet UILabel *objId;

@end

wich means you must use a xib.

So you first have to use

DetailViewController *detailView = [[DetailViewController alloc] initWithNibName:@"DetailViewXIBName" bundle:nil]; 

then try to affect the value to objIdLabel once the controller is pushed:

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath{

    DetailViewController *detailView = [[DetailViewController alloc] initWithNibName:@"DetailViewXIBName" bundle:nil]; 

    [self.navigationController pushViewController:detailView animated:YES];

    detailView.objIdLabel.text = [[self.objArray objectAtIndex:indexPath.row] objId];

    NSLog(@"Object Selection: Report");
    NSLog(@"Label Text      : %@", detailView.objId.text);
    NSLog(@"Id of object    : %@", [[self.objArray objectAtIndex:indexPath.row] objId]);

} 

You have to affect the text after [super viewDidLoad]; is called.

These are just assumptions as we only have limited information

Answer 2

The output of the UILabel 's text field is null because the label property is not actually linked to anything until the view is loaded. In this case, it wont happen until after you push the view. In order to get around this, you can just have a property on your DetailViewController that holds the string until the view appears.

Then in viewDidLoad or viewWillAppear in DetailViewController , you can set the text on objIdLabel to the property that contains the string

Answer 3

It may push a blank view controller because you are not strongly holding onto the detailView.

Try adding:

@property (nonatomic, strong) DetailViewController *detailView;

to the main ViewController, then in the didSelectItemAtIndexPath method:

// Implement action for object selection
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath{

    self.detailView = [[DetailViewController alloc] init];

    self.detailView.objIdLabel.text = [[self.objArray objectAtIndex:indexPath.row] objId];

    [self.navigationController pushViewController:self.detailView animated:YES];
}

Answer 4

It turns out this comes down to a xib's lifecycle. It is true, as many of you suggested, that the detailView is not actually linked with all of the items associated with it (like the objId UILabel ) until it's loaded.

I ended up fixing this by creating a new singleton model object to hold onto the selected object. I was then able to set the selected object on the singleton from my main view controller and call it from the detailView to set my objId on ViewDidLoad .