Jimmery Jimmery - 2 months ago 14
Objective-C Question

Reused cells in a UICollectionView show multiple UIImageViews when they should only show one

Im having a problem with my

UICollectionView
. Initially it displays fine, showing a grid of cells, each cell with a single
UIImageView
. These
UIImageViews
are showing PNGs with transparency that are stored in the app's bundle.

My problem is, once the
UICollectionView
has been scrolled, some of the cells seem to be corrupt.

A corrupt cell shows multiple images stacked on top of each other, the top most image is the one it should be showing, and images underneath are the ones that should be used in other cells.

My best guess is that this has something to do with the way cells inside a
UICollectionView
are reused, but I am open to suggestions.

This is the delegate code I use for creating cells within the
UICollectionView
:

// creates the individual cells to go in the menu view
- (UICollectionViewCell*) collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {

// create collection view cell
UICollectionViewCell * cell=[collectionView dequeueReusableCellWithReuseIdentifier:@"cell" forIndexPath:indexPath];

// create a uiview where we can place all views that need to go into this cell
UIView * contents=[[UIView alloc] initWithFrame:cell.contentView.bounds];
[contents setBackgroundColor:[UIColor clearColor]];
[cell.contentView addSubview:contents];

// add a button image
NSString * buttonPath=[[NSBundle mainBundle] pathForResource:@"button" ofType:@"png" inDirectory:[[buttons objectAtIndex:indexPath.row] objectForKey:@"name"]];
UIImage * button=[UIImage imageWithContentsOfFile:buttonPath];
UIImageView * buttonView=[[UIImageView alloc] initWithImage:button];
[buttonView setContentMode:UIViewContentModeScaleAspectFit];
[buttonView setFrame:contents.bounds];
[contents addSubview:buttonView];

// set tag to the indexPath.row so we can access it later
[cell setTag:indexPath.row];

// add interactivity
UITapGestureRecognizer * tap=[[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(onButtonTapped:)];
[tap setNumberOfTapsRequired:1];
[cell addGestureRecognizer:tap];

// return the cell
return cell;

}


I can provide more code if required.

How can I stop the cells from corrupting?

Answer

The problem is that you keep adding views to the UICollectionViewCell as they are being reused automagically by the UICollectionView. So the old UIImageView's are still on the cell as you are adding one more as the cellForItemAtIndexPath: is called.

DO NOT USE addSubview:!

Instead you could make a custom cell with all the views you want already in them. So that when the cellForItemAtIndexPath: is called you only need to set the contents of this CustomCollectionViewCell instead.

This way it will certainly stop being corrupted.


How to build a CustomCell.

Step1: Create the .h & .m class.

CustomCell.h

#import <UIKit/UIKit.h>

@interface CustomCell : UICollectionViewCell
{
    UIImageView *imageView;
}

@property (nonatomic, retain) UIImageView *imageView; //this imageview is the only thing we need right now.

@end

CustomCell.m

#import "CustomCell.h"

@implementation CustomCell

@synthesize imageView;

- (id)initWithFrame:(CGRect)aRect
{
    self = [super initWithFrame:aRect];
    {
         //we create the UIImageView in this overwritten init so that we always have it at hand.
         imageView = [UIImageView alloc] init];
         //set specs and special wants for the imageView here.
         [self addSubview:imageView]; //the only place we want to do this addSubview: is here!

         //You wanted the imageView to react to touches and gestures. We can do that here too.
         UITapGestureRecognizer * tap=[[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(onButtonTapped:)];
        [tap setNumberOfTapsRequired:1];
        [self addGestureRecognizer:tap];


        //We can also prepare views with additional contents here!
        //just add more labels/views/whatever you want.
    }
    return self;
}

-(void)onButtonTapped:(id)sender
{
    //the response to the gesture.
    //mind that this is done in the cell. If you don't want things to happen from this cell.
    //then you can still activate this the way you did in your question.

}

Step2: Import it! Now that we created the CustomCell we can import it in the class we want to use it in.

Step3: Use it in action!

// creates the individual cells to go in the menu view
- (CustomCell*) collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {

    // create collection view cell
    CustomCell *cell = (CustomCell *)[collectionView dequeueReusableCellWithReuseIdentifier:@"CustomCell" forIndexPath:indexPath]; //this is the place where the CustomCell does his magic.
    //Make sure to use the CustomCellReuseId that you register in the viewdidload/loadview (step4)

    // add a button image
    NSString * buttonPath=[[NSBundle mainBundle] pathForResource:@"button" ofType:@"png" inDirectory:[[buttons objectAtIndex:indexPath.row] objectForKey:@"name"]];

    cell.imageView.image = [UIImage imageWithContentsOfFile:buttonPath]; //place the image on the CustemCell.imageView as we prepared.

    // set tag to the indexPath.row so we can access it later
    [cell setTag:indexPath.row]; //we don't need this to access the cell but I left this in for your personal want.

/*
 * we can now do this from the CustomCell as well!
 *
    // add interactivity
    UITapGestureRecognizer * tap=[[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(onButtonTapped:)];
    [tap setNumberOfTapsRequired:1];
    [cell addGestureRecognizer:tap];
*/
    // return the cell
    return cell;

}

Step4: Register the cell to the collectionView

in the viewDidLoad / loadView add this line:

[_collectionView registerClass:[CustomCell class] forCellWithReuseIdentifier:@"CustomCell"];

Step5: Enjoy! Your CustomCell is done. Now do whatever you like and don't forget to get some coffee too.

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