In this episode I’ll show you how to build a UICollectionView from scratch in Xcode 6. The class is available for both iPhone and iPad since iOS 6. If you know how to build a UITableView then building a UICollectionView will be familiar to you.
I’ll start with a single view application, delete the ViewController class and start fresh with a UICollectionViewController. Next I’ll add a custom class for the UICollectionViewController and UICollectionViewCell and then we’ll hook it up in the storyboard.
By the end we’ll have a simple Collection View App which allows multiple selections. I’m going to use this project to build on with other features in the future.
Custom CollectionViewController Class
The template provides a few good starting points, but they need to be changed to work. First there’s the cell’s reuse identifier, conveniently added as a static at the top of the implementation file. It’s there so we only need to change this once in the file. Replace it with your own, and remember to make the same change in the storyboard:static NSString * const reuseIdentifier = @"Cell";
Next up is the viewDidLoad method. To make dequeuing cells easier, Apple have provided a registerClass method. If you don’t add your custom cell here, nothing will appear when you run the app. I found that simply commenting out the line works just as well.
The reason they provide this is so that the dequeueCellWithIdendifier method knows which custom cell class to instantiate (prior to iOS 6 it returned nil, but that check is no longer necessary).
I’m also adding multiple selections here, something that cannot be done in the storyboard.- (void)viewDidLoad {
[super viewDidLoad];
// EVIL: Register your own cell class (or comment this out)
[self.collectionView registerClass:[YourCustomCell class] forCellWithReuseIdentifier:reuseIdentifier];
// allow multiple selections
self.collectionView.allowsMultipleSelection = YES;
self.collectionView.allowsSelection = YES;
}
UICollectionViewDataSource
Much like with UITableViews, we need to provide the number of sections, as well as the number of items in each section:- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
return 1;
}
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
return self.cellData.count;
}If you don’t provide the sections method, it is assumed that you have one section. Usually we’d have some data and would return a count of how many items we have rather than fixed values here.
We also need to provide what’s in each cell. Here we can add data to labels, populate UIImageViews and many other things our collection view cells may need:- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
MyCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];
cell.textLabel.text = [self.cellData objectAtIndex:indexPath.row];
return cell;
}
UICollectionViewCell
With UITableViews there were four styles of table cells we could choose from out of the box. A collection view cell on the other hand is completely blank, and we’re expected to provide everything inside it. This means we need a custom UICollectionViewCell class for our project.
Anything we drag into the prototype cell in the storyboard can be hooked up to that custom class and the configured in the above cellForItemAtIndexPath method. Make sure any outlets are defined in the cell’s header file.
