Variable-Sized Items in UICollectionView
UICollectionView was added last year with iOS 6 and to this date I had no real chance to get acquainted with it since most of my apps were still supporting iOS 5. Doing a fresh app only supporting the latest iOS version finally allowed me to dig into it and share the journey with you.
The special scenario we want to look at today is how we could configure variable-sized collection view cells for items like tags. We want to have the cells adjust their size automatically based on the tag string and ideally we don’t want to have to write any layout code for determine the needed sizes.
Please forgive if the following has a few places where I stumbled. It is these temporary snags that I believe you learn from the most, so I left them in the final article.
Basic Training
We start out with a fresh project from the “Single View Application” template. On the ViewController.xib we add a UICollectionView to fill the entire view.
The Collection View – identical to it’s grandfather UITableView – has a delegate and a dataSource outlet which we connect to “File’s Owner”, which is the ViewController class. This puts our view controller also in charge of the collection view’s contents and interactions. We also need an outlet for a reference to the collection view, so we add that with the assistant editor. The outlet property we add to ViewController.h can be weak because being a subview of the ViewController’s view means it is also sufficiently retained.
Cells
For the content we need to design a prototype cell. If you’re using the collection view in a view controller’s XIB you have to register it for use in the collection view in code. If you add a collection view inside a storyboard you create prototype cells inside the collection view’s area and thus won’t have to register the cell identifier. Since we started without a story board, let’s proceed with the manual steps.
We create a prototype cell in Interface Builder, give it a white background, add a UILabel and add some constraints to link the edges with the superview’s edges, 5 points distance. From the Editor menu we choose “Size to Fit Content” for the Label because we want the label text to determine the item size. We’ll see later if that works as we think.
We set the Identifier to “TagCell” so that we can reference our design.
Here we hit the first snag. If you are not using story boards there is no way to get the collection view to use a Collection View Cell from the same NIB file. These are the two methods available for registering a cell design:
- – registerClass:forCellWithReuseIdentifier:
- – registerNib:forCellWithReuseIdentifier:
The first instantiates a certain class to be used for a given reuse identifier. This we would use if we created the cell’s view hierarchy entirely in code. The second requires a NIB and thus requires that the cell needs to be the only item in this NIB. This is the second important reason why it is way less work using Collection Views with storyboards, to avoid this hassle.
To remedy this we create a new “empty Interface Builder Document”, which we name TagCollectionViewCell. Thankfully we can simply CMD+X the cell design from one ViewController NIB and CMD+V it into the empty document.
Next we’ll try to register the cell for use in the collection view and want to see a number of items displayed.
Registering the Cell Design
We need to register the NIB to be used whenever we want a cell for identifier “TagCell”, so we add the following to ViewController.m in the viewDidLoad section.
UINib *cellNib = [UINib nibWithNibName:@"TagCollectionViewCell" bundle:nil];
[self.collectionView registerNib:cellNib forCellWithReuseIdentifier:@"TagCell"];
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We only need to implement more methods belonging to the UICollectionViewDataSource protocol to get something to show.
#pragma mark - UICollectionView
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
return 100;
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"TagCell" forIndexPath:indexPath];
return cell;
}
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Launching the app now, we see 100 items all using our – probably not award-winning – cell design.
The first observation to make is that all cells have the same size, 50×50 points to be exact. This comes from us not changing the default values in Interface Builder.
But before we proceed to giving the cells individual sizes, let’s first improve the design a bit. We remove the white background, add a new TagCollectionViewCell class and specify this as the cell’s class.
Prettier Cells
Instead of relying on iOS to fill a rectangle with the cell’s background color, instead we want to have a rounded rect around our labels.
- (void)drawRect:(CGRect)rect
{
// inset by half line width to avoid cropping where line touches frame edges
CGRect insetRect = CGRectInset(rect, 0.5, 0.5);
UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:insetRect cornerRadius:rect.size.height/2.0];
// white background
[[UIColor whiteColor] setFill];
[path fill];
// red outline
[[UIColor redColor] setStroke];
[path stroke];
}
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Note: You need to set the cell’s contentMode to redraw because otherwise a modification of its size would not trigger a redraw of this good looking background.
This results in the following look, with parts of the black background shining through. Note that we need to inset the outline drawing slightly because otherwise Quartz would clip the red circle on the outside sections where it touches the view’s frame.
The next challenge will be to size the cells appropriately for the current Label contents.
Individual Item Sizing
Of course we are not content with using 1) a static size for all items and b) want to have the size automatically adjust to the contents of our cells. Remember that we specified for the cell’s label to use the text size to size it. The edges of the background view are all distanced 5 points away from the edges of the label. So it would be awesome if there was a method to retrieve a cells actual size. Then it would be even more awesome to tell the collection view to use these individual sizes for the items.
A quick test in LLDB shows that the cell returns an intrinsicContentSize of -1, -1 which is the same as not defined. We add an outlet for the label to the TagCollectionViewCell’s header and then we find that conveniently the UILabel returns the size needed by the currently set text/font.
Since we know the amount of space around the constraints we can add that to the label’s intrinsic content size to derive the cell’s size.
// cache for margins configured via constraints in XIB
static CGSize _extraMargins = {0,0};
@implementation TagCollectionViewCell
- (CGSize)intrinsicContentSize
{
CGSize size = [self.label intrinsicContentSize];
if (CGSizeEqualToSize(_extraMargins, CGSizeZero))
{
// quick and dirty: get extra margins from constraints
for (NSLayoutConstraint *constraint in self.constraints)
{
if (constraint.firstAttribute == NSLayoutAttributeBottom || constraint.firstAttribute == NSLayoutAttributeTop)
{
// vertical spacer
_extraMargins.height += [constraint constant];
}
else if (constraint.firstAttribute == NSLayoutAttributeLeading || constraint.firstAttribute == NSLayoutAttributeTrailing)
{
// horizontal spacer
_extraMargins.width += [constraint constant];
}
}
}
// add to intrinsic content size of label
size.width += _extraMargins.width;
size.height += _extraMargins.height;
return size;
}
@end
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This approach gets the actual margin widths from the constrains we have set in Interface Builder. This allows us to keep tweaking the amounts in IB without having to change some constant or #define in code. Since we don’t plan to modify any constraints, we’re lazily setting the static _extraMargins the first time this method is called, as those are constant.
Now the tricky part is dequeue a cell for reuse from the collection view to get the size. This is tricky because the method that calls our dataSource’s collectionView cellForItemAtIndexPath: is also calling collectionView:layout:sizeForItemAtIndexPath:. This prohibits us from dequeuing a cell in the latter method because that would lead to an endless loop.
Some people get their data from a model object and then have a class method on the cell to calculate the needed size. However this doesn’t make use of any constraints which we want to be able to configure in Interface Builder. The smartest solution I could come up with for this chicken and egg problem is to have a separate instance of one such cell as a template and then use its freshly overridden intrinsicContentSize method.
@implementation ViewController
{
TagCollectionViewCell *_sizingCell;
}
- (void)viewDidLoad
{
[super viewDidLoad];
UINib *cellNib = [UINib nibWithNibName:@"TagCollectionViewCell" bundle:nil];
[self.collectionView registerNib:cellNib forCellWithReuseIdentifier:@"TagCell"];
// get a cell as template for sizing
_sizingCell = [[cellNib instantiateWithOwner:nil options:nil] objectAtIndex:0];
}
#pragma mark - UICollectionView
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
return 100;
}
- (void)_configureCell:(TagCollectionViewCell *)cell forIndexPath:(NSIndexPath *)indexPath
{
if (indexPath.row%2)
{
cell.label.text = @"A";
}
else if (indexPath.row%3)
{
cell.label.text = @"longer";
}
else
{
cell.label.text = @"much longer";
}
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
TagCollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"TagCell" forIndexPath:indexPath];
[self _configureCell:cell forIndexPath:indexPath];
return cell;
}
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
[self _configureCell:_sizingCell forIndexPath:indexPath];
return [_sizingCell intrinsicContentSize];
}
@end
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Right after registering the NIB for use by the collection view I’m creating an instance of the cell and put it into my _sizingCell IVAR. To get an actual item’s cell we have a _configureCell:forIndexPath: method which we call on a dequeued reusable cell instance. For sizing we apply the same method on our sizing cell so that we get the correct intrinsicContentSize for that.
Once More With Auto Layout
When I asked my Tweeps about how I could ask a cell for how much size it would want based on the set constraints,Martin Pilkington was quick to point me towards -systemLayoutSizeFittingSize:. At first I failed in its application, kept getting {0,0}. But after having written the above workaround I took a heart and tried it once more.
And it worked right away. All that’s needed is to replace this method:
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
[self _configureCell:_sizingCell forIndexPath:indexPath];
return [_sizingCell systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
}
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This method determines the layout size that fits the passed size the closest. Two standard values are available, UILayoutFittingCompressedSize for getting the smallest size based on the layout constraints, UILayoutFittingExpandedSize to get the largest.
With this in please we can actually get rid of all the intrinsicContentSize stuff we wrote earlier. Also we can fully use the layout constraints to further restrict individual parts of our item cell. Say that you want the label to not go below a certain width for very short labels, and if the label gets too unwieldy then we would want to have the tail truncated.
This results in the following result, proving that this is indeed what we want.
Isn’t that awesome? Let me stress that one more time: you do not need to calculate sizes in a cell class method, essentially duplicating the calculations auto layout would execute. You just ask the system .
Conclusion
In this blog post I have shown how you would create item cells for collections views coming from NIB files. An alternative – and in some aspects more convenient – method is to use the collection view in a storyboard. There the cell prototype would be positioned not in a separate XIB file, but right in the structure hierarchy of the collection view. Exploring how to get a sizing cell there is left as an exercise for the reader.
We found that UILabel conveniently exposes an intrinsicContentSize which is used by auto layout to size it if you specify that in Interface Builder. We explored a technique where we would use a cell instance as template for determine the optimal item size. Then we went one step further and employed a method that actually tells us the perfect size based on all layout constrains and intrinsic sizes.
The code for this example is available on the Cocoanetics Examples repo on GitHub.
