Resize UICollectionView cells after their data has

问题:

My UICollectionView cells contain UILabels with multiline text. I don't know the height of the cells until the text has been set on the label.

-(CGSize)collectionView:(UICollectionView *)collectionView
                 layout:(UICollectionViewLayout*)collectionViewLayout
 sizeForItemAtIndexPath:(NSIndexPath *)indexPath;

This was the initial method I looked at to size the cells. However, this is called BEFORE the cells are created out of the storyboard.

Is there a way to layout the collection and size the cells AFTER they have been rendered, and I know the actual size of the cell?

回答1:

I think your are looking for the invalidateLayout method you can call on the .collectionViewLayout property of your UICollectionView. This method regenerates your layout, which in your case means also calling -collectionView: layout: sizeForItemAtIndexPath:, which is the right place to reflect your desired item size. Jirune points the right direction on how to calculate them.

An example for the usage of invalidateLayout can be found here. Also consult the UICollectionViewLayout documentation on that method:

Invalidates the current layout and triggers a layout update.

Discussion:

You can call this method at any time to update the layout information. This method invalidates the layout of the collection view itself and returns right away. Thus, you can call this method multiple times from the same block of code without triggering multiple layout updates. The actual layout update occurs during the next view layout update cycle.

Edit:

For storyboard collection view which contains auto layout constraints, you need to override viewDidLayoutSubviews method of UIViewController and call invalidateLayout collection view layout in this method.

- (void)viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];
    [yourCollectionView.collectionViewLayout invalidateLayout];
}

回答2:

subclass UICollectionViewCell and override layoutSubviews like this

hereby you will anchor cell leading and trailing edge to collectionView

override func layoutSubviews() {
        super.layoutSubviews()

        self.frame = CGRectMake(0, self.frame.origin.y, self.superview!.frame.size.width, self.frame.size.height)
    }

回答3:

Hey in the above delegate method itself, you can calculate the UILabel size using the below tricky way and return the UICollectionViewCell size based on that calculation.

    // Calculate the expected size based on the font and 
    // linebreak mode of your label
    CGSize maximumLabelSize = CGSizeMake(9999,9999);

    CGSize expectedLabelSize = 
     [[self.dataSource objectAtIndex:indexPath.item] 
             sizeWithFont:[UIFont fontWithName:@"Arial" size:18.0f] 
        constrainedToSize:maximumLabelSize 
            lineBreakMode:NSLineBreakByWordWrapping];

回答4:

- (void)viewDidLoad {
    self.collectionView.prefetchingEnabled = NO;
}

In iOS 10, prefetchingEnabled is YES by default. When YES, the collection view requests cells in advance of when they will be displayed. It leads to crash in iOS 10