UICollectionViewCell using prototype sizes

问题:

I have an UICollectionView with three different prototype cells, each of which with different heights set via Storyboard. During runtime, the Collection View uses its own cell size, ignoring my Storyboard ones.

I am currently using collectionView:layout:sizeForItemAtIndexPath: with a couple conditionals to set each CGSize straight.

Is there a better way to set the cell sizes? I don't seem to be able to retrieve the Storyboard size each cell has, and CGSizeMake seems too hardcoded and not really flexible.

回答1:

It seems that there's currently no easy way to:

• Fetch UICollectionViewCell prototype cell sizes runtime from Storyboard(s).
• Manage sizes of prototype cells just in one place (rather than having to enter them in the Storyboard cell prototype and implement sizeForItemAtIndexPath).

A method proposed here (for UITableViews) does not work, because using dequeueReusableCellWithReuseIdentifier in sizeForItemAtIndexPath will cause an indefinite loop.

However, I've managed to do this the following way:

1. Add a unique (across all UICollectionViewCells in every storyboard) reuse identifier into each of your UICollectionView prototype cells in all Storyboards.

2. Add a Run script Build Phase to your project with the script that pulls out UICollectionViewCell frame sizes from all Storyboards.

   output=${PROJECT_DIR}/StoryboardPrototypeCellSizes.h
   printf "@{" > $output
   
   for storyboard in $(find ${PROJECT_DIR} -name "*.storyboard")
   do
       echo "Scanning storyboard $storyboard..."
       delimiter=
       for line in $(xpath $storyboard "//collectionViewCell/@reuseIdentifier[string-length()>0] | //collectionViewCell/rect" 2>&-)
       do
           case $line in
               reuseIdentifier*)
                   reuseIdentifier=$(sed 's/[^"]*"\([^"]*\)".*/\1/' <<< $line)
                   ;;
               width*)
                   if [ -n "$reuseIdentifier" ]; then
                       width=$(sed 's/[^"]*"\([^"]*\)".*/\1/' <<< $line)
                   fi
                   ;;
               height*)
                   if [ -n "$reuseIdentifier" ]; then
                       height=$(sed 's/[^"]*"\([^"]*\)".*/\1/' <<< $line)
                   fi
                   ;;
           esac
   
           if [ -n "$reuseIdentifier" ] && [ -n "$width" ] && [ -n "$height" ]; then
               printf "$delimiter@\"$reuseIdentifier\" : [NSValue valueWithCGSize:CGSizeMake($width, $height)]" >> $output
               unset reuseIdentifier
               unset width
               unset height
               delimiter=,\\n
           fi
       done
   done
   
   printf "};\n" >> $output
   

   This creates a header file called StoryboardPrototypeCellSizes.h with a following example content:

   @{@"TodayCell" : [NSValue valueWithCGSize:CGSizeMake(320, 80)],
   @"SpecialDayCell" : [NSValue valueWithCGSize:CGSizeMake(320, 42)],
   @"NameDayCell" : [NSValue valueWithCGSize:CGSizeMake(320, 30)]};
   

3. Add a helper method to return the UICollectionViewCell reuse identifier in the view controller controlling your UICollectionView:

   - (NSString *)cellReuseIdentifierAtIndexPath:(NSIndexPath *)indexPath
   {
       switch (indexPath.item) {
           case 0: return @"TodayCell";
           case 1: return @"SpecialDayCell";
           case 2: return @"NameDayCell";
       }
       return nil;
   }
   

4. Be sure to use the same reuse identifier in cellForItemAtIndexPath:

   - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
   {
       UICollectionViewCell *cell =
           [collectionView dequeueReusableCellWithReuseIdentifier:
               [self cellReuseIdentifierAtIndexPath:indexPath]
               forIndexPath:indexPath];
       ...
   

5. Finally implement sizeForItemAtIndexPath:

   - (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
   {
       NSDictionary *storyboardPrototypeCellSizes =
   #import "StoryboardPrototypeCellSizes.h"
   
        return [(NSValue *)storyboardPrototypeCellSizes[
                [self cellReuseIdentifierAtIndexPath:indexPath]
               ] CGSizeValue];
   }
   

This solution allows you to define UICollectionViewCell prototype cell sizes only once in the Storyboard(s) and also doesn't do any non-App-Store-compliant at runtime.

****Edit:**** You can also fetch UICollectionReusableView sizes by adding another script with the same content and replacing "collectionViewCell" with "collectionReusableView", and renaming the header file to, for example, StoryboardReusableViewSizes.h

回答2:

There is no better way to set cell sizes. Cell sizes are used in several places in UICollectionView - for positioning, for scroll indicator. And it is very important to receive them as quick as possible in case if user scrolls collection with thousands small cells for example. So create cell and ask it about its size is not an option. You have to implement collectionView:layout:sizeForItemAtIndexPath: and it should work quickly.

回答3:

Are you using flow layout in your UICollectionView? If yes, you can use sizeForItemAtIndexPath method of the UICollectionViewDelegateFlowLayout protocol to provide the size of a cell. If you don't have issues using OSS components in your app, RFQuiltLayout can be used to achieve this.