Caching Images in UICollectionViewCell Swift - ios

Question

I am trying to cache images to prevent it from reloading constantly and crashing the app. So, I went to look at apple's implementation of ImageCaching which was written in Objective-C and had to replicate in swift. But, when I run and I try to cache, my app cache with an error of fatal error: found nil while unwrapping an optional value
Swift Code
func updateCachedAssets() -> Void {
let isViewVisible: Bool = self.isViewLoaded() && self.view.window != nil
if !isViewVisible { return }
//The preheat window is twice the height of the visible rect
var preheatRect: CGRect = (self.collectionView?.bounds)!
preheatRect = CGRectInset(preheatRect, 0.0, -0.5 * CGRectGetHeight(preheatRect))
//Check if the collection view is showing an area that is significantly diferent to the last preheated area
let delta: CGFloat = abs(CGRectGetMidY(preheatRect) - CGRectGetMidY(self.prevoiusPreheatRect))
if delta > CGRectGetHeight((self.collectionView?.bounds)!)/3.0 {
//Compute the assets to start caching and to stop caching
let addedIndexPaths = NSMutableArray()
let removedIndexPaths = NSMutableArray()
self.computeDifferenceBetweenRect(self.prevoiusPreheatRect, andRect: preheatRect, removedHandler: { (removedRect) -> Void in
let indexPaths: NSArray = [self.collectionView! .aapl_indexPathsForElementsInRect(removedRect)]
removedIndexPaths.addObjectsFromArray(indexPaths as [AnyObject])
}, addedHandler: { (addedRect) -> Void in
let indexPaths: NSArray = [self.collectionView! .aapl_indexPathsForElementsInRect(addedRect)]
addedIndexPaths.addObjectsFromArray(indexPaths as [AnyObject])
})
//print("AssetAtIndex", self.assetsAtIndexPaths(addedIndexPaths))
let assetsToStartCaching: NSArray = self.assetsAtIndexPaths(addedIndexPaths)
let assetsToStopCaching: NSArray = self.assetsAtIndexPaths(removedIndexPaths)
//Update the assets the PHCachingImageManager is caching.
self.imageManager.startCachingImagesForAssets(assetsToStartCaching as! [PHAsset], targetSize: AssetGridThumbnailSize, contentMode: .AspectFill, options: nil)
self.imageManager.startCachingImagesForAssets(assetsToStopCaching as! [PHAsset], targetSize: AssetGridThumbnailSize, contentMode: .AspectFill, options: nil)
self.prevoiusPreheatRect = preheatRect
}
}
Objective-C
- (void)updateCachedAssets {
BOOL isViewVisible = [self isViewLoaded] && [[self view] window] != nil;
if (!isViewVisible) { return; }
// The preheat window is twice the height of the visible rect.
CGRect preheatRect = self.collectionView.bounds;
preheatRect = CGRectInset(preheatRect, 0.0f, -0.5f * CGRectGetHeight(preheatRect));
/*
Check if the collection view is showing an area that is significantly
different to the last preheated area.
*/
CGFloat delta = ABS(CGRectGetMidY(preheatRect) - CGRectGetMidY(self.previousPreheatRect));
if (delta > CGRectGetHeight(self.collectionView.bounds) / 3.0f) {
// Compute the assets to start caching and to stop caching.
NSMutableArray *addedIndexPaths = [NSMutableArray array];
NSMutableArray *removedIndexPaths = [NSMutableArray array];
[self computeDifferenceBetweenRect:self.previousPreheatRect andRect:preheatRect removedHandler:^(CGRect removedRect) {
NSArray *indexPaths = [self.collectionView aapl_indexPathsForElementsInRect:removedRect];
[removedIndexPaths addObjectsFromArray:indexPaths];
} addedHandler:^(CGRect addedRect) {
NSArray *indexPaths = [self.collectionView aapl_indexPathsForElementsInRect:addedRect];
[addedIndexPaths addObjectsFromArray:indexPaths];
}];
NSArray *assetsToStartCaching = [self assetsAtIndexPaths:addedIndexPaths];
NSArray *assetsToStopCaching = [self assetsAtIndexPaths:removedIndexPaths];
// Update the assets the PHCachingImageManager is caching.
[self.imageManager startCachingImagesForAssets:assetsToStartCaching
targetSize:AssetGridThumbnailSize
contentMode:PHImageContentModeAspectFill
options:nil];
[self.imageManager stopCachingImagesForAssets:assetsToStopCaching
targetSize:AssetGridThumbnailSize
contentMode:PHImageContentModeAspectFill
options:nil];
// Store the preheat rect to compare against in the future.
self.previousPreheatRect = preheatRect;
}
}
The last ran code before crashing as shown by the debugger
Swift
func assetsAtIndexPaths(indexPaths: NSArray) -> NSArray {
if indexPaths.count == 0 {return []}
let assets = NSMutableArray(capacity: indexPaths.count)
for indexPath in indexPaths {
let asset = self.assetsFetchResults[indexPath.item] as! PHAsset
assets.addObject(asset)
}
return assets
}
Objective-C
- (NSArray *)assetsAtIndexPaths:(NSArray *)indexPaths {
if (indexPaths.count == 0) { return nil; }
NSMutableArray *assets = [NSMutableArray arrayWithCapacity:indexPaths.count];
for (NSIndexPath *indexPath in indexPaths) {
PHAsset *asset = self.assetsFetchResults[indexPath.item];
[assets addObject:asset];
}
return assets;
}
Error
fatal error: unexpectedly found nil while unwrapping an Optional value
The last line before throwing the error. Which is not an optional value is
let asset = self.assetsFetchResults[indexPath.item] as! PHAsset which is in assetsAtIndexPaths method
Any help would be appreciated. Thanks

Answer 1

You should unwrap optional value.

Answer 2

If the crash is due to an undefined value in the Optional, the following will protect against that condition
if let asset = self.assetsFetchResults[indexPath.item] as? PHAsset {
assets.addObject(asset)
}
if you replace this code with it inside your for loop
let asset = self.assetsFetchResults[indexPath.item] as! PHAsset
assets.addObject(asset)
This is called Optional Binding as found in the Swift book:
You use optional binding to find out whether an optional contains a value, and if so, to make that value available as a temporary constant or variable. Optional binding can be used with if and while statements to check for a value inside an optional, and to extract that value into a constant or variable, as part of a single action. if and while statements are described in more detail in Control Flow.