问题
I know that there is already this question on SO, but I don't think that the given answer is satisfying/complete: How can IOS Photos app can show hundreds of photos in one screen?
What I want to achieve
I want something like the image selection in whatsapp (iOS) (see screenshot). When you open the camera, there is also a horizontal slider where you can see all images from your gallery.
What I tried
Right now I have the following code in my appDelegate:
let options = PHFetchOptions()
options.sortDescriptors = [
NSSortDescriptor(key: "creationDate", ascending: true)
]
if let results = PHAsset.fetchAssetsWithMediaType(.Image, options: options) {
results.enumerateObjectsUsingBlock { (object, idx, _) in
if let asset = object as? PHAsset {
Variables.assets.append(asset)
}
}
println(Variables.assets.count)
Variables.imageManager.startCachingImagesForAssets(Variables.assets, targetSize: CGSizeMake(viewWidth, viewWidth), contentMode: .AspectFill, options: self.requestOptions)
}
Later I load the images in a UITableViewController and call the following function on scroll:
func fetchPhotoAtIndex(index : Int) {
let asset = Variables.assets[Variables.assets.count - 1 - index] as PHAsset
Variables.imageManager.requestImageForAsset(asset, targetSize: CGSizeMake(self.viewWidth, self.viewWidth), contentMode: .AspectFill, options: self.requestOptions, resultHandler: { (image, _) in
println("\(asset.localIdentifier) \(asset.creationDate) ")
[...]
self.tableView.reloadData()
})
}
This works well but I have the problem that on every app start all my assets get cached. Is this the right approach for my problem? How can I display the gallery photos in realtime like whatsapp does?
回答1:
Refer this Photos Framework example In this project you should look into the file AAPLAssetGridViewController.m and how it handles caching based on your scroll position.
- (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));
// If scrolled by a "reasonable" amount...
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];
[self.imageManager startCachingImagesForAssets:assetsToStartCaching
targetSize:AssetGridThumbnailSize
contentMode:PHImageContentModeAspectFill
options:nil];
[self.imageManager stopCachingImagesForAssets:assetsToStopCaching
targetSize:AssetGridThumbnailSize
contentMode:PHImageContentModeAspectFill
options:nil];
self.previousPreheatRect = preheatRect;
}
}
//In your case this computeDifference method changes to handle horizontal scrolling
- (void)computeDifferenceBetweenRect:(CGRect)oldRect andRect:(CGRect)newRect removedHandler:(void (^)(CGRect removedRect))removedHandler addedHandler:(void (^)(CGRect addedRect))addedHandler
{
if (CGRectIntersectsRect(newRect, oldRect)) {
CGFloat oldMaxY = CGRectGetMaxY(oldRect);
CGFloat oldMinY = CGRectGetMinY(oldRect);
CGFloat newMaxY = CGRectGetMaxY(newRect);
CGFloat newMinY = CGRectGetMinY(newRect);
if (newMaxY > oldMaxY) {
CGRect rectToAdd = CGRectMake(newRect.origin.x, oldMaxY, newRect.size.width, (newMaxY - oldMaxY));
addedHandler(rectToAdd);
}
if (oldMinY > newMinY) {
CGRect rectToAdd = CGRectMake(newRect.origin.x, newMinY, newRect.size.width, (oldMinY - newMinY));
addedHandler(rectToAdd);
}
if (newMaxY < oldMaxY) {
CGRect rectToRemove = CGRectMake(newRect.origin.x, newMaxY, newRect.size.width, (oldMaxY - newMaxY));
removedHandler(rectToRemove);
}
if (oldMinY < newMinY) {
CGRect rectToRemove = CGRectMake(newRect.origin.x, oldMinY, newRect.size.width, (newMinY - oldMinY));
removedHandler(rectToRemove);
}
} else {
addedHandler(newRect);
removedHandler(oldRect);
}
}
- (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;
}
The image caching manager heats up with your assets of the desired size and then you can retrieve them from the image caching manager itself.
Hope it helps you.
来源：https://stackoverflow.com/questions/30533934/how-to-use-phcachingimagemanager