UICollectionView indexPathsForVisibleItems不会更新新的可见单元格 [英] UICollectionView indexPathsForVisibleItems don't update new visible cells
问题描述
我有一个带有CollectionView的ViewController。加载视图时，正确显示可见单元格（9个单元格）。当我向下滚动时，我想通过调用partnerCollectionView的indexPathsForVisibleItems来加载带有loadImagesForOnscreenRows的集合视图中的可见项。但是当loadImagesForOnscreenRows时，indexPathsForVisibleItems总是包含其中的前9个单元格，即使单元格10到18应该在屏幕上可见。我使用的代码是：
I have a ViewController with a CollectionView inside. When the view loads, the visible cells (9 cells) are shown correctly. When I scroll down, I want to load the visible items in the collectionview with loadImagesForOnscreenRows by calling the indexPathsForVisibleItems for partnerCollectionView. But when loadImagesForOnscreenRows the indexPathsForVisibleItems has allways the first 9 cells in it, even when cells 10 to 18 should be visible on the screen. The code I use is:
#import "PartnerListViewController.h"
#import "AppDelegate.h"
#import "Partner.h"
#import "ImageLoader.h"
#import "PartnerDetailViewController.h"
@interface PartnerListViewController ()
@end
@implementation PartnerListViewController
@synthesize lblTitle;
@synthesize partnerCollectionView;
@synthesize imageDownloadsInProgress;
@synthesize fetchedResultsController;
@synthesize managedObjectContext;
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view.
AppDelegate * appDelegate = (AppDelegate *) [[UIApplication sharedApplication] delegate];
managedObjectContext = [appDelegate managedObjectContext];
imageDownloadsInProgress = [NSMutableDictionary dictionary];
appDelegate = nil;
[self setupFetchedResultsController];
[partnerCollectionView reloadData];
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#pragma mark - Collection view data source
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
return [[fetchedResultsController sections] count];
}
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
id <NSFetchedResultsSectionInfo> sectionInfo = [[fetchedResultsController sections] objectAtIndex:section];
return [sectionInfo numberOfObjects];
}
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"PartnerCell";
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:CellIdentifier forIndexPath:indexPath];
Partner *partner = [self.fetchedResultsController objectAtIndexPath:indexPath];
UIImageView *imageView = (UIImageView *)[cell viewWithTag:100];
if (!partner.image)
{
imageView.image = [UIImage imageNamed:@"annotationMap.png"];
if (self.partnerCollectionView.dragging == NO && self.partnerCollectionView.decelerating == NO)
{
[self startDownload:partner.imageUrl forIndexPath:indexPath];
}
} else {
imageView.image = [UIImage imageWithData:partner.image];
}
UILabel *lblTitlePartner = (UILabel *)[cell viewWithTag:101];
lblTitlePartner.text = partner.title;
lblTitlePartner.font = [UIFont fontWithName:fontName size:10];
return cell;
}
#pragma mark - Table cell image support
- (void)startDownload:(NSString *)urlString forIndexPath:(NSIndexPath *)indexPath
{
NSLog(@"startDownload:%ld", (long)indexPath.row);
ImageLoader *imageLoader = [imageDownloadsInProgress objectForKey:indexPath];
imageLoader = [[ImageLoader alloc] init];
imageLoader.urlString = urlString;
imageLoader.indexPathTableView = indexPath;
imageLoader.delegate = (id)self;
[imageDownloadsInProgress setObject:imageLoader forKey:indexPath];
[imageLoader startDownload];
}
// this method is used in case the user scrolled into a set of cells that don't have their app icons yet
- (void)loadImagesForOnscreenRows
{
NSArray *visiblePaths = [self.partnerCollectionView indexPathsForVisibleItems];
NSMutableArray *rowsArray = [NSMutableArray arrayWithCapacity:[visiblePaths count]];
[visiblePaths enumerateObjectsUsingBlock:^(NSIndexPath *indexPath, NSUInteger idx, BOOL *stop) {
NSLog(@"loadImagesForOnscreenRows1%@", @(indexPath.item));
[rowsArray addObject:@(indexPath.item)];
}];
for (NSIndexPath *indexPath in visiblePaths)
{
NSLog(@"loadImagesForOnscreenRows2:%ld", (long)indexPath.row);
Partner *item = [self.fetchedResultsController objectAtIndexPath:indexPath];
if (!item.image) // avoid the app icon download if the app already has an icon
{
[self startDownload:item.imageUrl forIndexPath:indexPath];
}
}
}
// called by our ImageDownloader when an icon is ready to be displayed
- (void)imageLoaderDidFinishDownloading:(NSIndexPath *)indexPath
{
NSLog(@"imageLoaderDidFinishDownloading:%ld", (long)indexPath.row);
ImageLoader *imageLoader = [imageDownloadsInProgress objectForKey:indexPath];
if (imageLoader != nil)
{
// Save the newly loaded image
Partner *item = [self.fetchedResultsController objectAtIndexPath:indexPath];
item.image = UIImageJPEGRepresentation(imageLoader.image, 1.0);
[self performSelectorOnMainThread:@selector(saveItem) withObject:nil waitUntilDone:YES];
[self performSelectorOnMainThread:@selector(reloadData) withObject:nil waitUntilDone:YES];
}
}
- (void)saveItem
{
[self.managedObjectContext save:nil];
}
- (void)reloadData
{
[self.partnerCollectionView reloadData];
}
#pragma mark deferred image loading (UIScrollViewDelegate)
// Load images for all onscreen rows when scrolling is finished
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
if (!decelerate)
{
[self loadImagesForOnscreenRows];
}
}
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
[self loadImagesForOnscreenRows];
}
- (void)setupFetchedResultsController
{
// 1 - Decide what Entity you want
NSString *entityName = @"Partner"; // Put your entity name here
NSLog(@"Setting up a Fetched Results Controller for the Entity named %@", entityName);
// 2 - Request that Entity
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:entityName];
// 4 - Sort it if you want
request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"title" ascending:NO selector:@selector(localizedCaseInsensitiveCompare:)]];
// 5 - Fetch it
self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil];
NSError *error = nil;
if (![[self fetchedResultsController] performFetch:&error]) {
NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
abort();
}
}
@end
结果输出：
可见项目的初始显示
2013-09-02 06:45:21.940 [2564:c07] startDownload:0
2013-09-02 06:45:21.943 [2564:c07] imageLoaderDidFinishDownloading:0
2013-09-02 06:45:21.950 [2564:c07] startDownload:1
2013-09-02 06:45:21.951 [2564:c07] imageLoaderDidFinishDownloading:1
2013-09-02 06:45:21.958 [2564:c07] startDownload:2
2013-09-02 06:45:21.959 [2564:c07] imageLoaderDidFinishDownloading:2
2013-09-02 06:45:21.965 [2564:c07] startDownload:3
2013-09-02 06:45:22.063 [2564:c07] imageLoaderDidFinishDownloading:3
2013-09-02 06:45:22.072 [2564:c07] startDownload:4
2013-09-02 06:45:22.073 [2564:c07] imageLoaderDidFinishDownloading:4
2013-09-02 06:45:22.081 [2564:c07] startDownload:5
2013-09-02 06:45:22.082 [2564:c07] imageLoaderDidFinishDownloading:5
2013-09-02 06:45:22.089 [2564:c07] startDownload:6
2013-09-02 06:45:22.090 [2564:c07] imageLoaderDidFinishDownloading:6
2013-09-02 06:45:22.098 [2564:c07] startDownload:7
2013-09-02 06:45:22.099 [2564:c07] imageLoaderDidFinishDownloading:7
2013-09-02 06:45:22.104 [2564:c07] startDownload:8
2013-09-02 06:45:22.163 [2564:c07] imageLoaderDidFinishDownloading:8
滚动到项目10到19之后：
After scrolling to item 10 to 19:
2013-09-02 06:45:26.212 [2564:c07] loadImagesForOnscreenRows1:8
2013-09-02 06:45:26.212 [2564:c07] loadImagesForOnscreenRows1:0
2013-09-02 06:45:26.212 [2564:c07] loadImagesForOnscreenRows1:1
2013-09-02 06:45:26.212 [2564:c07] loadImagesForOnscreenRows1:6
2013-09-02 06:45:26.213 [2564:c07] loadImagesForOnscreenRows1:2
2013-09-02 06:45:26.213 [2564:c07] loadImagesForOnscreenRows1:3
2013-09-02 06:45:26.213 [2564:c07] loadImagesForOnscreenRows1:4
2013-09-02 06:45:26.213 [2564:c07] loadImagesForOnscreenRows1:5
2013-09-02 06:45:26.213 [2564:c07] loadImagesForOnscreenRows1:7
2013-09-02 06:45:26.214 [2564:c07] loadImagesForOnscreenRows2:8
2013-09-02 06:45:26.214 [2564:c07] loadImagesForOnscreenRows2:0
2013-09-02 06:45:26.214 [2564:c07] loadImagesForOnscreenRows2:1
2013-09-02 06:45:26.214 [2564:c07] loadImagesForOnscreenRows2:6
2013-09-02 06:45:26.214 [2564:c07] loadImagesForOnscreenRows2:2
2013-09-02 06:45:26.215 [2564:c07] loadImagesForOnscreenRows2:3
2013-09-02 06:45:26.215 [2564:c07] loadImagesForOnscreenRows2:4
2013-09-02 06:45:26.215 [2564:c07] loadImagesForOnscreenRows2:5
2013-09-02 06:45:26.215 [2564:c07] loadImagesForOnscreenRows2:7
滚动后可以看到，项目可见索引路径保持不变。有没有其他人遇到这个或想法解决方案？或者我是否对收集视图的某些原则不以为然？
As you can see after scrolling, the items visible index paths do stay the same. Has anybody else encountered this or an idea for a solution? Or am I unsterstanding some principle of collectionview wrong?
非常感谢提前！
亲切的问候，
Jan
Many thanks in advance! Kind regards, Jan
推荐答案
如果你的目标是iOS 8.0及以上，你应该使用 collectionView：willDisplayCell：forItemAtIndexPath：开始下载。如果使用iOS 7.0，则应继续使用 collectionView：cellForItemAtIndexPath：。
If you are targeting iOS 8.0 and above you should use collectionView:willDisplayCell:forItemAtIndexPath: to kick off your download. If using iOS 7.0 you should continue to use collectionView:cellForItemAtIndexPath:.
在 imageLoaderDidFinishDownloading：回调中，您应该检查相关索引路径是否仍然可见。如果是，则检索相应的单元格并更新其图像视图。如果单元格不可见，那么您的工作就完成了。每次映像完成时调用 -reloadData 都会执行大量昂贵的工作，如果您的用户当前正在滚动表并重置其内容，则可能会出现重大的UX问题。您也可能多次执行 UIImageJPEGRepresentation（）工作，如果您在 imageLoaderDidFinishDownloading中执行此操作，它将有助于您的滚动性能：然后缓存它。
In your imageLoaderDidFinishDownloading: callback you should check to see if the index path in question is still visible. If it is, retrieve the corresponding cell and update its image view. If the cell isn't visible, then your work is done. Calling -reloadData for every image completion is doing a lot of expensive work and could have significant UX issues if your user is currently mid-scroll of the table and you reset its contents. You are also potentially doing the UIImageJPEGRepresentation() work many times, it would help your scrolling performance if you did this work once in imageLoaderDidFinishDownloading: and then cached it.
因为看起来回调发生在后台线程上，所以确保你只操作 UICollectionView 。
Since it looks like the callback happens on a background thread make sure you only manipulate the UICollectionView from the main thread.
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