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[英]SDWebImage not setting images in UITableView cells until scroll in Xcode 9.0 beta
[英]SDWebImage does not load remote images until scroll
我正在使用SDWebImage库将远程图像加载到一个表视图中,该视图使用我创建的自定义单元类。 我只是用
[cell.imageView setImageWithURL:url placeholderImage:[UIImage imageNamed:@"loading.jpg"]];
in cellForRowAtIndexPath:现在的问题是它只在可见单元格中加载图像,而不是在屏幕上为我必须向上和向下滚动以加载图像的单元格。 有没有办法我可以加载所有图像,而无需滚动表格视图。 提前致谢!!
如果要预取行,可以响应UIScrollViewDelegate
方法以确定表滚动何时完成,从而触发行的预取。 你可以使用SDWebImagePrefetcher
执行预取(在我的原始答案中,我对这个有用的类有点不屑一顾,但它现在似乎工作得相当好):
- (void)viewDidLoad
{
[super viewDidLoad];
// the details don't really matter here, but the idea is to fetch data,
// call `reloadData`, and then prefetch the other images
NSURL *url = [NSURL URLWithString:kUrlWithJSONData];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
if (connectionError) {
NSLog(@"sendAsynchronousRequest error: %@", connectionError);
return;
}
self.objects = [NSJSONSerialization JSONObjectWithData:data options:0 error:nil];
[self.tableView reloadData];
[self prefetchImagesForTableView:self.tableView];
}];
}
// some of the basic `UITableViewDataDelegate` methods have been omitted because they're not really relevant
这是简单的cellForRowAtIndexPath
(不完全相关,但只是显示如果你使用SDWebImagePrefetcher
,你不必乱用cellForRowAtIndexPath
:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *cellIdentifier = @"Cell";
CustomCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
NSAssert([cell isKindOfClass:[CustomCell class]], @"cell should be CustomCell");
[cell.customImageView setImageWithURL:[self urlForIndexPath:indexPath] placeholderImage:nil];
[cell.customLabel setText:[self textForIndexPath:indexPath]];
return cell;
}
滚动完成时,这些UIScrollViewDelegate
方法会预取更多行
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
// if `decelerate` was true for `scrollViewDidEndDragging:willDecelerate:`
// this will be called when the deceleration is done
[self prefetchImagesForTableView:self.tableView];
}
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
// if `decelerate` is true, then we shouldn't start prefetching yet, because
// `cellForRowAtIndexPath` will be hard at work returning cells for the currently visible
// cells.
if (!decelerate)
[self prefetchImagesForTableView:self.tableView];
}
您显然需要实现预取例程。 这将获取可见单元格两侧的单元格的NSIndexPath
值,获取其图像URL,然后预取该数据。
/** Prefetch a certain number of images for rows prior to and subsequent to the currently visible cells
*
* @param tableView The tableview for which we're going to prefetch images.
*/
- (void)prefetchImagesForTableView:(UITableView *)tableView
{
NSArray *indexPaths = [self.tableView indexPathsForVisibleRows];
if ([indexPaths count] == 0) return;
NSIndexPath *minimumIndexPath = indexPaths[0];
NSIndexPath *maximumIndexPath = [indexPaths lastObject];
// they should be sorted already, but if not, update min and max accordingly
for (NSIndexPath *indexPath in indexPaths)
{
if (indexPath.section < minimumIndexPath.section || (indexPath.section == minimumIndexPath.section && indexPath.row < minimumIndexPath.row)) minimumIndexPath = indexPath;
if (indexPath.section > maximumIndexPath.section || (indexPath.section == maximumIndexPath.section && indexPath.row > maximumIndexPath.row)) maximumIndexPath = indexPath;
}
// build array of imageURLs for cells to prefetch
NSMutableArray *imageURLs = [NSMutableArray array];
indexPaths = [self tableView:tableView priorIndexPathCount:kPrefetchRowCount fromIndexPath:minimumIndexPath];
for (NSIndexPath *indexPath in indexPaths)
[imageURLs addObject:[self urlForIndexPath:indexPath]];
indexPaths = [self tableView:tableView nextIndexPathCount:kPrefetchRowCount fromIndexPath:maximumIndexPath];
for (NSIndexPath *indexPath in indexPaths)
[imageURLs addObject:[self urlForIndexPath:indexPath]];
// now prefetch
if ([imageURLs count] > 0)
{
[[SDWebImagePrefetcher sharedImagePrefetcher] prefetchURLs:imageURLs];
}
}
这些是为紧邻可见单元格之前的行以及紧跟可见单元格之后的行获取NSIndexPath
的实用方法:
/** Retrieve NSIndexPath for a certain number of rows preceding particular NSIndexPath in the table view.
*
* @param tableView The tableview for which we're going to retrieve indexPaths.
* @param count The number of rows to retrieve
* @param indexPath The indexPath where we're going to start (presumably the first visible indexPath)
*
* @return An array of indexPaths.
*/
- (NSArray *)tableView:(UITableView *)tableView priorIndexPathCount:(NSInteger)count fromIndexPath:(NSIndexPath *)indexPath
{
NSMutableArray *indexPaths = [NSMutableArray array];
NSInteger row = indexPath.row;
NSInteger section = indexPath.section;
for (NSInteger i = 0; i < count; i++) {
if (row == 0) {
if (section == 0) {
return indexPaths;
} else {
section--;
row = [tableView numberOfRowsInSection:section] - 1;
}
} else {
row--;
}
[indexPaths addObject:[NSIndexPath indexPathForRow:row inSection:section]];
}
return indexPaths;
}
/** Retrieve NSIndexPath for a certain number of following particular NSIndexPath in the table view.
*
* @param tableView The tableview for which we're going to retrieve indexPaths.
* @param count The number of rows to retrieve
* @param indexPath The indexPath where we're going to start (presumably the last visible indexPath)
*
* @return An array of indexPaths.
*/
- (NSArray *)tableView:(UITableView *)tableView nextIndexPathCount:(NSInteger)count fromIndexPath:(NSIndexPath *)indexPath
{
NSMutableArray *indexPaths = [NSMutableArray array];
NSInteger row = indexPath.row;
NSInteger section = indexPath.section;
NSInteger rowCountForSection = [tableView numberOfRowsInSection:section];
for (NSInteger i = 0; i < count; i++) {
row++;
if (row == rowCountForSection) {
row = 0;
section++;
if (section == [tableView numberOfSections]) {
return indexPaths;
}
rowCountForSection = [tableView numberOfRowsInSection:section];
}
[indexPaths addObject:[NSIndexPath indexPathForRow:row inSection:section]];
}
return indexPaths;
}
那里有很多,但实际上, SDWebImage
和它的SDWebImagePrefetcher
正在进行繁重的工作。
为了完整起见,我在下面提供了我原来的答案。
原始答案:
如果您想使用SDWebImage
进行预取,可以执行以下操作:
在setImageWithURL
调用中添加完成块:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath { NSLog(@"%s", __FUNCTION__); static NSString *cellIdentifier = @"Cell"; UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier]; TableModelRow *rowData = self.objects[indexPath.row]; cell.textLabel.text = rowData.title; [cell.imageView setImageWithURL:rowData.url placeholderImage:[UIImage imageNamed:@"placeholder.png"] completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType) { [self prefetchImagesForTableView:tableView]; }]; return cell; }
我必须承认我不喜欢在这里调用我的prefetcher
例程(我希望iOS有一些不错的didFinishTableRefresh
委托方法),但是它有效,即使它调用例程的次数比我真正想要的多。 我只是确保下面的例程确保它不会发出冗余请求。
无论如何,我编写了一个预取例程来查找接下来的十个图像:
const NSInteger kPrefetchRowCount = 10; - (void)prefetchImagesForTableView:(UITableView *)tableView { // determine the minimum and maximum visible rows NSArray *indexPathsForVisibleRows = [tableView indexPathsForVisibleRows]; NSInteger minimumVisibleRow = [indexPathsForVisibleRows[0] row]; NSInteger maximumVisibleRow = [indexPathsForVisibleRows[0] row]; for (NSIndexPath *indexPath in indexPathsForVisibleRows) { if (indexPath.row < minimumVisibleRow) minimumVisibleRow = indexPath.row; if (indexPath.row > maximumVisibleRow) maximumVisibleRow = indexPath.row; } // now iterate through our model; // `self.objects` is an array of `TableModelRow` objects, one object // for every row of the table. [self.objects enumerateObjectsUsingBlock:^(TableModelRow *obj, NSUInteger idx, BOOL *stop) { NSAssert([obj isKindOfClass:[TableModelRow class]], @"Expected TableModelRow object"); // if the index is within `kPrefetchRowCount` rows of our visible rows, let's // fetch the image, if it hasn't already done so. if ((idx < minimumVisibleRow && idx >= (minimumVisibleRow - kPrefetchRowCount)) || (idx > maximumVisibleRow && idx <= (maximumVisibleRow + kPrefetchRowCount))) { // my model object has method for initiating a download if needed [obj downloadImageIfNeeded]; } }]; }
在下载例程中,您可以检查图像下载是否已开始,如果没有,则启动它。 要使用SDWebImage
执行此操作,我在TableModelRow
类(支持表的各行的模型类)中保留了一个指向Web图像操作的weak
指针:
@property (nonatomic, weak) id<SDWebImageOperation> webImageOperation;
然后我将downloadImageIfNeeded
例程开始下载(如果它还没有)(你可以看到为什么这个weak
是如此重要......我正在检查这行是否已经有一个操作挂起,然后再启动另一个)。 我没有对下载的图像做任何事情(为了调试目的,记录下载已完成的事实),而是只是下载并让SDImageWeb
跟踪我的缓存图像,所以当cellForRowAtIndexPath
后来请求时当用户向下滚动图像时,它就在那里,准备好并等待。
- (void)downloadImageIfNeeded { if (self.webImageOperation) return; SDWebImageManager *imageManager = [SDWebImageManager sharedManager]; self.webImageOperation = [imageManager downloadWithURL:self.url options:0 progress:nil completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished) { NSLog(@"%s: downloaded %@", __FUNCTION__, self.title); // I'm not going to do anything with the image, but `SDWebImage` has now cached it for me }]; }
部分我认为首先调用imageManager.imageCache
实例方法queryDiskCacheForKey
可能会更健壮,但在进行一些测试之后,它看起来并不像需要的那样(而且无论如何, downloadWithURL
都为我们做了这个)。
我应该指出, SDImageWeb
库确实有一个SDWebImagePrefetcher
类(参见文档 )。 这个类的名称是非常有前途的,但看着代码,完全尊重一个优秀的库,这对我来说感觉不太健壮(例如,它是一个简单的URL列表,如果你再次这样做,它取消了先前的列表,没有“添加到队列”或类似的东西的概念)。 这是一个很有希望的概念,但在执行方面有点弱。 当我尝试它时,我的用户体验明显遭受了损失。
所以,我倾向于不使用SDWebImagePrefetcher
(至少它是改进的),并坚持我的基本预取技术。 它不是非常复杂,但似乎有效。
我只需解决这个确切的问题,并不想要预取器的开销。 内置的imageView属性必然会发生一些额外的引擎盖内容,这会阻止加载,因为新的UIImageView工作得很好。
如果您不介意(或已经)使用UITableViewCell的子类,我的解决方案非常干净:
这是我自己的代码的修改版本(这里没有记录是设置框架以匹配iOS Photo应用程序专辑封面的大小和位置):
YourTableCell.h
@interface YourTableCell : UITableViewCell
@property (nonatomic, strong) UIImageView *coverPhoto;
@end
YourTableCell.m
@implementation YourTableCell
@synthesize coverPhoto;
- (id)initWithStyle:(UITableViewCellStyle)style reuseIdentifier:(NSString *)reuseIdentifier
{
self = [super initWithStyle:style reuseIdentifier:reuseIdentifier];
if (self) {
self.imageView.image = nil;
self.coverPhoto = [[UIImageView alloc] init];
// Any customization, such as initial image, frame bounds, etc. goes here.
[self.contentView addSubview:self.coverPhoto];
}
return self;
}
//...
@end
YourTableViewController.m
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"Cell";
YourTableCell *cell = (YourTableCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
//...
[cell.coverPhoto setImageWithURL:coverUrl placeholderImage:nil options:SDWebImageCacheMemoryOnly];
//...
}
这是一个示例,您需要为此目的实现此目的。
你的UITableView委托:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
YourCustomTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"YourCustomTableViewCellReuseIdentifier"];
if (!cell)
{
cell = [[[YourCustomTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
reuseIdentifier:CellIdentifier];
}
NSString *imageURL = // ... get image url, typically from array
[cell loadImageWithURLString:imageURL forIndexPath:indexPath];
return cell;
}
您的自定义UITableViewCell .h文件 :
#import <UIKit/UIKit.h>
#import "UIImageView+WebCache.h"
#import "SDImageCache.h"
@interface YourCustomTableViewCell
{
NSIndexPath *currentLoadingIndexPath;
}
- (void)loadImageWithURLString:(NSString *)urlString forIndexPath:(NSIndexPath *)indexPath;
@end
您的自定义UITableViewCell .m文件 :
// ... some other methods
- (void)loadImageWithURLString:(NSString *)urlString forIndexPath:(NSIndexPath *)indexPath
{
currentLoadingIndexPath = indexPath;
[self.imageView cancelCurrentImageLoad];
[self.imageView setImage:nil];
NSURL *imageURL = [NSURL URLWithString:urlString];
[self.imageView setImageWithURL:imageURL
placeholderImage:nil
options:SDWebImageRetryFailed
completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType)
{
if (currentLoadingIndexPath != indexPath)
{
return;
}
if (error)
{
... // handle error
}
else
{
[imageView setImage:image];
}
}];
}
// ... some other methods
currentLoadingIndexPath
需要检测我们是否将此单元重用于另一个图像而不是用户滚动表视图时下载的图像。
我遇到了同样的问题,我发现UIImageView + WebCache在新下载时取消了上次下载。
我不确定这是否是作者的意图。 所以我在SDWebImage上编写了一个新的UIImageView category
。
易于使用:
[cell.imageView mq_setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]
groupIdentifier:@"customGroupID"
completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
}];
查看更多: ImageDownloadGroup
高级用法:
// create customGroup
MQImageDownloadGroup *customGroup = [[MQImageDownloadGroup alloc] initWithGroupIdentifier:@"tableViewCellGroup"];
customGroup.maxConcurrentDownloads = 99;
// add to MQImageDownloadGroupManage
[[MQImageDownloadGroupManage shareInstance] addGroup:customGroup];
// use download group
[cell.imageView mq_setImageWithURL:@"https://xxx"
groupIdentifier:@"tableViewCellGroup"
completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
}];
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