我SDWebImage库加载远程图片到一个表视图中我创建了一个自定义单元格类。
[cell.imageView setImageWithURL:url placeholderImage:[UIImage imageNamed:@"loading.jpg"]];
现在的问题是它加载在可见单元格的图像，而不是只为我要上下滚动，使它们加载的cell都位于屏幕之外。有没有什么办法可以加载所有的图像，而无需滚动表视图。 在此先感谢！
原文地址: http://stackoverflow.com/questions/15701556/sdwebimage-does-not-load-remote-images-until-scroll
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1. 如果你想预取行，你可以回应UIScrollViewDelegate方法，以确定何时台滚动完成时，触发的行的预取。您可以执行SDWebImagePrefetcher（在我原来的答复我是类的一个小不屑一顾，但它似乎工作相对现在好了）：
- (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
下面是简单的cellForRowAtIndexPath（不完全相关，但只是表明，如果SDWebImagePrefetcher CodeGo.net，你不必与周围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;
}
这些UIScrollViewDelegate滚动结束时，预取的方法更多的行
- (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];
}
你需要一个预取程序。这得到了NSIndexPath对于在可见单元格的每一侧的单元格的值，得到他们的图像URL，然后预取数据。
/** 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];
}
}
这些是用于获取NSIndexPath对于前可见单元格，以及那些跟随可见单元格的行：
/** 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;
}
有很多在那里，但在现实中，SDWebImage其SDWebImagePrefetcher是做繁重。 里面含有的缘故低于我原来的答复 原来的答案： 如果你想要做的与预取SDWebImage，你可以做如下所示： 添加块到您的setImageWithURL调用：
- (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，我真的不喜欢我打电话prefetcher常规这里（我希望iOS的有很好的didFinishTableRefresh但它的作品，即使它调用例程超过我真正想要的。我只是确保低于下面的程序可以确保它不会让多余的请求。 反正我写一个程序预取，看起来，比如说，未来十年的影像：
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];
}
}];
}
在下载程序中，您可以检查，看看图像下载已经开始，如果没有，则启动它。要做到这一点有SDWebImage，我保持weak指针在网页图像处理我的TableModelRow类（即备份我的表的各行的模型类）：
@property (nonatomic, weak) id<SDWebImageOperation> webImageOperation;
然后我有downloadImageIfNeeded程序开始下载，如果它还没有（你就会明白为什么做了weak是如此重要......我检查，看看这行已经有另一个开始之前的操作挂起）。我不这样做与下载图像（短的，用于调试目的，记录的事实，下载已完成），而只是下载，让任何事情SDImageWeb保持缓存图像的轨道，所以当cellForRowAtIndexPath后来要求形象向下滚动，它的存在，准备和等待。
- (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
}];
}
部分认为它可能更叫imageManager.imageCachequeryDiskCacheForKey优先，做测试后，它看起来并不像是一个需要（和downloadWithURL确实是这样）。 我要指出的是，SDImageWeb库有SDWebImagePrefetcher类（请参阅类的的是令人难以置信的前途，但看代码，与所有尊重一个原本优秀的库，这不会觉得很（例如，它的网址是一个简单的列表来获取，如果你再这样做，它取消了之前列表“添加到队列”或类似的东西）的概念。这是一个有前途的概念，但在执行有点弱，而当我试了一下，我UX遭受明显。 所以，我倾向于SDWebImagePrefetcher（直到它的改善，至少），并坚持我的预取技术。它并不十分复杂，但它似乎工作。
2. 这是一个例子，你需要这样你的目的。 您的UITableView的委托：
- (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;
}
，您的自定义的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
，您的自定义的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如果我们检测到该小区的另一幅图像，而不是将其下载滚动表视图图像所需。
3. 我必须解决这个确切的问题，不想预取的开销。必须有额外的下引擎罩的事情发生与防止装载内置ImageView的属性，一个新的UIImageView工作得很好。 我的解决方案是非常干净的，如果你不使用的UITableViewCell的子类介意（或已经）： 子类的UITableViewCell。 在你的子类，隐藏self.imageView。 创建您自己的UIImageView子视图，并设置该视图的形象。 下面是我自己的代码（这里是设置到了iOS照片应用程序的相册的大小和位置匹配覆盖）的修改版本： 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];
//...
}