SDWebImage图片预加载的实现

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];
 //...
}