``` 有时候我们需要在 TableView CollectionView 这样的控件滚动之前就想要加载一些东西 我这里做的是加载图片
我们想在当前的这些可见Cell前几个 或者后几个 的数据提前加载 这个时候就可以用到一下代码 OC的直接拷贝 我这里举例的是CollectionView 你只要把 CollectionView相关的数据源代码 换成 TableView的就可以了 获取算法还是一样的
// 停止滚动
- (void)scrollViewDidEndDecelerating:(UIScrollView*)scrollView
{
// if decelerate was true for scrollViewDidEndDragging:willDecelerate:
// this will be called when the deceleration is done
[selfprefetchImagesForCollectionView:self.collectionView];
}
// 停止拖拽
(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){
[selfprefetchImagesForCollectionView:self.collectionView];
}
}
/** 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)prefetchImagesForCollectionView:(SMCollectionView*)collectionView
{
NSArray*indexPaths = [collectionViewindexPathsForVisibleItems];
if([indexPathscount] ==0)return;
NSIndexPath*minimumIndexPath = indexPaths[0];
NSIndexPath*maximumIndexPath = [indexPathslastObject];
// they should be sorted already, but if not, update min and max accordingly
for(NSIndexPath*indexPathinindexPaths)
{
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 = [NSMutableArrayarray];
indexPaths =[selfcollectionView:collectionViewpriorIndexPathCount:kPrefetchRowCountfromIndexPath:minimumIndexPath];
for(NSIndexPath*indexPathinindexPaths) {
[imageURLsaddObject:[selfurlForIndexPath:indexPath]];
}
indexPaths = [selfcollectionView:collectionViewnextIndexPathCount:kPrefetchRowCountfromIndexPath:maximumIndexPath];
for(NSIndexPath*indexPathinindexPaths) {
[imageURLsaddObject:[selfurlForIndexPath:indexPath]];
}
// now prefetch
if([imageURLscount] >0)
{
[[SDWebImagePrefetchersharedImagePrefetcher]prefetchURLs:imageURLs];
}
}
/** 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 countThe number of rows to retrieve
- @param indexPath The indexPath where we're going to start (presumably the first visible indexPath)
*
- @returnAn array of indexPaths.
*/
- (NSArray)collectionView:(SMCollectionView)collectionView priorIndexPathCount:(NSInteger)count fromIndexPath:(NSIndexPath*)indexPath
{
NSMutableArray*indexPaths = [NSMutableArrayarray];
NSIntegerrow = indexPath.row;
NSIntegersection = indexPath.section;
for(NSIntegeri =0; i < count; i++) {
if(row ==0) {
if(section ==0) {
returnindexPaths;
}else{
section--;
row = [collectionViewnumberOfItemsInSection:section] -1;
}
}else{
row--;
}
[indexPathsaddObject:[NSIndexPathindexPathForRow:rowinSection:section]];
}
returnindexPaths;
}
/** 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 countThe number of rows to retrieve
- @param indexPath The indexPath where we're going to start (presumably the last visible indexPath)
*
@returnAn array of indexPaths.
*/
- (NSArray)collectionView:(SMCollectionView)collectionView nextIndexPathCount:(NSInteger)count fromIndexPath:(NSIndexPath*)indexPath
{
NSMutableArray*indexPaths = [NSMutableArrayarray];
NSIntegerrow = indexPath.row;
NSIntegersection = indexPath.section;
NSIntegerrowCountForSection = [collectionViewnumberOfItemsInSection:section];
for(NSIntegeri =0; i < count; i++) {
row++;
if(row == rowCountForSection) {
row =0;
section++;
if(section == [collectionViewnumberOfSections]) {
returnindexPaths;
}
rowCountForSection = [collectionViewnumberOfItemsInSection:section];
}
[indexPathsaddObject:[NSIndexPathindexPathForRow:rowinSection:section]];
}
returnindexPaths;
}
/**
*获取当前indexPath的ImageUrl
*
*@param indexPath indexPath description
*
*@return IamgeURL
*/
- (NSString)urlForIndexPath:(NSIndexPath)indexPath {
SMReadModel*readModel =self.dataArray[indexPath.section];
SMReadCartoonModel*readCartoonModel = readModel.readCartoonModels[indexPath.item];
returnreadCartoonModel.images;
}
```
