iphone – UICollectionView Cell Image随着GCD进入视图而改变

概述我需要调整大型本地存储的图像(包含在self.optionArray中),然后在collectionView中显示它.如果我只是显示它,iOS会尝试调整图像大小,因为我快速滚动导致与内存相关的崩溃. 在下面的代码中,collectionView将平滑滚动,但有时如果我滚动得非常快,则会显示一个不正确的图像,然后在滚动减速时更改为正确的图像.为什么不将cell.cellImage.image设置为n 我需要调整大型本地存储的图像(包含在self.optionArray中),然后在collectionVIEw中显示它.如果我只是显示它,iOS会尝试调整图像大小,因为我快速滚动导致与内存相关的崩溃.

在下面的代码中,collectionVIEw将平滑滚动,但有时如果我滚动得非常快,则会显示一个不正确的图像,然后在滚动减速时更改为正确的图像.为什么不将cell.cellImage.image设置为nil修复此问题？

- (UICollectionVIEwCell *)collectionVIEw:(UICollectionVIEw *)collectionVIEw cellForItemAtIndexPath:(NSIndexPath *)indexPath{    CustomTabbarCell *cell = [collectionVIEw dequeueReusableCellWithReuseIDentifIEr:@"CustomTabbarCell" forIndexPath:indexPath];    cell.cellimage.image = nil;            dispatch_queue_t queue = dispatch_get_global_queue(disPATCH_QUEUE_PRIORITY_HIGH,0ul);            dispatch_async(queue,^{                cell.cellimage.image = nil;                UIImage *test = [self.optionArray objectAtIndex:indexPath.row];                UIImage *localimage2 = [self imageWithImage:test scaledToSize:CGSizeMake(test.size.wIDth/5,test.size.height/5)];                dispatch_sync(dispatch_get_main_queue(),^{                    cell.cellimage.image = localimage2                    cell.cellTextLabel.text = @"";                    [cell setNeedsLayout];                });            });        }    return cell;    }- (UIImage *)imageWithImage:(UIImage *)image scaledToSize:(CGSize)newSize {    UIGraphicsBeginImageContextWithOptions(newSize,NO,0.0);    [image drawInRect:CGRectMake(0,newSize.wIDth,newSize.height)];    UIImage *newImage = UIGraphicsGetimageFromCurrentimageContext();    UIGraphicsEndImageContext();    return newImage;}

编辑：
我添加了另一个async来缓存第一个和nil并初始化了cell.image.我在初始快速向下滚动时遇到了同样的问题.然而,在卷轴上,它现在完美无瑕.

我补充说：

-(voID)createDictionary{    for (UIImage *test in self.optionArray) {        UIImage *shownImage = [self imageWithImage:test scaledToSize:CGSizeMake(test.size.wIDth/5,test.size.height/5)];        [localimageDict setobject:shownImage forKey:[NSNumber numberWithInt:[self.optionArray indexOfObject:test]]];    }}- (voID)vIEwDIDLoad{    [super vIEwDIDLoad];    if (!localimageDict) {        localimageDict = [[NSMutableDictionary alloc]initWithCapacity:self.optionArray.count];    }    else {        [localimageDict removeAllObjects];    }    dispatch_queue_t queue = dispatch_get_global_queue(disPATCH_QUEUE_PRIORITY_HIGH,0ul);    dispatch_async(queue,^{        [self createDictionary];    });}- (UICollectionVIEwCell *)collectionVIEw:(UICollectionVIEw *)collectionVIEw cellForItemAtIndexPath:(NSIndexPath *)indexPath{    CustomTabbarCell *cell = [collectionVIEw dequeueReusableCellWithReuseIDentifIEr:@"CustomTabbarCell" forIndexPath:indexPath];    cell.cellimage.image = nil;    cell.cellimage.image = [[UIImage alloc]init];        if ([localimageDict objectForKey:[NSNumber numberWithInt:indexPath.row]]) {            cell.cellimage.image = [localimageDict objectForKey:[NSNumber numberWithInt:indexPath.row]];            cell.cellTextLabel.text = @"";        }    else {        cell.cellimage.image = nil;        cell.cellimage.image = [[UIImage alloc]init];        dispatch_queue_t queue = dispatch_get_global_queue(disPATCH_QUEUE_PRIORITY_HIGH,0ul);        dispatch_async(queue,^{            UIImage *test = [self.optionArray objectAtIndex:indexPath.row];            UIImage *shownImage = [self imageWithImage:test scaledToSize:CGSizeMake(test.size.wIDth/5,test.size.height/5)];            [localimageDict setobject:shownImage forKey:[NSNumber numberWithInt:indexPath.row]];            dispatch_sync(dispatch_get_main_queue(),^{                cell.cellimage.image = shownImage;                cell.cellTextLabel.text = @"";                [cell setNeedsLayout];            });        });    }}return cell;

解决方法 仔细看看你的代码示例,我可以看到你的内存问题的根源.跳出来的最重要的问题是您似乎将所有图像保存在一个数组中.这需要非常大的内存(我推断你需要调整它们必须大的图像).

要减少应用程序的占用空间,您不应该维护一组UIImage对象.相反,只需维护一个URL或路径到您的图像,然后只在UI需要时动态创建UIImage对象(一个称为延迟加载的过程).一旦图像离开屏幕,就可以释放它(UICollectionVIEw,就像UItableVIEw为你做的很多清理工作一样,只要你不保持对图像的强引用).

应用程序通常只应维护当前可见图像的UIImage对象.您可能出于性能原因缓存这些已调整大小的图像(例如,使用NSCache),但是当内存不足时,将自动清除缓存.

好的是你显然已经精通异步处理.无论如何,实现可能如下所示：

- (UICollectionVIEwCell *)collectionVIEw:(UICollectionVIEw *)collectionVIEw cellForItemAtIndexPath:(NSIndexPath *)indexPath{    CustomTabbarCell *cell = [collectionVIEw dequeueReusableCellWithReuseIDentifIEr:@"CustomTabbarCell" forIndexPath:indexPath];    Nsstring *filename = [self.filenameArray objectAtIndex:indexPath.row]; // I always use indexPath.item,but if row works,that's great    UIImage *image = [self.thumbnailCache objectForKey:filename];          // you can key this on whatever you want,but the filename works    cell.cellimage.image = image;                                          // this will load cached image if found,or `nil` it if not found    if (image == nil)                                                      // we only need to retrIEve image if not found in our cache    {        dispatch_queue_t queue = dispatch_get_global_queue(disPATCH_QUEUE_PRIORITY_HIGH,^{            UIImage *test = [UIImage imageWithContentsOffile:filename];    // load the image here,Now that we kNow we need it            if (!test)            {                NSLog(@"%s: unable to load image",__FUNCTION__);                return;            }            UIImage *localimage2 = [self imageWithImage:test scaledToSize:CGSizeMake(test.size.wIDth/5,test.size.height/5)];            if (!localimage2)            {                NSLog(@"%s: unable to convert image",__FUNCTION__);                return;            }            [self.thumbnailCache setobject:localimage2 forKey:filename];   // save the image to the cache            dispatch_async(dispatch_get_main_queue(),^{                   // async is fine; no need to keep this background operation alive,waiting for the main queue to respond                // see if the cell for this indexPath is still onscreen; probably is,but just in case                CustomTabbarCell *updateCell = (ID)[collectionVIEw cellForItemAtIndexPath:indexPath];                if (updateCell)                {                    updateCell.cellimage.image = localimage2                    updateCell.cellTextLabel.text = @"";                    [updateCell setNeedsLayout];                }            });        });    }    return cell;}

这假定您定义了thumbnailCache的类属性,它是对您将在vIEwDIDLoad或其中任何位置初始化的NSCache的强引用.缓存是一种充分利用两个世界的方法,在内存中加载图像以获得最佳性能,但是当您遇到内存压力时它将被释放.

显然,我很乐意假设“哦,只需用一系列图像文件名替换你的图像阵列”,我知道你可能需要进入代码的一些不同部分才能使它工作,但是这个毫无疑问是你的记忆消耗的来源.很明显,你总是会遇到其他内存问题(保留周期等),但在你发布的代码片段中没有类似内容.

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