iphone - 当单元格具有 UIImageView subview 时，UITableView 会出现不稳定的滚动

Question

这一天的大部分时间里，这都让我发疯。

我有一个带有 UIImageViews 的 UITableView。这些 ImageView 在 tableview 的 cellForRow 函数中加载本地保存的 PNG 文件，这工作得很好，只是当其中包含图像的单元格滚动到视野中时，tableview 将停止滚动几分之一秒。我已经在 StackOverflow 和 google 上寻找答案，但没有找到答案 - 因此我们将不胜感激任何帮助。

这是我的 CellForRow 函数的代码:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {


    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"MyIdentifier"];



    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:@"MyIdentifier"] autorelease];
        cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
    }

    if([currSection isEqualToString:@"composer"]){

        MySlide *s = [slidesArray objectAtIndex:indexPath.row];

        cell.textLabel.hidden = YES;
        UIImageView *whiteView = [[UIImageView alloc] initWithFrame:CGRectMake((projectsTable.frame.size.width/2)-150, 4, 204.8, 153.6)];

        if([s.slideImage isEqualToString:@""] || s.slideImage == nil){
            //no custom image in this cell - go with default background image

            whiteView.image = [UIImage imageNamed:@"cellback2.png"];
            whiteView.backgroundColor = [UIColor whiteColor];
        }else{
            cell.layer.shouldRasterize = YES;
            cell.layer.rasterizationScale = [UIScreen mainScreen].scale;

            NSData *data = [[NSData alloc] initWithContentsOfFile:s.slideImage];

            UIImage *im = [[UIImage alloc] initWithData:data];


            whiteView.image = im;

            whiteView.image = [self imageWithImage:whiteView.image CovertToSize:CGSizeMake(204.8,153.6)];
            whiteView.backgroundColor = [UIColor whiteColor];

        }


        [cell.contentView addSubview:whiteView];

        [whiteView release];



        cell.accessoryType = UITableViewCellAccessoryNone;

    }


  return cell;  
}

Answer 1

需要进行一些更改，首先应在生成单元格时添加 UIImageView，而不是每次 tableView:cellForRowAtIndexPath: 时添加命中(正如@Vishy 建议的那样)。其次，您应该缓存从文档目录加载的图像([UIImage imageNamed:] 会自动为捆绑资源执行此操作)。

@interface MyViewController () {

    NSMutableDictionary *_imageCache;
}

@end


@implementation MyViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    // other viewDidLoad stuff...

    _imageCache = [[NSMutableDictionary alloc] init];
}

- (void)viewDidUnload {
    [super viewDidUnload];

    // other viewDidUnload stuff...

    [_imageCache release];
    _imageCache = nil;
}

- (void)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"MyIdentifier"];

    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:@"MyIdentifier"] autorelease];
        cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;

        UIImageView *whiteView = [[UIImageView alloc] initWithFrame:CGRectMake((projectsTable.frame.size.width/2)-150, 4, 204.8, 153.6)];
        whiteView.tag = 111;
        whiteView.backgroundColor = [UIColor whiteColor];

        [cell.contentView addSubview:whiteView];

        [whiteView release];
        cell.accessoryType = UITableViewCellAccessoryNone;
        cell.textLabel.hidden = YES;
    }

    UIImageView* iView = (UIImageView*) [cell.contentView viewWithTag:111];


    if([currSection isEqualToString:@"composer"]) {

        MySlide *s = [slidesArray objectAtIndex:indexPath.row];

        if([s.slideImage isEqualToString:@""] || s.slideImage == nil) {

            //no custom image in this cell - go with default background image
            iView.image = [UIImage imageNamed:@"cellback2.png"];
        }
        else {

            cell.layer.shouldRasterize = YES;
            cell.layer.rasterizationScale = [UIScreen mainScreen].scale;

            // use the image path as the cache key
            UIImage *theImage = [_imageCache objectForKey:s.slideImage];
            if (theImage == nil) {

                // load the image and save into the cache
                theImage = [UIImage imageWithContentsOfFile:s.slideImage];
                theImage = [self imageWithImage:theImage CovertToSize:CGSizeMake(204.8, 153.6)];

                [_imageCache setObject:theImage forKey:s.slideImage];
            }

            iView.image = theImage;
        }   
    }
}

@end

作为一般规则，tableView:cellForRowAtIndexPath: 是一种需要摆脱快速的方法，因此应尽可能避免从磁盘加载图像。