当单元格具有 UIImageView 子视图时，UITableView 经历断断续续的滚动

Question

这让我在一天中的大部分时间都快疯了。

我有一个带有 UIImageViews 的 UITableView。这些图像视图在 tableview 的 cellForRow 函数中加载本地保存的 PNG 文件，这可以正常工作，只是当带有图像的单元格滚动到视线中时，tableview 将停止滚动几分之一秒。我已经在 StackOverflow 和谷歌上搜索了答案，但我的回答很短 - 所以任何帮助都将不胜感激。

这是我的 CellForRow 函数代码：

- (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，首先应该在生成单元格时添加UIImageViews，而不是每次点击tableView:cellForRowAtIndexPath:（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

一般来说，tableView:cellForRowAtIndexPath: 是一种您需要退出快速的方法，因此应尽可能避免从磁盘加载图像。

Answer 2

如下修改代码...

- (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;
    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;

        iView.image = [UIImage imageWithContentsOfFile:s.slideImage];
        iView.image = [self imageWithImage:iView.image CovertToSize:CGSizeMake(204.8,153.6)];

    }   
  }
}

Answer 3

每个 UIImage 第一次显示时也会有延迟。有关在缓存时间而不是显示时间提示工作的详细信息，请参阅此帖子：

Setting image property of UIImageView causes major lag