iphone - Objective-C:每次上下滚动时，UITableView 单元格都会被覆盖

Question

我有一个通过 CoreData 填充的 UITableView，我刚刚注意到一些奇怪的事情。我在 UITable 中有大约 20 行左右，当我向下滚动表格并再次向上滚动时，单元格的标签被写在现有文本的顶部，并且每次我向下和向上滚动时都会继续这样做。我的 CellForRowAtIndexPath 代码是:

  // Customize the appearance of table view cells.
  - (UITableViewCell *)tableView:(UITableView *)tableView   cellForRowAtIndexPath:(NSIndexPath *)indexPath {
//Some constants ---
const NSInteger TOP_LABEL_TAG = 1001;
const NSInteger BOTTOM_LABEL_TAG = 1002;
UILabel *topLabel;
UILabel *bottomLabel;
const CGFloat LABEL_HEIGHT = 20;
//CGRect Imageframe = CGRectMake(7, 5, 35, 35);
//----



static NSString *CellIdentifier = @"Cell";

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
    cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
    cell.textLabel.font = [UIFont boldSystemFontOfSize:12];
}

//custom implementation of UIView for lazy loading of image 
//LazyUIImageView *lazy = [[LazyUIImageView alloc] initWithFrame:Imageframe];


//[cell addSubview:lazy];



Post *post = [fetchedResultsController objectAtIndexPath:indexPath];

NSSet *medias = post.PostMedia;
Media *media = [medias anyObject];
NSSet *thumbs  = media.MediaThumb;
Thumb *thumb = [thumbs anyObject];


UIImage *img = thumb.url;

if (img)
    cell.imageView.image = img;
else 
    cell.imageView.image = post.authorpic;

//The creation of the top label
topLabel =
[[[UILabel alloc]
  initWithFrame:
  CGRectMake(
             35 + 2.0 * cell.indentationWidth,
             0.5 * (tableView.rowHeight - 2 * LABEL_HEIGHT),
             tableView.bounds.size.width -
             45 - 4.0 * cell.indentationWidth
             - 35,
             LABEL_HEIGHT)]
 autorelease];
[cell.contentView addSubview:topLabel];

topLabel.tag = TOP_LABEL_TAG;
topLabel.backgroundColor = [UIColor clearColor];
topLabel.textColor = [UIColor colorWithRed:0.25 green:0.0 blue:0.0 alpha:1.0];
topLabel.highlightedTextColor = [UIColor colorWithRed:1.0 green:1.0 blue:0.9 alpha:1.0];
topLabel.font = [UIFont systemFontOfSize:[UIFont labelFontSize]];
//---------------------------------

//The creation of the bottom label
bottomLabel =
[[[UILabel alloc]
  initWithFrame:
  CGRectMake(
             35 + 2.0 * cell.indentationWidth,
             0.5 * (tableView.rowHeight - 2 * LABEL_HEIGHT) + LABEL_HEIGHT,
             tableView.bounds.size.width -
             35 - 4.0 * cell.indentationWidth
             - 35,
             LABEL_HEIGHT)]
 autorelease];
[cell.contentView addSubview:bottomLabel];
//--------------------------------

//
// Configure the properties for the text that are the same on every row
//
bottomLabel.tag = BOTTOM_LABEL_TAG;
bottomLabel.backgroundColor = [UIColor clearColor];
bottomLabel.textColor = [UIColor colorWithRed:0.25 green:0.0 blue:0.0 alpha:1.0];
bottomLabel.highlightedTextColor = [UIColor colorWithRed:1.0 green:1.0 blue:0.9 alpha:1.0];
bottomLabel.font = [UIFont systemFontOfSize:[UIFont labelFontSize] - 2];

//
// Create a background image view.
//
cell.backgroundView =
[[[UIImageView alloc] init] autorelease];
cell.selectedBackgroundView =
[[[UIImageView alloc] init] autorelease];;


topLabel.text = post.title;
bottomLabel.text = @"put something here";


//
// Set the background and selected background images for the text.
// Since we will round the corners at the top and bottom of sections, we
// need to conditionally choose the images based on the row index and the
// number of rows in the section.
//
UIImage *rowBackground;
UIImage *selectionBackground;
NSInteger sectionRows = [tableView numberOfRowsInSection:[indexPath section]];
NSInteger row = [indexPath row];
if (row == 0 && row == sectionRows - 1)  //all
{
    rowBackground = [UIImage imageNamed:@"topAndBottomRow.png"];
    selectionBackground = [UIImage imageNamed:@"topAndBottomRowSelected.png"];
}
else if (row == 0) //top
{
    rowBackground = [UIImage imageNamed:@"topRow.png"];
    selectionBackground = [UIImage imageNamed:@"topRowSelected.png"];
}
else if (row == sectionRows - 1) //bottom
{
    rowBackground = [UIImage imageNamed:@"bottomRow.png"];
    selectionBackground = [UIImage imageNamed:@"bottomRowSelected.png"];
}
else //middle
{
    rowBackground = [UIImage imageNamed:@"middleRow.png"];
    selectionBackground = [UIImage imageNamed:@"middleRowSelected.png"];
}
((UIImageView *)cell.backgroundView).image = rowBackground;
((UIImageView *)cell.selectedBackgroundView).image = selectionBackground;


//[lazy release];

return cell;

提前感谢您的帮助!

Answer 1

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

此行告诉您的应用重用现有的屏幕外单元格(如果存在)。所以基本上你正在使用一个已经存在的单元格并在 subview 已经存在的相同位置向它添加一个新的 subview 。在添加新的 subview 之前，您需要清除现有的 subview 。

更新回复您的评论:

jan 的代码将删除现有的 subview 。您可以将它添加到上面的代码中，因为您的 if 单元格的 else 子句是 nil 语句。

因此，如果您没有可用的可重用单元格 (cell == nil)，您的代码将创建一个新单元格，否则您的代码将从现有单元格中删除旧的 subview :

if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
        cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
    cell.textLabel.font = [UIFont boldSystemFontOfSize:12];
}
else {
    UIView* subview;
    while ((subview = [[[cell contentView] subviews] lastObject]) != nil)
      [subview removeFromSuperview];
}

实现相同目的的另一种更简洁的方法是:

if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
    cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
    cell.textLabel.font = [UIFont boldSystemFontOfSize:12];
}
else {
    [[[cell contentView] subviews] makeObjectsPerformSelector: @selector(removeFromSuperview)];
}