UITableView experiences choppy scrolling when cell has a UIImageView subview - iphone

Question

This has been driving me crazy for the better part of the day.
I've got a UITableView with UIImageViews. These imageviews load a locally saved PNG-file in the cellForRow-function of the tableview, and this works fine except the tableview will stop scrolling for a fraction of a second when a cell with an image in it scrolls into sight so to speak. I've trawled StackOverflow and google for an answer but I've come up short - so any help will be greatly appreciated.
Here is my code for the CellForRow-function:
- (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

There are a couple of changes to be made, first off the UIImageViews should be added when the cell is generated, rather than every time tableView:cellForRowAtIndexPath: is hit (as #Vishy suggests). Secondly you should cache the images you are loading from the documents directory ([UIImage imageNamed:] does this automatically for bundle resources).
#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
As a general rule, tableView:cellForRowAtIndexPath: is a method you need to get out of fast, so loading images from disk should be avoided wherever possible.

Answer 2

Change the code as per below...
- (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

There is also a delay the first time each UIImage is displayed. See this post for details on prompting the work at cache time rather than display time:
Setting image property of UIImageView causes major lag