UITableView - cell images changing while scrolling - ios

Hi my problem is that when I scroll TableView the image will appear in a wrong cell, after a few seconds the correct image appears.
here is my code
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
static NSString *CellIdentifier = #"Cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
// Configure the cell...
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] ;
}
[cell setOpaque:NO];
[cell setBackgroundColor: [UIColor clearColor]];
PlaceData *data = [tableData objectAtIndex:indexPath.row];
UILabel *nameLabel = (UILabel *)[cell viewWithTag:100];
UILabel *sciNameLabel = (UILabel *)[cell viewWithTag:200];
UIImageView *thumbnailImageView = (UIImageView *)[cell viewWithTag:300];
nameLabel.text = data.name;
sciNameLabel.text = data.scientific_name;
// get a dispatch queue
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
// this will start the image loading in bg
dispatch_async(concurrentQueue, ^{
NSURL *urlToPicture = [NSURL URLWithString:[NSString stringWithFormat:#"%#", data.thumbnail]];
NSData *imgData = [NSData dataWithContentsOfURL:urlToPicture options:0 error:nil];
// This will set the image when loading is finished
dispatch_async(dispatch_get_main_queue(), ^{
UIImage *tmpImage = [[UIImage alloc] initWithData:imgData];
thumbnailImageView.image = tmpImage;
//dispatch_release(concurrentQueue);
});
});
return cell;
}
please help me

You can try adding following code to your cellForRowAtIndexPath -
1) Assign an index value to your custom cell. For instance,
cell.tag = indexPath.row
2) On main thread, before assigning the image, check if the image belongs the corresponding cell by matching it with the tag.
dispatch_async(dispatch_get_main_queue(), ^{
if(cell.tag == indexPath.row) {
UIImage *tmpImage = [[UIImage alloc] initWithData:imgData];
thumbnailImageView.image = tmpImage;
}});
});

You are reusing old cells, which is a good thing. However, you are not initializing the image in the cell's image view, which is not such a good thing. What you're describing happens because an old cell, with an image that was already loaded for that cell, is used for the new cell. You are then loading that cell's image in the background (which, again, is good) but it takes a few moments for that image to fully load. In the meantime, the image that was already loaded on the old cell, is displayed (and that's the reason you're seeing a wrong image in that cell, for a few moments).
The solution? add either
thumbnailImageView.image = nil
or
thumbnailImageView.image = someDefaultImageWhileYourRealOneLoads
right before dispatch_queue_t concurrentQueue ....
That way, you won't see the old (irrelevant) image while the real one loads.
I hope this helps. Good luck.

As becauase your ImageView is being loaded in an async dispatch call which is NOT on the main thread and is being called in some other thread so there is a delay in fetching the data from the URL and then converting it to an UIImage. THis process takes a bit of time as you know but you are scrolling the tableview in a faster rate. And as you know cellForRowAtIndexPath reuses any cell that is out of the window so the cell that is being reused might NOT fetched the imagedata that it WAS TO RETRIEVE previously when it was in the Window. Thus it loads the wrong data and then again when async is fired for that specific cell the cell loads that image but there comes the delay.
To overcome this feature as Chronch pointed it out u can leave the imageview as nil OR you can use AFNetworking's own UIImageView catagory which has a superb class to load imageview images quite elegantly
I'll leave u a link to it AFNetworking

I would do all my data binding at - tableView:willDisplayCell:forRowAtIndexPath: only because at cellForRowAtIndexPath your cell hasn't been drawn yet.
Another solution you can use is AFNetworking like someone else mentioned before me.

Swift 3.0
DispatchQueue.main.async(execute: {() -> Void in
if cell.tag == indexPath.row {
var tmpImage = UIImage(data: imgData)
thumbnailImageView.image = tmpImage
}
})

cell.thumbnailimages.image=nil
cell.thumbnailimages.setImageWith(imageurl!)
I think these two lines solve your problem.

Related

cell Background images are downloaded but take 5 seconds to appear right after finishing download

I want a UITableView to appear instantly after being tapped. Inside each cell of the UITableView there is a background image which is taken from Flickr, so the images must be downloaded in a background thread.
So everything works perfectly, the images download successfully in a separate thread. The problem though is that you must wait 5 seconds after they finish downloading for the images to actually appear within each cell:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"rightMenuCell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];
UILabel *lblTitle = (UILabel *)[cell viewWithTag:1];
UIImageView *imgCube = (UIImageView *)[cell viewWithTag:2];
UIActivityIndicatorView *spinnerFlickr = (UIActivityIndicatorView *)[cell viewWithTag:4];
// Configure the cell...
NSDictionary *region = self.regions[indexPath.row];
//default hashtag is life
NSString *hashTag;
cell.detailTextLabel.text = [region valueForKeyPath:#"name"];
cell.detailTextLabel.hidden = TRUE;
lblTitle.text = [region valueForKeyPath:#"name"];
NSString *regionType = [region valueForKeyPath:#"region"];
if ([regionType isEqual: #"neighborhood"]) {
hashTag = #"houses";
cell.textLabel.text = #"neighborhood";
} else if ([regionType isEqual: #"locality"]) {
hashTag = #"urban";
cell.textLabel.text = #"locality";
} else {
hashTag = #"life";
cell.textLabel.text = #"administrative_area_level_2";
}
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),
^{
UIImageView *imgBackground = (UIImageView *)[cell viewWithTag:3];
[imgBackground setClipsToBounds:YES];
NSLog(#"loading image data...");
NSString *flickrURL = [NSString stringWithFormat:#"http://domfa.de/get_image/?text=%#&lat=%f&long=%f", hashTag, self.latitude, self.longitude];
NSData *image = [[NSData alloc] initWithContentsOfURL: [NSURL URLWithString: flickrURL]];
UIImage *cellBG = [[UIImage alloc] init];
cellBG = [UIImage imageWithData:image];
imgBackground.image = cellBG;
[imgBackground reloadInputViews];
NSLog(#"images loaded!");
imgBackground.image = cellBG;
[spinnerFlickr stopAnimating];
});
return cell;
}
Basically each cell gets the background image downloaded async but even after the download is complete I either have to tap a cell or wait 5 seconds and then 2/3 of the cells will then get an image loaded.
dispatch_get_global_queue() fetches a background queue, not the main queue. You can only do UI-related things in the main queue. Your image fetch should look something like this:
dispatch_async(dispatch_get_global_queue(…) ,
^{
// Do non-UI-related things like fetch your UIImage from the network
UIImage *image = [self fetchAndCacheImageAtURL:imageURL] ;
dispatch_async(dispatch_get_main_queue() ,
^{
// Assign your UIImage to your UIImageView
UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath] ;
if( cell )
cell.imageView.image = image ;
}) ;
}) ;
Notice that in the second dispatch_async() I fetch the cell again using -cellForRowAtIndexPath:. This is because by the time this code has run, the original cell may have been reused for a different index path (because, say, the user has been scrolling). In fact, there may be no cell at the current index path because that index path may have scrolled off screen. This, plus the check to see if cell is nil, ensures you're setting the image on the right cell for that index path.

Fetching data from url and displaying it in a tableview

In my app i have to fetch images from a json webservice and display them in a table view. While scrolling downwards all the images come correct order but while i revert backwards all the images get over each other. I am using [ tableview reloaddata] then too its happening.
This happened to me in my app, where I have a leaderboards table. It is definitely because the tables are being reused.
The easiest way to fix this is simply setting the cell's image to nil first, then downloading and displaying the new image.
There are definitely other ways to do this, but I can definitely vouch for this one, as I use it in my own app, and it works great!!
Here's how I solve the problem.
-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString * cellIdentifier = #"Cell";
UITableViewCell * cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
if (!cell)
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];
// Make sure you set the image for the cell to nil first
[cell.imageView setImage:nil];
// Then load your new image using the url and display it
dispatch_queue_t backgroundQueue = dispatch_queue_create("com.example.imagedownloadqueue", NULL);
dispatch_async(backgroundQueue, ^(void){
NSUrl * url = [NSUrl UrlWithString:#"www.example.com"]; // Url of image to download
UIImage * image = [UIImage imageWithData:[NSData dataWithContentsOfUrl:url]];
if (image)
{
dispatch_async(dispatch_get_main_queue(), ^(void) {
[cell.imageView setImage:image]; // Display image after it is downloaded
});
}
});
return cell;
}

Scrolling breaks in uitableview

I am having a problem viewing my tableview when i get the data of my cells from a server. If i do not use photos there is no breaks in the scrolling, but i want to use the images also. Can anyone knows how can i resolve this? I am getting the data from a plist in my server.
Here is the code of the image that makes the scrolling breaks (i am using custom style cell)
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"Cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
}
NSURL *URL = [[NSURL alloc] initWithString:[[self.content objectAtIndex:indexPath.row] valueForKey:#"imageName"]];
NSData *URLData = [[NSData alloc] initWithContentsOfURL:URL];
UIImage *img = [[UIImage alloc] initWithData:URLData];
UIImageView *imgView = (UIImageView *)[cell viewWithTag:100];
imgView.image = img;
....
If you mean that the scrolling stops and starts, this might be because if the images are loaded from a server (which might take a noticeable amount of time to do), executing on the main thread causes freezing.
If this is the case, the fix is to fetch the images in another thread. Fortunately iOS has a fairly easy to use multithreading system called Grand Central Dispatch.
Here's some example code:
dispatch_queue_t q = dispatch_queue_create("FetchImage", NULL);
dispatch_async(q, ^{
/* Fetch the image from the server... */
dispatch_async(dispatch_get_main_queue(), ^{
/* This is the main thread again, where we set the tableView's image to
be what we just fetched. */
});
});

UITableView Custom cells, built of UIViews, can be concurrent?

I am building my cellViews like this:
- (UITableViewCell *)cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString* cellIdentifier=#"cell";
UITableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
if(cell == nil)
{
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier] autorelease];
}
UIImageView cellView = [[UIImageView alloc] initWithFrame:rectCellFrame];
NSError* error=nil;
NSData* imageData = [NSData dataWithContentsOfURL:imageArray[indexPath.row] options:NSDataReadingUncached error:&error];
UIImage* theImage= [UIImage ImageWithData:imageData];
[cellView setImage:theImage];
[cell addSubView:cellView];
.
.
.
.
[cell addSubView:moreViews];
}
Since the loading time (even when the images are cached) is very slow, I need to make this concurrent. But I would like to still be using my code with UIViews/UIImageViews.
Is there a way for me to show a placeholder and when relevant, ie cellView is finished building from all subviews, update the image instead of the placeholder?
Sure. You can set up all the heavy slow code in a asynchronous task. It's often down when images need to be downloaded. I'm sure it's covered in at least 1 of the WWDC videos on Table Views, but I've no clue which one or how old it would be by now.
- (UITableViewCell *)cellForRowAtIndexPath:(NSIndexPath *)indexPath {
// place holder image for the moment
[cellView setImage:placeHolderImage];
// run code to get the real image in asynchronous task
dispatch_async(self.contextQueue, ^{
UIImage *realImage = [thingy imageFromTimeConsumingTask];
// update cell on main thread (you need to do all UI stuff on main thread)
dispatch_async(dispatch_get_main_queue(), ^{
[cellView setImage:realImage];
});
});
}

GCD jumbles the data on scrolling tableview

I'm using grand central dispatcher to load images from server but when i scroll the table the data, i.e. images, jumbles - means 1st image comes to some other place and like wise other images do.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"ItemImageCellIdentifier";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] ;
cell.selectionStyle=UITableViewCellSelectionStyleNone;
}
NSDictionary *item=[responseDictionary objectAtIndex:[indexPath row]];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0ul);
NSString *actionForUser=[item objectForKey:#"action"];
objc_setAssociatedObject(cell,
kIndexPathAssociationKey,
indexPath,
OBJC_ASSOCIATION_RETAIN);
dispatch_async(queue, ^{
if([actionForUser isEqualToString:like])
{
NSURL *url = [NSURL URLWithString:[item objectForKey:#"user_image"]];
NSData *data1 = [[NSData alloc] initWithContentsOfURL:url];
UIImage *image1 = [[UIImage alloc] initWithData:data1];
//userProfileimage
UIButton *userImageButton = [[UIButton alloc] initWithFrame:CGRectMake(10,5, 40,40)];
userImageButton.contentVerticalAlignment = UIControlContentVerticalAlignmentCenter;
userImageButton.contentHorizontalAlignment = UIControlContentHorizontalAlignmentCenter;
[userImageButton setBackgroundImage:image1 forState:UIControlStateNormal];
[userImageButton addTarget:self
action:#selector(userImageButtonclick:)
forControlEvents:UIControlEventTouchDown];
[cell.contentView addSubview:userImageButton];
}
});
return cell;
}
This is because by the time your async method has finished, cell has been recycled and used for a different index path, so you're updating the wrong cell.
At the point of update, get the cell reference by using the tableview's (not the data source method) cellForRowAtIndexPath: method. This will return the correct cell, or nil if the cell isn't on the screen any more. You can update this cell safely.
You should probably be adding the image data to your model as well so you aren't downloading it repeatedly.
As an example, instead of this line:
[cell.contentView addSubview:userImageButton];
You should have something like this:
UITableViewCell *cellToUpdate = [tableView cellForRowAtIndexPath:indexPath];
[cellToUpdate.contentView addSubview:userImageButton];
There are further problems with your code; you are not caching the images, you will be adding this subview every time this cell comes on screen, and if the cell is reused for a case where it doesn't need the button, the button will still be present. I have only addressed the "GCD jumbling" as described in your question.

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