I have a small problem, which seems to be really classic (mixed up cells when reused by dequeueReusableCellWithIdentifier) but I have already looked at similar questions and can't find an answer.
So, classic context: I have a UITableView with protype cells in the storyboard, one with an image (asynchronously fetched) and one with a UITextView.
Let's focus on the text cells as I get problems on both and so we know the network is not the cause.
When I scoll down and up again, cells get mixed up. I think it is because I set the text value according to my data inside the cellForRowAtIndexPath function but I'm not sure.
Here are my cells in my example:
---------------
| 1. "TEXT1" |
---------------
| |
| 2. IMAGE2 |
| |
---------------
| 3. "TEXT3" |
---------------
| 4. "TEXT4" |
---------------
| |
| 5. IMAGE5 |
| |
---------------
I scroll all the way down, everything is fine. When I bounce at bottom, cells 3 and 4 are reloaded (I put a NSLog in cellForRowAtIndexPathfunction). First cell 4 is reloaded, then cell 3 (according to the log).
The problem is, cells 3 and 4 both end up with "TEXT3" as value ! I mean, if I NSLog the value I give to the UITextField.text in the function, they don't have the same text value, but on the simulator's screen, they have.. ?
Here is my code:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
// Getting data for row from indexPath
Data *data = [self.dataArray objectAtIndex:indexPath.row];
NSString *cellIdentifier = [NSString stringWithFormat:#""];
if (data.type == 1) { // Text type
cellIdentifier = #"TextCell";
}
else if (data.type == 2){
cellIdentifier = #"PhotoCell";
}
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier forIndexPath:indexPath];
// Getting various cell elements
if (data.type == 1) {
// Text is 1000
UITextView *textView = (UITextView *)[tableView viewWithTag:1000];
textView.text = data.text;
}
else if (data.type == 2){
//STUFF FOR PHOTO TYPE OF DATA
}
return cell;
}
By the way, it can be useful to say that I get this error in the simulator on iOS6 but not on iOS7 (but maybe that's just different memory management..)
Thanks a lot for your help!
I had the same issue, and this is how I solved it.
In my CustomTableViewCell, I had the init method where I was setting values for labels etc.
so I added this condition in init.
code:
(void) initWithCellLabelString : (NSString *) cellLabelString
{
for (id subview in self.contentView.subviews) {
[subview removeFromSuperview];
}
//rest of init
}
Hope that helps.
EDIT
I better way of doing this is to override prepareForReuse in Custom table view cell class and set the values to nil. It might not be the recommended method, but I haven't experienced any performance issue.
I actually found the solution myself by testing...
My problem was that to access the text view in my prototype cell, I gave it a tag in the storyboard.
But then, when I wanted to edit the text in cellForRowAtIndexPath, I was using [tableView viewWithTag:myTag] while I should have been using [cell viewWithTag:myTag] , so I was editing, I guess, the last one?
All in all, it's good now.. And I was just looking at the wrong place since I was SURE it was about reusing the wrong cell or smth..
If anyone knows why it didn't happen in ios7, please share !
Related
I have a UITableView, and custom cells on it. On cell I have a UILabel, but before I set text to UILabel I did really hard work on text...like find the text in another text, highlight some words on it, and only then I set it to label. So when I scroll my list, it has delay because of this hard work. Any idea how to improve performance ? Maybe to do all hard work in another thread ??
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:
(NSIndexPath *)indexPath {
static NSString *customCellIdentifier =
#"CellIdentifier";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:
customCellIdentifier];
if (cell == nil) {
NSArray *nib = [[NSBundle mainBundle] loadNibNamed:#"CustomTableRow"
owner:self options:nil];
if (nib.count > 0) {
cell = self.customTableRow;
}
}
self.myLabel.text = [self giveMeTheTextThatINeed];
return cell;
}
[self giveMeTheTextThatINeed] - did a hard work on text that takes some time.
Make a new thread for every cell. this thread calls [self giveMeTheTextThatINeed:indexPath], and resets the label(s) in the cell. I'm assuming you can't get your data any faster, so you want to maintain the scrolling in the table and spin the hard work out to the thread. When the thread is finished, update the cell. You see this a lot in cells with a thumbnail image where the thumbnail only gets uploaded after a while, and is blank or has a placeholder there first.
Any way for you to precompute the values you'll need? In other words, start doing your "hard work" (in another thread) when the app starts, and store it somewhere so that, if it's ready, you can just grab it when the table view asks for it. It's hard to answer without more detail about what the hard work is and how much data we're talking about.
I don't know about just doing the hard work on another thread as you suggested, since you still have to give something to tableView:cellForRowAtIndexPath:. I suppose you could return some kind of template cell at first, and then update it with reloadRowsAtIndexPaths:withRowAnimation: when the hard work is done.
I believe the method giveMeTheTextThatINeed needs to take the current cell as one of the parameters (or another parameter dependent on the cell content), e.g.: [self giveMeTheTextThatINeed:indexPath]. Otherwise you could store the text as an instance variable and set it in all cells from that variable.
So, with that in mind, the easiest way is to store the result of the computation in an additional dictionary, where indexPath (or the other parameter) would be the key:
self.myLabel.text = [self->myDictionary objectForKey:indexPath];
Now, you could either pre-populate that dictionary before the cells are drawn (e.g. in viewWillAppear), or cache them once they are calculated so that they are not recalculated when the cells are scrolled, e.g.:
NSString* calculatedText = [self->myDictionary objectForKey:indexPath];
if(calculatedText == nil)
{
calculatedText = [self giveMeTheTextThatINeed:indexPath];
[self->myDicationary setValue:calculatedText forKey:indexPath];
}
self.myLabel.text = calculatedText;
This is related to another question of mine which wasn't answered in a helpful way (message when a UITableView is empty).
I'm trying to show an UIImage graphic that says You haven't saved any bookmarks over an UITableView when it's empty. I have NSNotification set-up so that when bookmarks are added or deleted, a message is sent so that the UITableView can be updated.
I've been trying to do it with this code. Why won't this work?
- (void)bookmarksChanged:(NSNotification*)notification
{
[self.tableView reloadData];
UIImageView* emptyBookmarks = [[UIImageView alloc] initWithFrame:CGRectMake(75, 100, 160, 57)];
emptyBookmarks.alpha = 1;
emptyBookmarks.image = [UIImage imageNamed:#"emptyBookmark.png"];
[self.view addSubview:emptyBookmarks];
[emptyBookmarks release];
if ([self.dataModel bookmarksCount] == 0)
{
emptyBookmarks.alpha = 1;
}
else
{
emptyBookmarks.alpha = 0;
}
}
I'm probably approaching this the wrong way... But if salvageable, what am I doing wrong?
When I initially have an empty bookmarks tableview, there's no image displayed. After I add a bookmark and then delete it, the image shows. Grrh.
Another way (and IMO the correct way) to do this is to manipulate the backgroundView property on the UITableView.
While making a single cell with a custom image cell would certainly works, I think it overly complicates the logic of your UITableViewController's data source. It feels like a kludge.
According to UITableView documentation:
A table view’s background view is automatically resized to match the
size of the table view. This view is placed as a subview of the table
view behind all cells , header views, and footer views.
Assigning an opaque view to this property obscures the background color
set on the table view itself.
While you probably don't want to just set it to your UIImageView, it is very easy to make a UIView that contains the UIImageView that you want.
Well first off if you were going to do it that way, you would need to reload the tableView after updating the image or model etc. and not before.
But you are probably making things more complicated than they need to be!
Why not just check to see if the data for section 0 and indexPath.row 0 are empty and if so in cellForRowAtIndexPath display a text message accordingly.
// First make sure there is always one row returned even if the dataModel is empty.
-(NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
NSInteger numRows = 0;
if ([self.dataModel lastObject]) {
// Return the number of rows in the section.
numRows = [self.dataModel count]; // etc.
}
if (numRows < 1) numRows = 1;
return numRows;
}
// Then display the data if there is some, otherwise a message if empty.
-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
static NSString *CellIdentifier = #"Cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:CellIdentifier];
}
if ([self.dataModel lastObject]) {
// setup the cell the normal way here.
} else { // the datasource is empty - print a message
cell.textLabel.text = nil;
cell.detailTextLabel.text = NSLocalizedString(#"You haven't saved any bookmarks", #"");
cell.detailTextLabel.textColor = [UIColor colorWithRed:0/255.0 green:0/255.0 blue:0/255.0 alpha:0.7];
cell.accessoryType = UITableViewCellAccessoryNone;
}
return cell;
}
Are you sure [self.dataModel bookmarksCount] is equal to 0 ?
While I agree that you are probably going about this the wrong way,
your image is allocated and added in your bookmark changed, your notification does not trigger when there are no bookmarks initially. Hence you don't see the image. Call the bookmar changed when your table view inits or appears.
Probably the best way to achieve this is to perform a check in your numberOfRowsInSection method to return 1 if your data source is empty. Then in cellForRowAtIndexPath check if your data source is empty and if it is, create a custom cell that contains whatever you want. In heightForRowAtIndexPath you need to return your custom cell height if your datasource is empty, but only if you want the cell larger than the default. At least that is how I would approach it.
when bookmarks count is nil add one to your row method:
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section{
int c;
c = bookmarks.count;
if(c == 0){
c = 1;
}
return c;
}
and then the same check again in your cellforrowatindexpath.
Another thing to be aware of in this situation is that if you're using core data and you're datasource is feeding off an entity, you will want to make sure your model matches. You can get some weird side-effect behavior in certain situations. This is especially true if you allow editing and core data has an empty model but you're tableview is still showing a cell.
I'm using a custom UITableViewCell. They load up great, but when they get reused instead of replacing the text in the labels, they are some how writing over the top of them. Any ideas how to stop this behaviour?
I assume I'm not reseting the view correctly before it gets reused. I'm currently empty the labels so they have just a blank #"" string. But I still get the old text plus the new (very messy).
I'm certain there's an obvious solution to this (presently i just don't reuse the cell, but this isn't best practice and is slow on old devices), so if someone can help I'd be very grateful.
Thanks
ED
As requested here is the method for amending the cell
static NSString *CellIdentifier = #"Cell";
TransactionCellView *cell = (TransactionCellView *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if(cell == nil) {
[[NSBundle mainBundle] loadNibNamed:#"TransactionCellView" owner:self options:nil];
cell = tblCell;
}
[cell resetCell]; // clears the labels
[cell setData:[dataArray objectAtIndex:[indexPath row]]; // replaces the data and updates the labels
return cell;
}
Solved this issue!!! Amazingly simple when you know what the problem is/was.
I was using IB and "Clear Graphics Context" was not selected on my UILabels. So that is why the next text was just overlaying the old.
THanks guys for trying to help.
I suspect that you are adding the label as a subview programatically in your cellForRowAtIndexPath method. You need to make sure that you are not adding that subview every time the cell is recycled. Instead create the label only when creating a new cell (not when recycling a cell) then assign a tag value to the label and then in the future, when it gets recycled, retrieve the label by tag value and change its text.
You are correct in that the cells are being reused. Your code should be set up roughly using the following pattern:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
NSString* identifier = #"someidentifier";
UITableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:identifier];
if(!cell) {
// create the cell and add all subviews to it
} else {
// update the cell and access appropriate subviews to modify what is displayed
}
return cell;
}
The cell will be created the first time the identifier is used. For all subsequent requests, the cell is pulled from the UITableView cache (via dequeueReusableCellWithIdentifier), and you can then access its subviews either by tag, index, type, or whatever mechanism you choose.
Somewhat related is that you can have multiple cell identifiers, which allows you to create multiple instances of different cells depending on the data that you have. In one of my projects, I have 4 different cells, each dependent upon the number of lines of data that they will display (anywhere between 1 and 4). This helps ensure a smooth scrolling experience regardless of how many lines the cell has since the renderer doesn't have to worry about dynamically changing the height of the cell on the fly.
Try this..
- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath {
for (UIView* subView in cell.contentView.subviews)
{
[subView removeFromSuperview];
}
// Your Code to customize cell goes here.
}
In my application I parsed the data through NSXMLParser and made separate class to store that data from which i usually display the data. Everything works fine in simulator except the the title which is display in table cell with image. Images appears properly but the title not appear properly.
This is my code:-
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"Cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
[[NSBundle mainBundle] loadNibNamed:#"TableCell3" owner:self options:NULL];
cell = nibLoadedCell;
}
NewsInfo *aNewsInfo = [appDelegate.newsArray objectAtIndex:indexPath.row];
titleLabel.text = aNewsInfo.title;
imageLabel.image = aNewsInfo.smallImageData;
return cell;
}
and in this way i configured my TableCell3.xib
Now i want my title data in 2 rows in label of the TableCell3.
I already used both the way through xib attributes settings or through code.
Code which i used instead of this xib attributes settings are:-
titleLabel.lineBreakMode = UILineBreakModeWordWrap;
titleLabel.numberOfLines = 2;
titleLabel.text = aNewsInfo.title;
Now Can any one help me to figure out this prob i want to display my title in two rows of the table view cell.
Thanks in Advance.
My Data is showing in this way...![enter image description here][2]
So i want to display this title fully it show's partially right now...
i want to display in two lines...
My guess is that you need to enlarge the label height.
Maybe the label height i not large enough to display 2 lines of text and that's could be the reason that you see only 1 line.
in interface builder try to enlarge the label height.
and maybe i am wrong ?
Good luck
Shani
Adjust frame of an UiLabel to have more space for two lines or decrease font size.
I'm extremely confused by the proper behavior of UITableView cell rendering. Here's the situation:
I have a list of 250 items that are loading into a table view, each with an image. To optimize the image download, I followed along with Apple's LazyTableImages sample code... pretty much following it exactly. Really good system... for reference, here's the cell renderer within the Apple sample code:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
// customize the appearance of table view cells
//
static NSString *CellIdentifier = #"LazyTableCell";
static NSString *PlaceholderCellIdentifier = #"PlaceholderCell";
// add a placeholder cell while waiting on table data
int nodeCount = [self.entries count];
if (nodeCount == 0 && indexPath.row == 0)
{
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:PlaceholderCellIdentifier];
if (cell == nil)
{
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle
reuseIdentifier:PlaceholderCellIdentifier] autorelease];
cell.detailTextLabel.textAlignment = UITextAlignmentCenter;
cell.selectionStyle = UITableViewCellSelectionStyleNone;
}
cell.detailTextLabel.text = #"Loading…";
return cell;
}
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil)
{
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle
reuseIdentifier:CellIdentifier] autorelease];
cell.selectionStyle = UITableViewCellSelectionStyleNone;
}
// Leave cells empty if there's no data yet
if (nodeCount > 0)
{
// Set up the cell...
AppRecord *appRecord = [self.entries objectAtIndex:indexPath.row];
cell.textLabel.text = appRecord.appName;
cell.detailTextLabel.text = appRecord.artist;
// Only load cached images; defer new downloads until scrolling ends
if (!appRecord.appIcon)
{
if (self.tableView.dragging == NO && self.tableView.decelerating == NO)
{
[self startIconDownload:appRecord forIndexPath:indexPath];
}
// if a download is deferred or in progress, return a placeholder image
cell.imageView.image = [UIImage imageNamed:#"Placeholder.png"];
}
else
{
cell.imageView.image = appRecord.appIcon;
}
}
return cell;
}
So – my implementation of Apple's LazyTableImages system has one crucial flaw: it starts all downloads for all images immediately. Now, if I remove this line:
//[self startIconDownload:appRecord forIndexPath:indexPath];
Then the system behaves exactly like you would expect: new images load as their placeholders scroll into view. However, the initial view cells do not automatically load their images without that prompt in the cell renderer. So, I have a problem: with the prompt in the cell renderer, all images load at once. Without the prompt, the initial view doesn't load. Now, this works fine in Apple sample code, which got me wondering what was going on with mine. It's almost like it was building all cells up front rather than just the 8 or so that would appear within the display. So, I got looking into it, and this is indeed the case... my table is building 250 unique cells! I didn't think the UITableView worked like this, I guess I thought it only built as many items as were needed to populate the table. Is this the case, or is it correct that it would build all 250 cells up front?
Also – related question: I've tried to compare my implementation against the Apple LazyTableImages sample, but have discovered that NSLog appears to be disabled within the Apple sample code (which makes direct behavior comparisons extremely difficult). Is that just a simple publish setting somewhere, or has Apple somehow locked down their samples so that you can't log output at runtime?
Thanks!
You certainly should not have an actual UITableViewCell instance for every row in the table. You should only see a few more instances than are visible in the UI. That is where your problem is. It doesn't have anything to do with the loading of images.
The only time I've seen a large number of cells instantiated when the cells where dequeued was when a coder had altered the frame of the tableview to make it much larger than the screen. The tableview retains enough cells from being dequeued to carpet its own frame regardless of what is visible. If the frame is to big then you get a lot of cells in queue.
NSLog does work in Apple examples so if you can't get NSLog output you've got something weird going on with the dev tools themselves.
You might want to shutdown Xcode and the simulator and restart and see if that clears up the odd behavior.
Oh my... mustISignUp is absolutely correct in saying "you definitely have a deeper underlying problem". I have variable-height table rows, and I was doing all height calculation and data storage on the rows themselves rather than on the data model that populated them. As a result, ALL cells were being created and populated by my heightForRowAtIndexPath method which was reading cell height from the cell objects. SO – lesson learned.
Thanks mustISignUp, and I love your username.
NSLog definitely isn't disabled within the Apple sample code. I don't know why you can't see it but you definitely have a deeper underlying problem.
Anyway, for your comparison:- if you have 6 rows on screen -tableView: cellForRowAtIndexPath: is called 6 times with index [0, 0] - [0, 5].
So, is that what you are seeing? How many times is -cellForRowAtIndexPath being called?