Using dequeueReusableCellWithIdentifier for custom cells - ios

Question

Let's say that I have
- (UITableViewCell*)tableView:(UITableView*) cellForRowAtIndexPath:(NSIndexPath*)indexPath
{
static NSString *cellID = #"Cell Identifier";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellID];
if (!cell)
{
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:cellID];
cell.selectionStyle = UITableViewCellSelectionStyleNone;
}
else
{
return cell;
}
UILabel * nameLabel = [[UILabel alloc] initWithFrame: CGRectMake( 0, 15, box.size.width, 19.0f)];
nameLabel.text = name;
[nameLabel setTextColor: [UIColor colorWithRed: 79.0f/255.0f green:79.0f/255.0f blue:79.0f/255.0f alpha:1.0f]];
[nameLabel setFont: [UIFont fontWithName: #"HelveticaNeue-Bold" size: 18.0f]];
[nameLabel setBackgroundColor: [UIColor clearColor]];
nameLabel.textAlignment = NSTextAlignmentCenter;
[cell addSubview: nameLabel];
}
What is that going to do?
If cell is not nil, and let's say you are at row 5, will it return the cell for row 5 with the exact text labels, etc?
Basically, my question is, if you have custom cells with labels, imageviews, etc. How do you use cellForRowAtIndexPath with dequeueReusableCellWithIdentifier?

Answer 1

You attempt to dequeue a cell. If the attempt failed (cell is nil), then you create a cell and configure it it's views (not the data inside the view). Afterwards, you populate the views with any data or settings that change cell-to-cell. Also, you should add any custom views to the cell's contentView, not the cell itself.
#define NAME_LABEL_TAG 1234
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *cellID = #"Cell Identifier";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellID];
if (!cell) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:cellID];
cell.selectionStyle = UITableViewCellSelectionStyleNone;
UILabel * nameLabel = [[UILabel alloc] initWithFrame: CGRectMake( 0, 15, box.size.width, 19.0f)];
nameLabel.tag = NAME_LABEL_TAG;
[nameLabel setTextColor: [UIColor colorWithRed: 79.0f/255.0f green:79.0f/255.0f blue:79.0f/255.0f alpha:1.0f]];
[nameLabel setFont: [UIFont fontWithName: #"HelveticaNeue-Bold" size: 18.0f]];
[nameLabel setBackgroundColor: [UIColor clearColor]];
nameLabel.textAlignment = NSTextAlignmentCenter;
[cell.contentView addSubview: nameLabel];
}
// Populate views with data and retrieve data for "name" variable
UILabel *nameLabel = (UILabel *)[cell.contentView viewWithTag:NAME_LABEL_TAG];
nameLabel.text = name;
// Return fully configured and populated cell
return cell;
}
If you have a complex cell, it's often easier to create it in Interface Builder and subclass UITableViewCell so you can have custom properties that refer to your Labels, Buttons, etc.

Answer 2

Yes, dequeueing a cell that you have already added those labels to will still have them and their text just as you left it when you created that particular cell.
Create a UITableViewCell subclass, let's call it MyTableViewCell that has properties holding the labels/imageViews/etc that it will need. Once you have either dequeued or alloc init'ed one of your MyTableViewCell, you can then set the text/images/etc on these properties. Like this:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
static NSString *CellIdentifier = #"identifier";
MyTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[MyTableViewCell alloc] initWithStyle:UITableViewStylePlain reuseIdentifier:CellIdentifier];
}
cell.nameLabel.text = name;
cell.imageView.image = anImage;
return cell;
}
One major issue with your method is the conditionals surrounding dequeueing and creating. In your method you only set up the cell the label when it is alloc init'ed (you instantly return a dequeued cell without formatting it). However, you want this set up to occur for both dequeued and manually instantiated cells. Notice how this happens in my method, the return statement is at the very bottom. This will ensure that both created and reused cells have the appropriate data.
EDIT: One important thing I left out, you will instantiate the properties of your cell in its initWithStyle: reuseIdentifier: method and add them as subviews to the cell. This is so when you go to set the text of the label (or whatever) in your cellForRowAtIndexPath method, it has already been created. Basically the cell manages creating its own views and the UITableView delegate only has to worry about filling those views with data.

Answer 3

UITableView at first ask you for number of expected cells. Then it's load through - tableView:cellForRowAtIndexPath: method cells which is will be displayed + some to smooth scrolling, more objects it doesn't create. Created objects (by user) stored in table view and you can access not used through dequeueReusableCellWithIdentifier: method. TableView ask user for modifying currently created cells when it's scrolling. If here is free object - take it from dequeueReusableCellWithIdentifier: another - create new one.

Answer 4

in order to not have to allocate every single table cell, a table only allocates what is needed. If only 8 cells fit on a page then a table view will only allocate 8 cells or 9 i don't remember if it has a padding. When you scroll a tableview and the cell goes off the page the cell is queued up to be re-used, instead of re-allocating a new cell the table view takes an existing one this process is called dequeue-ing. When you make/allocate your cells you give it an identifier, this identifier is used to retrieve a cell that is marked with that string.

Answer 5

You can either use this
- (id)dequeueReusableCellWithIdentifier:(NSString *)identifier forIndexPath:(NSIndexPath *)indexPath
This is available after iOS 6
Or you can also register your class somewhere in ViewDidLoad and use ResuseIdentifier, so you don't have to write the ResuseIdentifier part in CellForRowAtIndexpath
- (void)registerClass:(Class)cellClass forCellReuseIdentifier:(NSString *)identifier NS_AVAILABLE_IOS(6_0)
Hope this helps you...