iPhone - dequeueReusableCellWithIdentifier usage

Question

Hi,

I'm working on a iPhone app which has a pretty large UITableView with data taken from the web, so I'm trying to optimize its creation and usage.

I found out that dequeueReusableCellWithIdentifier is pretty useful, but after seeing many source codes using this, I'm wondering if the usage I make of this function is the good one.

Here is what people usually do:

UITableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:@"Cell"];

if (cell == nil) {
  cell = [[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:@"Cell"];

// Add elements to the cell
return cell;

And here is the way I did it:

NSString identifier = [NSString stringWithFormat:@"Cell %d", indexPath.row]: // The cell row

UITableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:identifier];

if (cell != nil)
  return cell;

cell = [[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:identifier];
// Add elements to the cell
return cell;

The difference is that people use the same identifier for every cell, so dequeuing one only avoids to alloc a new one.

For me, the point of queuing was to give each cell a unique identifier, so when the app asks for a cell it already displayed, neither allocation nor element adding have to be done.

In fine I don't know which is best, the "common" method ceils the table's memory usage to the exact number of cells it display, whislt the method I use seems to favor speed as it keeps all calculated cells, but can cause large memory consumption (unless there's an inner limit to the queue).

Am I wrong to use it this way? Or is it just up to the developper, depending on his needs?

Answer 1

I Think the first one ist the best (and as you said common) way to implement a UITableView. With your second way there will be memory allocated for every new cell which is displayed and no memory will be reused.

Answer 2

The purpose of dequeueReusableCellWithIdentifier is to use less memory. If the screen can fit 4 or 5 table cells, then with reuse you only need to have 4 or 5 table cells allocated in memory even if the table has 1000 entries.

In the second way there is no reuse. There is no advantage in the second way over just using an array of table cells. If your table has 1000 entries then you will have 1000 cells allocated in memory. If you are going to do that you would put them in an array and just index the array with the row number and return the cell. For small tables with fixed cells that may be an reasonable solution, for dynamic or large tables it is not a good idea.

Answer 3

As for cell identifier- Instead of just using "cell" for the identifier, and instead of using a unique identifier like the OP, could you use a "type-identifier"? For example, if my table had 3 types of cells- one with a very complicated sub-layout, one with just Style1, and one with Style2, should I identify those three all separately and then just rebuild them?

For example:

-(UITableViewCell*)tableView:(UITableView*)tableView cellForRowAtIndexPath:(NSIndexPath*)indexPath{

NSString ident = @"";
if(indexPath.section == 0) ident= @"complicated";
if(indexPath.section == 1) ident= @"style1";
if(indexPath.section == 2) ident = @"style2";

UITableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:ident];


if(cell == nil){


   if(ident == @"complicated"){
      cell = [[[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:ident] autorelease]; 
     // do excessive subview building
   }
   if(ident == @"style1"){
      cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyle1 reuseIdentifier:ident] autorelease]; 
   }

   if(ident == @"style2"){
      cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyle2 reuseIdentifier:ident] autorelease]; 
   }


}
if(ident == @"complicated"){
   // change the text/etc (unique values) of our many subviews
}
if(ident == @"style1"){
  [[cell textLabel] setText:@"Whatever"];
}
if(ident == @"style2"){
  [[cell textLabel] setText:@"Whateverelse"];
}

return cell; 

}

now this code probably won't run because i wrote it here but hopefully you get the idea.

I don't think Apple would have created the whole reusable cell idea with identifiers if they wanted all the identifiers to be "cell", don't you think?