Reusable Cells in UITableView

转自：http://www.eduoliveros.com/2009/07/reusable-cells-in-uitableview.html

It is a really well known issue that you need to “reuse” the cells to increase the performance of the table… what it is not so well explained is how this works in practice.

This is a snap of the code for reusing a UITableViewCell or loading the cell from a NIB file when there is no cell available for reusing:

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

static NSString *MyIdentifier = @"MyIdentifier";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];
if (cell == nil) {
NSArray *nib = [[NSBundle mainBundle] loadNibNamed:@"TestCell" owner:self options:nil];
cell = [nib objectAtIndex:1];
}
// Set up the cell
return cell;
}

The first time I saw this I thought the cell was loaded from the Nib file just once and then reused all the time, WRONG!!.
This way of thinking was causing me a lot of problems, basically how was the Table able to manage all the complexity of the views’ behaviour with just one cell… But once I read how all this works in fact, all came clear, crystal clear.

Some facts:
1. All the cells that are visible in the Table have its one UITableViewCell.
2. The UITableView only put cells in the reusable queue when they go outside the visual window.
3. In the first time, all the visible cells in the table are loaded using the Nib file (7,8, 10 times, depending on the height of the cells).
4. Once you start scrolling the table is when UITableView starts to put UITableViewCells in the reusable queue and can be reused in other positions of the table.

Why this behaviour is not explained in dequeueReusableCellWithIdentifier reference is really amazing. Probably because it’s evident… this entry is dedicated to those who are as dim as me.