What this does is provide the table view with a temporary estimate/placeholder for the row heights of cells that are not yet onscreen. Then, when these cells are about to scroll onscreenon screen, the actual row height will be calculated. To determine the actual height for each row, the table view automatically asks each cell what height its contentView needs to be based on the known fixed width of the content view (which is based on the table view's width, minus any additional things like a section index or accessory view) and the auto layout constraints you have added to the cell's content view and subviews. Once this actual cell height has been determined, the old estimated height for the row is updated with the new actual height (and any adjustments to the table view's contentSize/contentOffset are made as needed for you).
As of iOS 7, you can (and absolutely should) use the estimatedRowHeight property on the table view. What this does is provide the table view with a temporary estimate/placeholder for the row heights of cells that are not yet onscreen. Then, when these cells are about to scroll onscreenon screen, the actual row height will be calculated (by calling tableView:heightForRowAtIndexPath:), and the estimated height updated with the actual one.
If you've done all the above and are still finding that performance is unacceptably slow when doing the constraint solving in tableView:heightForRowAtIndexPath:, you'll unfortunately need to implement some caching for cell heights. (This is the approach suggested by Apple's engineers.) The general idea is to let the Auto LayoutAutolayout engine solve the constraints the first time, then cache the calculated height for that cell and use the cached value for all future requests for that cell's height. The trick of course is to make sure you clear the cached height for a cell when anything happens that could cause the cell's height to change -- primarily, this would be when that cell's content changes or when other important events occur (like the user adjusting the Dynamic Type text size slider).
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
// Determine which reuse identifier should be used for the cell at this
// index path, depending on the particular layout required (you may have
// just one, or may have many).
NSString *reuseIdentifier = ...;
// Dequeue a cell for the reuse identifier.
// Note that this method will init and return a new cell if there isn't
// one available in the reuse pool, so either way after this line of
// code you will have a cell with the correct constraints ready to go.
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:reuseIdentifier];
// Configure the cell with content for the given indexPath, for example:
// cell.textLabel.text = someTextForThisCell;
// ...
// Make sure the constraints have been set up for this cell, since it
// may have just been created from scratch. Use the following lines,
// assuming you are setting up constraints from within the cell's
// updateConstraints method:
[cell setNeedsUpdateConstraints];
[cell updateConstraintsIfNeeded];
// If you are using multi-line UILabels, don't forget that the
// preferredMaxLayoutWidth needs to be set correctly. Do it at this
// point if you are NOT doing it within the UITableViewCell subclass
// -[layoutSubviews] method. For example:
// cell.multiLineLabel.preferredMaxLayoutWidth = CGRectGetWidth(tableView.bounds);
return cell;
}
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
// Determine which reuse identifier should be used for the cell at this
// index path.
NSString *reuseIdentifier = ...;
// Use a dictionary of offscreen cells to get a cell for the reuse
// identifier, creating a cell and storing it in the dictionary if one
// hasn't already been added for the reuse identifier. WARNING: Don't
// call the table view's dequeueReusableCellWithIdentifier: method here
// because this will result in a memory leak as the cell is created but
// never returned from the tableView:cellForRowAtIndexPath: method!
UITableViewCell *cell = [self.offscreenCells objectForKey:reuseIdentifier];
if (!cell) {
cell = [[YourTableViewCellClass alloc] init];
[self.offscreenCells setObject:cell forKey:reuseIdentifier];
}
// Configure the cell with content for the given indexPath, for example:
// cell.textLabel.text = someTextForThisCell;
// ...
// Make sure the constraints have been set up for this cell, since it
// may have just been created from scratch. Use the following lines,
// assuming you are setting up constraints from within the cell's
// updateConstraints method:
[cell setNeedsUpdateConstraints];
[cell updateConstraintsIfNeeded];
// Set the width of the cell to match the width of the table view. This
// is important so that we'll get the correct cell height for different
// table view widths if the cell's height depends on its width (due to
// multi-line UILabels word wrapping, etc). We don't need to do this
// above in -[tableView:cellForRowAtIndexPath] because it happens
// automatically when the cell is used in the table view. Also note,
// the final width of the cell may not be the width of the table view in
// some cases, for example when a section index is displayed along
// the right side of the table view. You must account for the reduced
// cell width.
cell.bounds = CGRectMake(0.0, 0.0, CGRectGetWidth(tableView.bounds), CGRectGetHeight(cell.bounds));
// Do the layout pass on the cell, which will calculate the frames for
// all the views based on the constraints. (Note that you must set the
// preferredMaxLayoutWidth on multi-linemultiline UILabels inside the
// -[layoutSubviews] method of the UITableViewCell subclass, or do it
// manually at this point before the below 2 lines!)
[cell setNeedsLayout];
[cell layoutIfNeeded];
// Get the actual height required for the cell's contentView
CGFloat height = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
// Add an extra point to the height to account for the cell separator,
// which is added between the bottom of the cell's contentView and the
// bottom of the table view cell.
height += 1.0;
return height;
}
// NOTE: Set the table view's estimatedRowHeight property instead of
// implementing the below method, UNLESS you have extreme variability in
// your row heights and you notice the scroll indicator "jumping"
// as you scroll.
- (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath
{
// Do the minimal calculations required to be able to return an
// estimated row height that's within an order of magnitude of the
// actual height. For example:
if ([self isTallCellAtIndexPath:indexPath]) {
return 350.0;
} else {
return 40.0;
}
}
