Why tableview's delegate method heightForRowAtIndexPath: executes one more time? - ios

Question

my platform is ios8 and xcode 6.3.1
tableview's delegate like this:
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
return 3;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
return 1;
}
so, the delegate of heightForRowAtIndexPath: should be execute three times , but my code execute four, why ?
My code :
init tableView
- (void)setupTableView {
_selectTableView = [[UITableView alloc] initWithFrame:self.view.bounds style:UITableViewStylePlain];
_selectTableView.delegate = self;
_selectTableView.dataSource = self;
_selectTableView.separatorStyle = UITableViewCellSeparatorStyleNone;
[self.view addSubview:_selectTableView];
}
other delegate method:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
NSInteger section = indexPath.section;
static NSString *identified = #"selectCell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:identified];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:identified];
cell.selectionStyle = UITableViewCellSelectionStyleNone;
}
return [self cellWith:cell andSection:section];
}
- (UITableViewCell *)cellWith:(UITableViewCell *)cell andSection:(NSInteger)section {
....
return cell;
}
- (CGFloat)tableView:(UITableView *)tableView heightForFooterInSection:(NSInteger)section {
CGFloat height = 0;
if (section != SVCellTypeHot) {
height = 5;
}
return height;
}
- (UIView *)tableView:(UITableView *)tableView viewForFooterInSection:(NSInteger)section {
UIView *footerView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, kScreenSize.width, 5)];
[footerView setBackgroundColor:[UIColor lightGrayColor]];
return footerView;
}
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
CGFloat height = 0;
switch (indexPath.section) {
case SVCellTypeBanner:
{
height = kHeaderViewHeigth;
}
break;
case SVCellTypeRecommand:
{
height = kRecommandViewHeight;
}
break;
case SVCellTypeHot:
{
height = kHotViewHeight;
}
break;
default:
break;
}
return height;
}

Answer 1

heightForRowAtIndexPath allows the delegate to specify rows with varying heights. If this method is implemented, the value it returns overrides the value specified for the rowHeight property of UITableView for the given row. There is no guarentee that this method can only be called 'section count * item count' times in the UITableView. As you can tell from its name, it will calculate the height for the cells at IndexPath, so combining the re-using technique, this method will be called many times, as long as it needs to calculate the height for cell at IndexPath
So actually, it is a system behaviour to decide how many times it should be called and when. In your comment, it seems like something changed in indexPath {1-0} so heightForRowAtIndexPath is called twice for {1-0}. You might need to check have you changed any content that cause iOS to re-calculate the cell's height.
Without knowing more details, this is the best we can do to provide you some clues to debug. However, you should not rely on how many times it calls heightForRowAtIndexPath, again, this can be called at any time, as long as you scroll or change any frame inside that cell

Answer 2

heightForRowAtIndexPath: will execute as many times as it needs to. If you are scrolling, for example, it will execute as offscreen cells are about to come onscreen. That method should always be able to provide the correct height and you normally shouldn't be concerned with how often it's called. cellForRowAtIndexPath: executes 3 times as it should.