Calling dequeueReusableCellWithIdentifier:cellId inside heightForRowAtIndexPath - ios

Question

I am working on an ios application,
I have a normal table view. When calling heightForRowAtIndexPath I am doing the folowing
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
NSString *cellId = [self getCellIds][indexPath.row];
BaseTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellId];
return [cell calculateHeigh];
}
Basically I am dequeueing the cell because I have a function calculateHeigh inside every cell that will do the height calculation. this is working fine as intended however I have a concern:
Is it safe to call dequeueReusableCellWithIdentifier: inside the heightForRowAtIndexPath ? will it cause any issue?
EDIT:
Just to clarify why I did this, I have a big amount of custom cells with different identifier that needs to be loaded. and to avoid having a huge if-else statement in my heightForRowAtIndexPath I placed the getter of the cell height in the custom cell that way I just ask it to return it (no calculation is made there), I can't do it as a class method as I don't know which class, I can get the object from the identifier and not the class. And I want to avoid a big if-else just for code readability.
So my concern was with the dequeueReusableCellWithIdentifier: is it heavy to call it when getting the height? will it cause memory issues or lags? or is it worth to just do a bug if-else of use a dictionary?

Answer 1

First of all you should avoid any calculations in table drawing methods(such as heightForRow, cellForRow, etc). These methods are called a lot and although your table may be short and/or not complicated(with custom cells with a lot of labels, buttons and images) you should always try to optimize this drawing process or otherwise user will experience some nasty lag when scrolling.
So you should call some method to prepare data before calling 'reloadTable'
-(void)prepareMethod
{
//get only one cell to calculate all row heights
BaseTableViewCell *cell = [_myTableView dequeueReusableCellWithIdentifier:cellId];
for (NSDictionary* dataObj in _dataArray)
{
//loop through all rows data and set new property for row height
dataObj[#"rowHeight"] = [cell calculateHeigh];
}
}
And then when calling heightForRow just pass this value without any expensive operation(such as probably string calculations):
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
//always make sure you don't access unexisting array index
return ( indexPath.row < _dataArray.count ) ? _dataArray[indexPath.row][indexPath.row][#"rowHeight"] : 1.0;
}
Of course you don't need separate method just to populate row heights in your data array - you can populate this value when populating(formatting) your data array to avoid second array iteration. It all depends on your current implementation.
Just remember that expensive drawing methods(not only for table though) should always be as short as possible and just get data needed for drawing and draw. It's really so simple. If you need to make some complicated calculations do it before that(maybe in view init) so your data is prepared before actual drawing. This way your application will be working smoothly even with bigger tables(because no matter how big the table is, UITableViewController draws only visible cells).
Regards,
hris.to

Answer 2

I don't like to have big if statements in heightForRowAtIndexPath and accessing a cell using dequeueReusableCellWithIdentifier. Your approach getting cell height from each cell quite is reasonable. I believe your calculateHeigh return value depends on the table data you pass into the cell.
In BaseTableViewCell.h
+ (CGFloat)heightWithData:(id)data;
In BaseTableViewCell.m
+ (CGFloat)heightWithData:(id)data
{
//put your calculateHeigh logic here. I believe your calculateHeigh depends on the data each cell has.
}
Then you can do
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
return [BaseTableViewCell heightWithData:[self.tableData objectAtIndex:indexPath.row]];
}
If you do this, you don't need to access each cell object to get cell height.

Answer 3

You should not use this method to provide the calculation. Based on what I can see on your setup, you are calculating the height based on the values already on your cell. What happens is that the cell dequeue system will give you a cell to reuse, but because it's sharing cells from multiple index paths, that cell probably has data that belongs to a record of an index path different from the current one. Get the calculate height code and try to reproduce it inside the datasource callback you are using.