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Iphone - when to calculate heightForRowAtIndexPath for a tableview when each cell height is dynamic?

Asked 2011-01-27T23:29:56.950
42

I have seen this question asked many times but astoundingly, I have not seen a consistent answer, so I will give it a try myself:

If you have a tableview containing your own custom UITableViewCells that contain UITextViews and UILabels whose height must be determined at runtime, how are you supposed to determine the height for each row in heightForRowAtIndexPath?

The most obvious first idea is to calculate the height for each cell by calculating and then summing the heights of each view inside the cell inside of cellForRowAtIndexPath, and store that final total height for later retrieval.

This will not work however because cellForRowAtIndexPath is called AFTER heightForRowAtIndexPath.

The only thing I can think of is to do all the calculations inside viewDidLoad, create all the UITableViewCells then, calculate the cells height and store that in a custom field inside your UITableViewCell subclass, and put each cell in an NSMutableDictionary with the indexPath as the the key, and then simply retrieve the cell from the dictionary using the indexPath inside cellForRowAtIndexPath and heightForRowAtIndexPath, returning either the custom height value or the cell object itself.

This approach seems wrong though because it does not make use of dequeueReusableCellWithIdentifier, instead I would be loading all the cells at once into a dictionary in my controller, and the delegate methods would be doing nothing more than retrieving the correct cell from the dictionary.

I don't see any other way to do it though. Is this a bad idea - if so, what is the correct way to do this?

ios uitableview reloaddata heightforrowatindexpath
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2 Answers

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An approach I have used in the past is to create a class variable to hold a single instance of the cell you are going to be using in the table (I call it a prototype cell). Then in the custom cell class I have a method to populate the data and determine the height the cell needs to be. Note that it can be a simpler variant of the method to really populate the data - instead of actually resizing a UILabel in a cell for example, it can just use the NSString height methods to determine how tall the UILabel would be in the final cell and then use the total cell height (plus a border on the bottom) and UILabel placement to determine the real height. YOu use the prototype cell just to get an idea of where elements are placed so you know what it means when a label is going to be 44 units high.

In heightForRow: I then call that method to return the height.

In cellForRow: I use the method that actually populates labels and resizes them (you never resize the UITableView cell yourself).

If you want to get fancy, you can also cache the height for each cell based on the data you pass in (for instance it could just be on one NSString if that's all that determines height). If you have a lot of data that's often the same it may make sense to have a permanent cache instead of just in-memory.

You can also try estimating line count based on character or word count, but in my experience that never works - and when it goes wrong it usually messes up a cell and all the cells below it.

answered 2011-01-28T01:51:45.663
2

Really good question: looking for more insight on this as well.

Clarifying the issue:

  1. Height for Row is called Before the (cellForRowAtIndexPath)
  2. Most people calculate the height-type information within the CELL (cellForRowAtIndexPath).

Some of the solutions are surprisingly simple/effective:

  • solution 1: force the heightForRowAtIndexPath to calculate the the cell's specs. Massimo Cafaro Sept 9th

  • solution 2: do a first pass "standard size" for the cells, cache results when you do have cell heights, then reload the table using the new heights - Symmetric

  • solution 3: the other interesting answer seems to be the involving three20 but based on the answer it seems that there isn't a cell drawn in storyboard/xib which would make this "problem" much easier to solve.

answered 2012-12-04T04:59:18.377

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