Oct 09

UITableView Alternate Row Colours with NSFetchedResultsController

iOS, iPad, iPhone, objective-c, Uncategorized No Comments »

Ok, this might seem like a simple thing to do – and in most cases it is, unless you’re using core data and have some row additions coming in dynamically.

- (void) tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
    cell.backgroundColour = (indexPath.row % 2) ? [Constants darkTableCellBackgroundColor] : [Constants lightTableCellBackgroundColor];
}

You see the problem I had was, that row zero is white, row one is blue, row two is white….etc, and when I synced with my remote server and new data arrived, it was inserted at row zero  - which is white! The other rows were pushed down, so row zero and row one were both white! No more alternate. I could have just refreshed the UITableView – or the visible rows, but I didn’t want to do that! I wanted it to figure out the correct row colour from the insert.

I got it working by combining the row index with the total rows, using the inverse of one and XOR’ing the result! Sounds complicated? Look at this table:

A: Row B: Row Modulus (%2) C: Row Count D: Count Modulus (%2) E: Inverse Count Modulus F: XOR B ^ E
0 0 6 0 1 1
1 1 6 0 1 0
2 0 6 0 1 1
3 1 6 0 1 0
4 0 6 0 1 1
5 1 6 0 1 0

As can be seen above, I can get alternate row colours from Column B or Column F. However, if you then add another row at index zero – the row modulus can no longer be used, but Column F, still preserves the alternate row colours!

A: Row B: Row Modulus (%2) C: Row Count D: Count Modulus (%2) E: Inverse Count Modulus F: XOR B ^ E
0 0 7 1 0 0
0 0 6 0 1 1
1 1 6 0 1 0
2 0 6 0 1 1
3 1 6 0 1 0
4 0 6 0 1 1
5 1 6 0 1 0

This can be achieved with this bit of code:

- (void) tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
    int rowFlag = (indexPath.row % 2);
    int totalFlag = [self tableView:tableView numberOfRowsInSection:indexPath.section] % 2;
    cell.backgroundColor = (rowFlag ^ !totalFlag) ? [Constants darkTableCellBackroundColor] : [Constants lightTableCellBackroundColor];
}

Let me know if I’ve missed something obvious here and there was a more common way of doing this.

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