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Oct 09
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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.
