How to set the full width of separator in UITableView

Question

This worked for me on iOS 8.4 - 9.0 devices using Xcode 6.4 and Swift 1.2:

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    var cell = UITableViewCell()


    cell.preservesSuperviewLayoutMargins = false
    cell.separatorInset = UIEdgeInsetsZero
    cell.layoutMargins = UIEdgeInsetsZero

    return cell
}

Swift 5 Update:

cell.preservesSuperviewLayoutMargins = false
cell.separatorInset = UIEdgeInsets.zero
cell.layoutMargins = UIEdgeInsets.zero

In your UITableViewCell

Go to Attributes Inspector in your Interface Builder and simply change "15" to 0. Do this for all the cells you wish to change.

You may need to add [cell setLayoutMargins:UIEdgeInsetsZero]; to your tableViewCell

I got the answer from this post: iOS 8 UITableView separator inset 0 not working

Just add this code on your UITableViewController

-(void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if ([cell respondsToSelector:@selector(setSeparatorInset:)]) {
        [cell setSeparatorInset:UIEdgeInsetsZero];
    }

    if ([cell respondsToSelector:@selector(setLayoutMargins:)]) {
        [cell setLayoutMargins:UIEdgeInsetsZero];
    }
}

-(void)viewDidLayoutSubviews
{
    [super viewDidLayoutSubviews];
    if ([self.tableView respondsToSelector:@selector(setSeparatorInset:)]) {
        [self.tableView setSeparatorInset:UIEdgeInsetsZero];
    }

    if ([self.tableView respondsToSelector:@selector(setLayoutMargins:)]) {
        [self.tableView setLayoutMargins:UIEdgeInsetsZero];
    }
}

for Swift 3 :

override func viewDidLoad() {
  super.viewDidLoad()

  tableView.separatorInset = .zero
  tableView.layoutMargins = .zero
}