Correct way to setting a tag to all cells in TableView

Question

I'm using a button inside a tableView in which I get the indexPath.row when is pressed. But it only works fine when the cells can be displayed in the screen without scroll.

Once the tableView can be scrolleable and I scrolls throught the tableview, the indexPath.row returned is a wrong value, I noticed that initially setting 20 objects, for example Check is just printed 9 times no 20.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
   UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
   if (cell == nil) {
       cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];

       lBtnWithAction = [[UIButton alloc] initWithFrame:CGRectMake(liLight1Xcord + 23, 10, liLight1Width + 5, liLight1Height + 25)];
       lBtnWithAction.tag = ROW_BUTTON_ACTION;
       lBtnWithAction.titleLabel.font = luiFontCheckmark;
       lBtnWithAction.tintColor = [UIColor blackColor];
       lBtnWithAction.autoresizingMask = UIViewAutoresizingFlexibleLeftMargin;
       [cell.contentView addSubview:lBtnWithAction];
   }
   else 
   { 
       lBtnWithAction = (UIButton *)[cell.contentView viewWithTag:ROW_BUTTON_ACTION];
   }

//Set the tag
lBtnWithAction.tag = indexPath.row;
//Add the click event to the button inside a row
[lBtnWithAction addTarget:self action:@selector(rowButtonClicked:) forControlEvents:UIControlEventTouchUpInside];

//This is printed just 9 times (the the number of cells that are initially displayed in the screen with no scroll), when scrolling the other ones are printed
NSLog(@"Check: %li", (long)indexPath.row);

return cell;
}

To do something with the clicked index:

-(void)rowButtonClicked:(UIButton*)sender
{
    NSLog(@"Pressed: %li", (long)sender.tag);
}

Constants.h

#define ROW_BUTTON_ACTION 9

What is the correct way to get the indexPath.row inside rowButtonClicked or setting a tag when I have a lot of of cells in my tableView?

Answer 1

My solution to this kind of problem is not to use a tag in this way at all. It's a complete misuse of tags (in my opinion), and is likely to cause trouble down the road (as you've discovered), because cells are reused.

Typically, the problem being solved is this: A piece of interface in a cell is interacted with by the user (e.g. a button is tapped), and now we want to know what row that cell currently corresponds to so that we can respond with respect to the corresponding data model.

The way I solve this in my apps is, when the button is tapped or whatever and I receive a control event or delegate event from it, to walk up the view hierarchy from that piece of the interface (the button or whatever) until I come to the cell, and then call the table view's indexPath(for:), which takes a cell and returns the corresponding index path. The control event or delegate event always includes the interface object as a parameter, so it is easy to get from that to the cell and from there to the row.

Thus, for example:

UIView* v = // sender, the interface object
do {
    v = v.superview;
} while (![v isKindOfClass: [UITableViewCell class]]);
UITableViewCell* cell = (UITableViewCell*)v;
NSIndexPath* ip = [self.tableView indexPathForCell:cell];
// and now we know the row (ip.row)

---

_{[NOTE A possible alternative would be to use a custom cell subclass in which you have a special property where you store the row in cellForRowAt. But this seems to me completely unnecessary, seeing as indexPath(for:) gives you exactly that same information! On the other hand, there is no indexPath(for:) for a header/footer, so in that case I do use a custom subclass that stores the section number, as in this example (see the implementation of viewForHeaderInSection).]}

Answer 2

I agree with @matt that this is not a good use of tags, but disagree with him slightly about the solution. Instead of walking up the button's superviews until you find a cell, I prefer to get the button's origin, convert it to table view coordinates, and then ask the table view for the indexPath of the cell that contains those coordinates.

I wish Apple would add a function indexPathForView(_:) to UITableView. It's a common need, and easy to implement. To that end, here is a simple extension to UITableView that lets you ask a table view for the indexPath of any view that lies inside one of the tableView's cells.

Below is the key code for the extension, in both Objective-C and Swift. There is a working project on GitHub called TableViewExtension-Obj-C that illustrates the uses of the table view extension below.

EDIT

In Objective-C:

Header file UITableView_indexPathForView.h:

#import <UIKit/UIKit.h>
@interface UIView (indexPathForView)
- (NSIndexPath *) indexPathForView: (UIView *) view;
@end

UITableView_indexPathForView.m file:

#import "UITableView_indexPathForView.h"

@implementation UITableView (UITableView_indexPathForView)

- (NSIndexPath *) indexPathForView: (UIView *) view {
  CGPoint origin = view.bounds.origin;
  CGPoint viewOrigin = [self convertPoint: origin fromView: view];
  return [self indexPathForRowAtPoint: viewOrigin];
}

And the IBAction on the button:

- (void) buttonTapped: (UIButton *) sender {
  NSIndexPath *indexPath = [self.tableView indexPathForView: sender];
  NSLog(@"Button tapped at indexpPath [%ld-%ld]",
        (long)indexPath.section,
        (long)indexPath.row);
}

In Swift:

import UIKit

public extension UITableView {
  func indexPathForView(_ view: UIView) -> IndexPath? {
    let origin = view.bounds.origin
    let viewOrigin = self.convert(origin, from: view)
    let indexPath = self.indexPathForRow(at: viewOrigin)
    return indexPath
  }
}

I added this as a file "UITableView+indexPathForView" to a test project to make sure I got everything correct. Then in the IBAction for a button that is inside a cell:

func buttonTapped(_ button: UIButton) {
  let indexPath = self.tableView.indexPathForView(button)
  print("Button tapped at indexPath \(indexPath)")
}

I made the extension work on any UIView, not just buttons, so that it's more general-purpose.

The nice thing about this extension is that you can drop it into any project and it adds the new indexPathForView(_:) function to all your table views without having do change your other code at all.

Answer 3

You are running into the issue of cell-reuse.

When you create a button for the view you set a tag to it, but then you override this tag to set the row number to it.

When the cell get's reused, because the row number is longer ROW_BUTTON_ACTION, you don't reset the tag to the correct row number and things go wrong.

Using a tag to get information out of a view is almost always a bad idea and is quite brittle, as you can see here.

As Matt has already said, walking the hierarchy is a better idea.

Also, your method doesn't need to be written in this way. If you create your own custom cell, then the code you use to create and add buttons and tags isn't needed, you can do it in a xib, a storyboard, or even in code in the class. Furthermore, if you use the dequeue method that takes the index path, you will always get either a recycled cell, or a newly created cell, so there is no need to check that the cell returned is not nil.