Why does adding a gesture recognizer to UITableViewCell subclass in Interface Builder crash the app?

Question

I have a custom UITableViewCell subclass and I'm trying to add a pinch gesture recognizer using Interface Builder to one of the views. My app crashes with:

2016-09-11 17:37:22.425 MYAPPNAME[4619:1284144] *** Assertion failure in -[ULFeedView _dequeueReusableViewOfType:withIdentifier:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKit/UIKit-3512.60.12/UITableView.m:6539

I've tried different gesture recognizers (e.g. tap recognizers) and different subviews, but they all crash my app.

An interesting observation: It doesn't crash if I add the recognizer to a view programmatically at awakeFromNib.

Here is some methods that might be relevant:

-(void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath{
    [tableView deselectRowAtIndexPath:indexPath animated:YES];
    if(indexPath.section != SECTION_CONTENT){
        return; //index path section is NOT equal to SECTION CONTENT for the cell in question, so it will always return.
    }
    ...
 }

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
    switch (indexPath.section) {
        case SECTION_MAP:
        {
            ULMapCell *cell = [tableView dequeueReusableCellWithIdentifier:@"map" forIndexPath:indexPath];
            return cell; //the cell in question is this one, so it will always return this cell.
        }
     ...
}

UPDATE: I have no problems with registering nibs. It was already working perfectly before the gesture recognizer. Please stop telling me how to register nibs for table view, I already know that as a senior iOS developer.

UPDATE 2: I confirm that it is occuring only when I add it through Interface Builder and there is no problem if I add it anywhere programmatically.

Why would this be happening?

Answer 1

The objects in a nib are organised hierarchically. Often there is just one object at the top level: the root view (in your case that's the cell). However, nibs can contain multiple top-level objects. In fact, gesture recognizers are added to the nib as top-level objects.

Any code that loads a nib with multiple top-level objects needs to know how to deal with this. For example, the code loading the nib for a UITableViewCell could:

1. find the cell object in the array of top-level-objects
2. make sure there is no other cell (because it would be impractical to add a heuristic for which one to choose)
3. ignore all top-level UIGestureRecognizers since they are retained by the views they have been added to
4. make sure there is nothing else in the top-level objects array

Unfortunately, the way Apple chose to deal with multiple top-level objects when loading UITableViewCell and UICollectionViewCell nibs is to throw an exception. That's why we cannot add gesture recognisers to cells in Interface Builder.