Question

I’m dealing with a view based TableView in cocoa. In the tableview data source method I want to create three different cells based on type of object they are related to.
My idea was to create a generic id variable for the cell before the if statement and then cast it to the proper cell type inside the if when I do introspection on the object they are related to.
Unfortunately with this method xcode complains that the generic cell variable does not have the property I try to set inside. It doesn’t recognise the casting I did in the previous line.

I know this is a very basic question but what is the usual pattern to solve this problem in objective-c/cocoa.

Here is the code of the data source method:

- (NSView *)tableView:(NSTableView *)tableView viewForTableColumn:(NSTableColumn *)tableColumn row:(NSInteger)row 
{
    GeneralVector *vector = [self.vectors objectAtIndex:row];
    id cellView;
    if ([[self.vectors objectAtIndex:row] isKindOfClass:[Vector class]]) {
        cellView = (VectorTableCellView *)[tableView makeViewWithIdentifier:@"vectorCell" owner:self];
        cellView.nameTextField.stringValue = vector.name;
    } else if ([[self.vectors objectAtIndex:row] isKindOfClass:[NoiseVector class]]) {
        cellView = (NoiseVectorTableCellView *)[tableView makeViewWithIdentifier:@"noiseVectorCell" owner:self];
    } else {
        cellView = (ComputedVectorTableCellView *)[tableView makeViewWithIdentifier:@"computedVectorCell" owner:self];
    }

    return cellView;
}    

Answer 1

You don’t have to cast the assignment, since everything is by-definition a descendant of id in Objective-C. Instead, you need to cast where you dereference the property. So, change your code to look something more like this:

if( /* some check */ ) {
  // No need to cast here, we are all descended from 'id'
  cellView = [tableView makeViewWithIdentifier:@"vectorCell" owner:self];
  // Here, we need to cast because we need to tell the compiler how to format
  // the method call for the 'nameTextField' property, so it needs to know
  // some information about the class
  ((VectorTableCellView*)cellView).nameTextField.stringValue = vector.name;
} else if( /* some other check... */ {
  // and so on
}