CODEWITHSUNDEEP
iosobjective-cuitableview
Jordan H
Jordan H

Reputation: 55665

How to detect if an instance of a superclass is a subclass?

I am downcasting a UITableViewCell to a custom subclass I have created, then assigning that to a UITableViewCell instance. Later I need to know if the downcast has occurred. Basically I need to know if the cell is an instance of the subclass even though it is now defined as an instance of the superclass.

isKindOfClass won't do the trick because that is only true when it is an instance of that class or an instance of a class that inherits from that class, neither of which are technically true, even though the cell actually is an instance of the class but just examining the passed in variable it's impossible to know that.

How could one assign a subclass to a regular UITableViewCell variable and later know whether nor not it's an instance of the subclass?

UITableViewCell *cell;
CustomCellClass *customCell = (CustomCellClass *)[super tableView:tableView cellForRowAtIndexPath:indexPath];
cell = customCell;
//...
if ([cell isKindOfClass:[CustomCellClass class]]) {
    //this is never true but I need to run code here
}
return cell

Upvotes: 0

Views: 462

Answers (2)

Zigglzworth
Zigglzworth

Reputation: 6803

If the ultimate purpose of this is to use a function that is only available in the subclass then use:

If ([cell respondsToSelector:@selector(mysubclassFunction)] { // this cell is a subclass and responds to that function }

Upvotes: 1

Daniel Brim
Daniel Brim

Reputation: 507

You can't.

Either your object IS in the inheritance tree (is an instance) of CustomCellClass, or it's NOT. There are no in between states and there's no way to change that.

Now, it would help to show us your full code, so we can give you a better explanation, but here's a try:

Now, with inheritance, there are rules. One primary rule is that you can't 'downcast' an object. You can only 'upcast' it. If you create an NSObject, you can't cast. it into a UIView. You can, however, cast a UIView into an NSObject.

So, for your code, you can't cast cell to be a CustomCellClass because cell isn't starting out as a CustomCellClass. Very easy change, really!

Edit: if tableView:cellForRowAtIndexPath: returns a CustomCellClass object, then the following will work. But I don't recommend thinking about it as a "downcast." It is normally just called "casting".

What you want:

CustomCellClass *cell = (CustomCellClass *)[super tableView:tableView    cellForRowAtIndexPath:indexPath];

if ([cell isKindOfClass:[CustomCellClass class]]) {
    // This will be true because it was created as a CustomCellClass object.
}
return cell

Upvotes: 1

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