Swift alternative to respondsToSelector:

Question

I was trying to implement swift's alternative to the `respondsToSelector:` syntax that was also shown in the keynote. I have the following: ``` protocol CustomItemTableViewCellDelegate { func changeCount(sender: UITableViewCell, change: Int) } ``` and then later in the code I call ``` class CustomItemTableViewCell: UITableViewCell { var delegate: CustomItemTableViewCellDelegate ... override func touchesEnded(touches: NSSet!, withEvent event: UIEvent!) { ... delegate?.changeCount?(self, change: -1) } ... } ``` I get the following errors - `Operand of postfix '?' should have optional type; type is '(UITableViewCell, change:Int) -> ()'` - `Operand of postfix '?' should have optional type; type is 'CustomItemTableViewCellDelegate'` - `Partial application of protocol method is not allowed` What I am doing wrong? Thanks

Accepted Answer

You have two `?` operators, and they're both causing problems.

First, the one after `delegate` indicates that you want to unwrap an optional value, but your `delegate` property isn't declared that way. It should be:

```
var delegate: CustomItemTableViewCellDelegate?
```

Second, it looks like you want your `changeCount` protocol method to be optional. If you do, you need to both mark the protocol with the `@objc` attribute and mark the function with the `optional` attribute:

```
@objc protocol CustomItemTableViewCellDelegate {
    optional func changeCount(sender: UITableViewCell, change: Int)
}
```

(Note: Classes that conform to `@objc` protocols need to be `@objc` themselves. In this case you're subclassing an Objective-C class, so you're covered, but a new class would need to be marked with the `@objc` attribute.)

If you only want the delegate to be optional (that is, it's okay to not have a delegate, but all delegates need to implement `changeCount`), then leave your protocol as is and change that method call to:

```
delegate?.changeCount(self, change: -1)
```

Swift alternative to respondsToSelector:

Solution

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