Xcode 9.3.1中UICollectionViewDataSource的Fix It功能失效，提示Candidate is not a function

Let's break down why you ran into this weird issue with Xcode's Fix It tool, and why manually adding the protocol methods fixed everything:

1. It's likely a bug in Xcode 9.3.1 #

Xcode 9.3.1 is a pretty old version (released back in 2018), and its code assistance tools—including Fix It—had a number of known quirks and bugs. Specifically, when dealing with protocol conformance combined with IBOutlet properties that have a didSet closure, the Fix It logic often got confused. Instead of generating the required protocol method stubs, it would throw irrelevant warnings like "Candidate is not a function" because it couldn't properly map the protocol requirement to your extension.

2. The didSet on your IBOutlet threw off Xcode's analyzer #

You set collectionView.dataSource = self directly inside the didSet of your IBOutlet collection view. While this is a valid pattern, Xcode's static analyzer in version 9.3.1 didn't handle this scenario well when checking protocol conformance. The Fix It tool couldn't correctly associate your UICollectionViewDataSource extension with the data source assignment in the didSet, leading it to look for fixes in the wrong place (your IBOutlet definition) instead of generating the required methods in the extension.

3. UICollectionViewDataSource requires mandatory methods #

Remember that UICollectionViewDataSource is a non-optional protocol—you must implement two methods to satisfy conformance:

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    // Return your item count here
    return 0
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    // Dequeue and configure your cell here
    return UICollectionViewCell()
}

Fix It should have auto-generated these stubs for you, but due to the bugs mentioned above, it failed to do so. Once you added these methods manually, Xcode recognized that your class now conforms to the protocol, so all warnings and errors vanished.

Quick Recommendations #

• If you're stuck on Xcode 9.3.1 for compatibility reasons, just manually implement the required protocol methods whenever Fix It fails like this—it's reliable and avoids these tooling bugs.
• If possible, upgrade to a newer Xcode version (Xcode 10 or later). Apple fixed most of these code assistance issues in subsequent releases, and you'll get a much smoother development experience.

内容的提问来源于stack exchange，提问作者Michael Young