Swift中UIScrollView子类forwardingTarget未调用及代理方法异常问题

Hey there, let's tackle the two issues you're facing with your ReusableScrollView open-source project. I've run into similar message forwarding quirks with UIScrollView subclasses before, so here's what I think is going on and how to fix it:

1. Why forwardingTarget(for:) isn't being called #

The key issue here lies in how Objective-C/Swift's message forwarding pipeline works. When you override responds(to:) to return true for selectors your delegate handles, the runtime first expects your class to provide a valid method signature for that selector. If you don't, it skips the forwardingTarget(for:) step entirely and throws an unrecognized selector error.

Your current code only overrides responds(to:) and forwardingTarget(for:), but you're missing a critical override: methodSignature(for:). Here's how to fix that:

override open func methodSignature(for aSelector: Selector!) -> NSMethodSignature? {
    // First check if the delegate can handle the selector
    if let delegateSignature = _delegate?.methodSignature(for: aSelector) {
        return delegateSignature
    }
    // Fall back to superclass's signature
    return super.methodSignature(for: aSelector)
}

Without this, even if you return true in responds(to:), the runtime can't find a valid method signature for the selector, so it never gets to your forwardingTarget(for:) implementation.

Additionally, make sure your scroll view subclass doesn't accidentally implement any UIScrollViewDelegate methods itself. If it does, super.responds(to:) will return true, and the runtime won't trigger forwarding.

2. Why most UIScrollViewDelegate methods aren't detected by responds(to:) #

The fact that only scrollViewWillEndDragging(_:withVelocity:targetContentOffset:) is being caught suggests a few possible issues:

• Objective-C exposure of delegate methods: If your _delegate is a Swift class implementing UIScrollViewDelegate, ensure the methods are exposed to Objective-C. While methods conforming to Objective-C protocols are usually auto-exposed, edge cases (like non-@objc marked classes) can break this. Try adding @objcMembers to your delegate class, or explicitly marking each implemented delegate method with @objc:

  @objcMembers
  class MyScrollDelegate: NSObject, UIScrollViewDelegate {
      func scrollViewDidScroll(_ scrollView: UIScrollView) {
          // Your implementation
      }
      // Other delegate methods...
  }
  

• Selector matching issues: Double-check that the selectors passed to responds(to:) exactly match the ones your delegate implements. For example, scrollViewDidScroll(_:) in Swift maps to the Objective-C selector @selector(scrollViewDidScroll:). A mismatch (like missing the underscore in Swift) will make responds(to:) return false.
• Delegate lifecycle: Ensure your _delegate isn't being deallocated before the selector check. If it's a weak reference, confirm the delegate instance is retained elsewhere in your code. The fact that one method works suggests this isn't the main issue, but it's worth verifying.

One final note: When forwarding UIScrollView delegate methods, avoid method swizzling or overriding UIScrollView's internal behavior unnecessarily, as it can interfere with the message forwarding flow. Stick to the standard steps (responds -> method signature -> forwarding target) for consistent results.

Hope this helps! Feel free to share more details if you're still hitting issues, and I'm happy to contribute to your open-source project if needed.

内容的提问来源于stack exchange，提问作者PiotrDomo