Overriding the Delegate Property of UIScrollView Like UICollectionView Does

UIScrollView has a delegate property typed as UIScrollViewDelegate:

public protocol UIScrollViewDelegate : NSObjectProtocol {
    //...
}
open class UIScrollView : UIView, NSCoding {
    weak open var delegate: UIScrollViewDelegate?
    //...
}

UICollectionView overrides this property, but uses UICollectionViewDelegate instead:

public protocol UICollectionViewDelegate : UIScrollViewDelegate {
   //...
}

open class UICollectionView : UIScrollView, UIDataSourceTranslating {
    weak open var delegate: UICollectionViewDelegate?
    //...
}

However, when you try to override the UIScrollView delegate with a custom protocol in the same way:

protocol MyScrollViewDelegate : UIScrollViewDelegate {
    //...
}

class MyScrollView: UIScrollView {
    weak open var delegate: MyScrollViewDelegate?
    //...
}

You get a compiler error:

Property 'delegate' with type 'MyScrollViewDelegate?' cannot override a property with type 'UIScrollViewDelegate?'

So how can a custom UIScrollView subclass override the delegate property?

The answer is didSet.

private weak var myDelegate: MyScrollViewDelegate? = nil

override var delegate: UIScrollViewDelegate? {
    didSet {
        myDelegate = delegate as? MyScrollViewDelegate
    }
}

Supplementary note:

The Objective-C approach to overriding delegate works as follows:

1. Assign delegate to myDelegate.

2. Set the superclass's delegate to the subclass itself.

When a delegate method is called, the runtime looks for a method signature via methodSignatureForSelector. If the subclass doesn't implement it, it checks whether myDelegate does. The signature is returned to the system, which wraps it into an NSInvocation and passes it to forwardInvocation. If myDelegate implements the delegate method, it will be called.

When calling the superclass's delegate methods, respondsToSelector is called first to check whether the given SEL is implemented. If it is, the runtime proceeds to methodSignatureForSelector and forwardInvocation.

@interface MyOCScrollView () <MyOCScrollViewDelegate, UIScrollViewDelegate>

@property(nonatomic, weak)  id <MyOCScrollViewDelegate> myDelegate;

//...

@end

@implementation MyOCScrollView

@dynamic delegate;

//...

- (void)setDelegate:(id<MyOCScrollViewDelegate>)delegate {
    self.myDelegate = delegate != self ? delegate : nil;
    [super setDelegate:delegate ? self : nil];
}

/**
 Attempt to obtain a method signature
 */
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    NSMethodSignature *a = [super methodSignatureForSelector:aSelector];
    NSMethodSignature *b = [(id)self.myDelegate methodSignatureForSelector:aSelector];
    NSMethodSignature *result = a ? a : b;
    return result;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation {
    if ([(id)self.myDelegate respondsToSelector:anInvocation.selector]) {
        [anInvocation invokeWithTarget:(id)self.myDelegate];
    }
}

/**
 Returns YES if either self or the delegate implements the given selector
 */
- (BOOL)respondsToSelector:(SEL)aSelector {
    BOOL a = [super respondsToSelector:aSelector];
    BOOL c = [self.myDelegate respondsToSelector:aSelector];
    BOOL result = a || c;
    return result;
}

@end

Note: MyOCScrollView also conforms to <MyOCScrollViewDelegate>, which I believe is to suppress the warning in self.myDelegate = delegate != self ? delegate : nil;. However, this means NSMethodSignature *a = [super methodSignatureForSelector:aSelector]; will always return a non-nil object. In that case, you can simply check in forwardInvocation whether to forward the call. You could remove the <MyOCScrollViewDelegate> conformance and change the assignment to self.myDelegate = (id)delegate != self ? delegate : nil; — testing confirms this works fine as well, and the flow becomes easier to understand.