Subclass a zooming UIScrollView without delegate methods

问题

I want to implement a UIScrollView subclass to present some custom formatted content. I just set a model object property of the scroll view and it handles all the required layout and rendering to display the content.

This works fine, but now I'd like to include zooming. According to the documentation, to support zooming you have to set a delegate and implement the viewForZoomingInScrollView: method. I guess I could set the delegate to the scroll view itself and implement that method in the subclass. But doing that I would lose the ability to have an external delegate (like an encapsulating UIViewController) that can be notified about scroll events.

Assuming the documentation is right and there is absolutely no (documented) way to implement zooming without a delegate, how could I still retain the possibility of having a regular, unrelated delegate?

回答1:

I'd abuse the fact that I'm being a subclass (on purpose :P). So you can hack it. Really bad, and I should feel bad for proposing this solution.

@interface MyHackishScrollView: UIScrollView {
    id <UIScrollViewDelegate> ownDelegate;
}

@end

@implementation MyHackishScrollView

- (void)setDelegate:(id <UIScrollViewDelegate>)newDel
{
    ownDelegate = newDel;
    [super setDelegate:self];
}

- (UIView *)viewForScrollingInScrollView:(UIScrollView *)sv
{
    return whateverYouWant;
}

// and then implement all the delegate methods
// something like this:
- (void)scrollViewDidScroll:(UIScrollView *)sv
{
    [ownDelegate scrollViewDidScroll:self];
}

// etc.

@end

回答2:

Building upon H2CO3's suggestion of saving a hidden pointer to the real delegate and forwarding all incoming messages to it, I came up with the following solution.

Declare a private delegate variable to store a reference to the "real" delegate that is passed in to the setDelegate: method:

@interface BFWaveScrollView ()
@property (nonatomic, weak) id<UIScrollViewDelegate> ownDelegate;
@end

Set the delegate to self to be notified about scrolling events. Use super, so the original setDelegate: implementation is called, and not our modified one.

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        [super setDelegate:self];
    }
    return self;
}

Override setDelegate: to save a reference to the "real" delegate.

- (void)setDelegate:(id<UIScrollViewDelegate>)delegate {
    _ownDelegate = delegate;
}

When the UIScrollView tries to call a method of its delegate, it will first check to see if the delegate respondsToSelector:. We have to forward this to the real delegate if the selector is part of the UIScrollViewDelegate protocol (Don't forget to #import <objc/runtime.h>).

- (BOOL)selectorIsScrollViewDelegateMethod:(SEL)selector {
    Protocol *protocol = objc_getProtocol("UIScrollViewDelegate");
    struct objc_method_description description = protocol_getMethodDescription(
                                                   protocol, selector, NO, YES);
    return (description.name != NULL);
}

- (BOOL)respondsToSelector:(SEL)selector {
    if ([self selectorIsScrollViewDelegateMethod:selector]) {
        return [_ownDelegate respondsToSelector:selector] ||
               [super respondsToSelector:selector];
    }
    return [super respondsToSelector:selector];
}

Finally, forward all delegate methods to the real delegate that are not implemented in the subclass:

- (id)forwardingTargetForSelector:(SEL)selector {
    if ([self selectorIsScrollViewDelegateMethod:selector]) {
        return _ownDelegate;
    }
    return [super forwardingTargetForSelector:selector];
}

Don't forget to manually forward those delegate methods that are implemented by the subclass.

回答3:

Maybe this is easier to read and understand a couple of weeks later :) (sample code for intercepting locationManager:didUpdateLocations: in a subclass)

Other than that the same handling for setting self as delegate to the superclass and intercepting setDelegate in order to save the user's delegate to mDelegate.

EDIT:

-(BOOL)respondsToSelector:(SEL)selector {
    if (sel_isEqual(selector, @selector(locationManager:didUpdateLocations:)))
        return true;
    return [mDelegate respondsToSelector:selector];
}

- (id)forwardingTargetForSelector:(SEL)selector {
    if (sel_isEqual(selector, @selector(locationManager:didUpdateLocations:)))
        return self;
    return mDelegate;
}

来源：https://stackoverflow.com/questions/15670491/subclass-a-zooming-uiscrollview-without-delegate-methods