ios - 拦截子类中的 Objective-C 委托(delegate)消息

Question

我有一个 UIScrollView 的子类，我需要在其中对滚动行为做出内部响应。然而，viewcontroller 仍然需要监听滚动委托(delegate)回调，所以我不能在我的组件中直接窃取委托(delegate)。

有没有办法保留名为“delegate”的属性并只听取沿它发送的消息，或者以某种方式在内部劫持 delegate 属性并在运行一些代码后向外转发消息？

Answer 1

为避免手动覆盖所有委托(delegate)方法，您可以使用消息转发。我只是使用中间代理类实现了同样的事情，如下所示:

MessageInterceptor.h

@interface MessageInterceptor : NSObject {
    id receiver;
    id middleMan;
}
@property (nonatomic, assign) id receiver;
@property (nonatomic, assign) id middleMan;
@end

MessageInterceptor.m

@implementation MessageInterceptor
@synthesize receiver;
@synthesize middleMan;

- (id)forwardingTargetForSelector:(SEL)aSelector {
    if ([middleMan respondsToSelector:aSelector]) { return middleMan; }
    if ([receiver respondsToSelector:aSelector]) { return receiver; }
    return [super forwardingTargetForSelector:aSelector];
}

- (BOOL)respondsToSelector:(SEL)aSelector {
    if ([middleMan respondsToSelector:aSelector]) { return YES; }
    if ([receiver respondsToSelector:aSelector]) { return YES; }
    return [super respondsToSelector:aSelector];
}

@end

MyScrollView.h

#import "MessageInterceptor.h"

@interface MyScrollView : UIScrollView {
    MessageInterceptor * delegate_interceptor;
    //...
}

//...

@end

MyScrollView.m(已编辑，感谢 jhabbott):

@implementation MyScrollView

- (id)delegate { return delegate_interceptor.receiver; }

- (void)setDelegate:(id)newDelegate {
    [super setDelegate:nil];
    [delegate_interceptor setReceiver:newDelegate];
    [super setDelegate:(id)delegate_interceptor];
}

- (id)init* {
    //...
    delegate_interceptor = [[MessageInterceptor alloc] init];
    [delegate_interceptor setMiddleMan:self];
    [super setDelegate:(id)delegate_interceptor];
    //...
}

- (void)dealloc {
    //...
    [delegate_interceptor release];
    //...
}

// delegate method override:
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    // 1. your custom code goes here
    // 2. forward to the delegate as usual
    if ([self.delegate respondsToSelector:@selector(scrollViewDidScroll:)]) {
        [self.delegate scrollViewDidScroll:scrollView];
    }
}

@end

使用这种方法，MessageInterceptor 对象会自动将所有委托(delegate)消息转发到常规委托(delegate)对象，除了您在自定义子类中覆盖的消息。 p>