在子类中拦截 Objective-C 委托消息

Question

我有一个 UIScrollView 的子类，我需要在其中内部响应滚动行为。但是，视图控制器仍然需要监听滚动委托回调，所以我不能直接在我的组件中窃取委托。

有没有办法保留名为“delegate”的属性并只收听沿它发送的消息，或者以某种方式在内部劫持委托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方法，您可以使用消息转发。我刚刚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 对象将自动将所有委托消息转发到常规委托对象，除了您在自定义子类中覆盖的那些。

Answer 2

e.James 的帖子为大多数观点提供了出色的解决方案。但是对于像 UITextField 和 UITextView 这样的依赖于键盘的视图，它往往会导致无限循环的情况。为了摆脱它，我用一些额外的代码来修复它，检查选择器是否包含在特定的协议中。

WZProtocolInterceptor.h

#import <Foundation/Foundation.h>

@interface WZProtocolInterceptor : NSObject
@property (nonatomic, readonly, copy) NSArray * interceptedProtocols;
@property (nonatomic, weak) id receiver;
@property (nonatomic, weak) id middleMan;

- (instancetype)initWithInterceptedProtocol:(Protocol *)interceptedProtocol;
- (instancetype)initWithInterceptedProtocols:(Protocol *)firstInterceptedProtocol, ... NS_REQUIRES_NIL_TERMINATION;
- (instancetype)initWithArrayOfInterceptedProtocols:(NSArray *)arrayOfInterceptedProtocols;
@end

WZProtocolInterceptor.m

#import  <objc/runtime.h>

#import "WZProtocolInterceptor.h"

static inline BOOL selector_belongsToProtocol(SEL selector, Protocol * protocol);

@implementation WZProtocolInterceptor
- (id)forwardingTargetForSelector:(SEL)aSelector
{
    if ([self.middleMan respondsToSelector:aSelector] &&
        [self isSelectorContainedInInterceptedProtocols:aSelector])
        return self.middleMan;

    if ([self.receiver respondsToSelector:aSelector])
        return self.receiver;

    return [super forwardingTargetForSelector:aSelector];
}

- (BOOL)respondsToSelector:(SEL)aSelector
{
    if ([self.middleMan respondsToSelector:aSelector] &&
        [self isSelectorContainedInInterceptedProtocols:aSelector])
        return YES;

    if ([self.receiver respondsToSelector:aSelector])
        return YES;

    return [super respondsToSelector:aSelector];
}

- (instancetype)initWithInterceptedProtocol:(Protocol *)interceptedProtocol
{
    self = [super init];
    if (self) {
        _interceptedProtocols = @[interceptedProtocol];
    }
    return self;
}

- (instancetype)initWithInterceptedProtocols:(Protocol *)firstInterceptedProtocol, ...;
{
    self = [super init];
    if (self) {
        NSMutableArray * mutableProtocols = [NSMutableArray array];
        Protocol * eachInterceptedProtocol;
        va_list argumentList;
        if (firstInterceptedProtocol)
        {
            [mutableProtocols addObject:firstInterceptedProtocol];
            va_start(argumentList, firstInterceptedProtocol);
            while ((eachInterceptedProtocol = va_arg(argumentList, id))) {
                [mutableProtocols addObject:eachInterceptedProtocol];
            }
            va_end(argumentList);
        }
        _interceptedProtocols = [mutableProtocols copy];
    }
    return self;
}

- (instancetype)initWithArrayOfInterceptedProtocols:(NSArray *)arrayOfInterceptedProtocols
{
    self = [super init];
    if (self) {
        _interceptedProtocols = [arrayOfInterceptedProtocols copy];
    }
    return self;
}

- (void)dealloc
{
    _interceptedProtocols = nil;
}

- (BOOL)isSelectorContainedInInterceptedProtocols:(SEL)aSelector
{
    __block BOOL isSelectorContainedInInterceptedProtocols = NO;
    [self.interceptedProtocols enumerateObjectsUsingBlock:^(Protocol * protocol, NSUInteger idx, BOOL *stop) {
        isSelectorContainedInInterceptedProtocols = selector_belongsToProtocol(aSelector, protocol);
        * stop = isSelectorContainedInInterceptedProtocols;
    }];
    return isSelectorContainedInInterceptedProtocols;
}

@end

BOOL selector_belongsToProtocol(SEL selector, Protocol * protocol)
{
    // Reference: https://gist.github.com/numist/3838169
    for (int optionbits = 0; optionbits < (1 << 2); optionbits++) {
        BOOL required = optionbits & 1;
        BOOL instance = !(optionbits & (1 << 1));

        struct objc_method_description hasMethod = protocol_getMethodDescription(protocol, selector, required, instance);
        if (hasMethod.name || hasMethod.types) {
            return YES;
        }
    }

    return NO;
}

这里是 Swift 2 版本：

//
//  NSProtocolInterpreter.swift
//  Nest
//
//  Created by Manfred Lau on 11/28/14.
//  Copyright (c) 2014 WeZZard. All rights reserved.
//

import Foundation

/**
`NSProtocolInterceptor` is a proxy which intercepts messages to the middle man 
which originally intended to send to the receiver.

- Discussion: `NSProtocolInterceptor` is a class cluster which dynamically
subclasses itself to conform to the intercepted protocols at the runtime.
*/
public final class NSProtocolInterceptor: NSObject {
    /// Returns the intercepted protocols
    public var interceptedProtocols: [Protocol] { return _interceptedProtocols }
    private var _interceptedProtocols: [Protocol] = []

    /// The receiver receives messages
    public weak var receiver: NSObjectProtocol?

    /// The middle man intercepts messages
    public weak var middleMan: NSObjectProtocol?

    private func doesSelectorBelongToAnyInterceptedProtocol(
        aSelector: Selector) -> Bool
    {
        for aProtocol in _interceptedProtocols
            where sel_belongsToProtocol(aSelector, aProtocol)
        {
            return true
        }
        return false
    }

    /// Returns the object to which unrecognized messages should first be 
    /// directed.
    public override func forwardingTargetForSelector(aSelector: Selector)
        -> AnyObject?
    {
        if middleMan?.respondsToSelector(aSelector) == true &&
            doesSelectorBelongToAnyInterceptedProtocol(aSelector)
        {
            return middleMan
        }

        if receiver?.respondsToSelector(aSelector) == true {
            return receiver
        }

        return super.forwardingTargetForSelector(aSelector)
    }

    /// Returns a Boolean value that indicates whether the receiver implements 
    /// or inherits a method that can respond to a specified message.
    public override func respondsToSelector(aSelector: Selector) -> Bool {
        if middleMan?.respondsToSelector(aSelector) == true &&
            doesSelectorBelongToAnyInterceptedProtocol(aSelector)
        {
            return true
        }

        if receiver?.respondsToSelector(aSelector) == true {
            return true
        }

        return super.respondsToSelector(aSelector)
    }

    /**
    Create a protocol interceptor which intercepts a single Objecitve-C 
    protocol.

    - Parameter     protocols:  An Objective-C protocol, such as
    UITableViewDelegate.self.
    */
    public class func forProtocol(aProtocol: Protocol)
        -> NSProtocolInterceptor
    {
        return forProtocols([aProtocol])
    }

    /**
    Create a protocol interceptor which intercepts a variable-length sort of
    Objecitve-C protocols.

    - Parameter     protocols:  A variable length sort of Objective-C protocol,
    such as UITableViewDelegate.self.
    */
    public class func forProtocols(protocols: Protocol ...)
        -> NSProtocolInterceptor
    {
        return forProtocols(protocols)
    }

    /** 
    Create a protocol interceptor which intercepts an array of Objecitve-C 
    protocols.

    - Parameter     protocols:  An array of Objective-C protocols, such as
    [UITableViewDelegate.self].
    */
    public class func forProtocols(protocols: [Protocol])
        -> NSProtocolInterceptor
    {
        let protocolNames = protocols.map { NSStringFromProtocol($0) }
        let sortedProtocolNames = protocolNames.sort()
        let concatenatedName = sortedProtocolNames.joinWithSeparator(",")

        let theConcreteClass = concreteClassWithProtocols(protocols,
            concatenatedName: concatenatedName,
            salt: nil)

        let protocolInterceptor = theConcreteClass.init()
            as! NSProtocolInterceptor
        protocolInterceptor._interceptedProtocols = protocols

        return protocolInterceptor
    }

    /**
    Return a subclass of `NSProtocolInterceptor` which conforms to specified 
        protocols.

    - Parameter     protocols:          An array of Objective-C protocols. The
    subclass returned from this function will conform to these protocols.

    - Parameter     concatenatedName:   A string which came from concatenating
    names of `protocols`.

    - Parameter     salt:               A UInt number appended to the class name
    which used for distinguishing the class name itself from the duplicated.

    - Discussion: The return value type of this function can only be
    `NSObject.Type`, because if you return with `NSProtocolInterceptor.Type`, 
    you can only init the returned class to be a `NSProtocolInterceptor` but not
    its subclass.
    */
    private class func concreteClassWithProtocols(protocols: [Protocol],
        concatenatedName: String,
        salt: UInt?)
        -> NSObject.Type
    {
        let className: String = {
            let basicClassName = "_" +
                NSStringFromClass(NSProtocolInterceptor.self) +
                "_" + concatenatedName

            if let salt = salt { return basicClassName + "_\(salt)" }
                else { return basicClassName }
        }()

        let nextSalt = salt.map {$0 + 1}

        if let theClass = NSClassFromString(className) {
            switch theClass {
            case let anInterceptorClass as NSProtocolInterceptor.Type:
                let isClassConformsToAllProtocols: Bool = {
                    // Check if the found class conforms to the protocols
                    for eachProtocol in protocols
                        where !class_conformsToProtocol(anInterceptorClass,
                            eachProtocol)
                    {
                        return false
                    }
                    return true
                    }()

                if isClassConformsToAllProtocols {
                    return anInterceptorClass
                } else {
                    return concreteClassWithProtocols(protocols,
                        concatenatedName: concatenatedName,
                        salt: nextSalt)
                }
            default:
                return concreteClassWithProtocols(protocols,
                    concatenatedName: concatenatedName,
                    salt: nextSalt)
            }
        } else {
            let subclass = objc_allocateClassPair(NSProtocolInterceptor.self,
                className,
                0)
                as! NSObject.Type

            for eachProtocol in protocols {
                class_addProtocol(subclass, eachProtocol)
            }

            objc_registerClassPair(subclass)

            return subclass
        }
    }
}

/**
Returns true when the given selector belongs to the given protocol.
*/
public func sel_belongsToProtocol(aSelector: Selector,
    _ aProtocol: Protocol) -> Bool
{
    for optionBits: UInt in 0..<(1 << 2) {
        let isRequired = optionBits & 1 != 0
        let isInstance = !(optionBits & (1 << 1) != 0)

        let methodDescription = protocol_getMethodDescription(aProtocol,
            aSelector, isRequired, isInstance)

        if !objc_method_description_isEmpty(methodDescription)
        {
            return true
        }
    }
    return false
}

public func objc_method_description_isEmpty(
    var methodDescription: objc_method_description)
    -> Bool
{
    let ptr = withUnsafePointer(&methodDescription) { UnsafePointer<Int8>($0) }
    for offset in 0..<sizeof(objc_method_description) {
        if ptr[offset] != 0 {
            return false
        }
    }
    return true
}

Answer 3

实际上，这对我有用：

@implementation MySubclass {
    id _actualDelegate;
}

// There is no need to set the value of _actualDelegate in an init* method
- (void)setDelegate:(id)newDelegate {
    [super setDelegate:nil];
    _actualDelegate = newDelegate;
    [super setDelegate:(id)self];
}

- (id)delegate {
    return self;
}

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

- (BOOL)respondsToSelector:(SEL)aSelector {
    return [super respondsToSelector:aSelector] || [_actualDelegate respondsToSelector:aSelector];
}
@end

...使子类成为 e.James 给出的绝妙答案中的消息拦截器。

Answer 4

是的，但是您必须覆盖the docs 中的每个委托方法。基本上，创建第二个委托属性并实现委托协议。当您的委托方法被调用时，请处理好您的业务，然后从刚刚运行的委托方法中对您的第二个委托调用相同的方法。例如。

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    // Do stuff here
    if ([self.delegate2 respondsToSelector:@selector(scrollViewDidScroll:)]) {
        [self.delegate2 scrollViewDidScroll:scrollView];
    }
}