ios使用NSProxy实现消息转发

/*    NSProxy.h
    Copyright (c) 1994-2019, Apple Inc. All rights reserved.
*/

#import <Foundation/NSObject.h>

@class NSMethodSignature, NSInvocation;

NS_ASSUME_NONNULL_BEGIN

NS_ROOT_CLASS
@interface NSProxy <NSObject> {
    __ptrauth_objc_isa_pointer Class    isa;
}

+ (id)alloc;
+ (id)allocWithZone:(nullable NSZone *)zone NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
+ (Class)class;

- (void)forwardInvocation:(NSInvocation *)invocation;
- (nullable NSMethodSignature *)methodSignatureForSelector:(SEL)sel NS_SWIFT_UNAVAILABLE("NSInvocation and related APIs not available");
- (void)dealloc;
- (void)finalize;
@property (readonly, copy) NSString *description;
@property (readonly, copy) NSString *debugDescription;
+ (BOOL)respondsToSelector:(SEL)aSelector;

- (BOOL)allowsWeakReference API_UNAVAILABLE(macos, ios, watchos, tvos);
- (BOOL)retainWeakReference API_UNAVAILABLE(macos, ios, watchos, tvos);

// - (id)forwardingTargetForSelector:(SEL)aSelector;

@end

NS_ASSUME_NONNULL_END

@interface TargetProxy : NSProxy


/// 初始化方法，保存两个真实对象
/// @param object1 第一个真实对象
/// @param object2 第二个真实对象
- (instancetype)initWithObject1:(id)object1 object2:(id)object2;

@end

@implementation TargetProxy {

    // 保存需要将消息转发到的第一个真实对象
    // 第一个真实对象的方法调用优先级会比第二个真实对象的方法调用优先级高
    id _realObject1;
    // 保存需要将消息转发到的第二个真实对象
    id _realObject2;
}

- (instancetype)initWithObject1:(id)object1 object2:(id)object2 {
    _realObject1 = object1;
    _realObject2 = object2;
    
    return self;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    // 获取 _realObject1 中 sel 的方法签名
    NSMethodSignature *signature = [_realObject1 methodSignatureForSelector:sel];
    // 如果 _realObject1 中有该方法，那么返回该方法的签名
    // 如果没有，返回 _realObject1 方法签名
    if (signature) {
        return signature;
    }
    // 获取 _realObject1 中的 sel 的方法签名
    signature = [_realObject2 methodSignatureForSelector:sel];
    return signature;
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    // 获取拥有该方法的真实对象
    id target = [_realObject1 methodSignatureForSelector:[invocation selector]] ? _realObject1 : _realObject2;

    // 执行方法
    [invocation invokeWithTarget:target];
}

- (void)testTargetProxy {
    NSMutableString *string = [NSMutableString string];
    NSMutableArray *array = [NSMutableArray array];
    
    id proxy = [[TargetProxy alloc] initWithObject1:string object2:array];
    [proxy appendString:@"This "];
    [proxy appendString:@"is "];
    [proxy addObject:string];
    [proxy appendString:@"a "];
    [proxy appendString:@"test!"];
    
    NSLog(@"The string is length is: %@", [proxy valueForKey:@"length"]);
    NSLog(@"count should be 1, it is %ld", [proxy count]);
    
    if ([[proxy objectAtIndex:0] isEqualToString:@"This is a test!"]) {
        NSLog(@"Appending successful.");
    } else {
        NSLog(@"Appending failed,, got: '%@'", proxy);
    }
}