NSProxy

一、官方文档

An abstract superclass defining an API for objects that act as stand-ins for other objects or for objects that don’t exist yet.
定义对象API的抽象超类，该对象充当其他对象或尚不存在的对象的替身。

通常，发送给代理的消息被转发到实际对象，或者导致代理加载(或将自身转换为)实际对象。NSProxy的子类可以用来实现透明的分布式消息传递(例如，NSDistantObject)，或者用于延迟实例化那些创建起来代价昂贵的对象。

作为一个抽象类，它不提供初始化方法，并且在接收到它不响应的任何消息时会引发异常。因此具体的子类必须提供一个初始化或创建方法，并覆盖forwardInvocation(:)和methodSignatureForSelector:方法来处理它自己没有实现的消息。子类的forwardInvocation(:)实现应该执行处理调用所需的任何操作，比如通过网络转发调用，或者加载实际对象并将调用传递给它。methodSignatureForSelector:需要为给定消息提供参数类型信息;一个子类的实现应该能够确定它需要转发的消息的参数类型，并且应该相应地构造一个NSMethodSignature对象。更多信息，请参见NSDistantObject、NSInvocation和NSMethodSignature类规范。

二、接口文档

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

#import 

@class NSMethodSignature, NSInvocation;

NS_ASSUME_NONNULL_BEGIN

NS_ROOT_CLASS
@interface NSProxy  {
    __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

func finalize()
The garbage collector invokes this method on the receiver before disposing of the memory it uses.

func forwardInvocation(NSInvocation)
Passes a given invocation to the real object the proxy represents.

methodSignatureForSelector:
Raises NSInvalidArgumentException. Override this method in your concrete subclass to return a proper NSMethodSignature object for the given selector and the class your proxy objects stand in for.

NSInvocation

An Objective-C message rendered as an object.

NSInvocation的使用

iOS给某个对象的发消息方式有两种:

• performSelector:withObject
• 通过创建NSInvocation调用某个对象的方法

代码实现：

 - (void)viewDidLoad {
    [super viewDidLoad];
    //NSInvocation;用来包装方法和对应的对象，它可以存储方法的名称，对应的对象，对应的参数,
    /*
     NSMethodSignature：签名：再创建NSMethodSignature的时候，必须传递一个签名对象，签名对象的作用：用于获取参数的个数和方法的返回值
     */
    //创建签名对象的时候不是使用NSMethodSignature这个类创建，而是方法属于谁就用谁来创建
    NSMethodSignature*signature = [ViewController instanceMethodSignatureForSelector:@selector(sendMessageWithNumber:WithContent:)];
    //1、创建NSInvocation对象
    NSInvocation*invocation = [NSInvocation invocationWithMethodSignature:signature];
    invocation.target = self;
    //invocation中的方法必须和签名中的方法一致。
    invocation.selector = @selector(sendMessageWithNumber:WithContent:);
    /*第一个参数：需要给指定方法传递的值
           第一个参数需要接收一个指针，也就是传递值的时候需要传递地址*/
    //第二个参数：需要给指定方法的第几个参数传值
    NSString*number = @"1111";
    //注意：设置参数的索引时不能从0开始，因为0已经被self占用，1已经被_cmd占用
    [invocation setArgument:&number atIndex:2];
    NSString*number2 = @"啊啊啊";
    [invocation setArgument:&number2 atIndex:3];
    //2、调用NSInvocation对象的invoke方法
    //只要调用invocation的invoke方法，就代表需要执行NSInvocation对象中制定对象的指定方法，并且传递指定的参数
    [invocation invoke];
}
 - (void)sendMessageWithNumber:(NSString*)number WithContent:(NSString*)content{
    NSLog(@"电话号%@,内容%@",number,content);
}

三、NSProxy使用

1、避免循环引用

#import 

@interface WeakProxy : NSProxy

+ (id)proxyWithTarget:(id)target;
- (instancetype)initWithTarget:(id)target;

@end

#import "WeakProxy.h"

@interface WeakProxy ()
@property (nonatomic, weak) id target;
@end

@implementation WeakProxy

- (instancetype)initWithTarget:(id)target {
    _target = target;
    return self;
}

+ (id)proxyWithTarget:(id)target {
    return [[self alloc] initWithTarget:target];
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    SEL selector = [invocation selector];
    if ([_target respondsToSelector:selector]) {
        [invocation invokeWithTarget:_target];
    }
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [_target methodSignatureForSelector:sel];
}

@end

 let timer = Timer(timeInterval: interval, target: WeakProxy.proxy(withTarget: self), selector: #selector(timerCallback), userInfo: nil, repeats: repeats)
        RunLoop.current.add(timer, forMode: RunLoop.Mode.common)

2、NSProxy面向切面

3、实现多继承