一个应用 ，一般都会有很多对象，对象与对象之间避免不了进行消息传递。说到消息传递，肯定是相互的，即由A传到B，也可以由B传的A。

iOS下，有以下消息传递方式：

1. 属性（Get/Set）
2. 方法（Public）
3. KVO
4. Target-Action
5. 通知（NSNotification）
6. 代理（DataSource/Delegate,其实是Protocol）
7. Block

这篇文章主要介绍Target-Action，其他方法这里暂不介绍。

先来看以下Target-Action在系统提供的Framework中都有哪些使用。

1. UIKit.framework 
   有UIControl以及其子类（比如UIButton、UISwitch等）

   UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
   btn.frame = CGRectMake(0.0, 0.0, 100.0, 50.0);
   [btn addTarget:self action:@selector(clickButton:) forControlEvents:UIControlEventTouchUpInside];
   [self.view addSubview:btn];

2. Foundation.framework 
   有NSNotification、NSTimer

   [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(test) name:@"TEST" object:nil];

   [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(test) userInfo:nil repeats:NO];

   上面提及到的示例，相信大家都有所了解，这里不多介绍，直接提需求，如果我们想自己实现Target-Action该如何做？ 
   直接上代码

方案1：

#import <Foundation/Foundation.h>
//
@interface Manager : NSObject
//
-(void)addTarget:(id)aTarget selector:(SEL)aSelector;
@end

#import "Manager.h"
//
@interface Manager ()
//
@property (nonatomic, weak) id target;
@property (nonatomic, assign) SEL selector;
@end
//
@implementation Manager
//
-(id)init {
    self = [super init];
    if (self) {
        //
    }
    return self;
}
//
 (void)addTarget:(id)aTarget selector:(SEL)aSelector {
    _target = aTarget;
    _selector = aSelector;
}
@end

方案2

#import <Foundation/Foundation.h>
//
@interface Manager : NSObject
//
-(void)addTarget:(id)aTarget selector:(SEL)aSelector forEvent:(NSInteger)aEvent;
-(void)removeTarget:(id)aTarget forEvent:(NSInteger)aEvent;
@end

#import "Manager.h"
//
@interface Manager ()
//
@property (nonatomic, strong) NSMutableDictionary *targetDict;
@end
//
@implementation Manager
//
-(id)init {
    self = [super init];
    if (self) {
        _targetDict = [[NSMutableDictionary alloc] initWithCapacity:0];
    }
    return self;
}
//
-(void)addTarget:(id)aTarget selector:(SEL)aSelector forEvent:(NSInteger)aEvent {
    NSString *key = [NSString stringWithFormat:@"%zd",aEvent];
    NSMutableDictionary *dict = [[NSMutableDictionary alloc] initWithCapacity:0];
    [dict setValue:aTarget forKey:@"target"];
    [dict setValue:NSStringFromSelector(aSelector) forKey:@"action"];
    [_targetDict setValue:dict forKey:key];
}
//
-(void)removeTarget:(id)aTarget forEvent:(NSInteger)aEvent {
    NSString *key = [NSString stringWithFormat:@"%zd",aEvent];
    if ([[_targetDict allKeys] containsObject:key]) {
        [_targetDict removeObjectForKey:key];
    }
}
@end

方案3

#import <Foundation/Foundation.h>
NS_ASSUME_NONNULL_BEGIN

@interface WeakProxy : NSProxy
//
@property (nullable, nonatomic, weak, readonly) id target;
//
-(instancetype)initWithTarget:(id)target;
+(instancetype)proxyWithTarget:(id)target;

NS_ASSUME_NONNULL_END
@end

#import "WeakProxy.h"

@implementation WeakProxy

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

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

- (id)forwardingTargetForSelector:(SEL)selector {
    return _target;
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    void *null = NULL;
    [invocation setReturnValue:&null];
}

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

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

#import <Foundation/Foundation.h>

@interface Manager : NSObject

- (void)addTarget:(id)aTarget selector:(SEL)aSelector forEvent:(NSInteger)aEvent;
- (void)removeTarget:(id)aTarget forEvent:(NSInteger)aEvent;

@end

#import "Manager.h"
#import "WeakProxy.h"

@interface Manager ()

@property (nonatomic, strong) NSMutableDictionary *targetDict;

@end

@implementation Manager

- (id)init {
    self = [super init];
    if (self) {
        _targetDict = [[NSMutableDictionary alloc] initWithCapacity:0];
    }
    return self;
}

- (void)addTarget:(id)aTarget selector:(SEL)aSelector forEvent:(NSInteger)aEvent {
    WeakProxy *proxy = [WeakProxy proxyWithTarget:aTarget];

    NSString *key = [NSString stringWithFormat:@"%zd",aEvent];
    NSMutableDictionary *dict = [[NSMutableDictionary alloc] initWithCapacity:0];
    [dict setValue:proxy forKey:@"target"];
    [dict setValue:NSStringFromSelector(aSelector) forKey:@"action"];
    [_targetDict setValue:dict forKey:key];
}

- (void)removeTarget:(id)aTarget forEvent:(NSInteger)aEvent {
    NSString *key = [NSString stringWithFormat:@"%zd",aEvent];
    if ([[_targetDict allKeys] containsObject:key]) {
        [_targetDict removeObjectForKey:key];
    }
}

@end

好啦，到这我想到的解决方案就是这些了。有兴趣的可以比较方案1、2、3的区别！！