NSProxy

一、什么是NSProxy

（1）NSProxy是一個(gè)抽象的基類甘有，是根類巾兆，與NSObject類似；

（2）NSProxy和NSObject都實(shí)現(xiàn)了<NSObject>協(xié)議棍现；

（3）提供了消息轉(zhuǎn)發(fā)的通用接口。

查看NSProxy類：

nsproxy.png

二伪朽、NSProxy和NSObject消息傳遞的異同

1轴咱、NSObject消息傳遞的流程：

（1）NSObject收到消息會(huì)先去緩存列表查找SEL汛蝙，若是找不到烈涮，就到自身方法列表中查找朴肺，依然找不到就順著繼承鏈進(jìn)行查找，依然找不到的話坚洽，那就是unknown selector戈稿，進(jìn)入消息轉(zhuǎn)發(fā)程序。

（2）調(diào)用+(BOOL)resolveInstanceMethod: 其返回值為BOOL類型讶舰，表示這個(gè)類是否通過class_addMethod新增一個(gè)實(shí)例方法用以處理該 unknown selector鞍盗，也就是說在這里可以新增一個(gè)處理unknown selector的方法，若不能跳昼，則繼續(xù)往下傳遞（若unknown selector是類方法般甲，那么調(diào)用 +(BOOL)resolveClassMethod:方法）

（3）調(diào)用-(id)forwardingTargetForSelector: 這是第二次機(jī)會(huì)處理unknown selector，即轉(zhuǎn)移給一個(gè)能處理unknown selector的其它對(duì)象鹅颊，若返回一個(gè)其它的執(zhí)行對(duì)象敷存，那消息從objc_msgSend (id self, SEL op, ...) 重新開始，若不能堪伍，則返回nil锚烦，并繼續(xù)向下傳遞，最后的一次消息處理機(jī)會(huì)帝雇。

（4）調(diào)用-(NSMethodSignature *)methodSignatureForSelector: 返回?cái)y帶參數(shù)類型涮俄、返回值類型和長(zhǎng)度等的selector簽名信息 NSMethodSignature對(duì)象，Runtime會(huì)基于NSMethodSignature實(shí)例構(gòu)建一個(gè)NSInvocation對(duì)象尸闸，作為回調(diào)forwardInvocation:的入?yún)ⅰ?/h6>

（5）調(diào)用-(void)forwardInvocation:這一步可以對(duì)傳進(jìn)來的NSInvocation進(jìn)行一些操作彻亲，把尚未處理的消息有關(guān)的全部細(xì)節(jié)都封裝在NSInvocation中，包括selector室叉，目標(biāo)(target)和參數(shù)等睹栖。

2、NSProxy消息傳遞的流程：

（1）接收到unknown selector后茧痕，直接回調(diào)methodSignatureForSelector:方法野来，返回?cái)y帶參數(shù)類型、返回值類型和長(zhǎng)度等的selector簽名信息 NSMethodSignature對(duì)象踪旷，Runtime會(huì)基于NSMethodSignature實(shí)例構(gòu)建一個(gè)NSInvocation對(duì)象曼氛，作為回調(diào)forwardInvocation:的入?yún)ⅰ?/h6>

（2）調(diào)用-(void)forwardInvocation:這一步可以對(duì)傳進(jìn)來的NSInvocation進(jìn)行一些操作，把尚未處理的消息有關(guān)的全部細(xì)節(jié)都封裝在NSInvocation中令野，包括selector舀患，目標(biāo)(target)和參數(shù)等。

三气破、NSProxy用法

1聊浅、實(shí)現(xiàn)多繼承

場(chǎng)景：西紅柿作為一種常見的食物，既是水果又是蔬菜。用程序語言來說低匙，西紅柿類應(yīng)當(dāng)既繼承水果類旷痕，又繼承蔬菜類，但是在OC中只有單繼承顽冶，如何模擬實(shí)現(xiàn)多繼承呢欺抗？

首先识窿，實(shí)現(xiàn)Fruit和Vegetable類：

########水果類#########
////水果類：Fruit.h
@protocol FruitProtocol <NSObject>
@property (nonatomic, copy) NSString *texture;
@property (nonatomic, copy) NSString *name;
- (void)fruitDesc;
@end

@interface Fruit : NSObject
@end

////水果類：Fruit.m
@interface Fruit() <FruitProtocol>
@end

@implementation Fruit
//告訴編譯器至耻、texture屬性的setter、getter方法由編譯器來生成婶溯、同時(shí)用_texture來合成成員變量
@synthesize texture = _texture;
@synthesize name = _name;

- (void)fruitDesc
{
    NSLog(@"%@间景，作為水果能夠促進(jìn)消化佃声。口感：%@", _name, _texture);
}
@end


########蔬菜類#########
////蔬菜類：Vegetable.h
@protocol VegatableProtocol <NSObject>
@property (nonatomic, copy) NSString *anotherName;
- (void)vegetableDesc;
@end

@interface Vegetable : NSObject
@end

////蔬菜類：Vegetable.m
@interface Vegetable() <VegatableProtocol>
@end

@implementation Vegetable
@synthesize anotherName = _anotherName;

- (void)vegetableDesc
{
    NSLog(@"%@倘要，作為蔬菜可提供多種維生素秉溉。", _anotherName);
}
@end

其次，讓代理TomatoProxy類遵循協(xié)議<FruitProtocol, VegatableProtocol>碗誉。通過重寫forwardInvocation:和methodSignatureForSelector:方法將消息轉(zhuǎn)給TomatoProxy類來響應(yīng)召嘶。methodSignatureForSelector:方法，返回的是一個(gè)NSMethodSignature類型哮缺，來描述給定selector的參數(shù)和返回值類型弄跌。返回nil，表示proxy不能識(shí)別指定的selector尝苇。forwardInvocation:方法將消息轉(zhuǎn)發(fā)到對(duì)應(yīng)的對(duì)象上铛只。

########西紅柿類#########
////TomatoProxy.h
@interface TomatoProxy : NSProxy <FruitProtocol, VegatableProtocol>
+ (instancetype)tomatoProxy;
@end

////TomatoProxy.m
@implementation TomatoProxy
{
    Fruit *_fruit;
    Vegetable *_vegetable;
    NSMutableDictionary *_targetMethodsHashMap;
}

+ (instancetype)tomatoProxy
{
    return [[TomatoProxy alloc] init];
}

//創(chuàng)建init方法
- (instancetype)init
{
    //初始化需要代理的對(duì)象(target)，和存儲(chǔ)對(duì)象方法的哈希表
    _targetMethodsHashMap = @{}.mutableCopy;
    _fruit = [[Fruit alloc] init];
    _vegetable = [[Vegetable alloc] init];
    
    [self registerMethodsForTarget:_fruit];
    [self registerMethodsForTarget:_vegetable];
    
    return self;
}

- (void)forwardInvocation:(NSInvocation *)invocation
{
    SEL aSel = invocation.selector;
    NSString *methodKey = NSStringFromSelector(aSel);
    id targetObject = _targetMethodsHashMap[methodKey];
    if (targetObject && [targetObject respondsToSelector:aSel]) {
        [invocation invokeWithTarget:targetObject];
    }
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel
{
    NSString *methodName = NSStringFromSelector(sel);
    id targetObject = _targetMethodsHashMap[methodName];
    if (targetObject && [targetObject respondsToSelector:sel]) {
        return [targetObject methodSignatureForSelector:sel];
    }
    return nil;
}

/**
 將目標(biāo)對(duì)象的所有方法添加到哈希表中
 @param aTarget 目標(biāo)對(duì)象
 key 方法名
 value 對(duì)象地址
 */
- (void)registerMethodsForTarget:(id)aTarget
{
    unsigned int count = 0;
    Method *methodList = class_copyMethodList([aTarget class], &count);
    
    for(int i = 0; i < count; i++) {
        Method aMethod = methodList[i];
        SEL aSel = method_getName(aMethod);
        const char *methodName = sel_getName(aSel);
        [_targetMethodsHashMap setObject:aTarget forKey:[NSString stringWithUTF8String:methodName]];
    }
    free(methodList);
}

@end

最后糠溜，在main函數(shù)中就可以通過TomatoProxy類訪問Fruit類和Vegetable類的屬性和方法了淳玩。

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // 1、使用NSProxy模擬多繼承非竿。
        // TomatoProxy同時(shí)可以具備Fruit和Vegetable類的特性
        TomatoProxy *tomato = [TomatoProxy tomatoProxy];
        //設(shè)置Fruit類的屬性蜕着，調(diào)用Fruit類的方法
        tomato.name = @"西紅柿";
        tomato.texture = @"酸酸甜甜";
        [tomato fruitDesc];
        //設(shè)置vegetable類的屬性，調(diào)用vegetable類的方法
        tomato.anotherName = @"番茄";
        [tomato vegetableDesc];
    }
    return 0;
}

//打印結(jié)果：
2019-03-21 15:34:15.247663+0800 NSProxyTest[44704:13499339] 西紅柿红柱，作為水果能夠促進(jìn)消化承匣。口感：酸酸甜甜
2019-03-21 15:38:04.690131+0800 NSProxyTest[44704:13499339] 番茄锤悄，作為蔬菜可提供多種維生素

2韧骗、利用NSProxy解決NSTimer循環(huán)引用

//SecondViewController.m
@interface SecondViewController ()
@property (nonatomic, strong) NSTimer *timer;
@end

@implementation SecondViewController
- (void)viewDidLoad
{
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    [self startTimer];
}
- (void)startTimer
{
    self.timer = [NSTimer timerWithTimeInterval:3.0
                                         target:self
                                       selector:@selector(timerInvoked:)
                                       userInfo:nil
                                        repeats:YES];
    [[NSRunLoop mainRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];
}
- (void)timerInvoked:(NSTimer *)timer
{
    NSLog(@"======== timer ========");
}
- (void)dealloc
{
    [self.timer invalidate];
    self.timer = nil;
    NSLog(@"SecondViewController dealloc");
}
@end

上面代碼中，從ViewController push到SecondViewController之后零聚，開啟一個(gè)定時(shí)器袍暴，當(dāng)頁(yè)面pop回ViewController之后些侍，定時(shí)器未被銷毀。其原因如下圖所示：

timer_cycle.png

稍作修改：

////TimerWeakProxy.h
@interface TimerWeakProxy : NSProxy
/**
 創(chuàng)建代理對(duì)象
 @param target 被代理的對(duì)象
 @return 代理對(duì)象
 */
+ (instancetype)proxyWithTarget:(id)target;
@end

////TimerWeakProxy.m
@interface TimerWeakProxy ()
@property (weak, nonatomic, readonly) id target; //被代理的對(duì)象為weak
@end

@implementation TimerWeakProxy
+ (instancetype)proxyWithTarget:(id)target
{
    return [[self alloc] initWithTarget:target];
}
- (instancetype)initWithTarget:(id)target
{
    _target = target;
    return self;
}
- (void)forwardInvocation:(NSInvocation *)invocation
{
    SEL sel = [invocation selector];
    if (_target && [_target respondsToSelector:sel]) {
        [invocation invokeWithTarget:_target];
    }
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
    return [_target methodSignatureForSelector:aSelector];
}
@end

////SecondViewController.m
- (void)startTimer
{
    self.timer = [NSTimer timerWithTimeInterval:3.0
                                         target:[TimerWeakProxy proxyWithTarget:self]
                                       selector:@selector(timerInvoked:)
                                       userInfo:nil
                                        repeats:YES];
    [[NSRunLoop mainRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];
}

weakProxy_timer.png

3政模、實(shí)現(xiàn)協(xié)議分發(fā)器（一對(duì)多代理）

在OC中娩梨，delegate一般用于對(duì)象之間一對(duì)一的通信，但是有時(shí)候我們希望可以有多個(gè)對(duì)象同時(shí)響應(yīng)一個(gè)protocol览徒。基于這種需求我們可以利用NSProxy實(shí)現(xiàn)一個(gè)消息分發(fā)器颂龙，將protocol中需要實(shí)現(xiàn)的方法交由分發(fā)器轉(zhuǎn)發(fā)給多個(gè)遵循protocol的對(duì)象习蓬，從而實(shí)現(xiàn)一對(duì)多的delegate。

一對(duì)一delegate的工作模式：

one_to_one_delegate.png

協(xié)議分發(fā)器的工作模式：

one_to_multi_delegate.png

簡(jiǎn)言之措嵌，第一步：將需要遵循的protocol及遵循該protocol的對(duì)象交由分發(fā)器躲叼；第二步：分發(fā)器將消息轉(zhuǎn)發(fā)給可以響應(yīng)protocol中方法的多個(gè)delegate對(duì)象；第三步：每個(gè)delegate對(duì)象分別實(shí)現(xiàn)protocol中聲明的方法企巢。

協(xié)議分發(fā)器的實(shí)現(xiàn)：

// 定義一個(gè)方法
//struct objc_method_description {
//SEL _Nullable name;               /**< The name of the method */
//char * _Nullable types;           /**< The types of the method arguments */
//};
//protocol_getMethodDescription: 為指定的method或者protocol返回一個(gè)method description structure
//protocol_getMethodDescription(Protocol * _Nonnull proto, SEL _Nonnull aSel, BOOL isRequiredMethod, BOOL isInstanceMethod)
//
struct objc_method_description MethodDescriptionForSelWithProtocol(Protocol *aProtocol, SEL aSel)
{
    struct objc_method_description desc = protocol_getMethodDescription(aProtocol, aSel, YES, YES);
    if (desc.types) {
        return desc;
    }
    desc = protocol_getMethodDescription(aProtocol, aSel, NO, YES);
    if (desc.types) {
        return desc;
    }
    return (struct objc_method_description) {NULL, NULL};
};

// 判斷selector是否屬于某一Protocol
BOOL ProtocolContainsSelector(Protocol *aProtocol, SEL aSel)
{
    return MethodDescriptionForSelWithProtocol(aProtocol, aSel).types ? YES : NO;
}

//*********** 協(xié)議消息轉(zhuǎn)發(fā)器 ***********//
@interface ProtocolMessageDispatcher ()
@property (nonatomic, strong) Protocol *prococol; //協(xié)議
@property (nonatomic, strong) NSArray *targets; //協(xié)議實(shí)現(xiàn)者對(duì)象（多個(gè)）
@end

@implementation ProtocolMessageDispatcher

+ (id)dispatchProtocol:(Protocol *)aProtocol withTargets:(NSArray *)theTargets
{
    return [[ProtocolMessageDispatcher alloc] initWithProtocol:aProtocol withTargets:theTargets];
}

- (instancetype)initWithProtocol:(Protocol *)aProtocol withTargets:(NSArray *)theTargets
{
    self.prococol = aProtocol;
    NSMutableArray *valideObjects = @[].mutableCopy;
    for (id object in theTargets) {
        if ([object conformsToProtocol:aProtocol]) {
            objc_setAssociatedObject(object, _cmd, self, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
            [valideObjects addObject:object];
        }
    }
    self.targets = valideObjects.copy;
    return self;
}

- (BOOL)respondsToSelector:(SEL)aSelector
{
    if (!ProtocolContainsSelector(self.prococol, aSelector)) {
        return [super respondsToSelector:aSelector];
    }
    for (id object in self.targets) {
        if ([object respondsToSelector:aSelector]) {
            return YES;
        }
    }
    return NO;
}

/*
返回?cái)y帶參數(shù)類型枫慷、返回值類型和長(zhǎng)度等的selector簽名信息 NSMethodSignature對(duì)象，Runtime會(huì)基于NSMethodSignature實(shí)例構(gòu)建一個(gè)NSInvocation對(duì)象浪规，作為回調(diào)forwardInvocation:的入?yún)?*/
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel
{
    if (!ProtocolContainsSelector(self.prococol, sel)) {
        return [super methodSignatureForSelector:sel];
    }
    struct objc_method_description methodDesc = MethodDescriptionForSelWithProtocol(self.prococol, sel);
    return [NSMethodSignature signatureWithObjCTypes:methodDesc.types];
}

/*
這一步可以對(duì)傳進(jìn)來的NSInvocation進(jìn)行一些操作或听，把尚未處理的消息有關(guān)的全部細(xì)節(jié)都封裝在NSInvocation中，包括selector笋婿，目標(biāo)(target)和參數(shù)等
*/
- (void)forwardInvocation:(NSInvocation *)invocation
{
    SEL aSel = invocation.selector;
    if (!ProtocolContainsSelector(self.prococol, aSel)) {
        [super forwardInvocation:invocation];
        return;
    }
    for (id object in self.targets) {
        if ([object respondsToSelector:aSel]) {
            [invocation invokeWithTarget:object]; //object是最后真正處理invocation的對(duì)象
        }
    }
}

使用協(xié)議分發(fā)器：

/// self和TableDelegate都成為tableView的代理
/// 當(dāng)點(diǎn)擊cell時(shí) self和TableDelegate中的方法都會(huì)被調(diào)用
self.tableView.delegate = [ProtocolMessageDispatcher dispatchProtocol:@protocol(UITableViewDelegate) withTargets:@[self, [TableDelegate new]]];

///點(diǎn)擊cell時(shí)誉裆，兩個(gè)delegate對(duì)象的方法都會(huì)執(zhí)行：
2019-04-03 17:02:31.733479+0800 NSProxyDemo[42445:13711860] -[DispatchMessageController tableView:didSelectRowAtIndexPath:]
2019-04-03 17:02:31.733687+0800 NSProxyDemo[42445:13711860] -[TableDelegate tableView:didSelectRowAtIndexPath:]

參考資料：

https://developer.apple.com/library/archive/samplecode/ForwardInvocation/Listings/main_m.html#//apple_ref/doc/uid/DTS40008833-main_m-DontLinkElementID_4
http://yehuanwen.github.io/2016/10/18/nsproxy/
http://www.olinone.com/?p=643
http://www.reibang.com/p/8e700673202b
http://www.reibang.com/p/b0f5fd3e4b7c
http://ggghub.com/2016/05/11/%E5%88%A9%E7%94%A8NSProxy%E8%A7%A3%E5%86%B3NSTimer%E5%86%85%E5%AD%98%E6%B3%84%E6%BC%8F%E9%97%AE%E9%A2%98/
NSProxy和NSObject：http://www.reibang.com/p/5bfcc32c21c0
NSProxy與UIKit框架：https://mazyod.com/blog/2014/03/10/nsproxy-with-uikit/#Enter-the-NSProxy-Class
NSProxy與KVO：https://stackoverflow.com/questions/9054970/nsproxy-and-key-value-observing