探究ReactiveCocoa 底层KVO封装流程

一、对比原生KVO,初识ReactiveCocoa的KVO

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我们先来看一段代码,通过触屏来动态修改视图背景色

@interface ViewController ()
@property (nonatomic, strong)UIColor * bgColor;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.

    //1/Normal KVO
    [self normalKVO];

    //2/RACKVO
    [self racObserver];
}

#pragma mark normalKVO
- (void)normalKVO {
    [self addObserver:self forKeyPath:@"bgColor" options:(NSKeyValueObservingOptionNew) context:nil];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    self.view.backgroundColor = [change objectForKey:NSKeyValueChangeNewKey];;
}

- (void)dealloc {
    [self removeObserver:self forKeyPath:@"bgColor"];
}

#pragma mark racKVO
- (void)racObserver {
    [RACObserve(self, bgColor) subscribeNext:^(id  _Nullable x) {
        self.view.backgroundColor = (UIColor *)x;
    }];
}

#pragma mark touch change

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    CGFloat red = arc4random() % 256 / 255.0;
    CGFloat blue = arc4random() % 256 / 255.0;
    CGFloat green = arc4random() % 256 / 255.0;

    UIColor * randomColor = [UIColor colorWithRed:red green:green blue:blue alpha:1];
    self.bgColor = randomColor;
}

@end

从上面步骤我们可以看出原生的KVO使用分为三个步骤:

  1. 添加监听
  2. 实现监听的代理方法
  3. 移除监听

但是RACKVO只是用了非常简单的一段代码就实现了以上的这三个步骤,去掉了胶水代码,真正的做到了面向业务开发,那它是怎么实现的呢,接下来我们来一层层分析

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二、深入RAC底层逐层探究KVO实现

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  1. 点击RACObserver找到这个宏
#define _RACObserve(TARGET, KEYPATH) \
({ \
    __weak id target_ = (TARGET); \
    [target_ rac_valuesForKeyPath:@keypath(TARGET, KEYPATH) observer:self]; \
})

继续点进去,
我们会进入NSObject+RACPropertySubscribing.m文件下的


- (RACSignal *)rac_valuesAndChangesForKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options observer:(__weak NSObject *)weakObserver {
    NSObject *strongObserver = weakObserver;
    keyPath = [keyPath copy];

    NSRecursiveLock *objectLock = [[NSRecursiveLock alloc] init];
    objectLock.name = @"org.reactivecocoa.ReactiveObjC.NSObjectRACPropertySubscribing";

    __weak NSObject *weakSelf = self;

    RACSignal *deallocSignal = [[RACSignal
        zip:@[
            self.rac_willDeallocSignal,
            strongObserver.rac_willDeallocSignal ?: [RACSignal never]
        ]]
        doCompleted:^{

            [objectLock lock];
            @onExit {
                [objectLock unlock];
            };
        }];

    return [[[RACSignal
        createSignal:^ RACDisposable * (id<RACSubscriber> subscriber) {

            [objectLock lock];

            @onExit {
                [objectLock unlock];
            };

            __strong NSObject *observer __attribute__((objc_precise_lifetime)) = weakObserver;
            __strong NSObject *self __attribute__((objc_precise_lifetime)) = weakSelf;

            if (self == nil) {
                [subscriber sendCompleted];
                return nil;
            }

            return [self rac_observeKeyPath:keyPath options:options observer:observer block:^(id value, NSDictionary *change, BOOL causedByDealloc, BOOL affectedOnlyLastComponent) {
                [subscriber sendNext:RACTuplePack(value, change)];
            }];
        }]
        takeUntil:deallocSignal]
        setNameWithFormat:@"%@ -rac_valueAndChangesForKeyPath: %@ options: %lu observer: %@", RACDescription(self), keyPath, (unsigned long)options, RACDescription(strongObserver)];
}

我们会发现其中有一个deallocSignal,见名知意,我们先猜这个信号大概是在delloc的时候调用的,至于怎么调用的我们搁在一边;重点来了,return这段代码是重点,我们能够从中发现return的是一个信号RACSignal对象,并且这个signal有一个依赖前提:takeUntil:deallocSignal,KVO取值会一直取到VC释放,当这个VC释放之后,也就没有必要去取值了,也就是说deallocSignal这个信号在VC释放之前会一直执行,VC释放之后功能也会跟着失效,这里我们可以猜出,RACKVO封装思路中,最后一步的释放时机应该是在这里。

好,我们接着分析中间部分的代码,可以看出的是,万物皆信号---RACKVO使用了信号量来处理监听,结合之前信号量生命周期(传送门https://www.jianshu.com/p/bd4fff21d9b7),此处创建了信号,然后把这个信号return了出去,在外面subscribeNext订阅信号,外面订阅信号并同时调用了初始化保存的这个block代码块,代码块里进行completed操作取消订阅,取消订阅之前,在一个这样的代码块中做了订阅者的sendNext操作,这样信号量的生命周期是完整的,但是我们的KVO操作到现在还没有看见,那么只可能在这步操作隐藏了封装的内容

[self rac_observeKeyPath:keyPath options:options observer:observer block:^(id value, NSDictionary *change, BOOL causedByDealloc, BOOL affectedOnlyLastComponent) {
                [subscriber sendNext:RACTuplePack(value, change)];
            }];

也就是return的这部分代码,我们接下来继续分析这部分代码:通过订阅信号时保存的sendNext代码块,把监听到的change值传出去,也就是我们在VC那一个block的调用部分,
重点来了:
点击进去我们能够看到一段很长的代码,前面的一大堆处理略过,来看重点部分,


RACKVOTrampoline *trampoline = [[RACKVOTrampoline alloc] initWithTarget:self observer:strongObserver keyPath:keyPathHead options:trampolineOptions block:^(id trampolineTarget, id trampolineObserver, NSDictionary *change) {

        if ([change[NSKeyValueChangeNotificationIsPriorKey] boolValue]) {
            [firstComponentDisposable() dispose];

            if ((options & NSKeyValueObservingOptionPrior) != 0) {
                block([trampolineTarget valueForKeyPath:keyPath], change, NO, keyPathHasOneComponent);
            }

            return;
        }

        if (value == nil) {
            block(nil, change, NO, keyPathHasOneComponent);
            return;
        }

        RACDisposable *oldFirstComponentDisposable = [firstComponentSerialDisposable swapInDisposable:[RACCompoundDisposable compoundDisposable]];
        [oldFirstComponentDisposable dispose];

        addDeallocObserverToPropertyValue(value);

        if (keyPathHasOneComponent) {
            block(value, change, NO, keyPathHasOneComponent);
            return;
        }

        addObserverToValue(value);
        block([value valueForKeyPath:keyPathTail], change, NO, keyPathHasOneComponent);
    }];
---------------------------------------------------------------
---------------------------------------------------------------

    NSObject *value = [self valueForKey:keyPathHead];
    if (value != nil) {
        addDeallocObserverToPropertyValue(value);

        if (!keyPathHasOneComponent) {
            addObserverToValue(value);
        }
    }

    if ((options & NSKeyValueObservingOptionInitial) != 0) {
        id initialValue = [self valueForKeyPath:keyPath];
        NSDictionary *initialChange = @{
            NSKeyValueChangeKindKey: @(NSKeyValueChangeSetting),
            NSKeyValueChangeNewKey: initialValue ?: NSNull.null,
        };
        block(initialValue, initialChange, NO, keyPathHasOneComponent);
    }

    RACCompoundDisposable *observerDisposable = strongObserver.rac_deallocDisposable;
    RACCompoundDisposable *selfDisposable = self.rac_deallocDisposable;

    [observerDisposable addDisposable:disposable];
    [selfDisposable addDisposable:disposable];

    return [RACDisposable disposableWithBlock:^{
        [disposable dispose];
        [observerDisposable removeDisposable:disposable];
        [selfDisposable removeDisposable:disposable];
    }];

上面一部分代码可以按分割线分成上下两部,可以看出上部分是KVO实现监听的部分,下面一部分是处理销毁的逻辑。

我们先分析监听上部分这段代码的逻辑,上面这段代码块还是只做中间层传值,RAC又封装了一个中间层对象RACKVOTrampoline,并且由这个对象实现了KVO的监听。点击就进入了RACKVOTrampoline对象的.m实现文件,下面是这个.m的全部代码,这部分代码的解析我直接写在代码中便于分析:

#import "RACKVOTrampoline.h"
#import "NSObject+RACDeallocating.h"
#import "RACCompoundDisposable.h"
#import "RACKVOProxy.h"

@interface RACKVOTrampoline ()

@property (nonatomic, readonly, copy) NSString *keyPath;
@property (nonatomic, readonly, copy) RACKVOBlock block;
@property (nonatomic, readonly, unsafe_unretained) NSObject *unsafeTarget;
@property (nonatomic, readonly, weak) NSObject *weakTarget;
@property (nonatomic, readonly, weak) NSObject *observer;

@end

@implementation RACKVOTrampoline

#pragma mark Lifecycle

- (instancetype)initWithTarget:(__weak NSObject *)target observer:(__weak NSObject *)observer keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(RACKVOBlock)block {
    NSCParameterAssert(keyPath != nil);
    NSCParameterAssert(block != nil);

    NSObject *strongTarget = target;
    if (strongTarget == nil) return nil;

    self = [super init];

    _keyPath = [keyPath copy];

    _block = [block copy];
    _weakTarget = target;
    _unsafeTarget = strongTarget;
    _observer = observer;

    ////1.此处是系统原生的的KVO方法,添加监听,RAC又做了额外的处理,又封装了一个单例中间层对象RACKVOProxy,把当前的vc和keypath,并由RACKVOProxy来监听RACKVOTrampoline的keyPath属性,相当于把代理移交给了这个RACKVOProxy单例中间层对象

    [RACKVOProxy.sharedProxy addObserver:self forContext:(__bridge void *)self];

    [strongTarget addObserver:RACKVOProxy.sharedProxy forKeyPath:self.keyPath options:options context:(__bridge void *)self];

    [strongTarget.rac_deallocDisposable addDisposable:self];
    [self.observer.rac_deallocDisposable addDisposable:self];

    return self;
}

- (void)dealloc {
    [self dispose];
}

#pragma mark Observation

//3/释放代码,当前RACKVOTrampoline对象在销毁的时候,会进行移除单例中间层监听对象RACKVOProxy,这里通过信号量生命周期分析得出,信号在销毁的时候,会调用这个dispose,然后取消信号的调用同时取消监听移除RACKVOProxy代理者
- (void)dispose {
    NSObject *target;
    NSObject *observer;

    @synchronized (self) {
        _block = nil;

        // The target should still exist at this point, because we still need to
        // tear down its KVO observation. Therefore, we can use the unsafe
        // reference (and need to, because the weak one will have been zeroed by
        // now).
        target = self.unsafeTarget;
        observer = self.observer;

        _unsafeTarget = nil;
        _observer = nil;
    }

    [target.rac_deallocDisposable removeDisposable:self];
    [observer.rac_deallocDisposable removeDisposable:self];

    [target removeObserver:RACKVOProxy.sharedProxy forKeyPath:self.keyPath context:(__bridge void *)self];
    [RACKVOProxy.sharedProxy removeObserver:self forContext:(__bridge void *)self];
}

//2、此处是系统原生的KVO代理实现,并且通过Block把KVO监听到的值传出去- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if (context != (__bridge void *)self) {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
        return;
    }

    RACKVOBlock block;
    id observer;
    id target;

    @synchronized (self) {
        block = self.block;
        observer = self.observer;
        target = self.weakTarget;
    }

   //在传出值得做了判断,target不存在的时候,就不传值出去了。否则就把改变的值传出去,通过三次的block代码块回传,传到VC的subscribeNext订阅保存的代码块里,供开发者使用!

    if (block == nil || target == nil) return;

    block(target, observer, change);
}

@end

这样一来,整个流程就很清楚了,RACKVO的设计,首先是集成RACDisposable的子类RACKVOTrampoline,把要监听的对象和keyPath传入封装的信号的子类,实现原生KVO监听,并且考虑到了整体架构的灵活度,又实现了RACKVOProxy类来移交监听,在RACKVOTrampoline系统KVO代理中,利用代码块把改变的值,通过订阅信号时保存的block传出去,在开发者层面上,我们只能看到逻辑紧凑并且简单易用的使用部分。

设计者设计的时候,实现了很多NSObject的分类,但是并不是提供给所有对象使用的,这就是中间层变量的好处了,通过中间层对象单独实现这些分类,整个框架和思路灵活度非常高,代码没有耦合部分,这也是我们需要学习的细节,以后我们在架构项目和设计项目的时候,可以利用这种中间层变量的思想,既能解耦代码,灵活度又非常高,这也是一个好的架构师必备的技能思想。

最后再来顺便瞅瞅RACProxy:
下面是对RACProxy代码部分的分析,主要是初始化了一个表,把observer和context以keyValue的形式存在表里,然后添加的时候设置到表里,移除的时候用key移除,这样PACProxy这个中间层的使用就很灵活,能用于RAC的任何类,可以做到多重*使用并且利用中间层设计完全可以避免循环引用问题

#import "RACKVOProxy.h"

@interface RACKVOProxy()

@property (strong, nonatomic, readonly) NSMapTable *trampolines;
@property (strong, nonatomic, readonly) dispatch_queue_t queue;

@end

@implementation RACKVOProxy

+ (instancetype)sharedProxy {
    static RACKVOProxy *proxy;
    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{
        proxy = [[self alloc] init];
    });

    return proxy;
}

- (instancetype)init {
    self = [super init];

    _queue = dispatch_queue_create("org.reactivecocoa.ReactiveObjC.RACKVOProxy", DISPATCH_QUEUE_SERIAL);
    _trampolines = [NSMapTable strongToWeakObjectsMapTable];

    return self;
}

- (void)addObserver:(__weak NSObject *)observer forContext:(void *)context {
    NSValue *valueContext = [NSValue valueWithPointer:context];

    dispatch_sync(self.queue, ^{
        [self.trampolines setObject:observer forKey:valueContext];
    });
}

- (void)removeObserver:(NSObject *)observer forContext:(void *)context {
    NSValue *valueContext = [NSValue valueWithPointer:context];

    dispatch_sync(self.queue, ^{
        [self.trampolines removeObjectForKey:valueContext];
    });
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    NSValue *valueContext = [NSValue valueWithPointer:context];
    __block NSObject *trueObserver;

    dispatch_sync(self.queue, ^{
        trueObserver = [self.trampolines objectForKey:valueContext];
    });

    if (trueObserver != nil) {
        [trueObserver observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

@end

下面是整个RACKVO设计思路总结图,调来调去,花了我整整一下午时间(=@__@=)

探究ReactiveCocoa 底层KVO封装流程

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