RxSwift源码分析(17)——deallocating

在前面曾经探索过销毁流程和方式，其实还有一种方式：

button.rx.tap
.takeUntil(vc.rx.deallocating)
.subscribe { (event) in
    print("页面释放时销毁序列")                
}

不过今天想分析的不是takeUntil，而是deallocating，deallocating的探索过程会跟之前不太一样，先来看看例子：

vc.rx.deallocating
.subscribe { (event) in
    print("页面将要释放")                
}

1. 看看序列的创建，首先创建了DeallocatingProxy类，再返回序列（proxy.messageSent.asObservable()）：

vc.rx.deallocating

extension Reactive where Base: AnyObject {
    ...
    public var deallocating: Observable<()> {
        return self.synchronized {
            do {
                let proxy: DeallocatingProxy = try self.registerMessageInterceptor(deallocSelector)
                return proxy.messageSent.asObservable()
            }
            catch let e {
                return Observable.error(e)
            }
        }
    }
    
    fileprivate func registerMessageInterceptor<T: MessageInterceptorSubject>(_ selector: Selector) throws -> T {
        ...
        let targetImplementation = RX_ensure_observing(self.base, selector, &error)
        if targetImplementation == nil {
            throw error?.rxCocoaErrorForTarget(self.base) ?? RxCocoaError.unknown
        }

        subject.targetImplementation = targetImplementation!

        return subject
    }

}

2. registerMessageInterceptor函数的代码看上去有点复杂，重点在于RX_ensure_observing，内部竟然是OC代码：

IMP __nullable RX_ensure_observing(id __nonnull target, SEL __nonnull selector, NSErrorParam error) {
    __block IMP targetImplementation = nil;
    @synchronized(target) {
        @synchronized([target class]) {
            [[RXObjCRuntime instance] performLocked:^(RXObjCRuntime * __nonnull self) {
                targetImplementation = [self ensurePrepared:target
                                               forObserving:selector
                                                      error:error];
            }];
        }
    }

    return targetImplementation;
}

[[RXObjCRuntime instance] performLocked:^(RXObjCRuntime * __nonnull self) { }只是确保调用安全：

3. 接着看block里调用的方法，这个方法的代码真的很多，只好直接找重点swizzleDeallocating：

@implementation RXObjCRuntime
...
-(IMP __nullable)ensurePrepared:(id __nonnull)target forObserving:(SEL __nonnull)selector error:(NSErrorParam)error {
    ...
    if (selector == deallocSelector) {
        Class __nonnull deallocSwizzingTarget = [target class];
        IMP interceptorIMPForSelector = [self interceptorImplementationForSelector:selector forClass:deallocSwizzingTarget];
        if (interceptorIMPForSelector != nil) {
            return interceptorIMPForSelector;
        }

        if (![self swizzleDeallocating:deallocSwizzingTarget error:error]) {
            return nil;
        }

        interceptorIMPForSelector = [self interceptorImplementationForSelector:selector forClass:deallocSwizzingTarget];
        if (interceptorIMPForSelector != nil) {
            return interceptorIMPForSelector;
        }
    }
    ...
}
...
@end

4. 看看swizzleDeallocating的实现，关键来了，转眼跳转到了宏，这写法就容易眼花了，得慢慢一一对应（我把参数一一分开，大家应该容易看懂点）：

@implementation RXObjCRuntime (InfrastructureMethods)
...
SWIZZLE_INFRASTRUCTURE_METHOD(
    void,
    swizzleDeallocating,
    ,
    deallocSelector,
    DEALLOCATING_BODY
)

@end

// infrastructure method

#define NO_BODY(...)

#define SWIZZLE_INFRASTRUCTURE_METHOD(
    return_value, 
    method_name, 
    parameters, 
    method_selector, 
    body, 
...
)               \
SWIZZLE_METHOD(
    return_value, 
    -(BOOL)method_name:(Class __nonnull)class parameters error:(NSErrorParam)error        \
        {                                                                                                              \
            SEL selector = method_selector; , //留意这里只是第二个参数
    body, 
    NO_BODY, 
    __VA_ARGS__
)                                              \


// common base

#define SWIZZLE_METHOD(
    return_value, 
    method_prototype, 
    body, 
    invoked_body, 
    ...
)                                          \
method_prototype                                                                                                         \
    __unused SEL rxSelector = RX_selector(selector);                                                                     \
    IMP (^newImplementationGenerator)(void) = ^() {                                                                          \
    ...
    };                                                                                                                   \
                                                                                                                         \
    IMP (^replacementImplementationGenerator)(IMP) = ^(IMP originalImplementation) {                                     \
    ...
    };                                                                                                                   \
                                                                                                                         \
    return [self ensureSwizzledSelector:selector                                                                         \
                                ofClass:class                                                                            \
             newImplementationGenerator:newImplementationGenerator                                                       \
     replacementImplementationGenerator:replacementImplementationGenerator                                               \
                                  error:error];                                                                          \
 }                                                                                                                       \

5. 最后发现宏的重点是通过调用ensureSwizzledSelector交换两个方法的IMP函数指针，实现ensureSwizzledSelector的代码也是很多，继续抓重点class_addMethod：

-(BOOL)ensureSwizzledSelector:(SEL __nonnull)selector
                      ofClass:(Class __nonnull)class
   newImplementationGenerator:(IMP(^)(void))newImplementationGenerator
replacementImplementationGenerator:(IMP (^)(IMP originalImplementation))replacementImplementationGenerator
                        error:(NSErrorParam)error {
    ...
    IMP newImplementation = newImplementationGenerator();

    if (class_addMethod(class, selector, newImplementation, encoding)) {
        [self registerInterceptedSelector:selector implementation:newImplementation forClass:class];

        return YES;
    }
    ...
}

@end

• 来到这里梳理一下思路，deallocating其实就是利用runtime交换方法来达到监听的目的。

6. 这么一大圈才完成了DeallocatingProxy的创建流程，然后才是序列（proxy.messageSent.asObservable()）：

fileprivate final class DeallocatingProxy
    : MessageInterceptorSubject
    , RXDeallocatingObserver {
    ...
    let messageSent = ReplaySubject<()>.create(bufferSize: 1)
    ...
    @objc func deallocating() {
        self.messageSent.on(.next(()))
    }

    deinit {
        self.messageSent.on(.completed)
    }
}

接着可以看见DeallocatingProxy的deallocating()函数会发送响应出去，根据RxSwift核心逻辑，最终来到外面的响应闭包：

.subscribe { (event) in
    print("页面将要释放")                
}

7. 但是一直没看见deallocating()的调用，这里需要我们回顾一下上面的流程，因为它在这里调用了：

#define DEALLOCATING_BODY(...)                                                        \
    id<RXDeallocatingObserver> observer = objc_getAssociatedObject(self, rxSelector); \
    if (observer != nil && observer.targetImplementation == thisIMP) {                \
        [observer deallocating];                                                      \
    }

所以当监听的对象销毁时，便执行DeallocatingProxy的deallocating()发送响应，最后DeallocatingProxy销毁时，便发送完成信号销毁序列。