RAC實踐採坑指北

時間 2019-12-08

標籤 rac 實踐简体版

原文原文鏈接

因爲新入職的團隊使用的是RAC，所以須要熟悉一下RAC的類圖和大體的實現。類圖大體以下：web

RACSequence

和Cocoa內置的集合對象(NSArray，NSSet)相似，內部不能包含nil，是RACStream（一個抽象類，用於表示爲信號流的值）的子類，RACSequence是拉力驅動(被動)的數據流，所以默認是惰性求值，而且當調用map或falttenMap之類的方法時，block對內部的對象求值只會進行一次。借用RAC官方Demo安全

NSArray *strings = @[ @"A", @"B", @"C" ];
    RACSequence *sequence = [strings.rac_sequence map:^(NSString *str) {
        NSLog(@"%@", str);
        return [str stringByAppendingString:@"_"];
    }];
    
    // Logs "A" during this call.
    NSString *concatA = sequence.head;
    
    // Logs "B" during this call.
    NSString *concatB = sequence.tail.head;
    
    // Does not log anything.
    NSString *concatB2 = sequence.tail.head;
    
    RACSequence *derivedSequence = [sequence map:^(NSString *str) {
        return [@"_" stringByAppendingString:str];
    }];
    //  Does not log anything, and concatA2 value is A_ ,NOT _A_
    NSString *concatA2 = sequence.head;
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RACSignal

RACSignal是專一於解決經過訂閱信號來異步進行事件傳輸 RAC是線程安全的，所以能夠在任意線程進行signal發送，可是一個訂閱者只能串行的處理一個信號，而不能併發的處理多個信號。所以-subscribeNext:error:completed:的 block不須要進行synchronized。bash

bind

利用一段代碼來測試bind函數的調用順序,因爲代碼結構複雜，因此在bind模塊對應的block都會標有數字，方便描述調用順序。網絡

RACSignal *sourceSig = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        //doSomething
        //...
        //...block1
        NSLog(@"\nbegin---\n%@\n---end",@"dosomething");
        [subscriber sendNext:@"hello world"];
//        [subscriber sendCompleted];
        return nil;
    }];
    
    RACSignal *bindSig = [sourceSig bind:^RACStreamBindBlock{
        //block2
        return ^(id value, BOOL *stop) {
            //block3
            //這裏對value進行處理
            return [RACSignal return:value];
        };
    }];
    
    [bindSig subscribeNext:^(id x) {
        //block4
        NSLog(@"\nbegin---\n%@\n---end",x);
    }];
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1.createSignal：的做用是將傳的:^RACDisposable *(id<RACSubscriber> subscriber)這個block存到sourceSig的didSubscribe字段中（block1）併發

2.bind:經過調用createSignal：返回一個新的信號bindSig，bind: 的參數是一個沒有入參，返回值爲RACStreamBindBlock的block(block2)。 RACStreamBindBlock入參和出參以下：異步

typedef RACSignal * _Nullable (^RACSignalBindBlock)(ValueType _Nullable value, BOOL *stop);
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經過改變傳入進來的Value（也就是改變block3的內部實現），從而實現了flattenMap:,skip:,takeUntilBlock:,distinctUntilChanged：等高級操做。async

- (RACSignal *)bind:(RACSignalBindBlock (^)(void))block {
    //返回bindSig，並將block保存至didSubscribe
	return [[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
		//省略didSubscribe內部代碼
	}] setNameWithFormat:@"[%@] -bind:", self.name];
}

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3.當bindSig 調用subscribeNext：，生成一個RACSubscriber，並將nextBlock保存在_next中ide

- (RACDisposable *)subscribeNext:(void (^)(id x))nextBlock {
	NSCParameterAssert(nextBlock != NULL);
	
	RACSubscriber *o = [RACSubscriber subscriberWithNext:nextBlock error:NULL completed:NULL];
	return [self subscribe:o];
}
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而後bindSig調用subscribe：，入參就是這個subscribe函數

4.在subcribe：中，調用bindSig保存的didSubscribe ,執行一長串代碼（block5）測試

return [[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
                //block5
		RACStreamBindBlock bindingBlock = block();
                 //這裏的self是sourceSig
		NSMutableArray *signals = [NSMutableArray arrayWithObject:self];
            
		RACCompoundDisposable *compoundDisposable = [RACCompoundDisposable compoundDisposable];
        
		void (^completeSignal)(RACSignal *, RACDisposable *) = ^(RACSignal *signal, RACDisposable *finishedDisposable) {
		        //block6
			BOOL removeDisposable = NO;

			@synchronized (signals) {
				[signals removeObject:signal];

				if (signals.count == 0) {
					[subscriber sendCompleted];
					[compoundDisposable dispose];
				} else {
					removeDisposable = YES;
				}
			}

			if (removeDisposable) [compoundDisposable removeDisposable:finishedDisposable];
		};

		void (^addSignal)(RACSignal *) = ^(RACSignal *signal) {
	        	//block7
			@synchronized (signals) {
				[signals addObject:signal];
			}

			RACSerialDisposable *selfDisposable = [[RACSerialDisposable alloc] init];
			[compoundDisposable addDisposable:selfDisposable];
                        //4.訂閱newSig，而後將newSig的值傳給bindSig的訂閱者，執行block8
			RACDisposable *disposable = [signal subscribeNext:^(id x) {
			        //block8
			        //這裏是subscriber對應的是bindSig
				[subscriber sendNext:x];
				//5.而後執行block4
			} error:^(NSError *error) {
				[compoundDisposable dispose];
				[subscriber sendError:error];
			} completed:^{
				@autoreleasepool {
					completeSignal(signal, selfDisposable);
				}
			}];

			selfDisposable.disposable = disposable;
		};

		@autoreleasepool {
			RACSerialDisposable *selfDisposable = [[RACSerialDisposable alloc] init];
			[compoundDisposable addDisposable:selfDisposable];
                         //1.先執行block1，而後執行block9
			RACDisposable *bindingDisposable = [self subscribeNext:^(id x) {
				// Manually check disposal to handle synchronous errors.
				//block9
				if (compoundDisposable.disposed) return;

				BOOL stop = NO;
				//對sourceSig傳的值進行處理，再包裝在新值（可爲nil)簡稱newSig
				//2.再執行block3
				id signal = bindingBlock(x, &stop);

				@autoreleasepool {
				    //3.假如block3返回的sig不爲nil執行block7
					if (signal != nil) addSignal(signal);
				    //假如block3返回的sig爲nil或者stop指針爲YES，執行block6
					if (signal == nil || stop) {
						[selfDisposable dispose];
						completeSignal(self, selfDisposable);
					}
				}
			} error:^(NSError *error) {
				[compoundDisposable dispose];
				[subscriber sendError:error];
			} completed:^{
				@autoreleasepool {
					completeSignal(self, selfDisposable);
				}
			}];

			selfDisposable.disposable = bindingDisposable;
		}

		return compoundDisposable;
	}] setNameWithFormat:@"[%@] -bind:", self.name];
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總結一下bind的做用：生成一個新的信號bindSig，訂閱源信號sourceSig，當sourceSig發送一個值時，bindSig經過訂閱收到這個值後,根據上層傳的RACStreamBindBlock轉換value,發送給bindSig的subscriber。

ATTENTION

因爲RACSignal是冷信號，因此每次有新的訂閱都會觸發反作用(對應的block)，這意味着 singal對應的block會執行屢次。

__block int missilesToLaunch = 0;

// Signal that will have the side effect of changing `missilesToLaunch` on
// subscription.
RACSignal *processedSignal = [[RACSignal return:@"missiles"]
	map:^(id x) {
		missilesToLaunch++;
		return [NSString stringWithFormat:@"will launch %d %@", missilesToLaunch, x];
	}];

// This will print "First will launch 1 missiles"
[processedSignal subscribeNext:^(id x) {
	NSLog(@"First %@", x);
}];

// This will print "Second will launch 2 missiles"
[processedSignal subscribeNext:^(id x) {
	NSLog(@"Second %@", x);
}];
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假如想冷信號執行一次，就得轉換成熱信號。好比網絡請求確定只須要一次就好，因此在業務場景中經過multicast使用，能夠避免冷信號的的屢次調用

// This signal starts a new request on each subscription.
RACSignal *networkRequest = [RACSignal createSignal:^(id<RACSubscriber> subscriber) {
    AFHTTPRequestOperation *operation = [client
        HTTPRequestOperationWithRequest:request
        success:^(AFHTTPRequestOperation *operation, id response) {
            [subscriber sendNext:response];
            [subscriber sendCompleted];
        }
        failure:^(AFHTTPRequestOperation *operation, NSError *error) {
            [subscriber sendError:error];
        }];

    [client enqueueHTTPRequestOperation:operation];
    return [RACDisposable disposableWithBlock:^{
        [operation cancel];
    }];
}];

// Starts a single request, no matter how many subscriptions `connection.signal`
// gets. This is equivalent to the -replay operator, or similar to
// +startEagerlyWithScheduler:block:.
// single中除了Subject以外的都是冷信號，Subject是熱信號。
RACMulticastConnection *connection = [networkRequest multicast:[RACReplaySubject subject]];
[connection connect];

[connection.signal subscribeNext:^(id response) {
    NSLog(@"subscriber one: %@", response);
}];

[connection.signal subscribeNext:^(id response) {
    NSLog(@"subscriber two: %@", response);
}];
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當咱們須要在nextBlock以前須要加一些反作用代碼，就能夠調用-doNext，這時候會先調用這裏的block，再調用subscriber的sendNext。

UI事件

RAC(self.label,text,@"nil的值") = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        __block int i = 0;
        [[self.button rac_signalForControlEvents:UIControlEventTouchDown] subscribeNext:^(id x) {
            i ++;
            if (i > 3) {
                [subscriber sendNext:nil];
            }
            else {
                [subscriber sendNext:@"123"];
            }
            
        }];
        return nil;
        }];
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通知

當咱們用RAC來改寫NSNotification的時候用rac_addObserverForName: 好比咱們須要監聽網絡狀態時

//當網絡發生變化後，RAC這個宏會進行keypath綁定，會將self.NetWorkStatus 賦予新值，這時其餘利用RACObserve會收到這個變化並做出對應改
    RAC(self, NetWorkStatus) = [[[[NSNotificationCenter defaultCenter]
                                   rac_addObserverForName:kRealReachabilityChangedNotification object:nil]
                                  map:^(NSNotification *notification) {
                                      return @([notification.object currentReachabilityStatus]);
                                  }]
                                distinctUntilChanged];

    //RACObserve接受新值並訂閱信號
    [RACObserve(self , NetWorkStatus) subscribeNext:^(NSNumber *networkStatus) {
        
        @strongify(self);
        if (networkStatus.integerValue == RealStatusNotReachable || networkStatus.integerValue == RealStatusUnknown) {
            [self.viewModel showErrorView];
        }else{
            [self.viewModel request];
        }
    }];  

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協議

@weakify(self);
    [[self
      rac_signalForSelector:@selector(webViewDidStartLoad:)
      fromProtocol:@protocol(WebViewDelegate)]
    	subscribeNext:^(RACTuple *tuple) {
            @strongify(self)
            if (tuple.first == self.webView){
                dispatch_main_async_safe(^{
                    [self showStatusWithMessage:@"Loading..."];
                });
            }
        }];
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網絡事件（耗時事件）

__block int callCount = 0;
    這裏由於訂閱了兩次，因此會調用兩次block，所以假如是io類操做，最好將networkSig包裝成RACSubject而後經過multicast廣播
    self.networkSig = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        __block int i = 0;
        callCount ++;
        //打印兩次
        NSLog(@"\nbegin---\n callCount ==%d\n---end",callCount );
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.f * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            i++;
            [subscriber sendNext:@(i)];
        });
        return nil;
    }];
    
    
    
    [self.networkSig subscribeNext:^(id x) {
        NSLog(@"\nbegin---\nfirst i ==== %@\n---end", x);
    }];
    
    [self.networkSig subscribeNext:^(id x) {
        NSLog(@"\nbegin---\nsecond i ==== %@\n---end", x);
    }];
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改進後:

__block int callCount = 0;
    self.networkSig = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        __block int i = 0;
        callCount ++;
        //只會打印一次
        NSLog(@"\nbegin---\n callCount ==%d\n---end",callCount );
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.f * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            i++;
            [subscriber sendNext:@(i)];
        });
        return nil;
    }];
    
    RACSubject *subject = [RACSubject subject];
    RACMulticastConnection *multicastConnection = [self.networkSig multicast:subject];
    [multicastConnection connect];
    
    [multicastConnection.signal subscribeNext:^(id x) {
        NSLog(@"\nbegin---\nfirst i ==== %@\n---end", x);
    }];
    
    [multicastConnection.signal subscribeNext:^(id x) {
        NSLog(@"\nbegin---\nsecond i ==== %@\n---end", x);
    }];
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KVO

//實現self.navigationItem.title 和 self.viewModel.title的單向綁定
 RAC(self.navigationItem,title) = RACObserve(self.viewModel, title);
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RACCommand

建立RACCommand的時候須要返回一個signal，當調用execute:,signal必須調用sendCompleted或sendError:,command才能進行下次execute:

初學者可能會想固然以下寫代碼

//1.先綁定self.button的keypath:enable
    RAC(self.button,enabled) = [RACSignal combineLatest:@[self.userNameField.rac_textSignal,self.passwordField.rac_textSignal]
                                                 reduce:^id(NSString *userName,NSString *password){
                                                     return @(userName.length >= 8 && password.length >= 6);
                                                 }];
    //2.而後設置button的點擊事件
    self.button.rac_command = [[RACCommand alloc] initWithSignalBlock:^RACSignal *(id input) {
        return [self login];
    }];
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這時候運行程序的時候報錯

這是由於 RAC()這個宏和 button.rac_command都會調用 setKeyPath:onObject:nilValue:這個方法。首次調用時，會經過objc_setAssociatedObject將keypath保存起來，當重複調用相同的keypath的時候會觸發 NSCAssert 正確的作法是

RACSignal *buttonEnabled = [RACSignal combineLatest:@[self.userNameField.rac_textSignal,self.passwordField.rac_textSignal]
                                                 reduce:^id(NSString *userName,NSString *password){
                                                     return @(userName.length >= 8 && password.length >= 6);
                                                 }];
    self.button.rac_command = [[RACCommand alloc] initWithEnabled:buttonEnabled signalBlock:^RACSignal *(id input) {
        return [self login];
    }];
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