ReactiveCocoa 中 集合类RACSequence 和 RACTuple底层实现分析
ywllc290
8年前
<p><img src="https://simg.open-open.com/show/50bad789711f3b06b8e8f84aaf11d2c6.jpg"></p> <h3>前言</h3> <p>在OOP的世界里使用FRP的思想来编程，光有函数这种一等公民，还是无法满足我们一些需求的。因此还是需要引用变量来完成各式各样的类的操作行为。</p> <p>在前几篇文章中详细的分析了RACStream中RACSignal的底层实现。RACStream还有另外一个子类，RACSequence，这个类是RAC专门为集合而设计的。这篇文章就专门分析一下RACSequence的底层实现。</p> <h3>目录</h3> <ul> <li>1.RACTuple底层实现分析</li> <li>2.RACSequence底层实现分析</li> <li>3.RACSequence操作实现分析</li> <li>4.RACSequence的一些扩展</li> </ul> <h3>一. RACTuple底层实现分析</h3> <p>在分析RACSequence之前，先来看看RACTuple的实现。RACTuple是ReactiveCocoa的元组类。</p> <p><img src="https://simg.open-open.com/show/f5eb97e71a9ca72b8ad5b40e178c6373.png"></p> <p>1. RACTuple</p> <pre> <code class="language-objectivec">@interface RACTuple : NSObject <NSCoding, NSCopying, NSFastEnumeration> @property (nonatomic, readonly) NSUInteger count; @property (nonatomic, readonly) id first; @property (nonatomic, readonly) id second; @property (nonatomic, readonly) id third; @property (nonatomic, readonly) id fourth; @property (nonatomic, readonly) id fifth; @property (nonatomic, readonly) id last; @property (nonatomic, strong) NSArray *backingArray; @property (nonatomic, copy, readonly) RACSequence *rac_sequence; // 这个是专门为sequence提供的一个扩展 @end</code></pre> <p>RACTuple的定义看上去很简单，底层实质就是一个NSArray，只不过封装了一些方法。RACTuple继承了NSCoding, NSCopying, NSFastEnumeration这三个协议。</p> <pre> <code class="language-objectivec">- (id)initWithCoder:(NSCoder *)coder { self = [self init]; if (self == nil) return nil; self.backingArray = [coder decodeObjectForKey:@keypath(self.backingArray)]; return self; } - (void)encodeWithCoder:(NSCoder *)coder { if (self.backingArray != nil) [coder encodeObject:self.backingArray forKey:@keypath(self.backingArray)]; }</code></pre> <p>这里是NSCoding协议。都是对内部的backingArray进行decodeObjectForKey:和encodeObject: 。</p> <pre> <code class="language-objectivec">- (instancetype)copyWithZone:(NSZone *)zone { // we're immutable, bitches! <---这里是原作者的注释 return self; }</code></pre> <p>上面这是NSCopying协议。由于内部是基于NSArray的，所以是immutable不可变的。</p> <pre> <code class="language-objectivec">- (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state objects:(id __unsafe_unretained [])buffer count:(NSUInteger)len { return [self.backingArray countByEnumeratingWithState:state objects:buffer count:len]; }</code></pre> <p>上面是NSFastEnumeration协议，快速枚举也都是针对NSArray进行的操作。</p> <pre> <code class="language-objectivec">// 三个类方法 + (instancetype)tupleWithObjectsFromArray:(NSArray *)array; + (instancetype)tupleWithObjectsFromArray:(NSArray *)array convertNullsToNils:(BOOL)convert; + (instancetype)tupleWithObjects:(id)object, ... NS_REQUIRES_NIL_TERMINATION; - (id)objectAtIndex:(NSUInteger)index; - (NSArray *)allObjects; - (instancetype)tupleByAddingObject:(id)obj;</code></pre> <p>RACTuple的方法也不多，总共就6个方法，3个类方法，3个实例方法。</p> <p>先看类方法：</p> <pre> <code class="language-objectivec">+ (instancetype)tupleWithObjectsFromArray:(NSArray *)array { return [self tupleWithObjectsFromArray:array convertNullsToNils:NO]; } + (instancetype)tupleWithObjectsFromArray:(NSArray *)array convertNullsToNils:(BOOL)convert { RACTuple *tuple = [[self alloc] init]; if (convert) { NSMutableArray *newArray = [NSMutableArray arrayWithCapacity:array.count]; for (id object in array) { [newArray addObject:(object == NSNull.null ? RACTupleNil.tupleNil : object)]; } tuple.backingArray = newArray; } else { tuple.backingArray = [array copy]; } return tuple; }</code></pre> <p>先看这两个类方法，这两个类方法的区别在于是否把NSNull转换成RACTupleNil类型。根据入参array初始化RACTuple内部的NSArray。</p> <p><img src="https://simg.open-open.com/show/2c85090f116654ae38dedf9937e06106.jpg"></p> <p>RACTuplePack( ) 和 RACTuplePack_( )这两个宏的实现也是调用了tupleWithObjectsFromArray:方法</p> <pre> <code class="language-objectivec">#define RACTuplePack(...) \ RACTuplePack_(__VA_ARGS__) #define RACTuplePack_(...) \ ([RACTuple tupleWithObjectsFromArray:@[ metamacro_foreach(RACTuplePack_object_or_ractuplenil,, __VA_ARGS__) ]])</code></pre> <p>这里需要注意的是RACTupleNil</p> <pre> <code class="language-objectivec">+ (RACTupleNil *)tupleNil { static dispatch_once_t onceToken; static RACTupleNil *tupleNil = nil; dispatch_once(&onceToken, ^{ tupleNil = [[self alloc] init]; }); return tupleNil; }</code></pre> <p>RACTupleNil是一个单例。</p> <p>重点需要解释的是另外一种类方法：</p> <pre> <code class="language-objectivec">+ (instancetype)tupleWithObjects:(id)object, ... { RACTuple *tuple = [[self alloc] init]; va_list args; va_start(args, object); NSUInteger count = 0; for (id currentObject = object; currentObject != nil; currentObject = va_arg(args, id)) { ++count; } va_end(args); if (count == 0) { tuple.backingArray = @[]; return tuple; } NSMutableArray *objects = [[NSMutableArray alloc] initWithCapacity:count]; va_start(args, object); for (id currentObject = object; currentObject != nil; currentObject = va_arg(args, id)) { [objects addObject:currentObject]; } va_end(args); tuple.backingArray = objects; return tuple; }</code></pre> <p>这个类方法的参数是可变参数类型。由于用到了可变参数类型，所以就会用到va_list，va_start，va_arg，va_end。</p> <pre> <code class="language-objectivec">#ifndef _VA_LIST_T #define _VA_LIST_T typedef __darwin_va_list va_list; #endif /* _VA_LIST_T */ #ifndef _VA_LIST typedef __builtin_va_list va_list; #define _VA_LIST #endif #define va_start(ap, param) __builtin_va_start(ap, param) #define va_end(ap) __builtin_va_end(ap) #define va_arg(ap, type) __builtin_va_arg(ap, type)</code></pre> <ol> <li>va_list用于声明一个变量，我们知道函数的可变参数列表其实就是一个字符串，所以va_list才被声明为字符型指针，这个类型用于声明一个指向参数列表的字符型指针变量，例如：va_list ap;//ap:arguement pointer</li> <li>va_start(ap,v),它的第一个参数是指向可变参数字符串的变量，第二个参数是可变参数函数的第一个参数，通常用于指定可变参数列表中参数的个数。</li> <li>va_arg(ap,t),它的第一个参数指向可变参数字符串的变量，第二个参数是可变参数的类型。</li> <li>va_end(ap) 用于将存放可变参数字符串的变量清空（赋值为NULL)。</li> </ol> <p>剩下的3个实例方法都是对数组的操作，没有什么难度。</p> <p>一般使用用两个宏，RACTupleUnpack( ) 用来解包，RACTuplePack( ) 用来装包。</p> <pre> <code class="language-objectivec"> RACTupleUnpack(NSString *string, NSNumber *num) = [RACTuple tupleWithObjects:@"foo", @5, nil]; RACTupleUnpack(NSString *string, NSNumber *num) = RACTuplePack(@"foo",@(5)); NSLog(@"string: %@", string); NSLog(@"num: %@", num); /* 上面的做法等价于下面的 */ RACTuple *t = [RACTuple tupleWithObjects:@"foo", @5, nil]; NSString *string = t[0]; NSNumber *num = t[1]; NSLog(@"string: %@", string); NSLog(@"num: %@", num);</code></pre> <p>关于RACTuple还有2个相关的类，RACTupleUnpackingTrampoline，RACTupleSequence。</p> <p>2. RACTupleUnpackingTrampoline</p> <pre> <code class="language-objectivec">@interface RACTupleUnpackingTrampoline : NSObject + (instancetype)trampoline; - (void)setObject:(RACTuple *)tuple forKeyedSubscript:(NSArray *)variables; @end</code></pre> <p>首先这个类是一个单例。</p> <pre> <code class="language-objectivec">+ (instancetype)trampoline { static dispatch_once_t onceToken; static id trampoline = nil; dispatch_once(&onceToken, ^{ trampoline = [[self alloc] init]; }); return trampoline; }</code></pre> <p>RACTupleUnpackingTrampoline这个类也就只有一个作用，就是它对应的实例方法。</p> <pre> <code class="language-objectivec">- (void)setObject:(RACTuple *)tuple forKeyedSubscript:(NSArray *)variables { NSCParameterAssert(variables != nil); [variables enumerateObjectsUsingBlock:^(NSValue *value, NSUInteger index, BOOL *stop) { __strong id *ptr = (__strong id *)value.pointerValue; *ptr = tuple[index]; }]; }</code></pre> <p>这个方法里面会遍历入参数组NSArray，然后依次取出数组里面每个value 的指针，用这个指针又赋值给了tuple[index]。</p> <p>为了解释清楚这个方法的作用，写出测试代码：</p> <pre> <code class="language-objectivec"> RACTupleUnpackingTrampoline *tramp = [RACTupleUnpackingTrampoline trampoline]; NSString *string; NSString *string1; NSString *string2; NSArray *array = [NSArray arrayWithObjects:[NSValue valueWithPointer:&string],[NSValue valueWithPointer:&string1],[NSValue valueWithPointer:&string2], nil]; NSLog(@"调用方法之前 string = %@,string1 = %@,string2 = %@",string,string1,string2); [tramp setObject:[RACTuple tupleWithObjectsFromArray:@[(@"foo"),(@(10)),@"32323"]] forKeyedSubscript:array]; NSLog(@"调用方法之后 string = %@,string1 = %@,string2 = %@",string,string1,string2);</code></pre> <p>输出如下：</p> <pre> <code class="language-objectivec">调用方法之前 string = (null),string1 = (null),string2 = (null) 调用方法之后 string = foo,string1 = 10,string2 = 32323</code></pre> <p>这个函数的作用也就一清二楚了。但是平时我们是很少用到[NSValue valueWithPointer:&string]这种写法的。究竟是什么地方会用到这个函数呢？全局搜索一下，找到了用到这个的地方。</p> <p>在RACTuple 中两个非常有用的宏：RACTupleUnpack( ) 用来解包，RACTuplePack( ) 用来装包。RACTuplePack( )的实现在上面分析过了，实际是调用tupleWithObjectsFromArray:方法。那么RACTupleUnpack( ) 的宏是怎么实现的呢？这里就用到了RACTupleUnpackingTrampoline。</p> <p style="text-align:center"><img src="https://simg.open-open.com/show/494d574da6f6b98a1e37ff96a110162f.png"></p> <pre> <code class="language-objectivec">#define RACTupleUnpack_(...) \ metamacro_foreach(RACTupleUnpack_decl,, __VA_ARGS__) \ \ int RACTupleUnpack_state = 0; \ \ RACTupleUnpack_after: \ ; \ metamacro_foreach(RACTupleUnpack_assign,, __VA_ARGS__) \ if (RACTupleUnpack_state != 0) RACTupleUnpack_state = 2; \ \ while (RACTupleUnpack_state != 2) \ if (RACTupleUnpack_state == 1) { \ goto RACTupleUnpack_after; \ } else \ for (; RACTupleUnpack_state != 1; RACTupleUnpack_state = 1) \ [RACTupleUnpackingTrampoline trampoline][ @[ metamacro_foreach(RACTupleUnpack_value,, __VA_ARGS__) ] ]</code></pre> <p>以上就是RACTupleUnpack( ) 具体的宏。看上去很复杂。还是写出测试代码分析分析。</p> <pre> <code class="language-objectivec"> RACTupleUnpack(NSString *string, NSNumber *num) = RACTuplePack(@"foo",@(10));</code></pre> <p>把上述的代码编译之后的代码贴出来：</p> <pre> <code class="language-objectivec"> __attribute__((objc_ownership(strong))) id RACTupleUnpack284_var0; __attribute__((objc_ownership(strong))) id RACTupleUnpack284_var1; int RACTupleUnpack_state284 = 0; RACTupleUnpack_after284: ; __attribute__((objc_ownership(strong))) NSString *string = RACTupleUnpack284_var0; __attribute__((objc_ownership(strong))) NSNumber *num = RACTupleUnpack284_var1; if (RACTupleUnpack_state284 != 0) RACTupleUnpack_state284 = 2; while (RACTupleUnpack_state284 != 2) if (RACTupleUnpack_state284 == 1) { goto RACTupleUnpack_after284; } else for (; RACTupleUnpack_state284 != 1; RACTupleUnpack_state284 = 1) [RACTupleUnpackingTrampoline trampoline][ @[ [NSValue valueWithPointer:&RACTupleUnpack284_var0], [NSValue valueWithPointer:&RACTupleUnpack284_var1], ] ] = ([RACTuple tupleWithObjectsFromArray:@[ (@"foo") ?: RACTupleNil.tupleNil, (@(10)) ?: RACTupleNil.tupleNil, ]]);</code></pre> <p>转换成这样就比较好理解了。RACTupleUnpack_after284: 是一个标号。RACTupleUnpack_state284初始值为0，在下面while里面有一个for循环，在这个循环里面会进行解包操作，也就是会调用setObject:forKeyedSubscript:函数。</p> <p>在循环里面，</p> <pre> <code class="language-objectivec">[RACTupleUnpackingTrampoline trampoline][ @[ [NSValue valueWithPointer:&RACTupleUnpack284_var0], [NSValue valueWithPointer:&RACTupleUnpack284_var1], ] ]</code></pre> <p>这里就是调用了[NSValue valueWithPointer:&string]的写法。</p> <p>至此，RACTupleUnpackingTrampoline这个类的作用也已明了，它是被作用设计出来用来实现神奇的RACTupleUnpack( ) 这个宏。</p> <p>当然RACTupleUnpackingTrampoline这个类的setObject:forKeyedSubscript:函数也可以使用，只不过要注意写法，注意指针的类型，在NSValue里面包裹的是valueWithPointer，(nullable const void *)pointer类型的。</p> <p>3. RACTupleSequence</p> <p>这个类仅仅只是名字里面带有Tuple而已，它其实是继承自RACSequence。</p> <p>需要分析这个类的原因是因为RACTuple里面有一个拓展的属性rac_sequence。</p> <pre> <code class="language-objectivec">- (RACSequence *)rac_sequence { return [RACTupleSequence sequenceWithTupleBackingArray:self.backingArray offset:0]; }</code></pre> <p>还是先看看RACTupleSequence的定义。</p> <pre> <code class="language-objectivec">@interface RACTupleSequence : RACSequence @property (nonatomic, strong, readonly) NSArray *tupleBackingArray; @property (nonatomic, assign, readonly) NSUInteger offset; + (instancetype)sequenceWithTupleBackingArray:(NSArray *)backingArray offset:(NSUInteger)offset; @end</code></pre> <p>这个类是继承自RACSequence，而且只有这一个类方法。</p> <p>tupleBackingArray是来自于RACTuple里面的backingArray。</p> <pre> <code class="language-objectivec">+ (instancetype)sequenceWithTupleBackingArray:(NSArray *)backingArray offset:(NSUInteger)offset { NSCParameterAssert(offset <= backingArray.count); if (offset == backingArray.count) return self.empty; RACTupleSequence *seq = [[self alloc] init]; seq->_tupleBackingArray = backingArray; seq->_offset = offset; return seq; }</code></pre> <p>RACTupleSequence这个类的目的就是把Tuple转换成Sequence。Sequence里面的数组就是Tuple内部的backingArray。offset从0开始。</p> <h3>二. RACSequence底层实现分析</h3> <p><img src="https://simg.open-open.com/show/5ee0c8cdf5308b6e5a1bd26a85a1838b.jpg"></p> <pre> <code class="language-objectivec">@interface RACSequence : RACStream <NSCoding, NSCopying, NSFastEnumeration> @property (nonatomic, strong, readonly) id head; @property (nonatomic, strong, readonly) RACSequence *tail; @property (nonatomic, copy, readonly) NSArray *array; @property (nonatomic, copy, readonly) NSEnumerator *objectEnumerator; @property (nonatomic, copy, readonly) RACSequence *eagerSequence; @property (nonatomic, copy, readonly) RACSequence *lazySequence; @end</code></pre> <p>RACSequence是RACStream的子类，主要是ReactiveCocoa里面的集合类。</p> <p>先来说说关于RACSequence的一些概念。</p> <p>RACSequence有两个很重要的属性就是head和tail。head是一个id，而tail又是一个RACSequence，这个定义有点递归的意味。</p> <pre> <code class="language-objectivec"> RACSequence *sequence = [RACSequence sequenceWithHeadBlock:^id{ return @(1); } tailBlock:^RACSequence *{ return @[@2,@3,@4].rac_sequence; }]; NSLog(@"sequence.head = %@ , sequence.tail = %@",sequence.head ,sequence.tail);</code></pre> <p>输出：</p> <pre> <code class="language-objectivec">sequence.head = 1 , sequence.tail = <RACArraySequence: 0x608000223920>{ name = , array = ( 2, 3, 4 ) }</code></pre> <p>这段测试代码就道出了head和tail的定义。更加详细的描述见下图：</p> <p style="text-align:center"><img src="https://simg.open-open.com/show/32c9cfba51d6ad87e53bf14af9cda75c.png"></p> <p>上述代码里面用到了RACSequence初始化的方法，具体的分析见后面。</p> <p>objectEnumerator是一个快速枚举器。</p> <pre> <code class="language-objectivec">@interface RACSequenceEnumerator : NSEnumerator @property (nonatomic, strong) RACSequence *sequence; @end</code></pre> <p>之所以需要实现这个，是为了更加方便的RACSequence进行遍历。</p> <pre> <code class="language-objectivec">- (id)nextObject { id object = nil; @synchronized (self) { object = self.sequence.head; self.sequence = self.sequence.tail; } return object; }</code></pre> <p>有了这个NSEnumerator，就可以从RACSequence的head一直遍历到tail。</p> <pre> <code class="language-objectivec">- (NSEnumerator *)objectEnumerator { RACSequenceEnumerator *enumerator = [[RACSequenceEnumerator alloc] init]; enumerator.sequence = self; return enumerator; }</code></pre> <p>回到RACSequence的定义里面的objectEnumerator，这里就是取出内部的RACSequenceEnumerator。</p> <pre> <code class="language-objectivec">- (NSArray *)array { NSMutableArray *array = [NSMutableArray array]; for (id obj in self) { [array addObject:obj]; } return [array copy]; }</code></pre> <p>RACSequence的定义里面还有一个array，这个数组就是返回一个NSArray，这个数组里面装满了RACSequence里面所有的对象。这里之所以能用for-in，是因为实现了NSFastEnumeration协议。至于for-in的效率，完全就看重写NSFastEnumeration协议里面countByEnumeratingWithState: objects: count: 方法里面的执行效率了。</p> <p>在分析RACSequence的for-in执行效率之前，先回顾一下NSFastEnumerationState的定义，这里的属性在接下来的实现中会被大量使用。</p> <pre> <code class="language-objectivec">typedef struct { unsigned long state; //可以被自定义成任何有意义的变量 id __unsafe_unretained _Nullable * _Nullable itemsPtr; //返回对象数组的首地址 unsigned long * _Nullable mutationsPtr; //指向会随着集合变动而变化的一个值 unsigned long extra[5]; //可以被自定义成任何有意义的数组 } NSFastEnumerationState;</code></pre> <p>接下来要分析的这个函数的入参，stackbuf是为for-in提供的对象数组，len是该数组的长度。</p> <pre> <code class="language-objectivec">- (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state objects:(__unsafe_unretained id *)stackbuf count:(NSUInteger)len { // 定义完成时候的状态为state = ULONG_MAX if (state->state == ULONG_MAX) { return 0; } // 由于我们需要遍历sequence多次，所以这里定义state字段来记录sequence的首地址 RACSequence *(^getSequence)(void) = ^{ return (__bridge RACSequence *)(void *)state->state; }; void (^setSequence)(RACSequence *) = ^(RACSequence *sequence) { // 释放老的sequence CFBridgingRelease((void *)state->state); // 保留新的sequence，把sequence的首地址存放入state中 state->state = (unsigned long)CFBridgingRetain(sequence); }; void (^complete)(void) = ^{ // 释放sequence，并把state置为完成态 setSequence(nil); state->state = ULONG_MAX; }; // state == 0是第一次调用时候的初始值 if (state->state == 0) { // 在遍历过程中，如果Sequence不再发生变化，那么就让mutationsPtr指向一个定值，指向extra数组的首地址 state->mutationsPtr = state->extra; // 再次刷新state的值 setSequence(self); } // 将会把返回的对象放进stackbuf中，因此用itemsPtr指向它 state->itemsPtr = stackbuf; NSUInteger enumeratedCount = 0; while (enumeratedCount < len) { RACSequence *seq = getSequence(); // 由于sequence可能是懒加载生成的，所以需要防止在遍历器enumerator遍历到它们的时候被释放了 __autoreleasing id obj = seq.head; // 没有头就结束遍历 if (obj == nil) { complete(); break; } // 遍历sequence，每次取出来的head都放入stackbuf数组中。 stackbuf[enumeratedCount++] = obj; // 没有尾就是完成遍历 if (seq.tail == nil) { complete(); break; } // 取出tail以后，这次遍历结束的tail，即为下次遍历的head，设置seq.tail为Sequence的head，为下次循环做准备 setSequence(seq.tail); } return enumeratedCount; }</code></pre> <p>整个遍历的过程类似递归的过程，从头到尾依次遍历一遍。</p> <p>再来研究研究RACSequence的初始化：</p> <pre> <code class="language-objectivec">+ (RACSequence *)sequenceWithHeadBlock:(id (^)(void))headBlock tailBlock:(RACSequence *(^)(void))tailBlock; + (RACSequence *)sequenceWithHeadBlock:(id (^)(void))headBlock tailBlock:(RACSequence *(^)(void))tailBlock { return [[RACDynamicSequence sequenceWithHeadBlock:headBlock tailBlock:tailBlock] setNameWithFormat:@"+sequenceWithHeadBlock:tailBlock:"]; }</code></pre> <p>初始化RACSequence，会调用RACDynamicSequence。这里有点类比RACSignal的RACDynamicSignal。</p> <p>再来看看RACDynamicSequence的定义。</p> <pre> <code class="language-objectivec">@interface RACDynamicSequence () { id _head; RACSequence *_tail; id _dependency; } @property (nonatomic, strong) id headBlock; @property (nonatomic, strong) id tailBlock; @property (nonatomic, assign) BOOL hasDependency; @property (nonatomic, strong) id (^dependencyBlock)(void); @end</code></pre> <p>这里需要说明的是此处的headBlock，tailBlock，dependencyBlock的修饰符都是用了strong，而不是copy。这里是一个很奇怪的bug导致的。在 https://github.com/ReactiveCocoa/ReactiveCocoa/issues/505 中详细记录了用copy关键字会导致内存泄露的bug。具体代码如下：</p> <pre> <code class="language-objectivec">[[[@[@1,@2,@3,@4,@5] rac_sequence] filter:^BOOL(id value) { return [value intValue] > 1; }] array];</code></pre> <p>最终发现这个问题的人把copy改成strong就神奇的修复了这个bug。最终整个ReactiveCocoa库里面就只有这里把block的关键字从copy改成了strong，而不是所有的地方都改成strong。</p> <p>原作者 Justin Spahr-Summers 大神对这个问题的 最终解释 是：</p> <p>Maybe there's just something weird with how we override dealloc, set the blocks from a class method, cast them, or something else.</p> <p>所以日常我们写block的时候，没有特殊情况，依旧需要继续用copy进行修饰。</p> <pre> <code class="language-objectivec">+ (RACSequence *)sequenceWithHeadBlock:(id (^)(void))headBlock tailBlock:(RACSequence *(^)(void))tailBlock { NSCParameterAssert(headBlock != nil); RACDynamicSequence *seq = [[RACDynamicSequence alloc] init]; seq.headBlock = [headBlock copy]; seq.tailBlock = [tailBlock copy]; seq.hasDependency = NO; return seq; }</code></pre> <p>hasDependency这个变量是代表是否有dependencyBlock。这个函数里面就只把headBlock和tailBlock保存起来了。</p> <pre> <code class="language-objectivec">+ (RACSequence *)sequenceWithLazyDependency:(id (^)(void))dependencyBlock headBlock:(id (^)(id dependency))headBlock tailBlock:(RACSequence *(^)(id dependency))tailBlock { NSCParameterAssert(dependencyBlock != nil); NSCParameterAssert(headBlock != nil); RACDynamicSequence *seq = [[RACDynamicSequence alloc] init]; seq.headBlock = [headBlock copy]; seq.tailBlock = [tailBlock copy]; seq.dependencyBlock = [dependencyBlock copy]; seq.hasDependency = YES; return seq; }</code></pre> <p>另外一个类方法sequenceWithLazyDependency: headBlock: tailBlock:是带有dependencyBlock的，这个方法里面会保存headBlock，tailBlock，dependencyBlock这3个block。</p> <p>从RACSequence这两个唯一的初始化方法之间就引出了RACSequence两大核心问题之一，积极运算 和 惰性求值。</p> <p>1. 积极运算 和 惰性求值</p> <p>在RACSequence的定义中还有两个RACSequence —— eagerSequence 和 lazySequence。这两个RACSequence就是分别对应着积极运算的RACSequence和惰性求值的RACSequence。</p> <p>关于这两个概念最最新形象的比喻还是臧老师博客里面的这篇文章 聊一聊iOS开发中的惰性计算 里面写的一段笑话。引入如下：</p> <p>有一只小白兔，跑到蔬菜店里问老板：“老板，有100个胡萝卜吗？”。老板说：“没有那么多啊。”，小白兔失望的说道：“哎，连100个胡萝卜都没有。。。”。第二天小白兔又来到蔬菜店问老板：“今天有100个胡萝卜了吧？”，老板尴尬的说：“今天还是缺点，明天就能好了。”，小白兔又很失望的走了。第三天小白兔刚一推门，老板就高兴的说道：“有了有了，从前天就进货的100个胡萝卜到货了。”，小白兔说：“太好了，我要买2根！”。。。</p> <p>如果日常我们遇到了这种问题，就很浪费内存空间了。比如在内存里面开了一个100W大小的数组，结果实际只使用到100个数值。这个时候就需要用到惰性运算了。</p> <p>在RACSequence里面这两种方式都支持，我们来看看底层源码是如何实现的。</p> <p>先来看看平时我们很熟悉的情况——积极运算。</p> <p><img src="https://simg.open-open.com/show/2be49a21db39229745b98425b772b534.png"></p> <p>在RACSequence中积极运算的代表是RACSequence的一个子类RACArraySequence的子类——RACEagerSequence。它的积极运算表现在其bind函数上。</p> <pre> <code class="language-objectivec">- (instancetype)bind:(RACStreamBindBlock (^)(void))block { NSCParameterAssert(block != nil); RACStreamBindBlock bindBlock = block(); NSArray *currentArray = self.array; NSMutableArray *resultArray = [NSMutableArray arrayWithCapacity:currentArray.count]; for (id value in currentArray) { BOOL stop = NO; RACSequence *boundValue = (id)bindBlock(value, &stop); if (boundValue == nil) break; for (id x in boundValue) { [resultArray addObject:x]; } if (stop) break; } return [[self.class sequenceWithArray:resultArray offset:0] setNameWithFormat:@"[%@] -bind:", self.name]; }</code></pre> <p>从上述代码中能看到主要是进行了2层循环，最外层循环遍历的自己RACSequence中的值，然后拿到这个值传入闭包bindBlock( )中，返回一个RACSequence，最后用一个NSMutableArray依次把每个RACSequence里面的值都装起来。</p> <p>第二个for-in循环是在遍历RACSequence，之所以可以用for-in的方式遍历就是因为实现了NSFastEnumeration协议，实现了countByEnumeratingWithState: objects: count: 方法，这个方法在上面详细分析过了，这里不再赘述。</p> <p>这里就是一个积极运算的例子，在每次循环中都会把闭包block( )的值计算出来。值得说明的是，最后返回的RACSequence的类型是self.class类型的，即还是RACEagerSequence类型的。</p> <p>再来看看RACSequence中的惰性求值是怎么实现的。</p> <p>在RACSequence中，bind函数是下面这个样子：</p> <pre> <code class="language-objectivec">- (instancetype)bind:(RACStreamBindBlock (^)(void))block { RACStreamBindBlock bindBlock = block(); return [[self bind:bindBlock passingThroughValuesFromSequence:nil] setNameWithFormat:@"[%@] -bind:", self.name]; }</code></pre> <p>实际上调用了bind: passingThroughValuesFromSequence:方法，第二个入参传入nil。</p> <pre> <code class="language-objectivec">- (instancetype)bind:(RACStreamBindBlock)bindBlock passingThroughValuesFromSequence:(RACSequence *)passthroughSequence { __block RACSequence *valuesSeq = self; __block RACSequence *current = passthroughSequence; __block BOOL stop = NO; RACSequence *sequence = [RACDynamicSequence sequenceWithLazyDependency:^ id { // 暂时省略 } headBlock:^(id _) { return current.head; } tailBlock:^ id (id _) { if (stop) return nil; return [valuesSeq bind:bindBlock passingThroughValuesFromSequence:current.tail]; }]; sequence.name = self.name; return sequence; }</code></pre> <p>在bind: passingThroughValuesFromSequence:方法的实现中，就是用sequenceWithLazyDependency: headBlock: tailBlock:方法生成了一个RACSequence，并返回。在sequenceWithLazyDependency: headBlock: tailBlock:上面分析过源码，主要目的是为了保存3个闭包，headBlock，tailBlock，dependencyBlock。</p> <p>通过调用RACSequence里面的bind操作，并没有执行3个闭包里面的值，只是保存起来了。这里就是惰性求值的表现——等到要用的时候才会计算。</p> <p>通过上述源码的分析，可以写出如下的测试代码加深理解。</p> <pre> <code class="language-objectivec"> NSArray *array = @[@1,@2,@3,@4,@5]; RACSequence *lazySequence = [array.rac_sequence map:^id(id value) { NSLog(@"lazySequence"); return @(101); }]; RACSequence *eagerSequence = [array.rac_sequence.eagerSequence map:^id(id value) { NSLog(@"eagerSequence"); return @(100); }];</code></pre> <p>上述代码运行之后，会输出如下信息：</p> <pre> <code class="language-objectivec">eagerSequence eagerSequence eagerSequence eagerSequence eagerSequence</code></pre> <p>只输出了5遍eagerSequence，lazySequence并没有输出。原因是因为bind闭包只在eagerSequence中真正被调用执行了，而在lazySequence中bind闭包仅仅只是被copy了。</p> <p>那如何让lazySequence执行bind闭包呢？</p> <pre> <code class="language-objectivec"> [lazySequence array];</code></pre> <p>通过执行上述代码，就可以输出5遍“lazySequence”了。因为bind闭包再次会被调用执行。</p> <p>积极运算 和 惰性求值在这里就区分出来了。在RACSequence中，除去RACEagerSequence只积极运算，其他的Sequence都是惰性求值的。</p> <p>接下来再继续分析RACSequence是如何实现惰性求值的。</p> <p><img src="https://simg.open-open.com/show/b0f18e7b33e562d09f64b51aa4e26c12.png"></p> <pre> <code class="language-objectivec">RACSequence *sequence = [RACDynamicSequence sequenceWithLazyDependency:^ id { while (current.head == nil) { if (stop) return nil; // 遍历当前sequence，取出下一个值 id value = valuesSeq.head; if (value == nil) { // 遍历完sequence所有的值 stop = YES; return nil; } current = (id)bindBlock(value, &stop); if (current == nil) { stop = YES; return nil; } valuesSeq = valuesSeq.tail; } NSCAssert([current isKindOfClass:RACSequence.class], @"-bind: block returned an object that is not a sequence: %@", current); return nil; } headBlock:^(id _) { return current.head; } tailBlock:^ id (id _) { if (stop) return nil; return [valuesSeq bind:bindBlock passingThroughValuesFromSequence:current.tail]; }];</code></pre> <p>在bind操作中创建了这样一个lazySequence，3个block闭包保存了如何创建一个lazySequence的做法。</p> <p>headBlock是入参为id，返回值也是一个id。在创建lazySequence的head的时候，并不关心入参，直接返回passthroughSequence的head。</p> <p>tailBlock是入参为id，返回值为RACSequence。由于RACSequence的定义类似递归定义的，所以tailBlock会再次递归调用bind:passingThroughValuesFromSequence:产生一个RACSequence作为新的sequence的tail。</p> <p>dependencyBlock的返回值是作为headBlock和tailBlock的入参。不过现在headBlock和tailBlock都不关心这个入参。那么dependencyBlock就是成为了headBlock和tailBlock闭包执行之前要执行的闭包。</p> <p>dependencyBlock的目的是为了把原来的sequence里面的值，都进行一次变换。current是入参passthroughSequence，valuesSeq就是原sequence的引用。每次循环一次就取出原sequence的头，直到取不到为止，就是遍历完成。</p> <p>取出valuesSeq的head，传入bindBlock( )闭包进行变换，返回值是一个current 的sequence。在每次headBlock和tailBlock之前都会调用这个dependencyBlock，变换后新的sequence的head就是current的head，新的sequence的tail就是递归调用传入的current.tail。</p> <p>RACDynamicSequence创建的lazyDependency的过程就是保存了3个block的过程。那这些闭包什么时候会被调用呢？</p> <pre> <code class="language-objectivec">- (id)head { @synchronized (self) { id untypedHeadBlock = self.headBlock; if (untypedHeadBlock == nil) return _head; if (self.hasDependency) { if (self.dependencyBlock != nil) { _dependency = self.dependencyBlock(); self.dependencyBlock = nil; } id (^headBlock)(id) = untypedHeadBlock; _head = headBlock(_dependency); } else { id (^headBlock)(void) = untypedHeadBlock; _head = headBlock(); } self.headBlock = nil; return _head; } }</code></pre> <p>上面的源码就是获取RACDynamicSequence中head的实现。当要取出sequence的head的时候，就会调用headBlock( )。如果保存了dependencyBlock闭包，在执行headBlock( )之前会先执行dependencyBlock( )进行一次变换。</p> <pre> <code class="language-objectivec">- (RACSequence *)tail { @synchronized (self) { id untypedTailBlock = self.tailBlock; if (untypedTailBlock == nil) return _tail; if (self.hasDependency) { if (self.dependencyBlock != nil) { _dependency = self.dependencyBlock(); self.dependencyBlock = nil; } RACSequence * (^tailBlock)(id) = untypedTailBlock; _tail = tailBlock(_dependency); } else { RACSequence * (^tailBlock)(void) = untypedTailBlock; _tail = tailBlock(); } if (_tail.name == nil) _tail.name = self.name; self.tailBlock = nil; return _tail; } }</code></pre> <p>获取RACDynamicSequence中tail的时候，和获取head是一样的，当需要取出tail的时候才会调用tailBlock( )。当有dependencyBlock闭包，会先执行dependencyBlock闭包，再调用tailBlock( )。</p> <p>总结一下：1. RACSequence的惰性求值，除去RACEagerSequence的bind函数以外，其他所有的Sequence都是基于惰性求值的。只有到取出来运算之前才会去把相应的闭包执行一遍。</p> <ol> <li>在RACSequence所有函数中，只有bind函数会传入dependencyBlock( )闭包，（RACEagerSequence会重写这个bind函数），所以看到dependencyBlock( )闭包一定可以推断出是RACSequence做了变换操作了。</li> </ol> <p>2. Pull-driver 和 Push-driver</p> <p style="text-align:center"><img src="https://simg.open-open.com/show/b138d2ba82bd98d769c25dc152c0181a.jpg"></p> <p>在RACSequence中有一个方法可以让RACSequence和RACSignal进行关联上。</p> <pre> <code class="language-objectivec">- (RACSignal *)signal { return [[self signalWithScheduler:[RACScheduler scheduler]] setNameWithFormat:@"[%@] -signal", self.name]; } - (RACSignal *)signalWithScheduler:(RACScheduler *)scheduler { return [[RACSignal createSignal:^(id<RACSubscriber> subscriber) { __block RACSequence *sequence = self; return [scheduler scheduleRecursiveBlock:^(void (^reschedule)(void)) { if (sequence.head == nil) { [subscriber sendCompleted]; return; } [subscriber sendNext:sequence.head]; sequence = sequence.tail; reschedule(); }]; }] setNameWithFormat:@"[%@] -signalWithScheduler: %@", self.name, scheduler]; }</code></pre> <p>RACSequence中的signal方法会调用signalWithScheduler:方法。在signalWithScheduler:方法中会创建一个新的信号。这个新的信号的RACDisposable信号由scheduleRecursiveBlock:产生。</p> <pre> <code class="language-objectivec">- (void)scheduleRecursiveBlock:(RACSchedulerRecursiveBlock)recursiveBlock addingToDisposable:(RACCompoundDisposable *)disposable { @autoreleasepool { RACCompoundDisposable *selfDisposable = [RACCompoundDisposable compoundDisposable]; [disposable addDisposable:selfDisposable]; __weak RACDisposable *weakSelfDisposable = selfDisposable; RACDisposable *schedulingDisposable = [self schedule:^{ if (disposable.disposed) return; void (^reallyReschedule)(void) = ^{ if (disposable.disposed) return; // 这里是递归 [self scheduleRecursiveBlock:recursiveBlock addingToDisposable:disposable]; }; // 这里实际上不需要__block关键字，但是由于Clang编译器的特性，为了保护下面的变量，所以加上了__block关键字 __block NSLock *lock = [[NSLock alloc] init]; lock.name = [NSString stringWithFormat:@"%@ %s", self, sel_getName(_cmd)]; __block NSUInteger rescheduleCount = 0; // 一旦同步操作执行完成，rescheduleImmediately就应该被设为YES __block BOOL rescheduleImmediately = NO; @autoreleasepool { recursiveBlock(^{ [lock lock]; BOOL immediate = rescheduleImmediately; if (!immediate) ++rescheduleCount; [lock unlock]; if (immediate) reallyReschedule(); }); } [lock lock]; NSUInteger synchronousCount = rescheduleCount; rescheduleImmediately = YES; [lock unlock]; for (NSUInteger i = 0; i < synchronousCount; i++) { reallyReschedule(); } }]; [selfDisposable addDisposable:schedulingDisposable]; } }</code></pre> <p>这段代码虽然长，但是拆分分析一下：</p> <pre> <code class="language-objectivec">__block NSUInteger rescheduleCount = 0; // 一旦同步操作执行完成，rescheduleImmediately就应该被设为YES __block BOOL rescheduleImmediately = NO;</code></pre> <p>rescheduleCount 是递归次数计数。rescheduleImmediately这个BOOL是决定是否立即执行reallyReschedule( )闭包。</p> <p>recursiveBlock是入参，它实际是下面这段闭包代码：</p> <pre> <code class="language-objectivec">{ if (sequence.head == nil) { [subscriber sendCompleted]; return; } [subscriber sendNext:sequence.head]; sequence = sequence.tail; reschedule(); }</code></pre> <p>recursiveBlock的入参是reschedule( )。执行完上面的代码之后开始执行入参reschedule( )的代码，入参reschedule( 闭包的代码是如下：</p> <pre> <code class="language-objectivec">^{ [lock lock]; BOOL immediate = rescheduleImmediately; if (!immediate) ++rescheduleCount; [lock unlock]; if (immediate) reallyReschedule(); }</code></pre> <p>在这段block中会统计rescheduleCount，如果rescheduleImmediately为YES还会继续开始执行递归操作reallyReschedule( )。</p> <pre> <code class="language-objectivec">for (NSUInteger i = 0; i < synchronousCount; i++) { reallyReschedule(); }</code></pre> <p>最终会在这个循环里面递归调用reallyReschedule( )闭包。reallyReschedule( )闭包执行的操作就是再次执行scheduleRecursiveBlock:recursiveBlock addingToDisposable:disposable方法。</p> <p>每次执行一次递归就会取出sequence的head值发送出来，直到sequence.head = = nil发送完成信号。</p> <p>既然RACSequence也可以转换成RACSignal，那么就需要总结一下两者的异同点。</p> <p>总结一下：</p> <p>RACSequence 和 RACSignal 异同点对比：</p> <p><img src="https://simg.open-open.com/show/cc50fca80d8a10fb69b81480fd96efc7.png"></p> <ol> <li>RACSequence除去RACEagerSequence，其他所有的都是基于惰性计算的，这和RACSignal是一样的。</li> <li>RACSequence是在时间上是连续的，一旦把RACSequence变成signal，再订阅，会立即把所有的值一口气都发送出来。RACSignal是在时间上是离散的，当有事件到来的时候，才会发送出数据流。</li> <li>RACSequence是Pull-driver，由订阅者来决定是否发送值，只要订阅者订阅了，就会发送数据流。RACSignal是Push-driver，它发送数据流是不由订阅者决定的，不管有没有订阅者，它有离散事件产生了，就会发送数据流。</li> <li>RACSequence发送的全是数据，RACSignal发送的全是事件。事件不仅仅包括数据，还包括事件的状态，比如说事件是否出错，事件是否完成。</li> </ol> <h3>三. RACSequence操作实现分析</h3> <p><img src="https://simg.open-open.com/show/b7ec1217dfa9bf18ba169d4df6575f34.jpg"></p> <p>RACSequence还有以下几个操作。</p> <pre> <code class="language-objectivec">- (id)foldLeftWithStart:(id)start reduce:(id (^)(id accumulator, id value))reduce; - (id)foldRightWithStart:(id)start reduce:(id (^)(id first, RACSequence *rest))reduce; - (BOOL)any:(BOOL (^)(id value))block; - (BOOL)all:(BOOL (^)(id value))block; - (id)objectPassingTest:(BOOL (^)(id value))block;</code></pre> <p>1. foldLeftWithStart: reduce:</p> <pre> <code class="language-objectivec">- (id)foldLeftWithStart:(id)start reduce:(id (^)(id, id))reduce { NSCParameterAssert(reduce != NULL); if (self.head == nil) return start; for (id value in self) { start = reduce(start, value); } return start; }</code></pre> <p>这个函数传入了一个初始值start，然后依次循环执行reduce( )，循环之后，最终的值作为返回值返回。这个函数就是折叠函数，从左边折叠到右边。</p> <p>2. foldRightWithStart: reduce:</p> <pre> <code class="language-objectivec">- (id)foldRightWithStart:(id)start reduce:(id (^)(id, RACSequence *))reduce { NSCParameterAssert(reduce != NULL); if (self.head == nil) return start; RACSequence *rest = [RACSequence sequenceWithHeadBlock:^{ return [self.tail foldRightWithStart:start reduce:reduce]; } tailBlock:nil]; return reduce(self.head, rest); }</code></pre> <p>这个函数和上一个foldLeftWithStart: reduce:是一样的，只不过方向是从右往左。</p> <p>3. objectPassingTest:</p> <pre> <code class="language-objectivec">- (id)objectPassingTest:(BOOL (^)(id))block { NSCParameterAssert(block != NULL); return [self filter:block].head; }</code></pre> <p>objectPassingTest:里面会调用RACStream中的filter:函数，这个函数在前几篇文章分析过了。如果block(value)为YES，就代表通过了Test，那么就会返回value的sequence。取出head返回。</p> <p>4. any:</p> <pre> <code class="language-objectivec">- (BOOL)any:(BOOL (^)(id))block { NSCParameterAssert(block != NULL); return [self objectPassingTest:block] != nil; }</code></pre> <p>any:会调用objectPassingTest:函数，如果不为nil就代表有value值通过了Test，有通过了value的就返回YES，反之返回NO。</p> <p>5. all:</p> <pre> <code class="language-objectivec">- (BOOL)all:(BOOL (^)(id))block { NSCParameterAssert(block != NULL); NSNumber *result = [self foldLeftWithStart:@YES reduce:^(NSNumber *accumulator, id value) { return @(accumulator.boolValue && block(value)); }]; return result.boolValue; }</code></pre> <p>all:会从左往右依次对每个值进行block( ) Test，然后每个值依次进行&&操作。</p> <p>6. concat:</p> <pre> <code class="language-objectivec">- (instancetype)concat:(RACStream *)stream { NSCParameterAssert(stream != nil); return [[[RACArraySequence sequenceWithArray:@[ self, stream ] offset:0] flatten] setNameWithFormat:@"[%@] -concat: %@", self.name, stream]; }</code></pre> <p>concat:的操作和RACSignal的作用是一样的。它会把原sequence和入参stream连接到一起，组合成一个高阶sequence，最后调用flatten“拍扁”。关于flatten的实现见前几篇RACStream里面的flatten实现分析。</p> <p>7. zipWith:</p> <pre> <code class="language-objectivec">- (instancetype)zipWith:(RACSequence *)sequence { NSCParameterAssert(sequence != nil); return [[RACSequence sequenceWithHeadBlock:^ id { if (self.head == nil || sequence.head == nil) return nil; return RACTuplePack(self.head, sequence.head); } tailBlock:^ id { if (self.tail == nil || [[RACSequence empty] isEqual:self.tail]) return nil; if (sequence.tail == nil || [[RACSequence empty] isEqual:sequence.tail]) return nil; return [self.tail zipWith:sequence.tail]; }] setNameWithFormat:@"[%@] -zipWith: %@", self.name, sequence]; }</code></pre> <p>由于sequence的定义是递归形式的，所以zipWith:也是递归来进行的。zipWith:新的sequence的head是原来2个sequence的head组合成RACTuplePack。新的sequence的tail是原来2个sequence的tail递归调用zipWith:。</p> <h3>四. RACSequence的一些扩展</h3> <p><img src="https://simg.open-open.com/show/137065fd1d6a4d407b20dc21619135e3.jpg"></p> <p>关于RACSequence有以下9个子类，其中RACEagerSequence是继承自RACArraySequence。这些子类看名字就知道sequence里面装的是什么类型的数据。RACUnarySequence里面装的是单元sequence。它只有head值，没有tail值。</p> <p><img src="https://simg.open-open.com/show/993a66d14dde978d3621d050ab84a51f.jpg"></p> <p>RACSequenceAdditions 总共有7个Category。这7个Category分别对iOS 里面的集合类进行了RACSequence的扩展，使我们能更加方便的使用RACSequence。</p> <p>1. NSArray+RACSequenceAdditions</p> <pre> <code class="language-objectivec">@interface NSArray (RACSequenceAdditions) @property (nonatomic, copy, readonly) RACSequence *rac_sequence; @end</code></pre> <p>这个Category能把任意一个NSArray数组转换成RACSequence。</p> <pre> <code class="language-objectivec">- (RACSequence *)rac_sequence { return [RACArraySequence sequenceWithArray:self offset:0]; }</code></pre> <p>根据NSArray创建一个RACArraySequence并返回。</p> <p>2. NSDictionary+RACSequenceAdditions</p> <pre> <code class="language-objectivec">@interface NSDictionary (RACSequenceAdditions) @property (nonatomic, copy, readonly) RACSequence *rac_sequence; @property (nonatomic, copy, readonly) RACSequence *rac_keySequence; @property (nonatomic, copy, readonly) RACSequence *rac_valueSequence; @end</code></pre> <p>这个Category能把任意一个NSDictionary字典转换成RACSequence。</p> <pre> <code class="language-objectivec">- (RACSequence *)rac_sequence { NSDictionary *immutableDict = [self copy]; return [immutableDict.allKeys.rac_sequence map:^(id key) { id value = immutableDict[key]; return RACTuplePack(key, value); }]; } - (RACSequence *)rac_keySequence { return self.allKeys.rac_sequence; } - (RACSequence *)rac_valueSequence { return self.allValues.rac_sequence; }</code></pre> <p>rac_sequence会把字典都转化为一个装满RACTuplePack的RACSequence，在这个RACSequence中，第一个位置是key，第二个位置是value。</p> <p>rac_keySequence是装满所有key的RACSequence。</p> <p>rac_valueSequence是装满所有value的RACSequence。</p> <p>3. NSEnumerator+RACSequenceAdditions</p> <pre> <code class="language-objectivec">@interface NSEnumerator (RACSequenceAdditions) @property (nonatomic, copy, readonly) RACSequence *rac_sequence; @end</code></pre> <p>这个Category能把任意一个NSEnumerator转换成RACSequence。</p> <pre> <code class="language-objectivec">- (RACSequence *)rac_sequence { return [RACSequence sequenceWithHeadBlock:^{ return [self nextObject]; } tailBlock:^{ return self.rac_sequence; }]; }</code></pre> <p>返回的RACSequence的head是当前的sequence的head，tail就是当前的sequence。</p> <p>4. NSIndexSet+RACSequenceAdditions</p> <pre> <code class="language-objectivec">@interface NSIndexSet (RACSequenceAdditions) @property (nonatomic, copy, readonly) RACSequence *rac_sequence; @end</code></pre> <p>这个Category能把任意一个NSIndexSet转换成RACSequence。</p> <pre> <code class="language-objectivec">- (RACSequence *)rac_sequence { return [RACIndexSetSequence sequenceWithIndexSet:self]; } + (instancetype)sequenceWithIndexSet:(NSIndexSet *)indexSet { NSUInteger count = indexSet.count; if (count == 0) return self.empty; NSUInteger sizeInBytes = sizeof(NSUInteger) * count; NSMutableData *data = [[NSMutableData alloc] initWithCapacity:sizeInBytes]; [indexSet getIndexes:data.mutableBytes maxCount:count inIndexRange:NULL]; RACIndexSetSequence *seq = [[self alloc] init]; seq->_data = data; seq->_indexes = data.bytes; seq->_count = count; return seq; }</code></pre> <p>返回RACIndexSetSequence，在这个IndexSetSequence中，data里面装的NSData，indexes里面装的NSUInteger，count里面装的是index的总数。</p> <p>5. NSOrderedSet+RACSequenceAdditions</p> <pre> <code class="language-objectivec">@interface NSOrderedSet (RACSequenceAdditions) @property (nonatomic, copy, readonly) RACSequence *rac_sequence; @end</code></pre> <p>这个Category能把任意一个NSOrderedSet转换成RACSequence。</p> <pre> <code class="language-objectivec">- (RACSequence *)rac_sequence { return self.array.rac_sequence; }</code></pre> <p>返回的NSOrderedSet中的数组转换成sequence。</p> <p>6. NSSet+RACSequenceAdditions</p> <pre> <code class="language-objectivec">@interface NSSet (RACSequenceAdditions) @property (nonatomic, copy, readonly) RACSequence *rac_sequence; @end</code></pre> <p>这个Category能把任意一个NSSet转换成RACSequence。</p> <pre> <code class="language-objectivec">- (RACSequence *)rac_sequence { return self.allObjects.rac_sequence; }</code></pre> <p>根据NSSet的allObjects数组创建一个RACArraySequence并返回。</p> <p>7. NSString+RACSequenceAdditions</p> <pre> <code class="language-objectivec">@interface NSString (RACSequenceAdditions) @property (nonatomic, copy, readonly) RACSequence *rac_sequence; @end</code></pre> <p>这个Category能把任意一个NSString转换成RACSequence。</p> <pre> <code class="language-objectivec">- (RACSequence *)rac_sequence { return [RACStringSequence sequenceWithString:self offset:0]; }</code></pre> <p>返回的是一个装满string字符的数组对应的sequence。</p> <h3> </h3> <p> </p> <p>来自：https://halfrost.com/reactivecocoa_racsequence_ractuple/</p> <p> </p>