Operation .h .m
和文章开头demo中的operation一样

// Prepare a value at +0 for return through a +0 autoreleasing convention.
id 
objc_retainAutoreleaseReturnValue(id obj)
{
    if (prepareOptimizedReturn(ReturnAtPlus0)) return obj;

    // not objc_autoreleaseReturnValue(objc_retain(obj)) 
    // because we don't need another optimization attempt
    return objc_retainAutoreleaseAndReturn(obj);
}

// Accept a value returned through a +0 autoreleasing convention for use at +1.
id
objc_retainAutoreleasedReturnValue(id obj)
{
    if (acceptOptimizedReturn() == ReturnAtPlus1) return obj;

    return objc_retain(obj);
}

@interface SSHttpOperation()
...
//网络回调的target
@property(nonatomic, weak)id target;
//外部在发网络请求的时候可以随意设置回调方法, 达到扩展性
@property(nonatomic, assign)SEL didFinishSelector;
...
@end

@implementation SSHttpOperation
- (void)notifyWithResult:(NSDictionary*)result error:(NSError*)error continueWhenCancel:(BOOL)continueWhenCancel {
	//网络请求回来, 进行外部业务回调
    ...
       if(target && [target respondsToSelector:didFinishSelector])  {
           NSMethodSignature *signature = [target methodSignatureForSelector:didFinishSelector];
           NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
           [invocation setTarget:target]; // 1
           [invocation setSelector:didFinishSelector];
           id wself = self;
           [invocation setArgument:&wself atIndex:2];
           [invocation setArgument:&result atIndex:3];
           [invocation setArgument:&logicError atIndex:4];
           [invocation setArgument:&userInfo atIndex:5];
           [invocation invoke];

       }
      ...
}
@end

Object A

[self.pgcActionManager changePGCAccount:mediaID likeStatus:!hasSubscribed extraTrack:extraTrack likeBlock:^(BOOL isLiked, NSError *error, NSArray<TTVUserAccount *> *recommendUsers) {
       // 一堆业务操作 再block的最后把这个manager置为nil
       ...
       self.pgcActionManager = nil; // 2
   }];

Manager

 
- (void)changePGCAccount:(NSString *)mediaID likeStatus:(BOOL)changeToLike likeBlock:(SSPGCActionManagerLikeBlock)likeBlock{
	...
	self.likeOperation = [SSHttpOperation httpOperationWithURLString:url getParameter:nil postParameter:parameterDict userInfo:userInfo];
    [_likeOperation setQueuePriority:NSOperationQueuePriorityHigh];
    [_likeOperation setFinishTarget:self selector:@selector(operation:finishedResult:error:userInfo:)];
    [SSOperationManager addOperation:_likeOperation];
}

#pragma mark - 网络回调
- (void)operation:(SSHttpOperation*)operation finishedResult:(NSDictionary*)result error:(NSError*)error userInfo:(id)userInfo{
 // 业务逻辑代码 xxx
 //3
 if (_likeBlock) {
	  _likeBlock(isFollow, followError, recommendUsersArray);
   }
 // 后续的业务逻辑代码
 // 4
 [TTVLoginManager showLoginForFollowWithResult:@"你的关注太多,登录保存一下吧" extraTrackDic:@{@"isOver":@(1)} position:self.position];
}

{
  id __weak obj1 = obj;
  NSLog(@"%@", obj1);
}
/* iOS5及之前编译器做法 */
id obj1;
objc_initWeak(&obj1, obj);
id tmp = objc_loadWeakRetained(&obj1);
objc_autorelease(tmp);//错误!!!mistake
NSLog(@"%@", tmp);
objc_destroyWeak(&obj1);

/* 现在的编译器做法*/
id obj = objc_msgSend(NSObject, "new");
id obj1;
objc_initWeak(&obj1, obj);
id tmp = objc_loadWeakRetained(obj1);//objc_loadWeakRetained would increment the reference count to ensure that tmp is alive in the NSLog statement.
NSLog(@"%@", obj1);
objc_release(tmp);
objc_destroyWeak(&obj1);
objc_storeStrong(&obj, 0);//release

-(id) autorelease
{
    return _objc_rootAutorelease(self);
}

id
_objc_rootAutorelease(id obj)
{
    assert(obj);
    return obj->rootAutorelease();
}

...

// Base autorelease implementation, ignoring overrides.
inline id 
objc_object::rootAutorelease()
{
    if (isTaggedPointer()) return (id)this;
    if (prepareOptimizedReturn(ReturnAtPlus1)) return (id)this;

    return rootAutorelease2();
}

...

__attribute__((noinline,used))
id 
objc_object::rootAutorelease2()
{
    assert(!isTaggedPointer());
    return AutoreleasePoolPage::autorelease((id)this);
}

public:
    static inline id autorelease(id obj)
    {
        assert(obj);
        assert(!obj->isTaggedPointer());
        id *dest __unused = autoreleaseFast(obj);
        assert(!dest  ||  dest == EMPTY_POOL_PLACEHOLDER  ||  *dest == obj);
        return obj;
    }
    ...
    static inline id *autoreleaseFast(id obj)
    {
        AutoreleasePoolPage *page = hotPage();
        if (page && !page->full()) {
            return page->add(obj);
        } else if (page) {
            return autoreleaseFullPage(obj, page);
        } else {
            return autoreleaseNoPage(obj);
        }
    }
    ...
        static __attribute__((noinline))
    id *autoreleaseNoPage(id obj)
    {
        // "No page" could mean no pool has been pushed
        // or an empty placeholder pool has been pushed and has no contents yet
        assert(!hotPage());

        bool pushExtraBoundary = false;
        if (haveEmptyPoolPlaceholder()) {
            // We are pushing a second pool over the empty placeholder pool
            // or pushing the first object into the empty placeholder pool.
            // Before doing that, push a pool boundary on behalf of the pool 
            // that is currently represented by the empty placeholder.
            pushExtraBoundary = true;
        }
        else if (obj != POOL_BOUNDARY  &&  DebugMissingPools) {
            // We are pushing an object with no pool in place, 
            // and no-pool debugging was requested by environment.
            _objc_inform("MISSING POOLS: (%p) Object %p of class %s "
                         "autoreleased with no pool in place - "
                         "just leaking - break on "
                         "objc_autoreleaseNoPool() to debug", 
                         pthread_self(), (void*)obj, object_getClassName(obj));
            objc_autoreleaseNoPool(obj);
            return nil;
        }
        else if (obj == POOL_BOUNDARY  &&  !DebugPoolAllocation) {
            // We are pushing a pool with no pool in place,
            // and alloc-per-pool debugging was not requested.
            // Install and return the empty pool placeholder.
            return setEmptyPoolPlaceholder();
        }

        // We are pushing an object or a non-placeholder'd pool.

        // Install the first page.
        AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
        setHotPage(page);
        
        // Push a boundary on behalf of the previously-placeholder'd pool.
        if (pushExtraBoundary) {
            page->add(POOL_BOUNDARY);
        }
        
        // Push the requested object or pool.
        return page->add(obj);
    }

__weak id obj;

...
[NSThread detachNewThreadSelector:@selector(createAndConfigObserverInSecondaryThread) toTarget:self withObject:nil];

- (void)createAndConfigObserverInSecondaryThread{
    __autoreleasing id test = [NSObject new];
    NSLog(@"obj = %@", test);
    obj = test;
    [[NSThread currentThread] setName:@"test runloop thread"];
    NSLog(@"thread ending");
}

static void tls_dealloc(void *p)
{
    if (p == (void*)EMPTY_POOL_PLACEHOLDER) {
        // No objects or pool pages to clean up here.
        return;
    }

    // reinstate TLS value while we work
    setHotPage((AutoreleasePoolPage *)p);

    if (AutoreleasePoolPage *page = coldPage()) {
        if (!page->empty()) pop(page->begin());  // pop all of the pools
        if (DebugMissingPools || DebugPoolAllocation) {
            // pop() killed the pages already
        } else {
            page->kill();  // free all of the pages
        }
    }
    
    // clear TLS value so TLS destruction doesn't loop
    setHotPage(nil);
}

void
_pthread_tsd_cleanup(pthread_t self)
{
#if !VARIANT_DYLD
	int j;

	// clean up dynamic keys first
	for (j = 0; j < PTHREAD_DESTRUCTOR_ITERATIONS; j++) {
		pthread_key_t k;
		for (k = __pthread_tsd_start; k <= self->max_tsd_key; k++) {
			_pthread_tsd_cleanup_key(self, k);
		}
	}

	self->max_tsd_key = 0;

	// clean up static keys
	for (j = 0; j < PTHREAD_DESTRUCTOR_ITERATIONS; j++) {
		pthread_key_t k;
		for (k = __pthread_tsd_first; k <= __pthread_tsd_max; k++) {
			_pthread_tsd_cleanup_key(self, k);
		}
	}
#endif // !VARIANT_DYLD
}

- (void)createAndConfigObserverInSecondaryThread{
    [[NSThread currentThread] setName:@"test runloop thread"];
    NSRunLoop *loop = [NSRunLoop currentRunLoop];
    CFRunLoopObserverRef observer;
    observer = CFRunLoopObserverCreate(CFAllocatorGetDefault(),
                                       kCFRunLoopAllActivities,
                                       true,      // repeat
                                       0xFFFFFF,  // after CATransaction(2000000)
                                       YYRunLoopObserverCallBack, NULL);
    CFRunLoopRef cfrunloop = [loop getCFRunLoop];
    if (observer) {
        
        CFRunLoopAddObserver(cfrunloop, observer, kCFRunLoopCommonModes);
        CFRelease(observer);
    }
    [NSTimer scheduledTimerWithTimeInterval:5 target:self selector:@selector(testAction) userInfo:nil repeats:YES];
    [loop run];
    NSLog(@"thread ending");
}

- (void)testAction{
    __autoreleasing id test = [NSObject new];
    obj = test;
    NSLog(@"obj = %@", obj);
}

static void __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__(CFRunLoopTimerCallBack func, CFRunLoopTimerRef timer, void *info) {
    if (func) {
        func(timer, info);
    }
    asm __volatile__(""); // thwart tail-call optimization
}

- (void)createAndConfigObserverInSecondaryThread{
    __autoreleasing id test = [NSObject new];
    NSLog(@"obj = %@", test);
    obj = test;
    [[NSThread currentThread] setName:@"test runloop thread"];
    NSRunLoop *loop = [NSRunLoop currentRunLoop];
    CFRunLoopObserverRef observer;
    observer = CFRunLoopObserverCreate(CFAllocatorGetDefault(),
                                       kCFRunLoopAllActivities,
                                       true,      // repeat
                                       0xFFFFFF,  // after CATransaction(2000000)
                                       YYRunLoopObserverCallBack, NULL);
    CFRunLoopRef cfrunloop = [loop getCFRunLoop];
    if (observer) {
        
        CFRunLoopAddObserver(cfrunloop, observer, kCFRunLoopCommonModes);
        CFRelease(observer);
    }
    
    CFRunLoopSourceRef source;
    CFRunLoopSourceContext sourceContext = {0, (__bridge void *)(self), NULL, NULL, NULL, NULL, NULL, NULL, NULL, &runLoopSourcePerformRoutine};
    source = CFRunLoopSourceCreate(NULL, 0, &sourceContext);
    CFRunLoopAddSource(cfrunloop, source, kCFRunLoopDefaultMode);
    runLoopSource = source;
    runLoop = cfrunloop;
    [loop run];
    NSLog(@"thread ending");
}

-(void)wakeupSource{
    //通知InputSource
    CFRunLoopSourceSignal(runLoopSource);
    //唤醒runLoop
    CFRunLoopWakeUp(runLoop);
    
}

...

void runLoopSourcePerformRoutine (void *info)
{
	  __autoreleasing id test = [NSObject new];
    obj = test;
    // 如果不对obj赋值，obj会一直持有createAndConfigObserverInSecondaryThread函数入口的那个object，那个object不受这里面的autoreleasepool影响。
    NSLog(@"obj is %@" , obj);
    NSLog(@"回调方法%@",[NSThread currentThread]);
}

- (BOOL) runMode: (NSString*)mode beforeDate: (NSDate*)date
{
  NSAutoreleasePool	*arp = [NSAutoreleasePool new];
  NSString              *savedMode = _currentMode;
  GSRunLoopCtxt		*context;
  NSDate		*d;

  NSAssert(mode != nil, NSInvalidArgumentException);

  /* Process any pending notifications.
   */
  GSPrivateNotifyASAP(mode);

  /* And process any performers scheduled in the loop (eg something from
   * another thread.
   */
  _currentMode = mode;
  context = NSMapGet(_contextMap, mode);
  [self _checkPerformers: context];
  _currentMode = savedMode;

  /* Find out how long we can wait before first limit date.
   * If there are no input sources or timers, return immediately.
   */
  d = [self limitDateForMode: mode];
  if (nil == d)
    {
      [arp drain];
      return NO;
    }

  /* Use the earlier of the two dates we have (nil date is like distant past).
   */
  if (nil == date)
    {
      [self acceptInputForMode: mode beforeDate: nil];
    }
  else
    {
      /* Retain the date in case the firing of a timer (or some other event)
       * releases it.
       */
      d = [[d earlierDate: date] copy];
      [self acceptInputForMode: mode beforeDate: d];
      RELEASE(d);
    }

  [arp drain];
  return YES;
}

obj = [[MyObject alloc] init];
[obj addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];

GSKVOInfo             *info;
GSKVOReplacement      *r;
NSKeyValueObservationForwarder *forwarder;
NSRange               dot;

setup();
[kvoLock lock];

// Use the original class
r = replacementForClass([self class]);

/*
 * Get the existing observation information, creating it (and changing
 * the receiver to start key-value-observing by switching its class)
 * if necessary.
 */
info = (GSKVOInfo*)[self observationInfo];
if (info == nil)
  {
    info = [[GSKVOInfo alloc] initWithInstance: self];
    [self setObservationInfo: info];
    object_setClass(self, [r replacement]);
  }

GSKVOReplacement *r;

setup();
[kvoLock lock];
r = (GSKVOReplacement*)NSMapGet(classTable, (void*)c);
if (r == nil)
  {
    r = [[GSKVOReplacement alloc] initWithClass: c];
    NSMapInsert(classTable, (void*)c, (void*)r);
  }
[kvoLock unlock];
return r;

original = aClass;

 /*
  * Create subclass of the original, and override some methods
  * with implementations from our abstract base class.
  */
 superName = NSStringFromClass(original);
 name = [@"GSKVO" stringByAppendingString: superName];
 template = GSObjCMakeClass(name, superName, nil);
 GSObjCAddClasses([NSArray arrayWithObject: template]);
 replacement = NSClassFromString(name);
 GSObjCAddClassBehavior(replacement, baseClass);

 /* Create the set of setter methods overridden.
  */
 keys = [NSMutableSet new];

 return self;

 unsigned int	count;
 Method	*methods;
 Class behavior_super_class = class_getSuperclass(behavior);
 
 	...
/* 一些不是很关键的代码 */

 /* Add instance methods */
 methods = class_copyMethodList(behavior, &count);
 BDBGPrintf("  instance methods from %s %u\n", class_getName(behavior), count);
 if (methods == NULL)
   {
     BDBGPrintf("    none.\n");
   }
 else
   {
     GSObjCAddMethods (receiver, methods, NO);
     free(methods);
   }

 /* Add class methods */
 methods = class_copyMethodList(object_getClass(behavior), &count);
 BDBGPrintf("  class methods from %s %u\n", class_getName(behavior), count);
 if (methods == NULL)
   {
     BDBGPrintf("    none.\n");
   }
 else
   {
     GSObjCAddMethods (object_getClass(receiver), methods, NO);
     free(methods);
   }

 /* Add behavior's superclass, if not already there. */
 if (!GSObjCIsKindOf(receiver, behavior_super_class))
   {
     GSObjCAddClassBehavior (receiver, behavior_super_class);
   }
 GSFlushMethodCacheForClass (receiver);

- (Class) class
{
  return class_getSuperclass(object_getClass(self));
}

- (void) setValue: (id)anObject forKey: (NSString*)aKey
{
  Class		c = [self class];
  void		(*imp)(id,SEL,id,id);

  imp = (void (*)(id,SEL,id,id))[c instanceMethodForSelector: _cmd];

  if ([[self class] automaticallyNotifiesObserversForKey: aKey])
    {
      [self willChangeValueForKey: aKey];
      imp(self,_cmd,anObject,aKey);
      [self didChangeValueForKey: aKey];
    }
  else
    {
      imp(self,_cmd,anObject,aKey);
    }
}

You should never rely on the isa pointer to determine class membership. Instead, you should use the class method to determine the class of an object instance.

我们说的objective-c是动态运行时语言是什么意思（when we call objective c is runtime language what does it mean）

NSString* testObject = [[NSData alloc] init];

[testObject stringByAppendingString:@"string"];  //1
[testObject base64EncodedDataWithOptions:NSDataBase64Encoding64CharacterLineLength]; //2

A variable is dynamically typed when the type of the object it points to is not 
checked at compile time. Objective-C uses the id data type to represent a variable 
that is an object without specifying what sort of object it is. This is referred 
to as dynamic typing.

Dynamic typing contrasts with static typing, in which the system explicitly 
identifies the class to which an object belongs at compile time. Static type 
checking at compile time may ensure stricter data integrity, but in exchange for 
that integrity, dynamic typing gives your program much greater flexibility. And 
through object introspection (for example, asking a dynamically typed, anonymous 
object what its class is), you can still verify the type of an object at runtime
and thus validate its suitability for a particular operation.

[1][Test instancesRespondToSelector:@selector(objFunc)];//YES
  [2][Test instancesRespondToSelector:@selector(classFunc)];//NO

  [3][Test respondsToSelector:@selector(objFunc)];//NO
  [4][Test respondsToSelector:@selector(classFunc)];//YES

  [5][test respondsToSelector:@selector(objFunc)];//YES
  [6][test respondsToSelector:@selector(classFunc)];//NO

SEL funcID = @select(func)；// 这个注册事件回调时常用，将方法转成SEL类型
SEL funcID = NSSelectorFromString(@"func"); // 根据方法名得到方法标识
NSString *funcName = NSStringFromSelector(funcID); // 根据SEL类型得到方法名字符串

Dynamic binding is determining the method to invoke at runtime instead of at 
compile time. Dynamic binding is also referred to as late binding. In Objective-C, 
all methods are resolved dynamically at runtime. The exact code executed is 
determined by both the method name (the selector) and the receiving object.

Dynamic binding enables polymorphism. For example, consider a collection of
 objects including Dog, Athlete, and ComputerSimulation. Each object has its own 
 implementation of a run method. In the following code fragment, the actual code 
 that should be executed by the expression [anObject run] is determined at 
 runtime. The runtime system uses the selector for the method run to identify the
  appropriate method in whatever the class of anObject turns out to be.

NSArray *anArray = [NSArray arrayWithObjects:aDog, anAthlete, aComputerSimulation, nil];

id anObject = [anArray objectAtIndex:(random()/pow(2, 31)*3)];

[anObject run];

2017-03-01 18:25:05.117 Hello[72930:2179247] <NSObject: 0x61000000a6e0>
2017-03-01 18:25:05.118 Hello[72930:2179247] (null)
2017-03-01 18:25:05.150 Hello[72930:2179247] (null)

__weak id reference = nil;
- (void)viewDidLoad {
    [super viewDidLoad];
    NSString *str = @"123456";
    reference = str;
    NSLog(@"%@", reference);
    // str是一个autorelease对象，设置一个weak的引用来观察它
}
- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    NSLog(@"%@", reference);
}
- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    NSLog(@"%@", reference);
}

2017-03-02 10:31:41.475 Hello[75799:2289076] 123456
2017-03-02 10:31:41.475 Hello[75799:2289076] 123456
2017-03-02 10:31:41.492 Hello[75799:2289076] 123456

#define Log(_var) ({ NSString *name = @#_var; NSLog(@"%@: %@ -> %p : %@ ", name, [_var class], _var, _var); })

__weak id references = nil;
- (void)viewDidLoad {
    [super viewDidLoad];

    NSString *str = @"123456789";
    NSString *bstr = [NSString stringWithFormat:@"123456789"];
    NSString *cstr = [str mutableCopy];
    NSNumber *xnum = [NSNumber numberWithInteger:8];
    references = cstr;
    self.num = xnum;
    Log(str);
    Log(bstr);
    Log(cstr);
}

2017-03-02 14:25:33.478 Hello[78655:2389611] str: __NSCFConstantString -> 0x10b52c308 : 123456789 
2017-03-02 14:25:33.478 Hello[78655:2389611] bstr: NSTaggedPointerString -> 0xa1ea1f72bb30ab19 : 123456789 
2017-03-02 14:25:33.478 Hello[78655:2389611] cstr: __NSCFString -> 0x608000074d00 : 123456789

Tip:
NSString *str = @“xxx”;其实创建了两个对象，指针str也是一个对象储存在栈内存中，由系统负责释放

__weak id references = nil;
- (void)viewDidLoad {
    [super viewDidLoad];
   
    NSString *str = @"12345678900";
    NSString *bstr = [NSString stringWithFormat:@"12345678900"];
    NSString *cstr = [str mutableCopy];
    references = bstr; //1
    // references = cstr;//2
    Log(str);
    Log(bstr);
    Log(cstr);
}
- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    NSLog(@"%@", references);
}
- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    NSLog(@"%p", references);
}

2017-03-02 17:27:12.637 Hello[81349:2493571] 12345678900
2017-03-02 17:27:13.187 Hello[81349:2493571] 0x0

2017-03-02 17:28:40.768 Hello[81398:2494728] (null)
2017-03-02 17:28:41.311 Hello[81398:2494728] 0x0

When returning from such a function or method, ARC retains the value at the point of evaluation of the return statement, then leaves all local scopes, and then balances out the retain while ensuring that the value lives across the call boundary. In the worst case, this may involve an autorelease, but callers must not assume that the value is actually in the autorelease pool.

ARC performs no extra mandatory work on the caller side, although it may elect to do something to shorten the lifetime of the returned value.

__weak id references = nil;
- (void)viewDidLoad {
    [super viewDidLoad];
    NSObject *object = [NSObject new];
    NSArray *arr = @[object]; //1
    //NSArray *arr = [NSArray arrayWithObjects:object, nil]; //2
    references = arr;
}
- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    NSLog(@"%@", references);
}
- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    NSLog(@"%p", references);
}

2017-03-02 17:48:19.667 Hello[81612:2505809] (null)
2017-03-02 17:48:20.216 Hello[81612:2505809] 0x0

2017-03-02 17:50:04.964 Hello[81664:2507484] (
    "<NSObject: 0x618000200480>"
)
2017-03-02 17:50:05.508 Hello[81664:2507484] 0x0

-bash: nvm: command not found

curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.25.4/install.sh | bash

source ~/.nvm/nvm.sh

ls -a ~/.nvm/versions/node

nvm use v4.4.5

hexo clean && hexo g && hexo d