在iOS9.0以前，通知中心對觀察者對象進行unsafe_unretained引用，當被引用的對象釋放時不會自動置爲nil，指針仍指向該內存空間，形成了野指針，引發EXC_BAD_ACCESS崩潰。git

在iOS9以後，不對觀察對象進行移除也不會形成崩潰，這是由於通知中心對觀察者作了弱引用，對象銷燬時會對對象的指針置空。在代碼編寫過程當中，基於嚴謹性，最好在註冊了通知以後，也要removeObserver。github

那麼通知中心是怎麼實現對觀察者的引用呢？要了解這一點，咱們先看一下通知的實現機制。因爲蘋果對Foundation源碼是不開源的，咱們具體就參考一下GNUStep的源碼實現。GNUStep的源碼地址爲：github.com/gnustep/lib…, 具體源碼能夠進行查看。數組

(void) addObserver: (id)observer selector: (SEL)selector name: (NSString*)name object: (id)object

添加觀察者以後，會建立Observation對象，存儲了觀察者，SEL等; Observation的結構爲：緩存

typedef	struct	Obs {
  id		observer;	/* Object to receive message.	*/
  SEL		selector;	/* Method selector.		*/
  struct Obs	*next;		/* Next item in linked list.	*/
  int		retained;	/* Retain count for structure.	*/
  struct NCTbl	*link;		/* Pointer back to chunk table	*/
} Observation;


typedef struct NCTbl {
  Observation		*wildcard;	/* Get ALL messages.		*/
  GSIMapTable		nameless;	/* Get messages for any name.	*/
  GSIMapTable		named;		/* Getting named messages only.	*/
  unsigned		lockCount;	/* Count recursive operations.	*/
  NSRecursiveLock	*_lock;		/* Lock out other threads.	*/
  Observation		*freeList;
  Observation		**chunks;
  unsigned		numChunks;
  GSIMapTable		cache[CACHESIZE];
  unsigned short	chunkIndex;
  unsigned short	cacheIndex;
} NCTable;

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NSNOtifocation維護了GSIMapTable表的結構，用於存儲Observation，分別是nameless,named,cache,nameless用於存儲沒有傳入名字的通知，named存儲傳入了名字的通知，cache用於快速緩存。這裏值得注意的是，nameless和named雖然同爲hash表，可是其結構以下：安全

在nameless表中：
GSIMapTable的結構以下
object : Observation
object : Observation
object : Observation

----------------------------
在named表中：
GSIMapTable結構以下:
name : maptable
name : maptable
name : maptable

maptable的結構以下
object : Observation
object : Observation
object : Observation

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執行以下代碼：bash

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleNotification) name:@"Test" object:nil];

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleNotification) name:@"Test" object:@"12"];

[[NSNotificationCenter defaultCenter] postNotificationName:@"Test" object:@"123" userInfo:nil];

通知發送以後，相同name,object相同或者object爲nil的觀察者，會接受到通知。

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通知是同步執行的，發送通知所在的線程和通知接收回調處理在同一線程進行處理，測試代碼以下markdown

NSLog(@"註冊通知 %@", [NSThread currentThread]);

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleNotification) name:@"Test" object:@"123"];

dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"發送通知 %@", [NSThread currentThread]);
        [[NSNotificationCenter defaultCenter] postNotificationName:@"Test" object:@"123" userInfo:nil];
});
    
- (void)handleNotification {
    NSLog(@"處理通知 %@", [NSThread currentThread]);
}

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執行結果：數據結構

註冊通知 <NSThread: 0x2821f4a80>{number = 1, name = main}less

發送通知 <NSThread: 0x2821f14c0>{number = 5, name = (null)}async

處理通知 <NSThread: 0x2821f14c0>{number = 5, name = (null)}**

那麼map表是怎麼對observer進行弱引用的呢，默認狀況下，數組對數組中的元素都是進行強持有，那麼咱們怎麼能夠實現弱引用呢？首先看一下Swift中對集合類型元素的弱引用

定義一個類：

class Pencil {
    var type: String
    var price: Double
    
    init(_ type: String, _ price: Double) {
        self.type = type
        self.price = price
    }
    
}
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class CXDWeakArray: NSObject {

    func testRetainCount()  {
        
        print("測試開始 \(CFGetRetainCount(Pencil("2B", 1.0) as CFTypeRef))")
        //測試開始  1
        
        let pencil2B = Pencil("2B", 1.0)
        let pencilHB = Pencil("HB", 2.0)
                
        print("對象初始化 \(CFGetRetainCount(pencil2B as CFTypeRef))")
        print("對象初始化 \(CFGetRetainCount(pencilHB as CFTypeRef))")
        //對象初始化  2
        //對象初始化   2
        
        let pencilBox = [pencil2B, pencilHB]
        
        print("強引用數組 \(CFGetRetainCount(pencil2B as CFTypeRef))")
        print("強引用數組 \(CFGetRetainCount(pencilHB as CFTypeRef))")
        //強引用數組   3
        //強引用數組   3
        
        }
}
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WeakArray類：

WeakArray<Element: AnyObject> {
    private var items: [WeakBox<Element>] = []
    
    init(_ elements: [Element]) {
        items = elements.map{ WeakBox($0) }
    }
}

extension WeakArray: Collection {
    var startIndex: Int { return items.startIndex }
    var endIndex: Int { return items.endIndex }
    
    subscript(_ index: Int) -> Element? {
        return items[index].unbox
    }
    
    func index(after idx: Int) -> Int {
        return items.index(after: idx)
    }
    
}
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測試代碼

let weakPencilBox1 = WeakArray([pencil2B, pencilHB])
        
        print("弱引用數組 \(CFGetRetainCount(pencil2B as CFTypeRef))")
        print("弱引用數組 \(CFGetRetainCount(pencilHB as CFTypeRef))")
        //弱引用數組   3
        //弱引用數組   3
        
        let firstElement = weakPencilBox1.filter {
            $0 != nil
        }.first
        
        print("元素類型 \(firstElement!!.type)")
        //元素類型   2B
        
        
        print("結束 \(CFGetRetainCount(pencil2B as CFTypeRef))")
        print("結束 \(CFGetRetainCount(pencilHB as CFTypeRef))")
        //結束   4
        //結束   3
        /**
         // 4 3 Note: 這裏的 4 是由於 firstElement 持有（Retain）了 pencil2B，致使其引用計數增 1
         */
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NSPointerArray

NSPointerArray是Array的一個替代品，主要區別在於它不存儲對象，而是存儲對象的指針這種類型的數組能夠管理弱引用也能夠管理強引用，取決於它是如何被初始化的。

NSPointerArray.weakObjects()

NSPointerArray.strongObjects()
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let weakPencilBox2 = NSPointerArray.weakObjects()
        
        let pencil2BPoiter =  Unmanaged.passUnretained(pencil2B).toOpaque()
        
        let pencilHBPoiter = Unmanaged.passUnretained(pencilHB).toOpaque()
        print("PoiterArray初始化 \(CFGetRetainCount(pencil2B as CFTypeRef))")
        print("PoiterArray初始化 \(CFGetRetainCount(pencilHB as CFTypeRef))")
        //PoiterArray初始化   4
        //PoiterArray初始化   3
        
        weakPencilBox2.addPointer(pencil2BPoiter)
        weakPencilBox2.addPointer(pencilHBPoiter)
        
        print("PoiterArray結束 \(CFGetRetainCount(pencil2B as CFTypeRef))")
        print("PoiterArray結束 \(CFGetRetainCount(pencilHB as CFTypeRef))")
        //PoiterArray結束   4
        //PoiterArray結束   3
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使用NSPointerArray對於存儲對象和保持弱引用很是有用，可是它不是類型安全的，編譯器沒法推斷隱含在NSPointerArray內部的對象的類型，由於它使用的是AnyObject型對象的指針。

不僅是數組，集合類型的數據結構對其中的元素默認都是強引用。

NSHashTable

//NSHashTable -NSSet
        let weakPencilSet = NSHashTable<Pencil>(options: .weakMemory)
        weakPencilSet.add(pencil2B)
        weakPencilSet.add(pencilHB)
        
        
        print("NSHashTable Set \(CFGetRetainCount(pencil2B as CFTypeRef))")
        print("NSHashTable Set \(CFGetRetainCount(pencilHB as CFTypeRef))")
        //NSHashTable Set   4
        //NSHashTable Set   3
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NSMapTable

class Eraser {
    var type: String
    
    init(_ type: String) {
        self.type = type
    }
}

     //NSMapTable -- NSDictionary
        let weakPencilDict = NSMapTable<Eraser, Pencil>(keyOptions: .strongMemory, valueOptions: .weakMemory)
        
        let paintingEraser = Eraser("Painting")
        
        weakPencilDict.setObject(pencil2B, forKey: paintingEraser)
        
        print("NSMapTable NSDictionary \(CFGetRetainCount(pencil2B as CFTypeRef))")
        print("NSMapTable NSDictionary \(CFGetRetainCount(pencilHB as CFTypeRef))")
        
        //NSMapTable NSDictionary   4
        //NSMapTable NSDictionary   3
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iOS通知底層實現原理

WeakArray類：

NSPointerArray

NSHashTable

NSMapTable