blocks|key|127600|text|要避免崩溃，请在使用该协议的Swift类中实现观察者方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|127601|实现必须在Swift类本身中，而不仅仅是协议扩展，因为选择器总是引用Objective-C方法，而协议扩展中的函数不能用作Objective-C选择器。但是，如果Swift类继承自Objective-C类，则Swift类中的方法可用作Objective-C选择器|127602|“If+your+Swift+class+inherits+from+an+Objective-C+class,+all+of+the+methods+and+properties+in+the+class+are+available+as+Objective-C+selectors.”|offset|length|127603|此外，在Xcode7.1中，当在addObserver调用中将self指定为观察者时，必须将其向下转换为AnyObject。|style|CODE|127604|protocol+A+{
++++func+doA()
}

extension+A+{
++++func+registerForNotification()+{
++++++++NSNotificationCenter.defaultCenter().addObserver(self+as!+AnyObject,
++++++++++++selector:+Selector("keyboardDidShow:"),
++++++++++++name:+UIKeyboardDidShowNotification,
++++++++++++object:+nil)
++++}

++++func+keyboardDidShow(notification:+NSNotification)+{
++++++++print("will+not+appear")
++++}
}

class+ViewController:+UIViewController,+A+{
++++override+func+viewDidLoad()+{
++++++++super.viewDidLoad()
++++++++self.registerForNotification()
++++++++triggerKeyboard()
++++}

++++func+triggerKeyboard(){
++++++++//+Some+code+that+makes+the+keyboard+appear
++++}

++++func+doA(){
++++}

++++func+keyboardDidShow(notification:+NSNotification)+{
++++++++print("got+the+notification+in+the+class")
++++}
}|code-block|syntax|javascript|127605|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.apple.com/library/ios/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithObjective-CAPIs.html#//apple_ref/doc/uid/TP40014216-CH4-ID35^0|0|0|0|40|0|0|G|B|V|4|1G|9|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|10|8|@]|9|@$F|11|G|12|1|13]]|A|$]]|$1|H|3|I|5|6|7|14|8|@$F|15|G|16|J|K]|$F|17|G|18|J|K]|$F|19|G|1A|J|K]]|9|@]|A|$]]|$1|L|3|M|5|N|7|1B|8|@]|9|@]|A|$O|P]]|$1|Q|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

To avoid the crash, implement the observer method in the Swift class that uses the protocol. 

The implementation has to be in the Swift class itself, not just the protocol extension, because a selector always refers to an Objective-C method, and a function within a protocol extension is not available as an Objective-C selector. Yet methods from a Swift class are available as Objective-C selectors if the Swift class inherits from an Objective-C class

<a href="https://developer.apple.com/library/ios/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithObjective-CAPIs.html#//apple_ref/doc/uid/TP40014216-CH4-ID35" rel="nofollow">“If your Swift class inherits from an Objective-C class, all of the methods and properties in the class are available as Objective-C selectors.”</a>

Also, in Xcode 7.1, <code>self</code> has to be downcast to <code>AnyObject</code> when specifying it as the observer in the <code>addObserver</code> call.

<pre><code>protocol A {
 func doA()
}

extension A {
 func registerForNotification() {
 NSNotificationCenter.defaultCenter().addObserver(self as! AnyObject,
 selector: Selector("keyboardDidShow:"),
 name: UIKeyboardDidShowNotification,
 object: nil)
 }

 func keyboardDidShow(notification: NSNotification) {
 print("will not appear")
 }
}

class ViewController: UIViewController, A {
 override func viewDidLoad() {
 super.viewDidLoad()
 self.registerForNotification()
 triggerKeyboard()
 }

 func triggerKeyboard(){
 // Some code that makes the keyboard appear
 }

 func doA(){
 }

 func keyboardDidShow(notification: NSNotification) {
 print("got the notification in the class")
 }
}
</code></pre>

blocks|key|2685176|text|我通过实现NSNotificationCenter的较新的-+addObserverForName:object:queue:usingBlock:方法并直接调用该方法解决了类似的问题。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2685177|extension+A+where+Self:+UIViewController++{
++++func+registerForNotification()+{
++++++++NSNotificationCenter.defaultCenter().addObserverForName(UIKeyboardDidShowNotification,+object:+nil,+queue:+nil)+{+[unowned+self]+notification+in
++++++++++++self.keyboardDidShow(notification)
++++++++}
++++}

++++func+keyboardDidShow(notification:+NSNotification)+{
++++++++print("This+will+get+called+in+protocol+extension.")
++++}
}|code-block|syntax|javascript|2685178|此示例将导致在协议扩展中调用keyboardDidShow。|2685179|entityMap^0|5|K|T|19|0|0|E|F|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

I solved a similar problem by implementing the newer <code>- addObserverForName:object:queue:usingBlock:</code> method of <code>NSNotificationCenter</code> and calling the method directly.

<pre><code>extension A where Self: UIViewController {
 func registerForNotification() {
 NSNotificationCenter.defaultCenter().addObserverForName(UIKeyboardDidShowNotification, object: nil, queue: nil) { [unowned self] notification in
 self.keyboardDidShow(notification)
 }
 }

 func keyboardDidShow(notification: NSNotification) {
 print("This will get called in protocol extension.")
 }
}
</code></pre>

This example will cause <code>keyboardDidShow</code> to be called in the protocol extension.

blocks|key|2897089|text|在Swift中使用选择器要求您的具体类必须继承自NSObject。要在协议扩展中强制执行这一点，您应该使用where。例如：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2897090|protocol+A+{
++++func+doA()
}

extension+A+where+Self:+NSObject+{
++func+registerForNotification()+{
++++++NSNotificationCenter.defaultCenter().addObserver(self,+selector:+Selector("keyboardDidShow:"),+name:+UIKeyboardDidShowNotification,+object:+nil)
++}

++func+keyboardDidShow(notification:+NSNotification)+{

++}
}|code-block|syntax|javascript|2897091|entityMap^0|1H|5|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

Using selectors in Swift requires that your concrete class must inherit from NSObject. To enforce this in a protocol extension, you should use <code>where</code>. For example:

<pre><code>protocol A {
 func doA()
}

extension A where Self: NSObject {
 func registerForNotification() {
 NSNotificationCenter.defaultCenter().addObserver(self, selector: Selector("keyboardDidShow:"), name: UIKeyboardDidShowNotification, object: nil)
 }

 func keyboardDidShow(notification: NSNotification) {

 }
}
</code></pre>

blocks|key|125098|text|除了詹姆斯·保兰托尼奥的回答。可以使用关联的对象来实现unregisterForNotification方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|125099|var+pointer:+UInt8+=+0

extension+NSObject+{
++++var+userInfo:+[String:+Any]+{
++++++++get+{
++++++++++++if+let+userInfo+=+objc_getAssociatedObject(self,+&pointer)+as?+[String:+Any]+{
++++++++++++++++return+userInfo
++++++++++++}
++++++++++++self.userInfo+=+[String:+Any]()
++++++++++++return+self.userInfo
++++++++}
++++++++set(newValue)+{
++++++++++++objc_setAssociatedObject(self,+&pointer,+newValue,+.OBJC_ASSOCIATION_RETAIN)
++++++++}
++++}
}

protocol+A+{}
extension+A+where+Self:+UIViewController+{

++++var+defaults:+NotificationCenter+{
++++++++get+{
++++++++++++return+NotificationCenter.default
++++++++}
++++}

++++func+keyboardDidShow(notification:+Notification)+{
++++++++//+Keyboard+did+show
++++}

++++func+registerForNotification()+{
++++++++userInfo["didShowObserver"]+=+defaults.addObserver(forName:+.UIKeyboardDidShow,+object:+nil,+queue:+nil,+using:+keyboardDidShow)
++++}

++++func+unregisterForNotification()+{
++++++++if+let+didShowObserver+=+userInfo["didShowObserver"]+as?+NSObjectProtocol+{
++++++++++++defaults.removeObserver(didShowObserver,+name:+.UIKeyboardDidShow,+object:+nil)
++++++++}
++++}
}|code-block|syntax|javascript|125100|entityMap^0|R|P|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

In addition to James Paolantonio's answer. A <code>unregisterForNotification</code> method can be implemented using associated objects.

<pre class="lang-Swift prettyprint-override"><code>var pointer: UInt8 = 0

extension NSObject {
 var userInfo: [String: Any] {
 get {
 if let userInfo = objc_getAssociatedObject(self, &amp;pointer) as? [String: Any] {
 return userInfo
 }
 self.userInfo = [String: Any]()
 return self.userInfo
 }
 set(newValue) {
 objc_setAssociatedObject(self, &amp;pointer, newValue, .OBJC_ASSOCIATION_RETAIN)
 }
 }
}

protocol A {}
extension A where Self: UIViewController {

 var defaults: NotificationCenter {
 get {
 return NotificationCenter.default
 }
 }

 func keyboardDidShow(notification: Notification) {
 // Keyboard did show
 }

 func registerForNotification() {
 userInfo["didShowObserver"] = defaults.addObserver(forName: .UIKeyboardDidShow, object: nil, queue: nil, using: keyboardDidShow)
 }

 func unregisterForNotification() {
 if let didShowObserver = userInfo["didShowObserver"] as? NSObjectProtocol {
 defaults.removeObserver(didShowObserver, name: .UIKeyboardDidShow, object: nil)
 }
 }
}
</code></pre>

blocks|key|126110|text|我使用NSObjectProtocol解决了它，如下所示：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|126111|@objc+protocol+KeyboardNotificaitonDelegate:+NSObjectProtocol+{
func+keyboardWillBeShown(notification:+NSNotification)
func+keyboardWillBeHidden(notification:+NSNotification)
}

extension+KeyboardNotificaitonDelegate+{

func+registerForKeyboardNotifications()+{
++++//Adding+notifies+on+keyboard+appearing
++++NotificationCenter.default.addObserver(self,+selector:+#selector(keyboardWillBeShown(notification:)),+name:+NSNotification.Name.UIKeyboardWillShow,+object:+nil)
++++NotificationCenter.default.addObserver(self,+selector:+#selector(keyboardWillBeHidden(notification:)),+name:+NSNotification.Name.UIKeyboardWillHide,+object:+nil)
}

func+deregisterFromKeyboardNotifications()+{
++++//Removing+notifies+on+keyboard+appearing
++++NotificationCenter.default.removeObserver(self,+name:+NSNotification.Name.UIKeyboardWillShow,+object:+nil)
++++NotificationCenter.default.removeObserver(self,+name:+NSNotification.Name.UIKeyboardWillHide,+object:+nil)
}
}|code-block|syntax|javascript|126112|entityMap^0|3|G|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

I solved it using <code>NSObjectProtocol</code> as below,

<pre><code>@objc protocol KeyboardNotificaitonDelegate: NSObjectProtocol {
func keyboardWillBeShown(notification: NSNotification)
func keyboardWillBeHidden(notification: NSNotification)
}

extension KeyboardNotificaitonDelegate {

func registerForKeyboardNotifications() {
 //Adding notifies on keyboard appearing
 NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillBeShown(notification:)), name: NSNotification.Name.UIKeyboardWillShow, object: nil)
 NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillBeHidden(notification:)), name: NSNotification.Name.UIKeyboardWillHide, object: nil)
}

func deregisterFromKeyboardNotifications() {
 //Removing notifies on keyboard appearing
 NotificationCenter.default.removeObserver(self, name: NSNotification.Name.UIKeyboardWillShow, object: nil)
 NotificationCenter.default.removeObserver(self, name: NSNotification.Name.UIKeyboardWillHide, object: nil)
}
}
</code></pre>

Let's consider the following code:

<pre><code>protocol A {
 func doA()
}

extension A {
 func registerForNotification() {
 NSNotificationCenter.defaultCenter().addObserver(self, selector: Selector("keyboardDidShow:"), name: UIKeyboardDidShowNotification, object: nil)
 }

 func keyboardDidShow(notification: NSNotification) {

 }
}
</code></pre>

Now look at a UIViewController subclass that implements A:

<pre><code>class AController: UIViewController, A {
 override func viewDidLoad() {
 super.viewDidLoad()
 self.registerForNotification()
 triggerKeyboard()
 }

 func triggerKeyboard() {
 // Some code that make key board appear
 }

 func doA() {
 }
}
</code></pre>

But surprisingly this crashes with an error: 

<blockquote>
 keyboardDidShow:]: unrecognized selector sent to instance
 0x7fc97adc3c60
</blockquote>

So should I implement the observer in the view controller itself? Can't it stay in the extension? 

Following things already tried.

making A a class protocol.
Adding keyboardDidShow to protocol itself as signature.

<pre><code>protocol A:class {
 func doA()
 func keyboardDidShow(notification: NSNotification)
}
</code></pre>

Swift make protocol extension a Notification observer

Objective-C

Swift

让我们考虑一下下面的代码：protocol A {    func doA()}extension A {  func registerForNotification() {      NSNotificationCenter.defaultCenter().addObserver(self, selector: Se...

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