问题描述
到目前为止我所学到的:在Objective-C中，您可以向任何对象发送任何消息.如果对象确实实现了正确的方法，它将被执行，否则什么都不会发生.这是因为在发送消息之前，Objective-C 会执行 respondsToSelector.
我希望到目前为止我是对的.
我做了一个小程序来测试每次移动滑块时调用的动作.同样为了测试，我将发送者设置为 NSButton 但实际上它是一个 NSSlider.现在我询问对象是否会响应 setAlternateTitle.虽然 NSButton 会做而 NSSlider 不会.如果我运行代码并自己执行 respondsToSelector ，它会告诉我对象不会响应该选择器.如果我测试其他类似 intValue 的东西，它会响应.所以到目前为止我的代码很好.
- (IBAction)sliderDidMove:(id)sender{NSButton *slider = 发件人；BOOL 响应 =[slider RespondsToSelector:@selector(setAlternateTitle)];如果(响应 == 是){NSLog(@"YES");}别的{NSLog(@"NO");}[slider setAlternateTitle:@"Hello World"];}但是当我真正发送 setAlternateTitle 消息时，程序会崩溃，我不确定为什么.发送消息之前不应该做一个respondsToSelector吗?
首先，方法的名称(其选择器)包括所有子部分和冒号字符，正如 mvds 所说.
其次，方法 -respondsToSelector: 不是由运行时调用的，它通常由用户调用(例如，您自己或想要知道委托是否响应的 API)协议的可选方法).
当你向一个对象发送消息时，运行时会在该对象的类中寻找该方法的实现(通过对象的isa指针).它等同于发送 -respondsToSelector: 尽管消息本身没有被调度.如果在类或其超类中找到该方法的实现，则会使用您传入的所有参数调用它.
如果不是，那么运行时会给消息第二次执行的机会.它将首先发送消息 + (BOOL)resolveInstanceMethod:(SEL)name 到对象的类:此方法允许您在运行时将方法添加到类中:如果此消息返回 YES，这意味着它可以重新发送消息.
如果不是它给消息第三次执行机会，它发送-(id)forwardingTargetForSelector:(SEL)aSelector和选择器，这个方法可以返回另一个可能能够执行的对象代表实际接收者响应选择器，如果返回的对象可以响应，则执行该方法并返回值，就好像它是由原始消息返回的一样.(注意:此功能从 OS X 10.6 或 iOS 4 开始可用.)
如果返回的对象是 nil 或 self(为了避免无限循环)，运行时会给消息第四次执行方法的机会......它发送消息 - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector 获取方法签名以构建调用.如果提供了一个调用，则通过消息 - (void)forwardInvocation:(NSInvocation *)anInvocation 发送调用.在这个方法中，你可以解析调用并构建其他消息以任何你想要的方式发送到其他目标，然后你可以设置调用的返回值......该值将作为原始消息的返回值.
最后，如果对象没有返回方法签名，那么运行时会发送消息- (void)doesNotRecognizeSelector:(SEL)aSelector 给你的对象，NSObject类中这个方法的实现抛出异常.
From what I have learned so far: In Objective-C you can send any message to any object. If the object does implement the right method it will be executed otherwise nothing will happen. This is because before the message is sent Objective-C will perform respondsToSelector.
I hope I am right so far.
I did a little program for testing where an action is invoked every time a slider is moved. Also for testing I set the sender to NSButton but in fact it is an NSSlider. Now I asked the object if it will respond to setAlternateTitle. While a NSButton will do and NSSlider will not. If I run the code and do respondsToSelector myself it will tell me the object will not respond to that selector. If I test something else like intValue, it will respond. So my code is fine so far.
- (IBAction)sliderDidMove:(id)sender
{
NSButton *slider = sender;
BOOL responds =
[slider respondsToSelector:@selector(setAlternateTitle)];
if(responds == YES)
{
NSLog(@"YES");
}
else
{
NSLog(@"NO");
}
[slider setAlternateTitle:@"Hello World"];
}
But when I actually send the setAlternateTitle message the program will crash and I am not exactly sure why. Shouldn't it do a respondsToSelector before sending the message?
First of all, the name of a method (its selector) includes all subparts and colon characters, as mvds said.
Second of all, the method -respondsToSelector: is not called by the runtime, it's usually called by the user (yourself or APIs that want to know if a delegate, for example, responds to an optional method of the protocol).
When you send a message to an object, the runtime will look for the implementation of the method in the class of the object (through the object's isa pointer). It's equivalent to sending -respondsToSelector: although the message itself is not dispatched. If the implementation of the method is found in the class or in its superclasses, it's called with all the arguments you passed in.
If not, then the runtime gives the message a second chance to be executed. It will start by sending the message + (BOOL)resolveInstanceMethod:(SEL)name to the class of the object: this method allows you to add the method at runtime to the class: if this message returns YES, it means it can redispatch the message.
If not it gives the message a third chance to be executed, it sends - (id)forwardingTargetForSelector:(SEL)aSelector with the selector, this method can return another object that may be able to respond to the selector on behalf of the actual receiver, if the returned object can respond, the method is executed and the value is returned as if it was returned by the original message. (Note: This is available beginning with OS X 10.6 or iOS 4.)
If the returned object is nil or self (to avoid infinite loops), the runtime gives the message a fourth chance to execute the method… It sends the message - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector to get a method signature in order to build an invocation. If one is provided then an invocation is sent through the message - (void)forwardInvocation:(NSInvocation *)anInvocation. In this method you can parse the invocation and build other messages to send to other targets in any ways you want, and then you can set the return value of the invocation… That value will act as the return value of the original message.
Finally, if no method signature is returned by the object, then the runtime sends the message - (void)doesNotRecognizeSelector:(SEL)aSelector to your object, the implementation of this method in NSObject class throws an exception.
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