Message forwarding is applicable to the implementation of operations to another class

1. -(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector  
2. {  
3.     NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];  
4.     if (!signature) {  
5.         signature = [self.displayLabel methodSignatureForSelector:aSelector];  
6.     }  
7.     return signature;  
8. }  
9.   
10. -(void)forwardInvocation:(NSInvocation *)anInvocation  
11. {  
12.     SEL selector = [anInvocation selector];  
13.     if ([self.displayLabel respondsToSelector:selector]) {  
14.         [anInvocation invokeWithTarget:self.displayLabel];  
15.     }  
16. }  

In the above example, a UIViewController contains the uilabel attribute displayLabel. If the UIViewController instance calls the [instance setText:@"string"] method, because the class does not implement the setText: method, through the above two lines of code, it will be forwarded and implemented by displayLabel.

Another: for the use of message forwarding, the following category expansion solves the crash caused by the operation of NSNull object (often encountered when the network data return is empty)

1. - (NSMethodSignature*)methodSignatureForSelector:(SEL)selector  
2. {  
3.     NSMethodSignature* signature = [super methodSignatureForSelector:selector];  
4.     if (!signature) {  
5.         for (NSObject *object in NSNullObjects) {  
6.             signature = [object methodSignatureForSelector:selector];  
7.             if (signature) {  
8.                 break;  
9.             }  
10.         }  
11.           
12.     }  
13.     return signature;  
14. }  
15.   
16. - (void)forwardInvocation:(NSInvocation *)anInvocation  
17. {  
18.     SEL aSelector = [anInvocation selector];  
19.       
20.     for (NSObject *object in NSNullObjects) {  
21.         if ([object respondsToSelector:aSelector]) {  
22.             [anInvocation invokeWithTarget:object];  
23.             return;  
24.         }  
25.     }  
26.       
27.     [self doesNotRecognizeSelector:aSelector];  
28. }  

For details, please refer to: https://github.com/caigee/iosdev_sample Lower
ClassForward