NSObject是大多数系统 api的基类，今天打开头文件看了一下，原来它还有很多现在都还不了解的地方。今天简单介绍一下它的forwardInvocation功能。
在obj-c中我们可以向一个实例发送消息，相当于c/c++ java中的方法调用，只不过在这儿是说发送消息，实例收到消息后会进行一些处理。比如我们想调用一个方法，便向这个实例发送一个消息，实例收到消息后，如果能respondsToSelector，那么就会调用相应的方法。如果不能respond一般情况下会crash。今天要的，就是不让它crash。
首先说一下向一个实例发送一个消息后，系统是处理的流程：
1. 发送消息如：[self startwork]
2. 系统会check是否能response这个消息
3. 如果能response则调用相应方法，不能则抛出异常
在第二步中，系统是如何check实例是否能response消息呢？如果实例本身就有相应的response,那么就会相应之，如果没有系统就会发出methodSignatureForSelector消息，寻问它这个消息是否有效？有效就返回对应的方法地址之类的，无效则返回nil。如果是nil就会crash， 如果不是nil接着发送forwardInvocation消息。
所以我们在重写methodSignatureForSelector的时候就人工让其返回有效实例。 文字说不清，还是用代码说明
我们定义了这样一个类
@interface TargetProxy : NSProxy {
id realObject1;
id realObject2;
}
- (id)initWithTarget1:(id)t1 target2:(id)t2;
@end
实现：
@implementation TargetProxy
- (id)initWithTarget1:(id)t1 target2:(id)t2 {
realObject1 = [t1 retain];
realObject2 = [t2 retain];
return self;
}
- (void)dealloc {
[realObject1 release];
[realObject2 release];
[super dealloc];
}
// The compiler knows the types at the call site but unfortunately doesn't
// leave them around for us to use, so we must poke around and find the types
// so that the invocation can be initialized from the stack frame.
// Here, we ask the two real objects, realObject1 first, for their method
// signatures, since we'll be forwarding the message to one or the other
// of them in -forwardInvocation:. If realObject1 returns a non-nil
// method signature, we use that, so in effect it has priority.
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
NSMethodSignature *sig;
sig = [realObject1 methodSignatureForSelector:aSelector];
if (sig) return sig;
sig = [realObject2 methodSignatureForSelector:aSelector];
return sig;
}
// Invoke the invocation on whichever real object had a signature for it.
- (void)forwardInvocation:(NSInvocation *)invocation {
id target = [realObject1 methodSignatureForSelector:[invocation selector]] ? realObject1 : realObject2;
[invocation invokeWithTarget:target];
}
// Override some of NSProxy's implementations to forward them...
- (BOOL)respondsToSelector:(SEL)aSelector {
if ([realObject1 respondsToSelector:aSelector]) return YES;
if ([realObject2 respondsToSelector:aSelector]) return YES;
return NO;
}
@end
现在我们还用这个类，注意向它发送的消息：
// Create a proxy to wrap the real objects. This is rather
// artificial for the purposes of this example -- you'd rarely
// have a single proxy covering two objects. But it is possible.
id proxy = [[TargetProxy alloc] initWithTarget1:string target2:array];
// Note that we can't use appendFormat:, because vararg methods
// cannot be forwarded!
[proxy appendString:@"This "];
[proxy appendString:@"is "];
[proxy addObject:string];
[proxy appendString:@"a "];
[proxy appendString:@"test!"];
NSLog(@"count should be 1, it is: %d", [proxy count]);
if ([[proxy objectAtIndex:0] isEqualToString:@"This is a test!"]) {
NSLog(@"Appending successful.");
} else {
NSLog(@"Appending failed, got: '%@'", proxy);
}
运行的结果是：
count should be 1, it is: 1
Appending successful.
TargetProxy声明中是没有appendString与addObject消息的，在这儿却可以正常发送，不crash，原因就是发送消息的时候，如果原本类没有这个消息响应的时候，转向询问methodSignatureForSelector，接着在forwardInvocation将消息重定向。 上面也说了多参数的消息是不能重定向的。这我还没测过。
reference:
http://cocoawithlove.com/2008/03/construct-nsinvocation-for-any-message.html