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
@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
// 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:
https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtForwarding.html#//apple_ref/doc/uid/TP40008048-CH105-SW2
http://developer.apple.com/library/mac/#samplecode/ForwardInvocation/Listings/main_m.html#//apple_ref/doc/uid/DTS40008833-main_m-DontLinkElementID_4
http://cocoawithlove.com/2008/03/construct-nsinvocation-for-any-message.html