NSObject中forwardInvocation消息重定向

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:

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