NSObject中forwardInvocation消息重定向

NSObject是大多數系統 api的基類，今天打開頭檔案看了一下，原來它還有很多現在都還不了解的地方。今天簡單介紹一下它的forwardInvocation功能。

在obj-c中我們可以向一個執行個體發送消息，相當于c/c++ java中的方法調用，隻不過在這兒是說發送消息，執行個體收到消息後會進行一些處理。比如我們想調用一個方法，便向這個執行個體發送一個消息，執行個體收到消息後，如果能respondsToSelector，那麼就會調用相應的方法。如果不能respond一般情況下會crash。今天要的，就是不讓它crash。

首先說一下向一個執行個體發送一個消息後，系統是處理的流程：

1. 發送消息如：[self startwork] 

2. 系統會check是否能response這個消息

3. 如果能response則調用相應方法，不能則抛出異常

在第二步中，系統是如何check執行個體是否能response消息呢？如果執行個體本身就有相應的response,那麼就會相應之，如果沒有系統就會發出methodSignatureForSelector消息，尋問它這個消息是否有效？有效就傳回對應的方法位址之類的，無效則傳回nil。如果是nil就會crash， 如果不是nil接着發送forwardInvocation消息。

是以我們在重寫methodSignatureForSelector的時候就人工讓其傳回有效執行個體。  文字說不清，還是用代碼說明

我們定義了這樣一個類

@interface TargetProxy : NSProxy {
    id realObject1;
    id realObject2;
}
 
- (id)initWithTarget1:(id)t1 target2:(id)t2;
 
@end
           

實作：

@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
           

現在我們還用這個類，注意向它發送的消息：

// 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);
    }
           

運作的結果是：

count should be 1, it is:  1

Appending successful.

TargetProxy聲明中是沒有appendString與addObject消息的，在這兒卻可以正常發送，不crash，原因就是發送消息的時候，如果原本類沒有這個消息響應的時候，轉向詢問methodSignatureForSelector，接着在forwardInvocation将消息重定向。 上面也說了多參數的消息是不能重定向的。這我還沒測過。

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