利用forwardInvocation实现消息重定向

 

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。不过貌似要实现这个要关掉ARC。