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。